textproduct: Gray - Portland

WHAT HAS CHANGED

Confidence has increased that southeastern New Hampshire is going to see heat index values greater then 95F Thursday afternoon. A Heat Advisory has been issued accordingly.

KEY MESSAGES

1. Heat and humidity build Thursday, lasting through Friday before a slow moving front brings cooler and drier air by late Friday night.

2. A couple of weak cold fronts bring a chance of thunderstorms on both Thursday and Friday afternoons and evenings. Some of these could be strong to severe.

3. Mostly dry with seasonable temperatures this weekend with a warming trend possible during the early to middle part of next week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION... Latest model guidance is all in good agreement on warm and sticky weather beginning Thursday as 850 mb temperatures climb to +16-18C under a low amplitude ridge that will be quickly swinging through the region. This supports surface temperatures in the upper 80s to 90F, with southwesterly flow providing an extra boost to valley locations that may push them more toward 91-92F via downsloping. The southwesterly winds are also going to bring elevated dewpoints (middle to upper 60s) that will push heat indices up above 90 in many locations. The southern New Hampshire portion of the Merrimack Valley may even see heat index values in 95+ range, so have gone ahead with a Heat Advisory for Eastern Hillsborough and Interior Rockingham. The sea breeze likely gets kept at bay, so these aforementioned warm temperatures should be realized right down to the coast. For areas north of the mountains, the front looks to begin to sag south of the International Border bringing increased cloud cover and showers/storms (detailed in Key Message 2). This stunts warming potential, limiting these locations to the low to mid 80s.

As the front continues to sink southward Friday cooler and drier air will be moving in behind it. As of now areas north of the mountains stand the best chance of seeing a more pleasant day with highs in the mid to upper 70s and dewpoints down into the upper 50s and low 60s. South of the mountains, 850 mb temperatures around +17C linger in southern zones so those locations are likely to see another day with temperatures climbing into the upper 80s to 90F. Elevated dewpoints linger too, but with temperatures a little bit lower on Friday, heat indices should be as well. Guidance is still struggling slightly with the timing of the front, but it looks like by late Friday night most locations should be seeing the relief with low temperatures dropping into the 50s and 60s.

KEY MESSAGE 2 DESCRIPTION... In addition to the heat and humidity, a couple of weak cold fronts will also bring chances for showers and thunderstorms during the afternoon to early evening hours each day for Thursday and Friday.

For Thursday, the higher overall chances for storms will be across the north in closer to proximity to the cold front itself, and some of these could be strong to severe with 30-35 kt of effective shear and around 500 J/kg of MUCAPE per the HREF ensemble mean (some deterministic guidance has more instability). Farther south, instability is forecast to be more in the 1000-1500 J/kg range, but coverage may be more limited because of lesser forcing, lesser shear, and some dry air aloft. However, if storms do develop in southern areas, they may carry more of a wind threat with the dry air aloft bringing more in the way of DCAPE. PWATs climbing over 1.50" and deepening warm cloud depths will also support efficient rain rates, and with flow roughly parallel the boundary, we'll have to monitor for a localized flash flood risk if any training can set up. This lines up well with a Marginal Risk for Excessive Rainfall from WPC in and around the mountains.

Most guidance has this front pushing offshore Friday morning with a secondary front approaching later in the day, and lift from this front should again allow showers and storms to develop in the afternoon. Today's 12Z guidance doesn't look to bring quite as much instability, but there still may be enough for a couple of strong to severe storms as shear parameters look fairly similar to Thursday (the higher potential looks to be south of the mountains this time).

KEY MESSAGE 3 DESCRIPTION... Surface high pressure builds toward New England over the weekend, but a 500mb shortwave may provide enough lift for a few showers on Saturday. Otherwise, dry conditions are forecast with fairly seasonable temperatures in the 80s.

For the early to middle part of next week, there is high confidence in a strong ridge of high pressure to become centered over the Central and Northern Plains and into the Great Lakes. Some expansion of the ridge is possible toward the Northeast to bring a warming trend, but we also may remain more on the eastern periphery to bring occasional chances of showers/storms ("ridge rollers") and more cloud cover.

AVIATION /18Z WEDNESDAY THROUGH MONDAY/

Through 18Z Thursday....VFR conditions continue through Thursday afternoon with LEB and HIE possible seeing localized IFR conditions after 06Z as valley fog develops again tonight. Any valley fog that forms should clear out after 12Z or 13Z with the prevailing condition turning back to VFR. Some TEMPO MVFR is possible in showers and thunderstorms Thursday afternoon.

Outlook... Thursday night: Mostly VFR with MVFR remaining possible as showers and storms linger. IFR fog is also possible again, especially in locations that see rain and valleys, but uncertainty is high regarding that.

Friday: Lingering IFR possible at LEB through around 13Z due to valley fog. VFR otherwise, but afternoon thunderstorms could bring TEMPO IFR to MVFR restrictions.

Saturday-Sunday: Mainly VFR. However, nighttime/early morning valley fog may bring IFR to LIFR restrictions, mainly at HIE and LEB.

Monday: TEMPO IFR to MVFR possible due to afternoon/evening SHRA/TSRA. Otherwise, VFR.

MARINE

Winds shift southwesterly tonight with that being the prevailing wind through Thursday. High pressure builds back over the Atlantic keeping wind gusts and waves generally below SCA thresholds. The exception may come Thursday night as high pressure moves off to the east and some 5ft seas creep into the outer coastal waters, but at this time the extent does not warrant an SCA.

Friday-Wednesday...SCA conditions are not expected through at least the weekend. A weak cold front crosses Friday with a brief wind shift to the north later in the day into Friday night. High pressure then builds across the Northeastern US over the weekend before shifting to the east early next week. As the high shifts east, south to southwest flow may approach SCA levels during the early to middle parts of next week.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...Heat Advisory from 11 AM to 8 PM EDT Thursday for NHZ012-013. MARINE...None.