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.