Late Evening/Overnight T'storm Potential

Throughout the day today hi-resolution computer forecast models have been in rather strong agreement and have shown strong consistency for the potential for a line or multiple lines of thunderstorms to work through portions of New England during the late evening and overnight hours:

Typically (especially this time of year) we see the likelihood or potential for thunderstorms vastly decrease after sunset as the atmosphere typically begins to rapidly stabilize, however, there are several atmospheric ingredients in place which will allow for these thunderstorms to maintain and in fact, some of these ingredients will also allow for the possibility for a few of these thunderstorms to be on the strong to severe side and pose a threat for hail and strong wind gusts.  One of these key ingredients is the presence of rather steep 700mb-500mb lapse rates (strong change of temperature with height between about 10,000' and 18,000' off the ground).  SPC mesoanalysis shows 700mb-500mb lapse rates are between 6.5 C/KM to 7 C/KM.  These values which may actually steepen more towards 7 C/KM which will help keep the atmosphere rather unstable through the evening and into the overnight, even despite the loss of daytime heating:

With the presence of steep lapse rates, surface temperatures still predominately in the 70's, and surface dewpoints near 60F, this will yield most-unstable cape values as high as 1000-1500+ J/KG through the overnight hours and the RAP computer forecast model shows this.  This will provide the "fuel" to keep the storms going as they move in from NY:

The focus for a lifting/forcing mechanism is an approaching upper-level trough and associated shortwave energy and attendant cold front.  As this trough progresses east and the cold front moves closer, we will begin to see winds aloft strengthen which will provide adequate shear to help storm updrafts remain organized.  SPC mesoanalysis shows 30-40 knots of 0-6km shear across New England with 40-50 knots off to the west.  These values will push east into the region as the trough/cold front approach:

Several bufkit profile soundings across the region show a rather favorable environment tonight for not only thunderstorms but the possibility for a few strong to severe t'storms.  Looking at a bufkit sounding from the 18z NAM computer forecast model for Windsor Locks, CT we see pretty decent cape within the 0-6km layer (almost 500 J/KG!) and rather impressive hail cape (573 J/KG!!) along with very strong shear (39 m/s):

What this indicates is any thunderstorms will be accompanied by a great deal of lightning and would also have the potential to produce hail along with strong winds.  The greatest potential for thunderstorms appears to be across southern VT, southern NH, down through western/central MA and into norther-central CT.  Not everyone will see thunderstorms but for those that do they could pack a quick punch!

Monday, May 1st, 2017 NY/PA Severe Weather Potential

Typically I confine my blog posts to either CT weather or southern New England weather, however, being a huge severe weather enthusiast and with a rather intriguing setup for later this afternoon across PA/NY I figured I'd throw up a blog post.

A vigorous piece of shortwave energy is slowly tracking to the northeast through the upper mid-West region.  Associated with this piece of shortwave energy is a trailing cold front with is draped south through the Ohio Valley and into the deep south and a warm front which currently just sits south of CT but is is pushing through the states of PA and NY.  As this piece of shortwave energy tracks off to the north and east the associated trough and cold front will push east and this will result in rather decent height falls across PA and NY which will aid in ascent and forcing:

There are some rather impressive elements involved with this system which is paving the potential for a potent severe weather event this afternoon, however, like with most severe weather setups here in the northeast there are numerous negatives in play and we will evaluate those as well within this post.  With this system are some very impressive dynamics aloft.  Currently working through the Ohio Valley region is a very impressive 80-100 knots 500mb (~18,000' ASL) jet streak!  This will continue to slowly work east into the PA and NY this afternoon and really enhance the degree of forcing:

In addition to very strong winds at 500mb winds at the 700mb level (~10,000') 850mb (~5000' ASL) and 925mb (~2,500' ASL) are expected to be very strong as well with 700mb winds expected to increase to between 60-80 knots, 850mb winds 40-50 knots, and 925mb winds 30-40 knots:

