Thunderstorms Possible Monday, June 19th, 2017; Some Could Be Strong to Severe

A warm front will continue to slowly push northeastward through southern New England during the day on Saturday and as this warm front pushes through a summer-like feel will return to the region (mainly in the sense of high humidity) as temperatures for Sunday and Monday away from the immediate coast are expected to push near the 80F mark (perhaps even a bit higher on Monday depending on cloud cover) with dewpoints climbing through the upper 60's to perhaps low 70's.  During the day on Monday a weather system will be approaching from the west as shortwave trough pushes east towards southern New England during the day as shortwave energy track northeastward into Canada from the upper mid-West.  This will swing a cold front towards the region.  Out ahead of the cold front the combination of near 80F surface temperatures, dewpoints around 70F, and strong winds aloft will all set the stage for the possibility of showers and thunderstorms to develop ahead of the front and the potential exists for severe of these to become strong to severe.  While scattered severe weather is possible I don't think we will see a widespread severe weather event here in southern New England.  A bit greater of a threat will exist to the south and west of southern New England.  While we do have some ingredients that will be in place to support some strong to severe thunderstorms on Monday we do have several inhibitors which will prevent a more widespread organized event from occurring.  We will analyze all below:

Low-level Moisture/Instability:

Ample low-level moisture will be in place for Monday as surface dewpoint temperatures are expected to be right around the 70F mark which will make it feel quite humid outside and precipitable water values (PWATS) surging upwards of 1.50'' to 2'' indicating a great deal of moisture will be in the air.  Despite rather poor mid-level lapse rates of 5.5 C/KM to 6 C/KM (temperature with height between 10,000'-18,000' only decreasing by 5.5C/KM to 6 C/KM) the degree of low-level moisture combined with surface temperatures around 80F should yield around 1500 J/KG of mixed-layer cape (measure of how unstable the atmosphere is).  The poor mid-level lapse rates will be one big inhibitor for a more widespread organized severe weather event.  The steeper the mid-level lapse rates, the greater the upwards acceleration of air parcels through the mid-troposphere.  Weak mid-level lapse rates can be overdone by very high dewpoints (dewpoints nearing the mid-70's or by exceptionally strong forcing):

These very high PWAT values may also be an inhibiting factor towards a more widespread and organized severe weather event.  While you do want the presence of rich low level-moisture (which comes in the form of high dewpoints) there is a such thing as too much.  When PWAT values begin exceeding the 1.5'' threshold and start nearing 2'' this can yield what is referred to as water loading.  For severe weather, one set of ingredients you want is rich low-level moisture but you want dry air or drier air between 10,000' to 18,000'.  When the moisture profile is rich through the mid-levels you get water loading and this ends up reducing a storm's updraft strength.

Wind Shear:

There will be no shortage of wind shear as rather unseasonably strong wind fields are expected to overspread the region on Monday ahead of the approaching cold front.  Computer forecast models indicate 40-50 knots of shear at 500mb (~18,000') out of the southwest overspreading southern New England by Monday afternoon, 700mb winds (~10,000') of 35-45 knots out of the southwest, and 850mb winds (~5,000') as strong as 35-45 knots out of the southwest as well.  These values are more than enough to help with the generation of organized thunderstorm clusters/lines and also enough to increase the potential for longer and more sustained updrafts and this is indicated by the combination of strong 700mb-500mb winds which could yield upwards of 40-50 knots of 0-6km shear.  Not only do these values indicate the potential for updraft organization/sustainability but they indicate the potential for some mid-level storm rotation as well.  While these values are quite strong we are lacking something here in southern New England and that's enhanced support for the core of the mid-level and upper-level jet stream.  On the 500mb map the area circled in red is where the core of the mid-level jet stream is (called a jet streak) and circled in blue is likely where the enhanced forcing from this jet streak is.  This area will run from portions of northern New England back southwest through parts of NY and down into PA and into the mid-Atlantic.  Enhanced forcing from jet streaks is something I find to be very critical to getting more organized severe weather outbreaks and this is just one more factor I foresee being an inhibiting factor here:

  
Also, notice how we have southwesterly shear from about 850mb up to 500mb?  (This actually rings too virtually from the surface to 250mb (~35,000').  This is going to yield shear which is parallel to the cold front and lead to storm motion which is parallel to the cold front.  This is typically not something which favors widespread severe weather outbreaks either.  This does indicate that storm mode will be linear (storms forming line segments) so strong to damaging wind gusts are the main threat but also indicates storm motion can be rather slow.  Given the high PWAT values, the combination of these high PWAT values and storm motion parallel to the flow/front may lead to not only training thunderstorms (multiple thunderstorms going over a single area) but this could lead to a threat for flash flooding and it's very possible the threat for flash flooding becomes more of a hazard than the threat for strong to damaging wind gusts here in southern New England.

This is sort of hard to show on a single image but one other negating factor will be rather weak height falls across the region, in fact, the heights may remain rather neutral.  Having strong height falls is yet another way to really enhance upward acceleration of parcels.

All in all for us here in southern New England, Monday afternoon into the evening should be rather active in terms of showers and thunderstorms with the main storm mode being multiple line segments.  Several of these thunderstorms could become strong to severe posing a threat for strong to damaging wind gusts.  While hail will be possible, it will be rather isolated due to poor lapse rates and moist mid-levels of the atmosphere.  This potential will rapidly decrease as you near the coast due to stabilizing influences from the cooler ocean waters.  Portions of northern New England back southwest across central/eastern NY and down into SE PA, MD, NJ, and DE greater potential will exist for clusters of strong to damaging winds.