textproduct: Gray - Portland

WHAT HAS CHANGED

The chance for strong to severe storms has shifted east to include portions of New Hampshire through this evening.

KEY MESSAGES

1. A cold front will bring the chance for showers and thunderstorms this afternoon across New Hampshire and western Maine, and again for southern Maine on Friday. Storms may contain gusty winds and small hail.

2. Lower dew points and pleasant temperatures for the weekend. Mostly dry with just a low chance of rain Sunday afternoon/night. Humidity rebounds by early next week, with uncertainty over temperatures.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

An elongated cold front approaches New England today, with showers and thunderstorms expected to develop ahead of it.

SW low level flow increases this morning as a ridge exits east. This will aid in the advection of moisture in the low levels this morning, with dewpoint temps in the lower 60s forecast. This will make it feel more humid today, but also provide additional fuel for cumulus development and eventual showers and thunderstorms.

Surface CAPE increases across NH and far western ME late this morning. HREF members keep a wide spread of CAPE values for this region, but there is more agreement that points towards the ME Midcoast and Kennebec Valley remaining more stable through this evening, thus primary focus will be points to the west.

Overlapping shear is most notable in the low levels (0-1km 15 to 25 kts) while deeper shear is marginal (0-6km 20 to 35 kts) owning to weak jet dynamics aloft. There is some improvement to LLJ velocity into the evening, but this is when daytime heating will be waning and limiting remaining storm strength.

While these variables strike a good balance for the potential of some stronger storms, weak lapse rates could limit storm severity and coverage. This leads to gusty winds/hail as the primary threat for this afternoon.

A few storms may continue into northern NH into the evening hours, with some isolated showers continuing into the overnight hours. The associated cold front will then move overhead, with daytime heating Friday again initiating showers and storms further into southern Maine and perhaps the NH Seacoast. The main moisture plume will have bisected the CWA, with the weakening front providing lift from the foothills east. CAMs show some redeveloping convection in this region to the coastal waters, although wind/hail threat appears lower at this time.

KEY MESSAGE 2 DESCRIPTION...

The cold front clears our region by early Saturday morning, setting us up for a fairly pleasant final weekend of meteorological Summer. Drying westerly flow out of Canada combined with a weak high pressure cresting over the region will usher in much drier conditions with dew points falling to the 40s/50s. Both Friday and Saturday night may have strong radiational cooling, with the potential for valley fog development.

Sunday will be a bit of a transition day, with some guidance ushering in showers to our northern zones later in the day while other outcomes keep it mostly dry. In any case expect a second day of low humidity with increasing clouds by afternoon, and the chance for showers in the north, potentially becoming more widespread by night. Highs will be near to slightly above seasonal averages both days.

Looking towards next week, the high pressure will shift south towards the Southeast U.S. coast. Combined with a deepening trough over Canada, this puts us back into an active storm track with likely a few disturbances riding on strong upper-level flow between these two features. Generally expect to see increasing humidity from Monday onward. Temperatures are more uncertain, as 500 mb height anomalies start to really tick up later into next week, but clouds/precipitation may put a limit on temperatures.

AVIATION /02Z THURSDAY THROUGH MONDAY/

Through 06z Friday...High pressure continues VFR conditions overnight. Patchy fog will be possible along the valleys (LEB and HIE) and around the coast at RKD, lifting by 12z to 14z Thurs morning. VFR Thursday with increasing southerly winds by early afternoon. SHRA with isolated TS develop across NH during the afternoon and move into western ME for the evening. There remains uncertainty how widespread SHRA/TS will be into ME in the evening, as activity is expected to lessen towards midnight local.

Outlook:

Thursday night-Friday: Generally MVFR conditions expected with showers. IFR conditions possible in fog at night.

Friday night: IFR conditions possible in valley fog at night, otherwise VFR.

Saturday-Sunday: Any early MVFR/IFR conditions should quickly improve to VFR Saturday with VFR the prevailing condition through the weekend.

Sunday night-Monday: MVFR conditions possible as ceilings thicken and lower and showers develop.

MARINE

Areas of marine fog/stratus will linger through the coastal waters as humidity increases today and Friday. Southerly winds remain over the waters through this period, with a few gusts to 25 kt this afternoon.

High pressure then builds across the waters for the weekend, bringing an extended period of fair conditions into early next week.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...None.