These strong winds aloft will work to help with forcing and storm organization.  In fact, given the magnitude of the 700mb winds this will yield 0-6km shear values as high of 50-60 knots which is not only more than favorable for storm organization (typically want to look for at least 30-35 knots) but for the possibility of rotating thunderstorms as well (typically want to look for at least 40-45 knots).  Another intriguing factor with regards to the winds aloft are the changes in wind direction between the 925mb level and the 700mb level.  700mb winds will be more out of the southwest with 925mb winds working out of the south (which will help to advect in higher dewpoints/moisture and theta-e air) and surface winds perhaps more southeasterly.  This change in direction with height is known as directional shear and is one ingredient to look for with regards to the potential of rotating thunderstorms.  Another aspect is how the winds increase with height.  This is known as speed shear and when combined with the directional shear will further enhance the potential for rotating thunderstorms this afternoon.  This combination will yield very large helicity values with 0-1km helicity expected to be as high as 150-250 m2s2, and 0-3km helicity values as high as 250-400 m2s2.  This is shown rather well on several bufkit profile soundings across PA and NY.  The 12z bufkit profile from the NAM computer forecast model for State College, PA shows this environment quite well with a rather large and elongated hodograph.  This indicates the presence of both directional and strong speed shear and indicates the potential for rotating thunderstorms:

Surface temperatures are expected to rise into the 70's to perhaps even lower 80's if enough sunshine can break through the clouds with dewpoints climbing into the lower to near mid-60's.  This combined with mid-level lapse rates will help contribute to a modestly unstable environment with 500-1000 J/KG of mixed-layer cape possible.  If more sunshine breaks out than forecasted and temperatures warm well into the 80's these instability values will be higher.  Typically when looking for higher magnitude severe weather events you would love to see values much higher (>2,000-2,500 J/KG), however, given how impressive the wind fields are aloft and degree of forcing, these values should be enough for thunderstorms to not only develop but have the potential to become strong to severe:

 
Extensive cloud cover is also yielding questions as to exactly how unstable the atmosphere will become, however, recent trends in satellite do show numerous breaks occurring.

As the day progresses showers and thunderstorms are expected to develop ahead of the cold front and push off the the northeast at 50-70+ mph.  Given the ingredients mentioned above, the potential will exist for thunderstorms to quickly become strong to severe and have the potential to produce damaging winds in excess of 60-70 mph, large hail, and perhaps even a few tornadoes, however, the tornado potential is a bit in question right now.  While we do have some directional shear in place this is confined to the the lower portion of the atmosphere.  While initially storms may develop more discretely, enhancing the likelihood of them developing supercell characteristics, they are expected to become linear rather quickly which could limit the tornado potential, however, embedded areas of rotation within line segments will be possible and would have to be watched:

  Another inhibitor on the tornado potential are rather high lifted condensation levels (LCL's).  LCL heights have been increasing up to around 1000 meters which is on the higher side when talking about tornado potential:

 
 Computer forecast soundings also show an inverted-v look which also favors against tornadoes but does increase the likelihood for damaging winds:

All in all today will be rather interesting to watch unfold.  Given everything mentioned above, thunderstorms are likely across parts of PA/NY later this afternoon into the evening hours.  Some of these thunderstorms will have the potential to become strong to severe and could produce damaging winds in excess of 60-70 mph, large hail upwards of golf ball sized, and perhaps even a few tornadoes, especially if any discrete cells get going.

Overnight Saturday into Sunday Snow Potential (3/18-3/19/17)

Can a weather forecast ever be easy?  Unfortunately in this field this answer to this question is more than likely not.  What makes weather forecasting rather unique (along with fun and stressful at the same time) is it is an imperfect science.  This is something that the general public doesn't understand I think and why they question how forecasts can be so widely ranged between forecasters and why there is always uncertainty.  Anyways, with this little rant out of the way we are faced with quite another complex situation tomorrow.  Unfortunately, due to time constraints I don't have ample enough time to really get into great depth of detail to fully explain the situation.

An amplifying "strengthening" trough will be digging into the northeastern United States during the day tomorrow and in response an area of surface low pressure will form somewhere south of Long Island (where exactly is still in question):

The biggest question at this stage is where exactly does the surface low develop?  There are substantial differences between the NAM computer forecast model and the GFS forecast model as shown below:

Both these images are valid for 2:00 AM Sunday morning from this morning's (12z) runs of each model.  You can certainly see there are substantial differences here.  The NAM barely grazes the southern coast of CT while the GFS brings what would be moderate snows all the way to the CT/MA border.  For what it's worth the European model (which I can't show) is very south and gives CT virtually nothing with the exception of some flurries.

