Substantial Pattern Change Expected as We Move into December

For those who love cold and the potential for lots of snow some good news exists as we move into and through the first week of December. For a while now, long-range teleconnection patterns have all indicated a major shift within the northern hemispheric pattern would occur which would favor major cold into the eastern United States as well as the possibility for increased storm chances.

Last night's 8-10 day mean 500mb height anomalies from the Euro (image on the left) and GFS (on the right) both indicate an extremely favorable pattern for some significant cold to pour into the eastern United States.  Both models indicate extreme blocking developing across the north-central Atlantic extending into Greenland (indicating a rather robust negative phase of the North Atlantic Oscillation (NAO) along with substantial ridging developing across the far western United States, extending northward into far western Canada and into Alaska (indicating a positive phase of the Pacific North-American teleconnection pattern (PNA) and negative phase of the Eastern Pacific Oscillation (EPO):

The configuration of the northern hemispheric pattern above is extremely favorable for very cold air to pour right into the eastern half of the United States.  There is a great deal of model support as well, especially within the GFS ensembles (unfortunately I don't have permission to post Euro ensemble data) for the AO, NAO, and EPO to tank and for the PNA to sharply rise:

While we recently are in a rather low state of angular atmospheric momentum (AAM) the forecasts are for the low state to relax with AAM hedging more towards the "circle of death".  Typically, when the atmosphere is in a really low AAM state, especially when La Nina forcing is present, this will favor troughs to dig into the western United States with ridging developing and strengthening across the southeastern United States. The extreme high latitude blocking, however, would work to suppress any ridge development across the southeastern United States:

There is also stratospheric support for blocking to develop as computer forecast models all indicate the likelihood for warming to occur within the stratosphere above the far north latitudes which will work to weaken the polar vortex and favor southward displacement. 

All in all the signals have been in place for quite some time on the potential for a rather significant pattern change to occur and one that will favor some major cold to spill into the eastern United States.  At the same time, this projected pattern will also become quite active.  With plenty of cold air available we'll have to certainly watch for snow events! One thing to also keep in mind is, if the blocking becomes as extreme or perhaps even more extreme than modeled, this could lead to a suppressed storm pattern meaning the storm track is well south of New England or well off the coast.  Regardless, for those who like interesting winter and root for snow and cold you may be in luck!

Significant Storm Update

Significant coastal storm has been dropping torrential downpours across the state all day and as the system continues to undergo rapid and further strengthening we are now beginning to introduce the strong wind aspect of the storm.  Numerous weather stations across the state, particularly across the coastline have reported wind gusts as high as 40-50 mph.  Flash flooding, downed trees, and power outages will continue to be the biggest threats as we move into the overnight hours.

As of this writing, our system is already very strong with a 984mb closed low just east of NJ.  Based on the trend of greatest pressure falls, the low should continue moving NNW and push into northern NJ and through SE NY.  Computer forecast models strengthen the system down to the mid-970 mb range which is very, very impressive:

With the last few updates there was big focus on the development and strength of the low-level jet streak and it appears that computer forecast models will be spot on with the extremely anomalous speed of the low-level jet with 925mb (~2500' off the ground) winds expected to exceed 90 knots and the 850mb (~5000' off the ground) low-level jet exceeding 100 knots.  This is extremely, extremely impressive and shows how much wind potential exists just off the ground.  The question just is how much of this wind energy is tapped into and how much of these winds are brought down to the surface.  Currently we see the low-level jet streak off to the SE of the state with 925mb winds as high as 65-75 knots and 850mb winds already as high as 80-90 knots.  Not only will these values continue to increase within the jet streak as the storm continues to strengthen but the core of the low-level jet max may even push into extreme SE parts of CT during the overnight hours.  If this does happen and any of this energy is tapped into, the potential will certainly increase for wind gusts along the coast to exceed 65-70 mph.  This likelihood will certainly exist just off to our east across RI and far SE MA:

The major concern over the next several hours outside of torrential downpours and flash flooding is with regards to the wind potential.  As of this writing, we're continuing to see winds increase, particularly along the coast.  Over 35,000 folks are without power as of this writing and this number should continue to rapidly rise.  The latest run of the Hi-Resolution Rapid Refresh (HRRR) computer forecast model shows very strong damaging wind gust potential, particularly across the CT coast.  the bufkit sounding for Groton, CT at 1:00 AM suggests sustained winds of 47 knots (54 mph) with gusts potentially upwards of 61 knots (70 mph).  Winds of this magnitude would yield scattered to widespread wind damage.  

