Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 281051 AAA AFDGYX Area Forecast Discussion...UPDATED National Weather Service Gray ME 651 AM EDT Wed Oct 28 2020 .SYNOPSIS... An area of light showers will move across southern portions of the area today, while the day remains relatively cool even outside of any precipitation. The approach of the remnants of Zeta and a storm system currently affecting the southern Plains will bring our next chances of precipitation. A generally strung out area of rain will move into the southern half of the forecast area Thursday afternoon. Thursday night rain will change to snow as colder air moves in from the north. While snow totals will be light, it will likely be the first accumulating snowfall of the season for many locations. Cold weather lasts through at least Saturday before high pressure shifts east and allows temperatures to slowly rebound. && .NEAR TERM /TODAY THROUGH TONIGHT/...
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Update...Minor changes to reflect latest observational trends. Light rain is being reported across Srn far N as CON...and no significant increase in intensity is expected this morning. Previous discussion...Zonal flow overhead today will allow a weak S/WV trof to zip thru the area. WAA ahead of it will lead to some showers along and N of the warm front. It remains pretty dry aloft thru the precip will probably tend more towards drizzle than rain at times. I opted for a relatively high PoP given the light QPF amounts expected. Clouds will try to clear from the W in the late afternoon. Tonight clouds will increase again from the W...which should keep temps from getting too cold.
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&& .SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/... Most impactful part of the forecast period expected late the first snowfall for many locations looks likely. Undeniably the trend in guidance has been to keep the remnants of Zeta and the ejecting upper low two separate features. As flow and confluence over the Canadian Maritimes is keeping the resulting system flatter. Less amplification is leading to less despite plenty of cold air advecting into the system snowfall looks to be light. Much of Thu should be dry...with precip moving into Srn zones in the late afternoon. Initially the air mass will be relatively mild...and so rain is expected. High pressure building to the N of the system will set up a feed of cold and dry that will gradually cool the column thru the night. Current forecast soundings have changeover occurring after midnight. The best lift looks to remain mainly S of the forecast area...but may sneak into the Monadnock region early Fri. Overall that limits potential heavy snow...and higher snowfall amounts. I also think that dry air on the Nrn edge of the system may limit QPF in those areas...and so ultimately a sharper edge to snowfall than my current forecast is possible. Trends today will be quite ensemble sensitivity has much of the forecast variance explained by a more amplified system. If we can see observed heights to the NE of the upper low come in lower than forecast we could see trends move back towards more amplification and higher snowfall amounts over parts of Srn NH and extreme SWrn ME. && .LONG TERM /FRIDAY THROUGH TUESDAY/... First things first, we start the long range forecast period wrapping up the storm that is the main story for this forecast package. By Friday morning, a broad low pressure area making up the post- tropical remnants of Zeta will be draped west-to-east over the Atlantic waters south of New England with upper level support in the form of a shortwave trough lagging behind as it approaches the coast. Upper level phasing between these features remains a key source of uncertainty with this storm with widely varying QPF solutions depending on how well forcing lines up over New England... as was discussed above the general trend has been toward weaker phasing. One feature I`m keeping a close eye on in models is a mid-level (around 7-800mb or so) trough axis and associated deformation/FGEN zone on the backside of the departing surface system. Should moisture remain available, this would give us one more ramp up of forcing Friday morning into mid-day, potentially resulting in another inch or two of wet snow before dry air filters in from the north later in the afternoon. As has been noted in prior AFDs... while the uncertainty in phasing is revealing a plethora of deterministic model solutions... model ensembles have been able to provide some confidence in some aspects of the forecast. By 12Z Friday, cold advection will be the rule through the column, so any sleety/mixed precipitation from the overnight period would switch to all rain or snow depending on how low- level temperatures shape up with the diurnal trend. Speaking of temp profiles... there is model consensus in a fairly deep isothermal layer of m5 to m10 C up through the mid- level lift zone, so there shouldn`t be that much of a fluff factor as is usually the case on the backside