One thing that has to be watched is the potential for what is called a norlun trough.  Typically a trough is oriented from a northerly position to a southerly position (this includes northwest to southeast, northeast to southwest).  A norlun, however, is a trough which orients from the southeast to the northwest with the main surface low to the south (a rather vague description here but again on a time constraint).  If this feature does setup it could act to draw in stronger moisture to part of the state along with lift which could yield the potential for some higher totals.

For now this is my forecast but limited time to view data and such so confidence not all that high  Hopefully I'll have an opportunity to better look at things later on:

Tuesday, March 14th, 2017 Blizzard Update

A rather impressive mid-March blizzard is set to strike the state of CT tomorrow.  While my forecasts have certainly been much lower with respect to everyone else (all my reasoning why is in my previous blog posts) there are several issues which I am worried about which is why I am much more conservative.  I've done enough scientific talk in the previous posts so I will refrain from that here and this post will just be a forecast.  However, I will briefly explain some of my concerns once again.
As we close on the event there actually continues to be some increasing spread within the computer forecast models which is rather unusual.  Typically they begin to converge.  There continues to be questions with exactly where the surface low pressure tracks and where the 850mb and 700mb lows track and this will be pivotal in determining where the heaviest banding occurs, where the highest snowfall totals occur, and if anyone sees any mixing.  I am also worried with dry air descending through the troposphere into the snowgrowth zone which would inhibit better snowflake production.  Very strong winds throughout the troposphere could also tear snowflakes apart as they descend towards the surface yielding smaller-sized flakes and poor ratios.  The speed is the system and how brief the strongest lift remains overhead is also a concern.  For the forecast:

• Snow begins to break out between 3 AM and 5 AM tomorrow morning.  
• The heaviest of the snow falls between the hours of 9:00 AM and 1:00 PM.  
• The likelihood at this time appears for the heaviest banding to perhaps hit extreme western CT during these hours.  Within these hours snowfall rates could approach 3'' per hour, however, could even briefly approach 4'' per hour.  The key for getting upwards or exceeding 18'' of snow would be for these rates to persist for a good 3-4 hours.  
• Winds will also be quite strong as well throughout the state with winds sustained between 25-35 mph with gusts upwards of 60 mph.  This will not only lead to blowing and drifting snow but lead to isolated pockets of tree damage and power outages.
• The combination of heavy snow and strong winds will yield blizzard conditions at times as well.
• Given degree of lift, thunder can't be ruled out. 
• There is a concern for mixing of sleet or even changing to rain across the southeastern part of the state (in fact some computer forecast models hint that mixing may even occur into north-central CT) so totals may be held back here. 
• After the heaviest snow moves out we will continue to see light to periods of moderate snow through the remainder of the afternoon (mix/rain across southeastern CT)
• Coastal flooding is also possible across southwestern CT along Long Island Sound.  

Tuesday, March 14th, 2017 Storm Discussion/Forecast

Computer forecast models continue to be in rather strong agreement that a coastal storm will provide major to significant impacts (in terms of accumulating snowfall and strong winds) across CT throughout the day on Tuesday.  While the agreement is strong on the impacts, there are still some uncertainties with regards to some of the details but they are becoming a bit more clearer as we close in on Tuesday.

One thing that is a bit uncertain at this time is the exact track this system will take and this holds true for the surface low as well as low pressure centers aloft as well.  If the surface low tracks further west and passes just southeast of CT this could allow for some milder air to work into portions of the state introducing the potential for mixed precipitation (sleet, freezing rain, rain).  A track just southeast of this position allows for colder air to remain in place and the likelihood for any mixing is extremely low.  As for the low pressure centers aloft, some guidance has suggested that the lows at 850mb and 700mb could track over or even just west of CT.  This track for the 850mb low would bring in warmer air up around 5,000' and increase the likelihood for mixed precipitation across portions of the state and this track for the 700mb low would actually allow for some drier air to work in around 10,000' and reduce snowfall intensity and rates.  If you're looking for lots of snow from this event you want all these lows to track just to the southeast of CT.

