Strong-to-Severe Convection Possible Late Afternoon/Evening 09/25/2018

Two windows will exist for thunderstorm development through Wednesday evening and the potential will also exist for a few strong-to-severe thunderstorms. The first window will develop later this afternoon into the early overnight hours as a warm front slowly lifts northward and the second window will be tomorrow evening as a cold front approaches.

A warm front is currently positioned south of Long Island back southwest into NJ. This warm front will continue to slowly lift northward through the afternoon. South of the warm front the airmass is characterized by rich theta-e air/low-level moisture and rather high dewpoints. The combination of rich theta-e air and dewpoints well into the 70's south of the warm front is yielding mixed-layer CAPE values between 500-100 J/KG. A very strong low-level jet is also present with 925mb winds in excess of 40 knots across CT. Forecast models indicate this could strengthen upwards of 45-50 knots through the afternoon.

With the warm front approaching CT and a strong low-level jet in place, strong speed and directional shear is present within the 0-6km layer which is yielding enlarged and curved hodographs. As the warm front lifts northward instability will begin to increase across portions of the state where the warm front lifts through. Forecast models indicate an overlap of the strongest wind shear and instability will occur across CT late afternoon into the early overnight hours. The image below is the forecast hodograph for Waterbury, CT from the 12z run of the NAM (on the left) and the HRRR (on the right) for early this evening. The NAM shows 0-3km helicity values just over 300 m2/s2 with 0-1km helicity values well over 200 m2/s2 (not shown) while the HRRR shows 0-3km helicity values over 400 m2/s2 with 0-1km helicity values just under 400 m2/s2 (not shown). What is also noteworthy is the 0-1km CAPE values just over 100 J/KG:

CAMS (convective allowing forecast models) have been rather consistent and aggressive in developing convection later on this afternoon and given the projected instability parameters and shear there is the possibility that any convection  may develop supercell characteristics. With this said we will have a window of opportunity for an isolated tornado/damaging wind gusts into the early overnight It is also possible that this convection may not contain thunder or lightning.

In addition to the potential for an isolated tornado/damaging wind gusts we will see localized pockets of flash flooding with heavy rain continuing through the day.

 A separate blog post will highlight and discuss the potential for severe weather tomorrow a bit later on.