textproduct: Gray - Portland

WHAT HAS CHANGED

Slightly increased snowfall amounts Tuesday with fast moving system, otherwise no significant forecast changes.

KEY MESSAGES

1. Winter is not ready to give up quite yet, with a fast moving, light snowfall possible Tuesday.

2. A warming trend begins midweek with above normal temperatures by the end of the work week. Then, the weekend begins with widespread chances for rain.

DISCUSSION

KEY MESSAGE 1 DESCRIPTION...

Cold front is ushering in some cooler and drier air for today and Tue. Of course this time of year 10 degrees below normal is still around 50 degrees for many areas. In the mtns however it will be cold enough for periods of snow showers thru this afternoon in upslope flow. The higher summits may end up with several inches of accumulation by the time it winds down. The feature to watch however is going to be the follow up shortwave trof diving out of Manitoba Tue. That wave will provide the forcing for ascent and the local area will already be underneath some very cold air aloft. Ensemble guidance is cold at 850 mb, but the anomalies get colder moving up thru 500 mb. The result is forecast soundings featuring very steep lapse rates. This will affect sensible weather in two ways. The first is that even if surface temps are above freezing Tue afternoon, it will be likely cool below freezing not very far off the surface. Any precip falling is more likely to remain snow with a very shallow near surface melting layer. The second is that steep lapse rates may allow for some strong lift. So while lift may be brief, it may also be intense and snowfall rates of an inch per hour are entirely reasonable based on the forecast soundings I am seeing. I do not anticipate much snow right now, but it may be more like snow squalls in nature that may lead to a higher impact, sub-advisory type event. There is reasonable confidence in the precip type/amounts and timing Tue afternoon, but the forecast is lower confidence in location. With a compact system and model runs varying the best forcing between southern NH and the lower Kennebec Valley I anticipate that some refining of snowfall totals will be needed even into this evening.

KEY MESSAGE 2 DESCRIPTION...

Snow showers taper off Tuesday night and cold air begins filling in behind the front. This will make for a cold Wednesday morning with temperatures expected to drop into the teens and low 20s. Strong high pressure than moves into the region centering itself over the Gulf of Maine which will kick start southwesterly flow at the surface. At the 500 mb level, as troughing departs, and on the northern periphery of the high we will see northwesterly flow continue, so while it will be warmer temperatures are likely going to stay in the 40s. Thursday is when we are going to see increased southerly flow through the column which is going to push 850 mb temperatures to around +1-3C, which should be good for 50s across much of the area. Of course the caveat with surface southerly flow this time of year is that the ocean is still very cold and the coastal plain likely ends up staying in the 40s as a result. High pressure pulls further away to the east Friday, but continued southerly flow aloft warms 850 mb temperatures to +5-7C resulting in temperatures in the 60s for most locations. With weaker onshore flow the warm temperatures should make it right down to the immediate coast, with the immediate coast having the greatest chance to stay stunted in the low 50s. Friday night also looks like it's going to be one of the mildest nights we have seen in a while with temperatures only dropping into the 40s.

A cold front begins to approach for Saturday which will bring rain chances along with it. Models are still not in the best agreement on timing. The Euro ensemble is pretty progressive with it at the moment starting rain early on Saturday and having it whip through in time to be done by dinner. The GEFS is a little longer duration starting it at about the the same time, but taking it into Saturday night. Models have come into agreement with trailing high pressure ending up over the Great Lakes Region, so even in the mountains this should be a rain event. Lastly, models are in reasonable agreement that the aforementioned high pressure moves into our area for Sunday making for a drier day.

AVIATION /06Z MONDAY THROUGH FRIDAY/

Largely VFR conditions expected, except in the mtns where MVFR CIGs persist in upslope flow. Some local IFR conditions are possible in snow showers north of the mtns including around HIE. Westerly wind gusts around 25 kt possible thru this afternoon. VFR conditions expected overnight.

Outlook... Tuesday: Fast moving area of light snow moves thru parts of the area Tue. Areas of IFR or lower possible during the day, mainly south of the mtns.

Tuesday night: Snow showers exit the area returning any terminals that saw restrictions back to VFR.

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.

MARINE

Gusty offshore winds are forecast to continue thru much of the day, especially outside of the bays. SCA conditions are expected thru late afternoon before winds and seas gradually diminish. While not likely, northeast winds Tue may be strong enough for a few 25 kt gusts outside of the bays.

Should see some fairly tranquil conditions on the waters through midweek as high pressure centers itself over the Gulf of Maine. However, Thursday high pressure begins to pull away and a tightening pressure gradient will see wind gusts 25-30 knots and seas 6-8 feet make a brief return. Wind gusts relax on Friday, but seas could stay elevated as low pressure returns to the region and a front crosses the waters on Saturday. Sub-SCA conditions should return Sunday as high pressure moves back over the waters.

GYX WATCHES/WARNINGS/ADVISORIES

ME...None. NH...None. MARINE...Small Craft Advisory until 2 PM EDT Monday for ANZ150-152-154. Small Craft Advisory until 8 AM EDT Monday for ANZ151-153.