One of the keys to developing these damaging gusts will be how the low-level temperature profile establishes over these next few hours.  Computer forecast model soundings show a weak unstable layer developing from the surface up through about 875mb which is noted by steepening lapse rates (strong temperature decrease with height) in this layer.  This is quite evident across coastal CT, however, the signal is much more weak inland.  This is one factor which could prevent widespread damaging wind gusts across inland CT.  

Breaking this down:

Torrential Downpours

Torrential downpours continue to batter the state of CT, however, a dry slot is beginning to work into the southwest part of the state now.  Torrential downpours will continue across central and eastern CT for the next few hours before the intensity begins to let up quite a bit.  Rainfall will transition from steady and heavy to more on and off.  Flash flooding continues to be a risk.

Strong to Damaging Wind Gusts

Sustained winds and wind gusts will continue to increase in magnitude over the next several hours as the low-level jet continues to strengthen in response to the rapid deepening of the low pressure center.  The worst of the winds will be confined to the CT coast where wind gusts at the height of the storm could exceed 65-70 mph.  Winds will become sustained as well between 40-45 mph.  Scattered to perhaps widespread tree damage is possible here along with widespread power outages which we are already beginning to see.  Further inland, strong to damaging wind gusts are possible as well, however, these will be more isolated in nature.  This will still yield isolated pockets of tree damage and power outages inland.  This potential will last through 2-3 AM before winds begin to subside.  However, late tomorrow morning the winds begin to rapidly increase again as the system departs.  With strong mixing expected and steep low-level lapse rates, we will be looking at statewide wind gusts upwards of 40-50 mph.  With a very wet ground this as well will lead to pockets of tree damage and further power outages.  

Sunday, October 29th, 2017 Dynamic Storm

Over the past several years there seems to be a trend with major storms and the end of October and in 2017 that trend continues as there is extremely high confidence in a significant storm system which will develop over the next few days.  We are looking at the potential for heavy rainfall which will result in flash flooding, strong to damaging wind gusts of 35-45 mph across the state with gusts upwards of 60+ mph across the southeastern part of the state, the possibility of convection (there may not be lightning associated with it so can't necessarily say thunderstorms) which could result in the locally higher rainfall amounts as well as increasing the likelihood for strong to damaging wind gusts, some coastal flooding, and on Monday when the system departs we will see strong to damaging wind gusts statewide with gusts upwards of 45-55 mph.  The rainfall extent, however, is a tad questionable at this time as there could be a dry slot which moves into the state.  What may transpire is a brief window of extremely heavy rainfall but then it tapers off.  

Over the past several days, computer forecast models have been in rather strong agreement on a rather dynamic storm system developing.  A slow moving upper-level trough will continue to undergo strong amplification (strengthening) over the next day and dig down into the Gulf coast states.  As this trough continues to amplify we will see extremely strong shortwave energy move overhead with strong positive vorticity advection.  Associated with this upper-level trough is a rather strong cold front as well:

As this trough continues to amplify and push east, it is going to capture what is now Tropical Depression Eighteen located near Cuba.  This tropical depression is expected to continue tracking to the north and pass just east of Florida and work up along the east coast.  At this time it will interact with the upper-level trough and cold front.  Once this interaction occurs, this will accelerate the system's strengthening and also add a great deal of moisture into the system.  It is this scenario right here which will lead to the threat for extremely heavy rainfall and strong to damaging winds.  

One of the more impressive aspects of this system will be the strength of the low-level wind fields at both 925mb (about 2500' off the ground) and 850mb (about 5000' off the ground).  The NAM/GFS/Euro all continue to indicate winds at these levels exceeding 90 knots which is beyond impressive.  With the winds at these levels coming from the SE this would transport a great deal of low-level moisture into the state.  In addition, some of these winds could also get transported down to the surface as well which also leads to the potential for strong to damaging wind gusts:

  

Something to watch as well is the track of the 700mb low as computer forecast models indicate this track will be to the west of CT.  A track like this would suggest drier air working in at some point during the system.  We see this happen sometimes during snow events and this essentially shuts off the heavier snows.  This could have an impact on the upper bounds of potential rainfall totals.  Regardless, we will certainly see a period of extremely heavy rainfall across the state of CT which will result in a widespread 1-3'' of rainfall.  The potential does exist for some isolated totals upwards of 6'', however, this is all dependent on any dry slot:

Now that we sort of looked a bit into the science behind this (there is so much more to discuss but I don't have the time and don't want to make this too long) we'll break down of each hazard.

