textproduct: Gray - Portland

WHAT HAS CHANGED

A Heat Advisory has been issued for most of New Hampshire and Western Maine.

Confidence continues to increase for a round of strong to severe thunderstorms in the Tuesday evening or overnight timeframe.

KEY MESSAGES

1. Heat builds today, with temperatures and humidity combining on Tuesday for heat indices of 95 to 100 degrees south of the mountains. Humidity abates Wednesday, but hot conditions linger into Thursday with highs in the upper 80s to low 90s.

2. Strong to severe thunderstorms track southeast across the area starting Tuesday evening into Tuesday night.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Warmer today as the center of high pressure moves east and WAA arrives on west to southwest winds. Temperatures will tend to preempt humidity (a factor for tomorrow), with interior temperatures climbing into the mid to upper 80s and a few 90 degree readings for southern NH. The SW winds at the surface once mixing begins will enable a seabreeze to work its way inland along the ME Midcoast and areas from Casco Bay north. These winds may act a little more suppressive for a seabreeze south of Casco Bay (ie the York and NH Seacoast). As a result, mid afternoon temps could retreat towards the upper 70s along the Midcoast.

The other feature of note Monday will be a weak embedded shortwave kicking off some showers and storms across Quebec that progress eastward towards the US/CAN border by late afternoon and early evening. What instability is available (especially surface based) will be waning, so think this will fall mainly as showers vs. thunder. A light wind and any remaining cloud cover should keep Monday night more mild vs. recent nights, with low only falling into the 60s areawide.

Monday night into Tuesday, record max climo 500mb heights across the Upper Midwest flatten, with low level winds ushering a plume of similarly rated 850mb temperatures towards the eastern Great Lakes and New England Tuesday afternoon. This plume should be moving quicker than our last heat episode, but will still result in a day where a Heat Advisory for most of the area will be needed.

Not only will 850mb temps around 20 to 22 C push mix down temps into the low to mid 90s, but a larger surge of low level moisture should raise surface dew point temps into the upper 60s to around 70 degrees. The result is widespread heat indices between 95 and 100 for Tuesday afternoon south of the mountains. Stronger background SW winds will make it more difficult for relief via seabreeze for coastal communities, especially for the Cumberland coast south.

Just as abruptly the push of heat and humidity arrived, a cold front w/ some convection (noted below) should tamp down max temps and humidity for Wednesday. However, there is some uncertainty how quickly this front progresses Tues night and a warm, mild night with lows in the 70s seems likely. This does create a opportunity to start very warm Wednesday morning, and daytime highs could easily climb back into the 90s for much of southern NH and far southern ME, albeit with less moisture/humidity. Given this trend, still see Tuesday as the primary day for heat related headlines, but still expect a followup hot day Wednesday.

Overnight lows Wed night and then Thurs night improve, falling into the 50s/60s and then upper 40s/50s respectively.

KEY MESSAGE 2 DESCRIPTION...

A strong shortwave trough and surface cold front will approach from the north on Tuesday before crossing Tuesday evening and night. This will serve as a lifting mechanism for the development of strong to severe thunderstorms Tuesday evening into the first half of Tuesday night. The strongest forcing for ascent is not likely to arrive until after sunset but despite this, latest CAMs and ensemble based guidance continue to show that the environment will remain favorable for severe storms even after daytime heating wanes.

Latest guidance including the RRFS suggest that storms will form across southern Quebec on Tuesday afternoon along the cold front before dropping south towards the Canadian Border as broken line segments on Tuesday evening (roughly between 5-8 pm). This activity is then likely to cross the region from north to south through the first half of Tuesday night. Forecast soundings show roughly between 1500-2000 J/KG of MUCAPE along with between 50-60 kts of 0-6km bulk shear. Several forecast models also show an elevated mixed layer (EML) with steep mid- lvl lapse rates of up to 7.5C/km. These parameters will support both large hail and damaging winds along with a conditional tornado threat should supercells exist. SPC has added an ENH Risk (level 3/5) for far northern zones near the International Border and maintains a SLGT Risk (level 2/5) across the remainder of the northern half of the forecast area, with a MRGL risk (level 1/5) across southern NH and coastal western ME.

AVIATION /01Z MONDAY THROUGH FRIDAY/

Through 06z Tuesday...Prevailing VFR through the morning. SW winds likely suppress afternoon seabreeze for PSM and perhaps PWM, but still could get a southerly push through to RKD. An incoming disturbance may bring a period of showers and slight chance TS this afternoon for points north and east of SFM to LEB. Brief MVFR conditions are possible in these.

Outlook:

Tuesday/Tuesday Night: VFR conditions through 18Z Tuesday with SW wind gusts up to 25 kts. SHRA and TSRA will then move from north to south Tuesday evening through the first half of Tuesday night, bringing localized LIFR restrictions. Storms may contain hail and damaging winds.

Wednesday/Wednesday Night: VFR conditions with daytime NW wind gusts up to 25 kts.

Thursday-Friday: VFR conditions with WNW flow prevailing.

MARINE

Below SCA conditions through this morning, but southerly gusts increase this afternoon 25 to 30 kt. As a result, wind wave should increase, to around 5 ft this evening.

SCA conditions likely continue outside of the bays Tuesday afternoon into Wednesday morning with SW wind gusts up to 25 kts and seas of 3- 6 ft. Otherwise winds and seas should remain below SCA criteria through Friday.

GYX WATCHES/WARNINGS/ADVISORIES

ME...Heat Advisory from 11 AM to 8 PM EDT Tuesday for MEZ012>014- 018>021-023-033. NH...Heat Advisory from 11 AM to 8 PM EDT Tuesday for NHZ002>015. MARINE...Small Craft Advisory from 4 PM this afternoon to 6 AM EDT Wednesday for ANZ150-152. Small Craft Advisory from 6 AM Tuesday to 6 AM EDT Wednesday for ANZ151.