textproduct: Gray - Portland

WHAT HAS CHANGED

625 AM Update...The only change to the going forecast for today was to add thunder to portions of eastern NH and central ME later this afternoon and evening as convective parameters and CAM lightning forecasts support isold thunder.

Previously...

No significant changes to the forecast with this update, except to beef up the wording of intensity of snowfall for brief periods during the afternoon and evening.

KEY MESSAGES

1. Snow returns today. It may be briefly heavy at times both during the afternoon and again in the evening.

2. A warming trend with direr weather begins midweek, with above normal temperatures by the end of the work week. We return to a more active pattern this weekend.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Potent, compact shortwave trof still forecast to affect the area today. It looks to be quite a dynamic little system with strong forcing for ascent. The shortwave itself will provide some of that, but very steep lapse are forecast to be present from afternoon thru evening. In some model guidance lapse rates approach 7.5 or 8 C/km in a near saturated environment. If this is realized, lift may be locally quite strong. The lapse rates will also serve to keep precip types mainly snow, especially if it is precipitating hard enough. If precip is really light it is possible some rain mixes in, but otherwise surface temps will not matter much. There are also two rounds of precip to watch. One with the initial shortwave arrival. This is mainly forecast to impact the southern half of the forecast area. Briefly moderate to heavy snow may lead to an inch or two accumulation, but totals may tend to be inconsistent rather than uniform across that area. The second will be with the secondary cold front and will tend more towards an organized band of snow squalls. Model guidance is showing fairly consistent intense snowfall rates north to south thru the line of snow showers. Accumulations of 1 to 2 inches in just a single hour are possible, during the later part of the evening commute. Given how steep lapse rates are forecast to be, I would not be surprised if there was some lightning as well. It is a tough call on potential headlines, as the lateness of the season and potential intensity of snowfall rates at times could lead to a higher impact, but sub- advisory event. At the same time there seems to be fair chance that some snow squall warnings may be needed during the evening hours.

KEY MESSAGE 2 DESCRIPTION...

Strong high pressure enters the region Wednesday which ushers in drier weather, but cooler air remains in place as northwesterly flow continues. Dewpoints look to drop into the single digits and teens with temperatures climbing into the 40s. This equates to minimum relative humidity values in the 20-30% range, but fortunately with high pressure moving overhead wind gusts should be light enough to preclude fire weather concerns. Thursday may be a different story as flow transitions to southwesterly and high pressure pulls off to the east. This results in a tightening pressure gradient which could bring wind gusts up to 20-25 mph. Southwesterly flow brings dewpoints up slightly, but with a clear well mixed day I expect them to stay below guidance and with temperatures into the mid to upper 50s outside of the coastal plain, another day of RH values 20-30% is not out of the question. This window will have to be watched closely for fire weather concerns.

Dewpoints come up quite a bit by Friday with continued southwesterly flow. Temperatures come up quite a bit as well with most locations seeing 60s right down to the immediate coast, but the immediate coast likely stays a little bit cooler thanks to a seabreeze. Friday ends up a very mild night with long range guidance shifting rain into the Friday night-Saturday morning window and trending the bulk of Saturday drier. This is still out in time and models are notorious for having trouble with the speed of fronts so we will continue to monitor the trends. There is great agreement however that high pressure takes control again Sunday. Ensembles are in reasonable agreement that this is short lived as it exits Sunday night and we return to a more active pattern heading into next week.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

VFR conditions forecast thru at least sunrise for much of the area. CIGs begin to lower in the mid morning and are most likely to drop to MVFR across the southern half of the forecast area. Where exactly it snows hard enough to reduce VIS to IFR or lower is a little harder to pin down, but at this time southern NH into southwestern Maine is most likely. Both during the afternoon and again in the evening there are short windows where snow may be heavy at times. MVFR CIGs are expected thru most of the day Tue before improving.

Outlook... Wednesday-Friday: VFR prevails for much of the week as we enter a period of quiet weather. HIE may begin to see MVFR become more prevalent for Friday.

Friday night: May see some expansion of MVFR ceilings outside of the mountains as rain approaches the area.

Saturday: MVFR/IFR restrictions possible through the morning as rain tapers off. The second half of the day should see improvement to VFR as high pressure moves in.

MARINE

Winds and seas expected to remain below SCA thresholds thru the afternoon. Late in the day northeast winds will start to gust to SCA. High pressure moves over the waters on Wednesday bringing tranquil conditions to the waters. This should generally be the case through the day Friday, with the exception of a brief period of SCA conditions Thursday afternoon and evening as high pressure pulling away tightens the pressure gradient over the waters. Friday night through Saturday night may see SCA waves as a cold front sweeps through. High pressure returns Sunday which should bring a brief return of sub-SCA conditions, but a return to an active weather pattern next week may bring SCA conditions back quickly.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory from 8 PM this evening to 8 AM EDT Wednesday for ANZ150>154.