Strong to Damaging wind Gust Potential

This potential will come in two different phases; one coming with the system Sunday and Sunday night and the other coming Monday morning through the afternoon on the backside of the system.  While the potential exists for widespread strong to damaging wind gusts across the state during the storm I think the greater potential for this will exist across southeastern CT where winds could gust as high as 35-45 mph with the potential for a few gusts up to 55 mph.  Elsewhere, gusts will be in the 25-35 mph range.  There are a couple of factors which I think will prevent the higher amounts being widespread Sunday night will be potential for either a low-level temperature inversion (or a few nearly isothermal...no temperature change with height) and the core of the low-level jet sliding east of the state.  Isolated pockets of wind damage will occur, especially across southeastern CT and this will lead to isolated power outages as well.  

For Monday, however, I think much of the state sees wind gusts upwards of 45-55 mph as the system departs.  Forecast soundings all show rather strong deep mixing taking place with fairly steep low-level lapse rates thanks to a quite a bit of sunshine.  This will lead to isolated to scattered pockets of wind damage and power outages.

This bufkit sounding from the latest GFS computer forecast model for Windsor Locks, CT 2:00 PM Monday afternoon highlights the strong wind potential suggesting winds of 30+ knots with gusts upwards of 40+ knots.  We see a well mixed atmosphere and steep low-level lapse rates present:

Heavy Rain/Flash Flooding Potential

While overall it has been quite dry the past few months, we are fresh off a rather impressive rain event just a few days ago which dropped 3-5'' of rain across the central and eastern part of the state.  With at least another 1-3'' of rainfall expected tomorrow this will result in some flash flooding.  Depending on the dry slot scenario, some areas could see upwards of 5-6'' of rainfall which would greatly increase the flash flooding potential for those locations.  

Thunderstorms/Convection

Due to rather poor mid-level lapse rates (weak decrease of temperature as you ascend through the atmosphere) the amount of instability will be quite weak.  This (along with rather warm 500mb temperatures) will make for a small likelihood of thunderstorms, however, the extent of the forcing and lift will be so significant that they can't be ruled out.  The significance of this is if there are any thunderstorms (or any embedded convective elements within the rain) this would vastly increase the threat for damaging wind gusts and higher rainfall totals on a local level.  

Tuesday's Severe Weather Potential

Tuesday continues to look rather active across southern New England as the potential exists for torrential downpours and strong to damaging wind gusts (thanks to a strengthening pressure gradient and with convection).  While the ground is relatively dry, the potential will exist for flash flooding as well with as much as 1-3'' of rainfall expected and perhaps some localized higher amounts as well. 

As discussed with yesterday's post, an amplifying trough digging into the Ohio Valley will swing a strong cold front towards southern New England late in the day.  A strengthening southerly flow will help transport an anomalously warm and moist low-level airmass into the region which will work to destabilize the atmosphere.  Associated with the trough will be very strong wind shear as well. 

The combination of a warm and moist low-level airmass, strong wind shear, weak instability, and strong forcing from the cold front and upper-level support will all contribute to periods of torrential downpours tomorrow along with the potential for thunderstorms which could produce strong to damaging wind gusts.  There are two windows for this potential tomorrow; 1) anytime during the afternoon hours and 2) late evening into the early overnight hours as the cold front arrives. 

The extent of the strong to severe thunderstorm potential tomorrow will all depend on exactly how unstable the airmass becomes and how deep (or how tall convection becomes).  The more unstable the airmass, the greater the strength of the updrafts associated with the thunderstorms.  Given how strong the wind shear aloft is, a stronger updraft enhances the likelihood for updrafts to be robust enough to penetrate upwards through the wind shear without being disrupted or toppled over.  If the updrafts are toppled over the storm(s) aren't able to grow tall enough to utilize the full potential of the atmosphere. 

Computer forecast models are in rather strong agreement that upwards of several hundred joules of mixed-layer CAPE (MLcape) will develop during the afternoon hours and persist into the early overnight hours.  While several hundred joules of MLcape is not overly impressive when discussing the possibility for strong to severe thunderstorms, given the degree of wind shear in place these values may be just enough to enhance the likelihood for strong to severe thunderstorms. 