of synoptic systems unless temperatures are significantly cooler (unlikely without also going dry) or better phasing occurs and more lift is realized in the DGZ higher up (trending less likely). Did hedge a little on the higher side of QPF/snowfall with this last burst as ensembles favor moist over dry low/mid- level profiles south of the Whites. As it sits right now, have additional snow up to about 1-2" through mid-day Friday on top of what falls Thursday night. A sharp northern terminus of precipitation remains in the forecast with highest QPF around 1" at the Mass border and less than 0.1" north of a line from around Gorham NH to Waldo Co ME... little change to the prior forecast other than to tighten it further. All in all a couple slushy inches looks to be a fair bet between Thursday night and Friday south of the Whites across into extreme southwest Maine with storm totals up to around 4", mainly in the higher terrain of southern New Hampshire. This would make for messy Friday morning travel as commuters remember how to drive in wintry conditions. Temperatures will hang around freezing through the day, likely depending on precipitation so regardless of if a wet snow or rain falls it`ll be a raw day especially as a north wind picks up over Maine and along the coast. Will also have to keep an eye on phasing and coastal amplification to see just how strong this north wind will be but for now have gusts to around 20-25 mph there, favoring the less-phased solution. The good news is that quiet (albeit cool) weather returns to the forecast area at least to start the weekend with upper level ridging and surface high pressure building into the northeast. High pressure passes overhead Friday night into early Saturday, providing what may end up being the coolest night of this season so far as temperatures plummet into the teens (north) and 20s... depending on how quickly we can scour out low level moisture in the wake of the messy late- week system. Temperatures rebound into the 30s and 40s on Halloween under clear skies with an increasing southerly flow into Sunday as high pressure moves offshore. Models are in good agreement that by Sunday afternoon a long-wave trough axis will extend from the Arctic down to the Gulf of Mexico Coast, setting up a broad warm and moist advection over the eastern CONUS. Locally this likely spells increasing onshore flow yielding increasing clouds, fog, and drizzle with temperatures limited by SSTs currently in the low-50s. A cold front associated with the long wave trough tracks into New England around the end of Sunday with increasing chances for rain showers, switching over to snow over the higher terrain which may persist through Monday in upslope flow. A significantly drier airmass filters in behind this front, then high pressure builds in through mid-week with a dry and cool forecast to start the work-week. One fly in the ointment is a little clipper- type system in cold advection which could deliver light showers on Tuesday... but confidence is low and impacts would be minor. Stuck close to the consensus blend for the forecast this weekend into next week given stronger than usual consensus in the synoptic pattern. && .AVIATION /12Z WEDNESDAY THROUGH SUNDAY/... Short Term...VFR conditions will give way to MVFR and eventually areas of IFR CIGs this morning ahead of passing S/WV trof. I do not expect much in the way of SHRA at terminals...but most likely across Srn NH including MHT and PSM. VFR conditions tonight into early Thu will quickly give way to IFR over Srn NH. Still a fair amount of uncertainty how far N that makes it...but any changeover to SN will lead to IFR or lower VSBY. Most likely window for that will be 06z to 15z Fri. Long Term...IFR or worse conditions in SN or RASN are expected for most terminals except for KHIE and perhaps KAUG/KRKD on Friday with a north wind. Conditions will improve to VFR from W to E as the system pulls away during the second half of Friday, prevailing through Saturday. Increasing onshore flow Sunday may lead to lowered ceilings and fog. && .MARINE... Short Term...Offshore flow will increase today but is expected to remain below SCA thresholds. As high pressure builds to the N thru Thu the NE flow will increase over the waters. CAA and favorable wind direction for overperforming some gale force gusts are in the forecast. A gale watch may be needed on subsequent shifts. Long Term...Marginal gale force winds over the outer waters gradually wind down Friday as low pressure passes south of the Gulf of Maine and pulls away into the North Atlantic. High pressure moves into the region by Saturday with winds weakening and seas falling below 5 ft. Another period of SCAs is likely at least over the waters with the next cold frontal passage late Sunday into Monday. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$ NEAR TERM...Legro SHORT TERM...Legro LONG TERM...Casey AVIATION...Casey/Legro MARINE...Casey/Legro is the U.S. government's official web portal to all federal, state and local government web resources and services.