textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes were made to the forecast at this time.

KEY MESSAGES

1. High pressure brings mostly dry weather and seasonably warm temperatures with comfortable humidity through today.

2. Hot and humid conditions bring Moderate to Major Heat Risk Monday- Wednesday with the peak occuring on Tuesday. Humidity abates the second half of the week, while temperatures continue to run above normal.

3. A cold front brings the threat for severe storms late Tuesday into Tuesday night.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Pleasant conditions will persist today as surface high pressure remains over New England. Mostly sunny skies are expected with high temperatures into the lower to middle 80s. Light surface flow will likely allow for a sea breeze to develop by early afternoon before pushing inland through the remainder of the afternoon and early evening.

Clear to partly cloudy skies tonight will allow for another night of radiational cooling with lows falling into the upper 40s across the northern valleys with middle to upper 50s elsewhere. Valley fog is once again possible as well.

KEY MESSAGE 2 DESCRIPTION...

Long wave ridging over the central U.S. will spread eastward Monday into Tuesday, allowing H85 temperatures to warm to around +20C with H925 temperatures nearing +30C. This will allow high temperatures to reach the lower to middle 90s on Monday with middle to upper 90s on Tuesday. Dew points will remain comfortable into the 50s/lower 60s on Monday before rising into the middle to upper 60s on Tuesday. This will allow afternoon heat indices to approach the 100 degree mark on Tuesday and therefore Heat Advisories will likely eventually be needed for portions of the forecast area. Overnight lows on Tuesday night will likely only be into the 70s, resulting in cumulative heat stress.

A drier airmass will arrive on Wednesday behind a cold front with comfortable humidity likely through the remainder of the week despite continued warm temperatures.

KEY MESSAGE 3 DESCRIPTION...

A rather robust trough will track across Quebec Tuesday (with an attendant surface low down to around 990 mb). Due to the upstream ridge over the central U.S., our region will be under northwest flow aloft. In advance of a surface cold front (which looks to arrive here Wednesday morning), we are expecting to see a rather favorable environment for severe storms over our region. The feature of particular interest for this setup is an elevated mixed layer (EML) moving over the region; steep mid- level lapse rates on the order of 7-7.5 C/Km. Combined with strong surface heating and dew points rising to the upper 60s/low 70s, plenty of instability will be available with MUCAPE values surging to 2000-3000 J/Kg, which is quite high even for July. What concerns me however is this instability looks to be paired with unusually strong wind energy. 0- 6 km bulk wind shear is on the order of 45-60 kts, while 0-3 km SRH is progged at 200-300 m2/s2, indicating very favorable speed and directional shear. This combination of instability and shear suggests a rather volatile parameter space for organized and long- lived severe storms, should they arrive, that would be capable of all severe hazards.

Major uncertainty remains on storm coverage/timing. Convection should initially fire well to our north (closer to the surface trough initiating boundary) during the afternoon. Storms, should they make it to northern New England, will likely be arriving in the evening, well past peak heating hours. This makes for a tricky forecast, as climatology typically favors weakening after the loss of daytime heating. However, the rare setup of having an EML and strong wind shear/forcing makes for an unusual scenario where storms may persist well into the night, unlike most other events. Given we are still around three days out, and CAMs are not into range yet, it still remains a wait and see for impacts. There is the chance the trough arrival could slow down, keeping the storm threat almost entirely in Canada. On the contrary, a faster trough arrival suggests convection may impact a larger swath of New England. SPC has maintained a 15% chance of severe, north of a Concord-Portland- Rockland line.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

Through 06Z Monday: VFR conditions expected. The exception is at KLEB and KHIE where valley FG is possible after 06Z Sunday through around 12Z Sunday and then again tonight. Light and variable winds are expected, although a sea breeze is likely at KPWM, KPSM, and KRKD with S winds at 5-15 kts. No LLWS is anticipated through the period.

Outlook:

Monday/Monday Night: VFR conditions.

Tuesday/Tuesday Night: VFR conditions during the daytime before possible SHRA/TSRA arrives late day or at night, bringing localized restrictions.

Wednesday-Thursday: VFR conditions likely.

MARINE

S winds may gust up to 25 kts late Monday into early Tuesday with seas outside of the bays building to 3-6 ft. W-SW winds may then approach 30 kts late Tuesday into Tuesday night with seas at 3-6 ft outside of the bays.

GYX WATCHES/WARNINGS/ADVISORIES

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