One of the more intriguing aspects of tomorrow's setup is that of the forecast hodographs.  A hodograph gives you a 'graphical' representation of wind speed with height in the atmosphere as well as wind direction with height in the atmosphere.  The below image shows the forecast hodograph for Waterbury, CT at 8:00 PM Tuesday night from both the NAM and GFS computer forecast models.  We see a rather long and somewhat curved look to the hodograph.  This indicates that not only is wind shear very strong aloft but there is a bit of directional wind shear (change of wind direction with height).  This is a strong indicator that the potential exists for a thunderstorms updraft to rotate if this potential is utilized.  Also on the hodograph is some forecasted CAPE values for different levels through the atmosphere.  Both models indicate over 200 J of 0-6km CAPE.  There are, however, some discrepancies with regards to the degree of 0-1km CAPE:

 
As tomorrow progresses we will have to watch and see exactly how unstable the atmosphere becomes.  If the atmosphere becomes as unstable as advertised the threat will exist for a few strong to severe thunderstorms, either in the form of discrete cells or within a low-topped squall line which develops ahead of the cold front.  If the degree of instability is less this potential would be vastly reduced.  However, if more instability develops (which can happen if mid-level lapse rates become a bit steeper, which some computer forecast models have hinted as a possibility) this potential would be greatly enhanced and a more significant/widespread event would be possible.  There are also some questions as to how much, if any, thunder/lightning will be associated with this activity.  It is still possible to get strong to damaging winds with the absence of thunder and lightning (which obviously would mean these wouldn't be thunderstorms) as even heavier downpours can transport stronger winds from aloft.  If the activity is associated with a great deal of thunder/lightning this would indicate rather deep (tall) thunderstorms which further enhances the severe threat as this indicates a higher likelihood for this activity to utilize the strong wind shear aloft.  What are we looking at?

• During the afternoon we will see periods of showers and perhaps a few thunderstorms.  The rain could come down heavy at times.  Strong wind gusts will be possible too with any showers and thunderstorms.
• Winds will be quite gusty during the day as well thanks to a strengthening pressure gradient as the system approaches.
• As we near evening the potential for heavier rain, thunderstorms, and strong to damaging winds will increase as the cold front approaches.  The timing of this will be from 7:00 PM to 11:00 PM.  For central and eastern southern New England the timing for this is pushed until after 12:00 AM and lasting through 6:00-7:00 AM Wednesday.  
• The main threats will be; torrential downpours leading to the possibility for flash flooding, especially in the typical flood-prone areas, strong to damaging wind gusts with any thunderstorms or heavier showers, and an isolated tornado, especially in any discrete cells.    
• Winds could potentially gust upwards of 45-55 mph with isolated gusts over 60 mph in the strongest of activity.  

Strong to Damaging Winds Possible Tuesday and Tuesday Night

A rather vigorous and amplifying (strengthening) upper-level trough will continue advancing eastward over the next few days.  As this trough approaches a strong cold front will approach the region.  Out ahead of this cold front the airmass will be anomalously warm and moist with daytime temperatures on Tuesday expected to push the 70°F mark with surface dewpoints expected to climb into the mid-60's.  The combination of such a warm and moist low-level airmass will help to contribute some rather robust (for the time of year) CAPE (Convective Available Potential Energy) values.  This combined with very strong wind shear in place will yield the potential for perhaps multiple rounds of low-topped convection yielding the potential for torrential downpours and strong to damaging wind gusts.  It is a bit unclear as to whether or not we will see much in the way of thunder or lightning given rather warm mid-level temperatures and how low-topped the convection may be, however, given the strength of the winds aloft even torrential downpours may transfer down strong to damaging wind gusts.  This potential will exist from early Tuesday afternoon lasing through the overnight hours into early Wednesday morning:

With such a warm and moist low-level airmass in place, the NAM and GFS computer forecast models both develop upwards of several hundred J/KG of surface-based CAPE during the afternoon on Tuesday and even keep this in place well until the overnight hours:

Computer forecast models also indicate that even upwards of several hundred J/KG of mixed-layer cape (MLcape) will develop as well on Tuesday with these values sticking around into the overnight hours.  This is rather impressive for this time of year. 

In addition to perhaps modest instability values, wind shear will be quite strong.  Computer forecast models are projecting a 90-105 knot 500mb mid-level jet streak to push towards the region late Tuesday placing southern New England in the right front quadrant of this jet.  This is a region of enhanced upper-level divergence which enhances upward vertical motion.  Along with instability, this feature could help fuel a low-topped line of convection just ahead of the front:

Bufkit soundings for across the region are rather ominous for Tuesday and Tuesday night and indicate the potential for strong to damaging wind gusts.  For lack of more appreciable time on my part we'll just look at the 12z NAM bufkit sounding for Waterbury, CT for 2:00 PM EDT Tuesday afternoon and 2:00 AM EDT Wednesday morning.  While the degree of CAPE is not something you would normally find impressive, given the degree of wind shear in place combined with strong lift from the cold front and the system, these CAPE values are more than enough to warrant the possibility for strong to damaging wind gusts:

2:00 PM EDT Tuesday

2:00 AM EDT Wednesday

While perhaps the degree of winds aloft is a bit weaker, the amount of CAPE is actually greater and actually is forecast to increase a bit more past 2:00 AM EDT:

What can we gather right now?

