NWS Forecast Discussion
For Fremont, NH
000 FXUS61 KGYX 271425 AAB AFDGYX Area Forecast Discussion...UPDATED National Weather Service Gray ME 1025 AM EDT Tue Oct 27 2020 .SYNOPSIS... Cool high pressure builds into the area today and moves to the east on Wednesday. Expect drier conditions with seasonably cool temperatures. Later this week, low pressure tracks eastward to the south of New England just as an Arctic cold front brings much colder air into the area from the north. Rain is likely to begin in southern areas on Thursday with rain changing to snow on Friday as the temperature cools. This will be the first accumulating snow of the season for many areas. Cold weather lasts through at least Saturday before high pressure shifts east and allows temperatures to slowly rebound. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1030 AM Update... Made a few changes to mainly the sky cover and temperature curve this morning. Overall the forecast is in good shape with highs in the upper 30s/lower 40s north to upper 40s/lower 50s south. 630 PM UPDATE... What I expect was the last band of light rain has exited the coast early this morning, so precipitation chances are on their way down. Made a few adjustments to the forecast based on this idea. ORIGINAL DISCUSSION... Winds have already shifted to the northwest across the area this morning and expect cool, dry air to continue moving in from the northwest today. We should even get rid of a good deal of the clouds for at least part of the day today especially over the coastal plain where downsloping conditions aid in mixing in the drier air. Temperatures will be a bit warmer behind the cold front today due to the deeper mixing. Southeast of the mountains the temperature should warm into the 50s, while northwest of the mountains the cold air will pile in and less mixing will occur, keeping temperatures in the low 40s. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... At the surface, high pressure will build in from the west overnight with calm conditions expected. This would normally set up ideal radiational cooling conditions as winds go calm and the temperature craters toward the dewpoint in the freshly drier air mass. However, we continue with a westerly flow aloft and embedded in that flow we expect a subtle shortwave trough to bring in some clouds and maybe even a few sprinkles to southern New Hampshire overnight. These clouds will be the dominant factor in low temperatures tonight. Areas to the north and east are more likely to stay clear and reach their full potential for cooling... into the low to mid 20s. Further to the south and west, clouds will be more widespread and buoy temperatures in the 30s most of the night. In between is where more uncertainty exists, with the temperature likely falling during the clear periods and holding steady or rising a bit when clouds pass by. High pressure shifts east on Wednesday with some weak warm advection bringing in some warmer temperatures aloft. We won`t be as well mixed on Wednesday as we were on Tuesday, though, with nearly all of the mixing coming as a result of the late October sun. Thus expect temperatures once again in the 40s and 50s for highs with dry conditions expected. Periods of cloudiness will also be around as the warm advection aloft begins. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Overview: Subtle but significant shift in guidance this evening has brought flatter solutions into play and introduced more uncertainty regarding potential snowfall. Chances for measurable snowfall still remain fairly high for late Oct...but guidance seems to falling into two camps...one with just minor measurable snow amounts and another with more significant accumulation. Impacts: Again the largest impacts of this forecast period revolve around how much and where snow accumulates. Trends this evening were decidedly towards lower snow amounts...but there remains plenty of support for more significant snowfall amounts and there is plenty of time for guidance to trend back in this direction. Forecast Details: Thu afternoon precip looks to pretty quickly enter SWrn portions of the forecast area. How quickly it spreads N...if at all...remains in question however. The most noticeable thing with guidance this evening is that the forecast appears to be bifurcating...between a nominal snowfall and a more substantial snowfall. In general the deterministic NWP featured a faster Zeta that sneaks off the East Coast ahead of the ejecting upper low...and ahead of the cold air. The result is a more strung out and flatter system...and less QPF. Less QPF means less QPF around when cold air surges into the system from the N. As a result snowfall amounts across guidance is generally lower due to unfavorable overlap between strongest lift and coldest air. The bifurcation also shows up in ensemble guidance...with the EPS generally more in favor of amplified solutions and the GEFS and CMC more in favor of faster and flatter. That being said...members of all ensembles show some larger snowfall events embedded among lighter events. Fewer and fewer members are showing no snowfall at all. This is again supported by the NBM probabilistic guidance...which has shown chances for measurable snow increase to 60 or 70 percent across the Srn half of the forecast area. The NBM has also shown chances for 4 to 8 inches of snow...a significant event for late Oct...increase as well albeit still less than 30 percent chance. This makes sense to me...as guidance does show cold air rushing into the system late Thu. This scenario would be favorable to flash precip over to heavy snow for a time...and the difference of an hour or two on the earlier side could mean several inches of snow. Much of this is dependent on the upper air pattern as it evolves when Zeta nears the Gulf Coast. We should have a better idea of this heading into Wed morning...and extra soundings from Gulf forecast offices will ensure it is well sampled. For now the most favorable overlap of all variables occurs across Srn NH. So I have the most snowfall there...generally 1 to 3 inches in the valleys and less as you head towards the Seacoast where there may be more mixing. I also have up to 3 to 5 inches in the higher terrain from the Whites down thru the Monadnocks as colder air will arrive there first. Beyond this system Fri and Fri night will be some of the coldest air of the season so far. Highs will likely struggle to reach 40 Fri afternoon...and we will fall well into the teens and 20s Fri night. && .AVIATION /15Z TUESDAY THROUGH SATURDAY/... Short Term...Conditions improve to VFR rapidly this morning as northwest winds bring in drier air. The exception to this will be on the northwest side of the mountains where those winds will be locking in the low level clouds for a while longer before the dry air finally breaks these clouds up this evening. VFR conditions expected tonight and Wednesday area wide as high pressure moves through the region. Some passing mid level clouds and a stray shower are possible in southern areas overnight. Long Term...As precip moves into the area the second half of Thu I expect widespread IFR conditions to overspread the area. Most of the area should start as RA given the relatively mild antecedent air...but precip is expected to transition to SN Thu night. LIFR conditions will develop S of the mtns especially in any snowfall. Precip tapering off Fri midday and warming temps will help to change any remaining precip back to SHRA and conditions will improve as flow becomes N and NW. && .MARINE... Short Term...Northwest winds behind a cold front may occasionally gust to 20KT or so today over the outer waters in a relatively well mixed environment. However, pressure gradient is rather weak so it is unlikely that a Small Craft Advisory will be needed. High pressure builds in from the west tonight and crests over the waters on Wednesday with light winds. Long Term...Low pressure developing S of New England will increase NE flow across the waters into Fri. A period of gale force gusts is possible outside of the bays very late Thu night into Fri afternoon. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$ NEAR TERM...Hanes SHORT TERM...Kimble LONG TERM...Legro