textproduct: Gray - Portland

WHAT HAS CHANGED

A heat advisory has been issued for portions of southeastern New Hampshire, southern and MidCoast Maine for Friday.

KEY MESSAGES

1. Hot and humid conditions continue into Friday. There will chances for showers and isolated thunderstorms through this evening, mainly across NH.

2. A mid level disturbance will bring a more robust chance for showers and thunderstorms Saturday.

3. Looking ahead to next week...the greatest threat for shower/storm activity will be Tuesday with a gradual cooling trend expected through the week.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Latest satellite and radar show some cumulus starting to blossom across the area with scattered showers and thunderstorms to the west across upstate NY. CAMs generally agree that much of the thunderstorm activity will remain west of the area as deep layer ridging holds over eastern Maine. However, instability is expected to build into this evening with SB CAPE climbing above 2000 J/kg across NH. This may allow for a few storms that form to the west to sustain themselves as they cross into NH. Any strong storm that can sustain itself will be capable of gusty winds and torrential rainfall given high PWATs in place, while the threat for severe storms and flash flooding is low. Any thunderstorm activity will diminish with the loss of heating later this evening. It will remain warm and humid tonight with lows in the upper 60s to low 70s with patchy fog possible across much of the area, especially in river valleys and near the coast.

Humidity will expand northward Friday as much of the area sees dew points rise into the upper 60s to low 70s. These high dew points combined with temperatures in the upper 80s to low 90s will bring heat indices into the mid 90s. I have issued a Heat Advisory across SE NH through the coastal plain of Maine where confidence is highest for heat indices greater than 95F. While there will be plenty of instability for thunderstorms the lack of forcing will keep coverage isolated at best with much of the area likely seeing mostly sunny skies. It will remain warm and humid Friday night with lows in the upper 60s.

KEY MESSAGE 2 DESCRIPTION...Shortwave embedded within warm southwest flow aloft will bring a more widespread chance for showers and thunderstorms on Saturday. The overall pattern shows modest flow aloft /H5 winds to 30kts/ with PWATs over 1.75" given the warm/humid airmass in place. There is good agreement through the ensemble/deterministic suite of measurable rain...but bigger questions remain on how much of the day will be rainy as there is the lack of a defined surface front to organize convection. There are a number of model solutions that bring convection in early...which limits the severe threat through the day...but would result in more of a washout with localized downpours and isolated flooding possible. Less morning convection would allow for more instability and a marginal severe weather threat given the aforementioned moderate wind fields aloft. Machine learning guidance indicates a severe wind threat which correlates well to the wind field aloft. At this range...will continue with likely/ categorical PoPs for much of the northern 2/3rds of the forecast area /best flow aloft/ with chance to the south. While it will remain very muggy...a decent amount of cloud cover will likely keep temperatures cooler than on Friday...with highs generally in the 80s throughout the area. Heat indices will likely still reach 90 south of the foothills.

KEY MESSAGE 3 DESCRIPTION...The week will generally be dominated by west northwest flow aloft with modest ensemble agreement for another precipitation event in the Tuesday/Tuesday night timeframe given a fast-moving shortwave embedded within this flow aloft. While there are differences in the surface details /particularly given influence from convection/ a frontal passage appears likely in this timeframe. There would be a conditional severe threat given robust mid level flow...but greater questions surround timing and thus instability potential. Will continue to message high chance to likely PoPs for this period and keep an eye on this timeframe for any potential severe / flash flooding threat...though a widespread event does not appear likely at this range. We turn quieter for Wednesday...with another...more substantial cold front arriving to end the week...bringing the potential for a more substantial cool down by next weekend. For the Tuesday-Thursday period...temperatures should generally be close to normal for this time of year...with highs in the 70s to 80s from north to south.

AVIATION /18Z THURSDAY THROUGH TUESDAY/

Through Friday night: Mainly VFR through this evening, while TSRA cannot be ruled out at LEB and HIE. Patchy fog is possible across much of the area tonight with fog likely in the valleys and near the coast. This will bring IFR/LIFR to LEB, HIE, RKD, and AUG with possible restrictions at PWM and CON. VFR is likely Friday with patchy fog again possible Friday night.

Outlook: Restrictions are likely throughout the area with showers and thunderstorms on Saturday with some fog development possible behind this activity Saturday night. VFR conditions are expected to dominate much of Sunday-Monday outside of a mountain shower/storm Sunday afternoon. A frontal system will bring additional showers and storms on Tuesday with an additional threat for restrictions.

MARINE

High pressure remains in control over the waters through Friday allowing for winds and seas to remain below SCA thresholds. Increasing moisture will likely bring fog over the waters, especially overnight.

Outlook: Winds and waves are expected to remain below SCA thresholds for Saturday through the middle of next week. Localized gusty winds due to thunderstorms will be possible on Saturday and again on Tuesday of next week.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Heat Advisory from 11 AM to 8 PM EDT Friday for MEZ014-018>028- 033. NH...Heat Advisory from 11 AM to 8 PM EDT Friday for NHZ010-012>014. Heat Advisory until 8 PM EDT this evening for NHZ012-013. MARINE...None.