Tuesday and Tuesday night has the potential to be rather active across southern New England in terms of periods of torrential downpours and the potential for strong to damaging wind gusts and this could come in multiple rounds with the first round of potential during the afternoon hours and the second round overnight as the cold front approaches.  If instability becomes even stronger, yielding the potential for more robust updrafts, the potential for thunder and lightning would increase which would increase the risk for strong to damaging wind gusts.  Right now the potential exists for winds to gust as high as 40-50 mph with gusts potentially to 60 mph.  These higher gusts would occur with the heavier downpours which would act to tap into those stronger winds aloft.  Isolated to scattered pockets of damage would be possible.  As we get closer the details will become much more clearer. 

Tracking Jose...Does Southern New England See Any Impacts?

With regards to (at the time of this writing) Tropical Storm Jose, I personally, may end up eating my words.  This week during my broadcast at school, I briefly touched upon Jose and I stated Jose would pose no threat to southern New England.  Well over the past several days really computer forecast models have been further indicating that not only will the beaches and coast see impacts from high surf but a good chunk of the region may see strong winds and heavy rains from Jose.  While this probability is on the table there is still a great deal of spread within the computer forecast models with regards to the exact track and there is still that possibility that Jose moves on out to sea with no impacts with the exception of high surf and swells.  In retrospect, do I, or should I regret my choice of words and be so quick to dismiss the possibility of an impact from Jose?  I guess the answer to this is yes and no.  These days in weather there is so much hype and this hype comes from numerous sources.  You have hype from some TV stations and this hype is generated to increase their ratings (higher ratings means more money), you have hype from hobbyists and people who study weather.  This is done to draw attention to themselves and get more "followers" on their social media platforms.  Then you have people who just base forecasts off each and every single model run.  This right here is not forecasting.  Forecasting is more than just looking at each model and each individual model run.  Forecasting is understanding persistence, understanding atmospheric patterns, understanding biases within each forecast model, understanding climatology and understanding what types of patterns can correlate to certain outcomes.  It is very difficult to get tropical systems to directly impact our region and there is a good reason for that.  It takes a very special weather pattern and a very specific interaction of numerous ingredients for these scenarios to happen.  Anyways, I'll end the long rant and we will look into all the specifics and what we need to watch for over the coming days with regards to Jose and where Jose may track.

The 5:00 PM advisory from September 15th by the National Hurricane Center (NHC) has Hurricane Jose just past the threshold between a tropical storm and a category 1 hurricane with maximum sustained winds of 75 mph (A tropical storm is classified as a hurricane when maximum sustained winds reach 73 mph) and well north and east of the Bahamas.  The cone of uncertainty indicates well...a great deal of uncertainty with the potential track.  The potential track ranges from a very close call with the east coast of the United States to a track well east of the United States.  Notice as the forecast position of Jose changes with each day the cone of uncertainty grows larger.  This is not all that uncommon, however, it is uncommon to see such a large spread.  This is due to the fact that the computer forecast models are really struggling with the track of Jose.  The reason for this is due to how the upper air pattern evolves over the next few days and this is something we will explore within this post.  Also notice that the NHC does expect Jose to re-strengthen back into a hurricane as denoted by the "H" within the track but they also forecast it to weaken back to a tropical storm as denoted by the "S":

 Exploring why the cone of uncertainty is so wide we will look at the latest ensemble means from this morning's runs of the GFS and European computer forecast models with the GFS on the left and the European on the right.  Each of those individual tracks are different solutions projected by each member from within that computer forecast model.  These maps are often referred to as "spaghetti" plots because they resemble that of spaghetti:

If these look messy well it's because they are.  What we can gather from this, however, is that there is rather strong consensus that Jose will come relatively close to the east coast, the question just is how close and what happens to Jose when nearing the east coast.  Some members bring Jose close enough to provide some impacts while other members just turn Jose right out to see, likely with minimal impact.

