Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 210432 AAD AFDGYX Area Forecast Discussion...UPDATED National Weather Service Gray ME 1132 PM EST Tue Feb 20 2018 .SYNOPSIS... Record breaking warm temperatures are expected across much of the region on Wednesday as deep southwesterly flow continues across the region ahead of an approaching cold front on Wednesday. This front will move through the region Wednesday night with cooler and drier air filtering into the area. Low pressure will pass south of the region with a chance for some light rain and snow over southern areas before another area of low pressure arrives late Friday and Friday night bringing a wintry mix to the region. The unsettled pattern looks to continue through the upcoming weekend. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... Update...Minor changes to account for latest observational trends. Previous discussion... High Impact Weather Potential: * Dense fog likely over areas with snowpack through tonight. * Elevated Ice Jam potential due to snowmelt and recent rainfall. Pattern: Early afternoon water vapor imagery reveals amplified trough/ridge pattern across the United States with an impressive moisture plume /PWATS +5-6 sigma/ between these features directed from the Gulf Coast north and east up the Ohio Valley...through the Great Lakes and across far northern New England. A look at the last 4 RAOBs at PWM reveals impressive warming aloft...with a slower warming trend at the surface...with some resistance to the warming aloft given a cold surface high over the Canadian maritimes...that continues to feed cool low level air into parts of northern New England. A look down at the surface pattern reveals low pressure moving north and east into the western Great Lakes. Through the near term forecast period...this low will continue to trek north and east...with its attendant warm front lifting north through the forecast area /at least aloft/. Forecast concerns center around precipitation potential surrounding this front as well as dense fog potential as increasingly warm/moist air flow over a retreating snowpack. Through this evening: A tale of two forecast areas late this afternoon with the warm front across southern NH...with mid/upper 60s to the south...and low clouds...some fog...and upper 30s/lower 40s to the north. Front will stop making northward progress through this evening....with fog north of the boundary beginning to thicken after sunset. Tonight: Low level jet forcing continues to transport increasing warmth and moisture above low level cold dome this evening and overnight as warm front lifts north. The northward lifting warm front should bring showers over my far northern zones to an end. To the south...the warm/moist advection will lead to an ideal setup for fog with moisture increasing with height in the first 1kft. Essentially...where the warm front hasn/t reached today appears likely to see renewed fog this evening which is expected to become dense given more moisture aloft tonight. Thus...will go ahead and issue a dense fog advisory for these areas and let evening shift adjust as fog becomes more widespread this evening. Otherwise...a mild night is in store...with lows only falling back to the 40s to around 50 in NH and upper 30s/lower 40s across western Maine. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... High Impact Weather Potential: * Record high temperatures likely on Wednesday Pattern: Low pressure east of James Bay will continue to move east through the day Wednesday as high pressure builds east into the Great Lakes region. A trailing cold front from this low will push through region Wednesday evening with high pressure associated drier airmass settling into the region Wednesday night. Primary forecast concerns will center around the potential for daily /monthly?/ record highs in the warm sector on Wednesday. Wednesday: While much of the area will again dawn to low clouds and fog...the warm front will have advanced to a location north of the forecast area. With strong low level jet just off the deck...expect the clearing process to be much more rapid than today and with T9s warming to +12-16C /and noting temperatures over MA today/ expect 70 to be in reach over southern NH and southwestern ME. Bigger question will be along the ME coast from Portland north and east where somewhat of an onshore component may limit highs somewhat. Even here...however...expect a more westerly component in the low level flow to develop by mid/late afternoon as the cold front approaches from the north and west...likely allowing coastal locations to warm /although probably not as warm as areas inland/. Given what occurred today...leaned more heavily towards the warmer MAV guidance. See climate section below for details on record highs. Shower chances will return to the mountains in the afternoon...although given weak convergence along this boundary and westerly flow...see little in the way of shower activity making it south of the mountains. Wednesday Night: Cold front will be along the coastline at 00Z Thursday...and slowly sink south overnight while stalling south of the region as it becomes increasingly parallel to the mid and upper level flow. The cold advection is not particularly impressive although surface dewpoints will dramatically fall from evening values in the 40s and 50s back into the teens and 20s by daybreak Thursday. Northwesterly winds should remain coupled through the night given the aforementioned cold advection and a collocated isallobaric component. So...while a significantly colder night is expected...lows will still remain above seasonal norms...teens north to lower/mid 30s along the coast. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Cold air continues to drain in from the north on Thursday with the surface high building in from the west. Highs will make it into the low 40s south of the mountains where a downslope component keeps things warm, especially in the morning. Temperatures may fall during the afternoon as the cold air continues to pour in. The first in a series of weak waves moves across southern New England on Thursday and may spread some light rain into southern New Hampshire during the day, with snow possible on the northern edge as temperatures continue to cool. Precipitation will be quite light. High pressure moves over the area Thursday night allowing some good radiational cooling conditions. Dewpoints in the teens suggest we will cool into the teens to perhaps single digits in the colder valleys. Fast moving wave crosses the area Friday night with the next round of precipitation expected. This will be primarily of the warm advection style precipitation as a warm front moves through. Southeast flow ahead of this feature, though, suggests that cold air damming will be a factor especially in interior western Maine. Much of the precipitation will fall as temperatures aloft are still cold enough for snow, but as the warm front moves through we may see some areas of freezing rain before it ends. Much warmer on Saturday behind the warm front with temperatures in the mid to upper 40s and a few 50s. This does not last too long as a cold front drops in from the north Saturday night bringing the colder air back in. The most robust of the waves this weekend will be the one arriving on Sunday. This will be tracking out of the Texas Panhandle and into the Great Lakes, with another surge of warm, moist air ahead of it. As the broader trough arrives in our area, the surface low will occlude with secondary low developing near the coastline, complicating the precipitation type forecast. A sustained easterly low level flow is expected ahead of the wave which is a strong signal of cold air damming being at play even into southern New Hampshire. This will likely keep temperatures near freezing even as temperatures aloft try to warm with the parent low moving to our northwest. But with the secondary low forming near the coast, we may hang on to deep enough cold air to keep much of the precipitation as snow, but if the warmer air moves in aloft we will see a switch to freezing rain. High pressure builds in Monday and Tuesday with temperatures near seasonal normals. && .AVIATION /06Z WEDNESDAY THROUGH SUNDAY/... Short Term... Summary: Deep southwesterly flow will continue to transport moist air across the region through tonight and Wednesday before a cold front pushes through the region Wednesday evening. High pressure will build into the region Wednesday night. Restrictions: Highly variable conditions early this afternoon with fog/low clouds scattering out over southern NH...but showers/fog continuing at HIE-AUG. Expect a bit more improvement through mid afternoon...then deterioration this evening with widespread VLIFR/LIFR in fog/low stratus except perhaps at MHT. Expect more rapid improvement to VFR on Wednesday morning with VFR continuing through Wednesday Night. Winds: Light/variable winds /5-10kt southerlies LEB-MHT-CON-PSM/ remaining light overnight before becoming southwesterly around 10kts for the day on Wednesday...shifting northwesterly 5-10kts Wednesday night. LLWS: Southwesterly winds at 1-1.5kft will increase to around 40kts this evening...promoting LLWS through the night. Winds aloft will strengthen through the day Wednesday which may prolong the LLWS threat despite some morning mixing. Any LLWS will come to an end Wednesday night as the cold front pushes through the area. Long Term...Several waves move through the region late this week and this weekend with the potential for lower clouds and precipitation with each of them. The first will primarily affect southern New Hampshire on Thursday. The next brings snow and rain on Friday. A larger system arrives on Sunday with more widespread snow possible. && .MARINE... Short Term... Southwesterly winds will increase on Wednesday /with seas also building as a result/...renewing small craft headlines for Wednesday/Wednesday night. Long Term...Winds becoming northerly behind the front Wednesday night and will be strong early before diminishing on Thursday. Several waves move across the area this weekend, the strongest being on Sunday. This could provide another chance for advisory level wind speeds. && .HYDROLOGY... Flood watch continues unchanged with the forecast this afternoon for the threat of ice jams over western New Hampshire. Rainfall amounts have been light...but dewpoints continue to climb and do expect a significant melt in the headwaters tonight and through Wednesday. Lesser threat exists to the east where less rain has fallen...there are less existing jams...and temperatures through tonight are cooler. && .CLIMATE... Record high temperatures are expected across the region on Wednesday. *** February 21 Records Temperatures*** Portland Concord Augusta 59F - 1953 64F - 1872 52F - 1981 ***Warmest February Temperatures*** Portland Concord Augusta 64F - Feb 26, 1957 69F - Feb 24, 2017 60F - Feb 18, 1981 && .GYX WATCHES/WARNINGS/ADVISORIES... ME...Dense Fog Advisory until 7 AM EST Wednesday for MEZ007>009- 012>014-018>028. NH...Flood Watch through Wednesday evening for NHZ001>003-005-007. Dense Fog Advisory until 7 AM EST Wednesday for NHZ001-002-004- 006-010-014. MARINE...Small Craft Advisory from 5 AM Wednesday to 5 AM EST Thursday for ANZ150>152-154. && $$ NEAR TERM...Legro

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