With this system becoming so strong there will be rather exceptional upward vertical motion which develops on the northwestern side of the low.  This will really work to enhance the development of snow and allow for a period of extremely heavy snowfall in which snowfall rates could approach 2-3'' per hour and even some 4'' per hour rates can't be ruled out.  The black and gray shadings of 800-600mb averaged frontogenesis suggest exceptional lift will push over a portion of the state on Tuesday:

Looking at bufkit profiles from this morning's run of the GFS computer forecast model for Waterbury, CT and Windsor Locks, CT we can also see the rather intense upward vertical motion as the GFS indicates as much as -40 to -50 units of omega within the dendritic snow growth zone!  This is very impressive and hints at the potential for those snowfall rates to approach 3'' per hour where the heaviest banding sets up:

The cards are certainly right on the table for a rather major/significant storm but the question is just how significant?  Well there are some factors we will be dealing with which could prevent this from being a situation where we are looking at 12-18'+ statewide.  One factor will be the speed of the storm.  It's not exactly a quick hitting storm (will last a solid 9-12 hours or so) but its not a very slow moving system and that will prevent the likelihood for any extreme snowfall totals (say several locations picking up 24"+ of snow).  If this was a longer duration storm the cards would be on the table for a swatch of 20-30'' of snow.

There is also some concern of some drier air which may sink into the middle and even lower levels of the atmosphere as the storm progresses and this shows up well on some forecast soundings.  For example, we will look at a sounding from the NAM computer forecast model for Waterbury, CT for 2:00 PM Tuesday afternoon.  Those three blue circles on the bottom left indicate that moderate to heavy snow will be falling.  However, if we look at the profile of the atmosphere we see a quite a bit of dry air (the further spaced apart the red (temperature line) line and green (dewpoint line) the drier the air is at that level):

What would this mean?  If the degree of dry air does indeed verify this could impact the snowfall rates in a way where the intensity is on the lighter side and the snow growth may not be all that stellar (smaller-sized flakes).  This will have to be watched to see if 1) We do indeed see drier air filter in and 2) If so, how quickly does that happen as this could really affect snowfall totals.

Another potential issue is subsidence (sinking motion which reduces intensity of precipitation or prevents it from reaching the ground) in and around the 850mb level (~5,000').  While there is no question with regards to the strength of upward vertical motion at 700mb (~10,000') this subsidence in the lower levels would likely reduce the intensity of the snowfall along with the snowfall rates.  Given the nature of this storm and how intense it will be there is guaranteed to be a zone of subsidence that sets up somewhere, it's just a question of where and that is not something that is very easy to forecast days out:

Winds with this system are expected to be rather strong as well and this is not good to keep big fluffy snowflakes intact as they fall towards the surface.  With winds both aloft and at the surface expected to be very strong, this could lead to snowflakes being ripped apart as they descend towards the ground.  This would yield smaller flakes and ones that are tougher to accumulate quickly as opposed to those bigger fluffier snowflakes.  These are a few things to take into account when making this forecast and to closely monitor over the next few days.

What can we expect?

• Snow begins to break out between 3 AM and 5 AM Tuesday morning and begin to quickly pickup in intensity.  
• The heaviest of the snowfall occurs between 9:00 AM and 1:00 PM.  This is also the time when the heaviest banding will setup somewhere within the state and this is where snowfall rates could approach 3'' per hour and perhaps even upwards of 4'' per hour.
• Given the timing of this system travel is not advised Tuesday morning.  Unlike with what happened Friday, the ground this time is much colder and we are looking at heavier snowfall rates and longer duration so the snow will stick to the roads.  
• Winds will become an issue as well as winds will be sustained between 20-30 mph with gusts upwards of 50-60 mph, especially down by the coast.  This will cause pockets of power outages and downed trees
• The storm begins ti push out during the early afternoon hours with some light snows sticking around into early evening.  
• Don't be shocked either if you hear some thunder given the degree of lift associated with the system

Before we get to the map just want to point something out with regards to that zone of 12-20'' totals.  Yes that is a rather big range, however, as the details with the banding become much more clear that range can be narrowed down.  Also this axis could shift as well depending on where the banding setsup and that is very, very difficult to pin down at this stage.  Based on some of the questions mentioned above, I think a widespread 8-12'' can be expected across the state, however, totals will be much higher in that area of banding.  There will also be an area of perhaps less than 8'' where the subsidence zone setusup:

Tracking Tuesday's Storm Potential

Computer forecast models have been in rather strong agreement on a potential major storm system to impact southern New England this upcoming Tuesday.  The agreement between the operational models and their ensembles have been rather impressive and this leads to a higher than average confidence in at least some sort of impact on Tuesday.  However, there are still numerous questions with regards to the details which is expected for this time frame.  Over the next few days as the details become more clear we will have a much better idea on how significant of an impact we are looking at.

