High Humidity Values to Return Along with Threat For Showers And T'storms

Humidity values will slowly begin to increase during the day on Sunday across southern New England as a warm front slowly pushes through the region.  As the warm front pushes through dewpoints are expected to climb into at least the lower 70's and that will make it feel very uncomfortable, especially when coupled with temperatures well in to the 80's.  The passage of the warm front will also set the stage for what looks to be an active 3-4 days of weather.  

Over the past several days, computer forecast models have hinted at a very anomalous (for July standards) trough developing and digging into the upper Mid-west/Great Lakes region in response to some very strong ridging developing across the western-tier of the United States.  

The deep digging trough will also for ridging to build along the eastern seaboard and western Atlantic Ocean.  This will work to lift a warm front northward through southern New England as the wind flow at the surface and lower levels of the atmosphere to become more southwesterly.  This will work to draw in ample moisture from the Gulf of Mexico.  Moisture from the Atlantic Ocean will also become en trained into the flow.  This will lead to the very high dewpoints described above.  

Sunday

Sunday will begin with drier air still in place, however, as the warm front approaches the dewpoints and humidity levels will slowly begin to rise from south to north across the region.  The one question at hand right now is, how quickly does the warm front push through?  This has some implications as some computer forecast models are more aggressive with how quickly the front pushes through and the result is a push of instability working into the region.  This would allow the potential for a few showers and t'storms to either develop across the region or move into western sections of southern New England late afternoon or during the evening.  Computer forecast models also show some weak energy moving through aloft which could be a focus for some development.  If this scenario were to unfold we would have to watch as any storm could become strong or perhaps severe.  However, this all depends on how quickly the front pushes through and how unstable the atmosphere were to become. 

Monday

By Monday everyone is southern New England will be under the influence of some very uncomfortable humidity values as dewpoints will be well into the lower 70's.  While last week we saw temperatures well into the upper 80's to even some 90's and dew points near 70F, this go around temperatures may not be so high as cloud cover is expected to hold temperatures back...more lower to mid 80's.  

A cold front will also be draped well west of southern New England, however, a pre-frontal trough is expected to develop across central NY and extend southward into PA.  This pre-frontal trough will be a focus for the development of showers and t'storms across central/eastern NY and PA.  The question here is will any portion of southern New England be close enough to the pre-frontal to see any development or will the pre-frontal be positioned east enough to where we see numerous development and activity across the region?  

On Monday computer forecast models are in agreement that with a combination of high dewpoints and some surface heating, this will lead to a fairly unstable atmosphere which would provide fuel for t'storms to develop.  Computer forecast models also have some pretty strong wind shear aloft thanks to the strong area of low pressure associated with the trough:

 

While the strongest 500mb winds will be confined to the Ohio Valley and western NY (closer to the cold front and trough), 500mb winds across southern New England will be in the 30-40 knot range with values increasing closer to 50 knots later in the day.  This will lead to vertical wind shear values of 35-45 knots across the region which is more than favorable for any thunderstorms that develop to become much better organized and for updrafts to sustain themselves.  

With high dewpoints and rich low-level moisture in place, this will lead to very high precipitable water values across the region, on the order of 2'':

On Monday, the combination of an unstable airmass and strong shear aloft ahead of a pre-frontal trough will lead to the development of showers and t'storms.  While the bulk of the activity may be confined to PA/NY, western.central sections of southern New England will also run the risk for activity, however, whether the threat begins early afternoon or later afternoon depends on timing and positioning of the pre-frontal trough.  The atmosphere will also be conducive for some of the t'storms to become strong to severe.  This would make a risk for strong to damaging winds and perhaps some hail.  We would also have to watch out for some rotating t'storms in any discrete t'storms.  

With the high precipitable water values, this means that any showers and t'storms will produce torrential downpours.  This will lead to the threat for flash flooding, especially across the typical flood prone areas and for any areas which are hit by multiple thunderstorms.  In fact, flash flooding will be a threat over the next several days.  

Tuesday

The forecast on Tuesday is much more uncertain as there are major questions with regards to timing and positioning of the cold front.  Computer forecast models also suggest a boundary may move through the region during the morning hours allowing for drier air to work into the lower levels of the atmosphere, especially across western/central southern New England.  This would mean the highest threat for t'storms on Tuesday could be across eastern RI and eastern MA.  Like Monday, any t'storms would have the potential to become strong to severe with risks for strong to damaging winds and some hail.  Flash flooding would be the #1 concern.  

All in all beginning tomorrow night lasting through Tuesday (perhaps lasting into Wednesday) we will run the risks for showers and t'storms to develop at any point during the morning, afternoon, evening, or overnight hours.  The threat for stronger or perhaps severe t'storms would be confined to afternoon/early evening hours as this is the times the atmosphere would be the most unstable.  Given the high precipitable water values mentioned above, any showers and t'storms will produce very torrential downpours and alot of rain would fall in a very short amount of time so flash flooding potential is on the higher side.  Some areas would be able to pick up a very quick 2''+ of rainfall very quickly.  

During this timeframe some areas will be hit much harder than others, as is always the case when you're dealing with thunderstorms so some areas may end up seeing as much as 3-4''+ of rain while other areas may be lucky to even see 0.50'' to 1'' of rainfall.  

We will continue to monitor the computer forecast models as new data becomes available and will continue to monitor the potential for strong to severe t'storms as well as flash flooding potential.