Before we explore further, you're probably wondering if we do receive direct impacts from Jose will it be a hurricane?  The answer to this question is it is very unlikely and there are some reasons for this.  For tropical storms to become hurricanes and become strong hurricanes they need 3 key ingredients; they need to traverse over very-warm ocean waters which have sea-surface temperatures of at least 80F or higher, they need to be in an area and move into an area of relatively weak wind shear aloft (strong wind shear tends to disrupt the circulations of tropical systems), and they need to be in an area of high moisture content (if dry air is drawn into the system the system will weaken).  We will first look at sea-surface temperatures (in degrees C) as of yesterday across the western Atlantic.  I have circled two areas.  The area circled in red will be waters in which Jose will move through.  Correlating the colors to the color bar at the bottom we are looking at water temperatures which range from about 27C to 29C or 80F to 84F.  With what I mentioned above, these water temperatures are certainly supportive for Jose to not only to keep its current strength but are supportive for some further strengthening.  The second circled area is water temperatures off the the Carolina coast extending northward along and just off the New England coast.  Water temperatures here are in the 20C to 22C range or 68F to 72F.  Obviously these temperatures are far below the 80F threshold.  This is one factor which will inhibit Jose from being a hurricane if it were to make landfall or come very close to our area:

    
We will now explore the wind shear over the western Atlantic.  On the map below, the green contours indicate areas of favorable (or weaker) wind shear (favorable meaning increased likelihood that a tropical system will strengthen) while red contours indicate areas of unfavorable (or stronger) wind shear (unfavorable meaning an increased likelihood for weakening).  I have circled where Jose is positioned currently.  Notice how Jose is in an area of favorable wind shear.  Also notice, however, Jose will be moving into a widespread area of much stronger wind shear:

Thus far we see that not only will Jose be lurking into much colder waters but Jose will also be lurking into an area composed of much stronger wind shear.  Two factors which will prevent Jose from being hurricane status if it were to come close to this region.

Finally, let's explore moisture content over the western Atlantic and to do this we will look at a current snapshot of water vapor imagery.  I have circled 3 areas where drier air exists.  The biggest area of interest is drier air working over much of the continental United States and some drier air east of Florida.  There is also some drier air to the east of Jose.  There is a possibility that some of this dry air could work into Jose and if that happen further weakening would likely occur:

Now let's discuss the upper air pattern and how the upper air pattern may evolve over the United States and Atlantic Ocean over the next several days as this right here is what will ultimately determine what path and track Jose will take.  The first thing we will look at is the projected forecast of what is known as the Pacific-North American teleconnection pattern (PNA).  The evolution of this teleconnection pattern strongly correlates to the development and evolution of the upper air pattern over North America.  This index has 3 phases; positive, negative, and neutral.  When the PNA is positive, this correlates to a trough (dip in the jet stream) across the eastern Pacific Ocean extending into western North America.  The result from this is a ridge (northward build in the jet stream) across the eastern United States extending into the western Atlantic.  The negative phase is just the opposite with ridging across the eastern Pacific extending into western North America with the result being a trough over the eastern United States extending into the western Atlantic Ocean.  When the signal is very weak the PNA is deemed to be neutral.  Currently (as shown in the image below) the state of the PNA is just all so slightly negative, however, it can be considered neutral.  Over the coming days, however, as ridging builds into the Gulf of Alaska this will result in a trough amplifying (or strengthening) across the western United States.  This will result in ridging building into the east coast and the PNA will become further negative:

This is typically not something you look for when talking about the idea of a tropical system working up the coast.  What you preferably want to see if this trough out in the west amplifying as it moves through the central part of the country and really amplify in the vicinity of the Ohio Valley region.  You also want the trough to begin taking on what is called a negative tilt (we won't go into details on what that means now since this post is already long enough) as the system is beginning to take a turn up the east coast.  When these conditions are met the trough does what is called "capturing" the system and enhances the likelihood for the storm to either hug the coast or make landfall along the coast.  However, there are other ways to get a track close to the coast or get a landfall along the coast.  While this section scenario is a bit more difficult it can happen.  This will be explored with below graphic.  The below graphic is looking at the 500mb height anomalies from this morning's run of the GFS for Monday morning.  The blue shadings represent where a trough is positioned and the red shadings indicate where a ridge is positioned.  I have placed 5 numbers across features in which their evolution will play crucial to what happens with Jose's path:

1) This is the ridging which is building into the Gulf of Alaska and this is a feature of the negative PNA state.  This promotes a trough into the western United States and we see this with #2.

2) The result of that ridging building into the Gulf of Alaska helps to promote and shape a trough digging into the western-tier of the United States.  The result from this is to build ridging into the eastern United States and we see this with #3.

