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.