What is the driver for this storm?  We are tracking two pieces of shortwave energy which computer forecast models are in strong agreement that these piece of energy will phase and the result will be explosive cyclogenesis just off the east coast:

The explosiveness of the cyclogenesis will be aided by an intensifying 500mb level jet streak of 100+ knots which develops and rounds the base of the upper trough.  This will allow for further amplification (strengthening) of the trough and enhance the rapid cyclogenesis of the surface low pressure:

Looking at each GFS computer forecast model ensemble from the 12z run of March 10th, we can see very strong consistency between each member for not only an impact from this system but the potential for rather significant impacts:

Over the next few days we will be watching several key features very closely and how the computer forecast models handle these features.  One major question as of right now is with regards to the exact track of the storm as this will vastly determine the significance of the impacts.  A storm track closer to the coast will increase the likelihood if ushering in warmer air into parts of the region which could result in reduced snowfall accumulations and risk for mixing and/or rain.  Of course there is still a shot this system goes completely out to sea, however, at this stage I think that sort of scenario is a bit unlikely.  One thing to watch for when determining how close to the coast a system will track is the behavior of the North American Oscillation over the next few days.  When the North American Oscillation transitions from a positive state (characterized by below-average 500mb height anomalies in and around Greenland) to a negative state (characterized by above-average height anomalies in and around Greenland) this yields what is called a "block" and this makes it rather difficult for a storm coming up the coast to go out to sea.  When the North Atlantic Oscillation transitions from a negative state to a positive state as a system is working up the coast this increases the likelihood the storm could scoot further off to the north and east.  Currently we have a North Atlantic Oscillation which is slightly positive and is expected to become more positive:

What this suggests is unless some other processes (such as a vast negative tilt to the system) occur this could mean a higher likelihood for a track a track which is more westerly as opposed to easterly.  This could mean mixing issues for a portion of southern New England.  

Outside of the track of the surface low, the tracks and development of the 925mb, 850mb, 700mb, and 500mb lows will be rather critical as well. Taking a look at the projected track of the 850mb low (as forecasted right now) the track is right over CT.  This sort of track would allow for warm air advection into a portion of southern New England which would lead to mixing issues:

The projected track of the 700mb low also raises some red flags.  The development and track is pretty much west and then over southern New England which is a huge signal for dry slotting:

These are some of the features we will be watching over the next few days but Tuesday certainly has potential to be quite the potent storm system which as potential to drop major snowfall accumulations.  Further updates will be posted as the situation becomes much more clear

Friday Snow Update

Not much changes from my previous forecast with the exception of decreasing snowfall totals a bit across the northern third of the state and increasing totals across the southeastern part of the state.  The highest totals should be confined to the southern third of the state as computer forecast models are converging on the idea that the strongest upward vertical motions and subsequent area of heaviest snows will occur here.  This is shown by looking at projected 700mb vertical velocity values from both the latest runs of the NAM and GFS computer forecast models:

Looking at a bufkit profile over Waterbury, CT from the latest GFS and NAM runs also shows the intense lift which will be across the southern part of the state:

Looking at bufkit profile over Windsor Locks, CT we see some vast differences in the profile and these differences strongly indicate the northern part of the state will see much less in the way of snow.  Both the NAM and GFS advertise  a brief period of stronger upward vertical motion which suggest the window here for more moderate snow is very limited:

  What can we expect tomorrow:

• Light snows begin to break out between 12:00 AM and 2:00 AM
• The most likelihood period for moderate to heavy snows (especially southern CT) will be between 4:00 AM and 10:00 AM tomorrow morning.  During this time snow rates could approach 1.5''/hr.  Perhaps upwards of 2''/hr for a brief time in the heaviest of bands.  
• Expect numerous delays on the roads tomorrow morning given the timing.  Travel is not advised tomorrow morning unless absolutely necessary.  
• The snow will begin winding down between 11:00 AM and noon with some light snow showers around for a few hours after that.
• Once the system passes a strong cold front pushes through and we turn windier and much colder.  
• Winds could gust upwards of 35-45 mph which could cause isolated pockets of power outages. 

Below is my updated map: