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.