textproduct: Gray - Portland

WHAT HAS CHANGED

Temperatures have trended down a few degrees for today and Friday.

KEY MESSAGES

1. Temperatures will run above normal today into Friday before a cold front crosses Friday evening.

2. Cooler, crisp conditions from the weekend onward. Sunday night- Monday brings the only window of opportunity for rain with a cold front.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Mid level ridging along the East Coast is looking less sturdy for today than guidance has portrayed over the past few forecast cycles. Short wave over Upstate NY will cross after day break and will likely maintain mostly cloudy skies. Latest water vapor imagery shows plenty of moisture traversing east across the Great Lakes region while much of model guidance suggests there will be at least partly sunny skies in the wake of the short wave. Cloud cover is then forecast to increase this afternoon as the moisture from the Great Lakes moves overhead. Overall, still looking at a warm day with temperatures hanging closer to the mid to low 70s. A few light showers cannot be ruled out across NH this afternoon while today will likely be mostly dry. Southwest flow tonight will aid in keeping temperatures in the 60s for lows and limit the potential for fog.

A cold front will approach from the NW Friday and cross during the afternoon and evening. A warm airmass will be in place ahead of the front and temperatures climbing into the 70s shouldn't be a problem. However, guidance has trended faster with the FROPA and increasing clouds have resulted in the downward trends in highs on Friday. The front will bring chances for showers and perhaps some instances of thunder across the area from late morning into the afternoon. Rainfall amounts are unimpressive with much of the area averaging less than a tenth of an inch while some locally higher amounts are possible in any convection.

KEY MESSAGE 2 DESCRIPTION...

Seasonably strong cold air advection behind Friday night's front as 850 mb temps nosedive to 2-5 C by Saturday evening, which will be quite an airmass change. Even more impressive is dew points falling from the 60s Friday to 30s on Saturday, in some locations a 30 degree drop within 18-24 hours! Highs Saturday will still reach the 60s with plenty of sunshine, but it will certainly be a very different airmass from Friday with a true crisp fall feel!

Very strong agreement on global guidance in another dynamic trough and attendant cold front approaching the region late Sunday then crossing through Monday. Similar to the first wave, a bit of a glancing blow with the surface low and attendant vort max well to the north up in Quebec. Guidance is currently split as ECMWF members show some southern wave energy clipping the southern half of the forecast area Sunday night, as well as some precipitation again Monday during the day in northern zones close to the best forcing along the front. GFS solutions meanwhile show these features splitting the region with a mostly dry frontal passage. For now the rainfall potential is still up in the air, but even with wetter solutions, totals are not likely to be impressive given a progressive setup combined with a lack of deep moisture. Most likely impacts will be increasing cloud cover Sunday and evening rain chances, with clearing conditions Monday afternoon. Both days should feature highs near 60 to mid 60s, but Monday may be cooler in the higher elevations as well as potentially breezy behind the front.

Very likely the coldest weather since the spring arrives Tuesday onward with 850 mb temperatures plunging to between 0 and -7C by Tuesday morning. Even 925 mb temperatures are near or slightly below freezing on guidance. If the boundary layer can successfully decouple with winds slacking, it is likely a significant chunk of the forecast area could see the first hard freeze of the season Tuesday and/or Wednesday. Even with full mixing/sunshine, highs may struggle to reach 60 Tuesday-Wednesday given the very chilly air aloft.

AVIATION /12Z THURSDAY THROUGH MONDAY/

Through 06Z Friday...Fog and low cigs with LIFR conditions to PWM, AUG, RKD, and LEB lingers into mid-morning. VFR likely prevails the rest of today while marine fog may linger near PWM and RKD into the afternoon. Mostly VFR Thursday night. Again, valley fog could develop at HIE and LEB, but more clouds ahead an approaching front may limit development. This front could bring a few showers to HIE late. LLWS is possible.

Outlook:

Friday: Areas of MVFR or lower possible in rain showers.

Saturday: VFR conditions expected. Local IFR or lower overnight in valley fog.

Sunday: VFR conditions expected. Local MVFR or lower conditions possible in rain showers.

Monday: VFR conditions expected.

Tuesday: VFR conditions expected. Westerly winds may gust up to 25 kt.

MARINE

High pressure remains centered south of New England today and will keep prevailing southwest winds across the waters. These begin to increase late today into Friday morning as a cold front approaches but are expected to remain below SCA levels. The cold front crosses Friday night shifting winds offshore with gusts 15 to 20 kts towards Saturday morning

Winds and seas are expected to remain below SCA thresholds through the weekend. Stronger CAA across the warm waters is likely to peak around Tuesday. This will be the most likely period for SCA conditions across the coastal waters and perhaps into the bays as well.

GYX WATCHES/WARNINGS/ADVISORIES

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