textproduct: Gray - Portland

WHAT HAS CHANGED

610 AM Update...The convection allowing models have shown varying coverages of thunderstorms this afternoon, but mainly in the central and northern zones. Highest PoPs continue to be in the north but did add some chancy PoPs elsewhere for this afternoon and evening.

KEY MESSAGES

1. Heat and humidity returns today. It is only expected to last through Friday before temperatures return to more seasonable readings.

2. As a cold front approaches the region showers and thunderstorms will be possible during the afternoon hours today and Friday. Some of these storms may be strong to severe with strong wind gusts the most likely hazard.

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...

Deep southwesterly flow will draw and heat and humidity back north today. There has not been much change in modeling since the last update, with guidance still favoring 850 mb temps in the +16 to +18C range. This is supportive of upper 80s to near 90 high temps. Southwesterly downsloping may allow the Merrimack Valley to tack on a couple extra degrees. Couple that with the humidity and apparent temps will be in the mid 60s from ASH/MHT eastward to the Seacoast. The Heat Advisory area looks good for now. Encroaching cloud cover will limit the northward extent of the heat.

This will be even more true of Fri. Mid level temps will get knocked back a degree or two, and as a result high temps will be more like the mid to upper 80s. Especially with the dewpoints remaining high, it will will still feel pretty steamy, but I do not anticipate the need for an additional Heat Advisory at this time.

KEY MESSAGE 2 DESCRIPTION...

Showers and thunderstorms will be possible thru Fri as the front slowly crosses the region. The thermodynamic environment will be fairly marginal, with weaker lapse rates aloft. However we will attempt to make up for that with the high heat and humidity south of the front. Bulk shear today should be at least on the order of 25 kt, and so any storm that does get going may stay organized. Largely unidirectional winds and drying aloft will support gusty to damaging wind as the primary hazard. At this time SPC only has a small portion of Coos County in the marginal risk for severe storms, but I could see that getting expanded south and eastward.

Similarly on Fri the threat will push farther south as the front continues to sag into the area. Overall the forecast soundings Fri have more moisture, so we can add torrential rainfall as another hazard. Once again marginal CAPE and shear profile will support isolated strong to severe storms. Wind remains the primary storm hazard. On both days various machine learning convective guidance indicates marginal risk of severe weather. So I could see the new Day 2 severe weather outlook including a marginal risk in the, or at the very least the need to be prepared for stronger storms Fri afternoon.

KEY MESSAGE 3 DESCRIPTION...

Surface high pressure will build over New England this weekend, allowing for warm temperatures and mainly dry conditions. A few diurnally driven scattered showers and thunderstorms cannot be ruled out though, especially on Saturday as an H5 s/wv trof slides across the region.

A long wave H5 ridge axis will gradually move east for the early to middle part of next week. Warm temperatures and mainly dry conditions are expected on Monday and Tuesday before an approaching frontal boundary and H5 s/wv trof moves towards the area Tuesday night into Wednesday, possibly resulting in showers and thunderstorms. The timing of this trof and front will play a large role in temperatures as well as the timing of greatest rain chances for the middle to latter half of next week.

AVIATION /10Z THURSDAY THROUGH MONDAY/

VFR conditions continue through this afternoon with LEB and HIE possible seeing localized IFR conditions after midnight as valley fog develops again tonight. This is more or less a persistence based forecast with a slight delay in timing due to drier conditions. Any valley fog that forms should clear out quickly after sunrise with the prevailing condition turning back to VFR. Some TEMPO MVFR is possible in showers and thunderstorms this afternoon but confidence is too low to add to TAFs at this time.

As the front gets closer there will be a growing threat for marine fog and stratus, especially towards Fri morning.

Outlook: Friday: VFR conditions expected to prevail over most of the area. IFR or lower possible in fog/stratus over the Midcoast. Local MVFR or lower possible in showers/thunderstorms during the afternoon across the southern half of the area.

Saturday-Sunday Night: Mainly VFR conditions expected. TEMPO restrictions are possible Saturday PM within possible SHRA/-TSRA.

MARINE

Winds and seas are expected to remain below SCA thresholds. Areas of fog likely begin to develop tonight and continue thru Fri until the front crosses the waters. Some strong thunderstorms are possible Fri afternoon over the nearshore waters.

Outlook:

A brief period of SCA conditions are possible Monday evening outside of the bays with S wind gusts up to 25 kts and seas of 3-5 ft. Otherwise, winds and seas will largely remain below SCA criteria through early next week.

GYX WATCHES/WARNINGS/ADVISORIES

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