3) The result of the trough digging into the west is for ridging to build across the eastern United States.

4) A trough is positioned across Greenland and this is indicative of the negative phase of what is called the North Atlantic Oscillation (NAO).  Sandwiched between #3 and #4 is that strong ridging across the eastern United States.  This also creates what is called a "block".  This block is something that could prevent Jose from turning out to sea and the only possible track would be right into the east coast with a potential landfall anywhere from the Carolina's to the New England coast.

5) This is a cut-off low positioned right over the central Atlantic Ocean and the configuration with what was explained in #'s 3 and 4 further enhance this blocking.

If you look closely between the #4 and #5 notice the area of "white".  This could be a focal point as well.  This would represent a "weakness" or a gap inbetween that trough across Greenland and the cut-off low.  This could act as an escape route for Jose.  If that trough in #4 is not as strong as advertised or the cut-off in #5 is weaker or displaced further east, this opens the gates for a sharp turn out to sea.  If, however, that trough in #4 is stronger or that cut-off in #5 is stronger or further west we now have a higher probability for a track towards the coast with a landfall somewhere.  How these features evolve over the next several days will be extremely telling as to what scenario we can expect.

If we were to see any impacts here in southern New England what can we expect?  Depending on what happens with the overall track and intensity of Jose the possibility exists for some strong winds, either with the system itself or due to a rather strong pressure gradient which may develop.  The latest forecast from the NHC yields anywhere from a 20% to 30% likelihood of tropical storm force winds (sustained winds >= 39 mph) across southern New England with 30% to 40% probabilities just to the south and east of Nantucket.  This could certainly yield the potential for not only some wind damage but some power outages as well...especially since the trees are still fully leaved.  In fact, if we do see a scenario where we are looking at sustained winds greater than 40 mph we could see an enhanced likelihood for wind damage and power outages:

In addition to the potential of strong winds the potential would also exist for some heavy rains and flash flooding.  This, aspect, however, will also depend on exact track and overall strength.  What is a given though will be the high surf and swells along the coast.  Entering these waters is not recommended.  Coastal flooding will be very possible too, especially during high tide cycles.

Finally, even if we do get the worst of Jose but Jose is a weak tropical storm this does not mean we can't get torrential rains, some flash flooding, or strong winds.  In fact, tropical storms alone can produce just as much rainfall as hurricanes.  Some of our biggest flooding events have occurred with tropical storms or remnants of tropical storms.  This isn't saying we will see this scenario its just a way to make it understood that tropical storm conditions can still pack a punch.

Over the next few days the details should begin to become much more clear as to what track Jose will take and what type of impacts we will see from Jose.  Stay tuned!

What's the Deal with Hurricane Irma?

Much of the news story over the past few weeks has centered on the state of TX as Hurricane Harvey brought catastrophic flooding to much of southeastern TX, substantial damage thanks to intense winds and flooding, and also numerous tornadoes.  Not very far past Harvey, attention has shifted out towards the deep tropical Atlantic where yet another intense hurricane lies.  Hurricane Irma has blossomed into a dangerous category 3 hurricane and continues to strengthen as it makes a push towards Cuba and the Islands.

 If you follow social media or even some news outlets you may have encountered postings or have heard about computer forecast models striking the east coast with Irma or even perhaps a hit here in southern New England.  In my opinion, posting these graphics so far out is very irresponsible.  While of course a hit along the east coast is possible or even a hit here in southern New England is possible, we're talking about something which is 7+ days out.  While we can forecast in this timeframe with rather decent confidence at times, when it comes to tropical system the forecast confidence decreases drastically and there is pretty much no confidence at all.  In my mind, the goal for a forecaster, whether that forecaster be a degreed meteorologist or a hobbyist is to inform the public in a responsible manner, not scare the public.  The later is something that is easily done nowadays as social media has blossomed and this is because so much information gets posted and when this is done irresponsibility the wrong idea gets out.

While we are still several-plus days away from any potential landfall along the east coast there are numerous factors which will come into play which will determine; 1) If Irma makes landfall along the east coast and 2) where Irma would make landfall. Within this post we will look at all of those factors.

