textproduct: Gray - Portland

WHAT HAS CHANGED

The forecast remains on track for this afternoon and evening as convection is initiating to our west and will move in over the next couple of hours. Storms may become strong to severe with gusty to damaging winds being the main hazard.

KEY MESSAGES

1. Warm and humid conditions continue into Friday south of the mountains. A few strong thunderstorms are possible as well.

2. Weekend weather looks slightly cooler and mostly dry, except for the chance for a few showers in southern NH and southwest ME on Saturday.

3. Mostly dry to start next week with increasing temperatures and humidity before the chance for showers and storms returning mid week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... A cold front continues to sink southward through the forecast area this evening providing the forcing for some strong to severe thunderstorms to develop. Any severe storm will be capable of producing damaging winds. The cold front ends up closer to the coast by Friday morning bringing relief from the humidity to locations in the mountains and foothills as drier air also sinks southward behind the front. Closer to the front, on the coastal plain and in southern New Hampshire, dew points remain elevated but temperatures do come down slightly as winds shift northwesterly. This is going to result in a continued mugginess as the upper 80s ambient temperatures feel more like low 90s. North of the mountains dewpoints should already be falling into the upper 50s and low 60s with ambient temperatures in the upper 70s and low 80s. Friday night looks like a good night to open the windows and flush out the heat and humidity as low temperatures fall into the 50s and 60s.

Severe weather potential, and coverage of any storms, looks low Friday as the main source of forcing is going to be continuing to sink southward, eventually exiting the area. However, it can not be completely ruled out. The 09/12Z HREF suggests 500-800 J/kg of CAPE still present along and near the front with around 25-30kts of bulk shear. This suggests organized storm development, but the mid-level lapse rates around 5 C/km is what is making me think coverage will be isolated. This is reflected in the 12Z CAMs as they show next to nothing developing. I don't think it will go quite that route, so I did keep low chance PoPs along the coast. Any storm that does develop has the fuel to grow, and with temperatures near 90F low level lapse rates would likely support strong winds.

KEY MESSAGE 2 DESCRIPTION...

Mostly dry conditions are expected for the weekend with high pressure nudging in from the northwest from Quebec. For Saturday afternoon, diurnally driven showers and thunderstorms will be possible south of the mountains due to a 500 mb shortwave trough crossing the region with precip mainly confined to places near and around southern portions of our CWA. Based on recent Hi-Res model guidance, I'd give these shower and thunderstorm chances on the lower side, but most models are hinting that these showers would stay confined to Rockingham and Strafford counties in NH and York county in ME. Regarding the temperatures, I would expect the mid 80s for the weekend, with 70s along the immediate coastline, with both afternoons featuring a sea breeze, lowering temperatures going into the evening hours.

KEY MESSAGE 3 DESCRIPTION...

Looking ahead to next week, high pressure looks to stick around at least until Tuesday morning. Noting this, heat looks to be building starting Monday with temperatures in the upper 80s with humidity on the lower side. Starting Tuesday, however, the heat and humidity look to build, with some models indicating temperatures in the middle of next week approaching the low to mid 90s away from the immediate coastline, with humidity levels making temperatures feel like the upper 90s as dewpoints approach the upper 60s to low 70s. On Tuesday or Wednesday, a shortwave trough looks to approach from our north, moving a front across our area which potentially results in shower and thunderstorm chances. In the coming days, trends will be monitored regarding the trough and front on how this impacts temperatures as well as rain chances for the middle to latter half of next week.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Through 18Z Friday...VFR continues through the remainder of the afternoon, with chances for showers and thunderstorms increasing this evening that may cause TEMPO MVFR conditions. Coverage of thunderstorms is not expected to be widespread so mention in the TAFs is limited. Fog development is higher confidence and may be fairly widespread with increasing moisture across the area. IFR conditions have been mentioned at most terminals for tonight as a result. Any fog should clear up around 12Z Friday with VFR expected to prevail through the afternoon. Coastal terminals may see some MVFR in afternoon showers and thunderstorms.

Outlook... Friday night: Mostly VFR conditions expected as drier air moves into the area behind a front.

Saturday: Brief instances of MVFR possible from -SHRA in the vicinity of the NH terminals and PWM. VFR otherwise.

Sunday-Monday: VFR expected.

Tuesday: Scattered showers and thunderstorms possible in the afternoon that may bring MVFR or IFR restrictions.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...Heat Advisory until 8 PM EDT this evening for NHZ012-013. MARINE...None.