textproduct: Gray - Portland

WHAT HAS CHANGED

No significant changes with this forecast update.

KEY MESSAGES

1. Temperatures trend upward on Monday with a chance of afternoon and evening showers.

2. Hot and humid conditions are likely Tuesday with widespread heat indices of 95 to 100 degrees south of the mountains. Humidity abates Wednesday while hot conditions linger into Thursday with highs in the upper 80s to low 90s.

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

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Diurnal cu will fade after sunset, but some we'll have some high clouds move in tonight. Winds will again be light and am not expecting the high clouds to curb radiational cooling. So low temperatures are again closer to the cooler MOS and mostly in the 50s. The exceptions are northern valleys could see the upper 40s, and parts of southern NH may stay in the lower 60s.

Temperatures start to climb on Monday as the center of high pressure continues to move east, and south to south winds carry a warmer air mass into the region. Good mixing and 850mb temperatures of +15C to +17C supports highs mostly in the mid to upper 80s, but there could be some 90F reading in southern NH and far SW ME. Fortunately, dewpoints should generally remain in the 50s, keeping heat index values close to the temperatures. The seabreeze should keep highs for the coastline in the upper 70s to lower 80s, especially on western ME.

The first part of the day will be dry, but models are honing in on a 500mb shortwave/vort max moving through in the afternoon and evening, that will bring a chance for showers with the higher of the chances across the northern half of NH and into western ME. Chances for thunderstorms on forecast soundings look pretty marginal, so have held off adding these to the forecast for the time bring.

A couple of showers may approach the far north Monday night, but it will otherwise be dry. Light S/SW flow and the slightly higher dewpoints will keep temps milder overnight, putting lows mostly in the 60s, except some 50s across the north. Patchy fog may develop in some areas overnight, especially for those that receive rain.

KEY MESSAGE 2 DESCRIPTION...

No significant changes in forecast thinking regarding a short period of heat and humidity Tuesday. Models remain in decent agreement that a piece of the hot air mass brewing over the northern Plains will advect into New England with T8s climbing to +20C to +23C. Low level southwest flow will advect higher dew point air into the region while mixing to around 850 mb should keep dew points from climbing into the oppressive territory of +70F. Nevertheless, highs well into the 90s combined with dew points in the mid to upper 60s will bring widespread heat indices into the 95-100 degree range south of the mountains.

A cold front will be approaching from the northwest Tuesday evening and will push offshore Wednesday morning. Winds ahead of the front will likely inhibit cooling Tuesday night leading to lows staying in the 70s south of the mountains. The front will drop dew points into 50s on Wednesday while the overall air mass remains very warm. Winds will be gusty Wednesday out of the WNW with peak gusts around 30 mph. The combination of the warm air mass and downsloping winds will push highs into the low 90s across southern NH and the coastal plain of Maine. Troughing will take up residence over the Northeast through the end of the week leading to a steady downward trend in temperatures.

KEY MESSAGE 3 DESCRIPTION...

Latest round of NWP models continue to show an environment conducive to severe thunderstorms over northern New England Tuesday afternoon into the first half of Tuesday night. The ECMWF EFI now shows the overlap of CAPE and shear will approach the 90th percentile south of the mountains and the 99th percentile north of the mountains. Available CAM guidance suggests storms will form along a cold front dropping south through Quebec Tuesday afternoon with a broken line of storms approaching the International Border Tuesday evening. There are some timing differences amongst models which will play a role in strength and coverage of storms as well as the potential for any storms to form ahead of the front. These finer detains will come into view within 48 hours of the event so the 12Z runs tomorrow should bring things into focus. At this time, deep layer shear in excess of 50 kts and MU CAPE in excess of 2000 J/kg will support super cell structures and steep mid level lapse rates around 7.5C/km will promote large hail. If a broken line of storms materializes as advertised by CAMs damaging winds will be the primary threat followed by hail. Modest curvature in low level wind fields also suggest a tornado cannot be ruled out. SPC maintains a Slight Risk across the northern half of the area and a Marginal through all but all of southern NH. SPC also has significant severe hatching to suggest that storms could be capable of winds to 75 mph and hail size to 2 inches.

Again, there is some variance in timing while a multi model consensus suggest storms will arrive at the International Border between 5 and 8 pm. Storms will quickly track SE across the area through midnight. Steep mid level lapse rates and strong height falls will allow storms to sustain strength after sunset with a gradual weakening trend through the first half of Tuesday night.

AVIATION /00Z MONDAY THROUGH FRIDAY/

Through 00Z Tuesday...Prevailing VFR through Monday morning. Chances appear too low for TAF inclusion of fog at HIE and LEB tonight. An incoming disturbance may bring a period of showers and slight chance TS Monday afternoon for points north and east of SFM to LEB. Brief MVFR conditions are possible in these.

Outlook...

Monday night: Mainly VFR, but patchy fog could bring localized IFR restrictions, especially for any sites that receive rain on Monday. A period of LLWS is possible early towards RKD.

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

Wednesday-Friday: VFR conditions likely.

MARINE

Through Monday night...High pressure center moves east of the waters tonight bringing a south to southwest wind flow. Winds increase Monday afternoon and evening, possibly bringing a period of SCA conditions through Monday night, mainly outside of the bays.

An approaching cold front will bring increasing SW flow Tuesday with gusts 25-30 kts by Tuesday afternoon. Seas will also build 5-7 feet late Tuesday through Tuesday night. The front crosses Wednesday morning shifting winds offshore while winds and seas likely drop below SCA thresholds. Winds generally stay WSW through the end of the week with conditions remaining below SCA thresholds.

GYX WATCHES/WARNINGS/ADVISORIES

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