Over the past few days there has been rather strong agreement between the computer forecast ensembles have been in rather strong agreement in bringing Hurricane Irma very close to the east coast of the United States.  It is likely this reason alone which has led many to be discussing the possibility of a hit along the east coast.  Below are ensembles from both the GFS and European computer forecast models showing the cluster of potential tracks.  The GFS ensemble mean is from this morning's run of the GFS while the European ensemble mean is from last night's run (as of this writing this morning's run was not yet out):

While both computer forecast models show a great deal of members showing a landfall somewhere in the southeastern United States, the European model has several members which actually take Irma and curve it out to sea.  While a landfall somewhere along the east coast is possible, a re-curve out to sea is just as possible.  The million dollar question is; what determines whether Irma makes landfall or whether Irma re-curves out to sea?  This is where we explore everything that will play a factor in what the ultimate outcome will be.

Believe it or not, one of the key pieces to all of this is thousands, and thousands of miles away well out in the western Pacific Ocean in the form of Typhoon Sanvu.  Extensive research has shown that re-curving typhoons out in the western Pacific play a substantial role in the evolution of the synoptic weather pattern across the eastern central/eastern Pacific ocean and across North America.  More times that not, tropical systems are quite large in nature.  Within these systems a great deal of condensation occurs and the process of condensation releases what is called latent heat.  When you have these large systems with a great deal of condensation, you're going to get a great amount of latent heat release.  In fact, so much latent heat can be released that it actually shapes the weather pattern across the areas mentioned above.  This can be illustrated by looking at this mornings run of the GFS computer forecast model.  Looking at the jet stream at 250mb (almost 40,000' off the ground)) notice the extensive ridging (massive northward build in the jet stream) which develops across Alaska, western Canada, and the western United States over the next 5-days:

The significance of this ridging developing is it typically correlates with a trough (dip in the jet stream) either across the central and/or eastern United States.  Looking back at the image, notice the troughing develop and amplify (strengthen) between the central and eastern United States.  The significance of this feature is, when dealing with land-falling tropical systems across the east coast, and especially southern New England, this feature is nearly a prominent feature in many land-falls.  However, there is also more to this.  In order to increase the likelihood of a land-fall with this sort of scenario, you typically want the trough to amplify and become what is called negatively tilted as the tropical system is beginning to make the turn up the coast.  When this all occurs, the negative-tilt of the trough 'captures' the system and tugs it right into the east coast.  A great example of such a scenario would be with what occurred with Hurricane Sandy.

The next feature to focus on is across the northern Atlantic Ocean.  Again, using this morning's run of the GFS computer forecast model we will focus on the large area of high pressure which becomes established over a large portion of the north Atlantic:

  The importance of this feature is it could act as a block and actually prevent the tropical system from re-curving out to sea.  In this scenario, the only possible track would likely be into the United States.  Like with the troughing scenario, however, there is also more to this.  If the extent of this high pressure cell shifts eastward, this now opens up an 'escape' route for the system to make a re-curve out to sea.  How this high pressure center evolves over the next several days will be extremely critical in what Irma does as far as track is concerned.

The latest forecast track of Irma shows Irma potentially making striking Puerto Rico, the Dominican Republic, and Haiti regions:

  Not only would this yield substantial destruction across portions of these areas but the topography, especially of Haiti would drastically alter the strength and organization of the system.  When tropical systems go over land, they weaken significantly and rapidly as they need the ingestion of warm and moist air in order to keep going.  Also, Haiti is home to some rather large mountains.  If a tropical system goes over a mountain, if the mountain(s) are large enough, they can vastly disrupt the circulation of the tropical system which causes it to weaken substantially.

Some other factors to consider with regards to Irma are the degree of wind shear out ahead of Irma as well as available atmospheric moisture.  Tropical systems tend to strengthen when they reside in areas with weak wind shear and high amounts of available moisture content.  Assessing current wind shear across the Atlantic Ocean and just off the east coast of the United States we have low wind shear present around Irma and across the majority of the projected path of Irma.  However, across the eastern United States and just off the east coast we have rather strong wind shear:

The forecasts are for this region of stronger shear to remain in place for at least the next several days.  If this holds true, if Irma does take a turn towards the east coast or comes up the east coast, Irma could weaken quite a bit because of this.

Current water vapor imagery does show some drier air out ahead of Irma.  This may not be all that significant, however, if Irma does ingest some of this drier air this could act as a weakening factor for Irma:

 At this juncture its still extremely too early to say for certain what the likelihood is for a landfall across the eastern United States or if we have to worry about a landfall here in southern New England.  At this time, however, my best guess would be either a landfall off the southeast coast (perhaps SC/NC/N FL) or either a sharp re-curve out to sea once Irma nears the east coast.  However, the uncertainties are still rather high and any possibility remains on the table at this point.  All the latest data will continue to be reviewed and over the coming days we'll have a much better idea as to what solutions are more likely and which aren't.