Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 072315 AFDGYX Area Forecast Discussion National Weather Service Gray ME 715 PM EDT Tue Apr 7 2020 .SYNOPSIS... A weak area of low pressure will pass south of the region tonight with a some light rain possible over southern New Hampshire towards daybreak. High pressure returns on Wednesday before the weather turns significantly more active on Thursday as low pressure develops over the Gulf of Maine with rain and mountain snow expected. High pressure returns on Friday into Saturday with temperatures a bit below normal for this time of year. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... 710 PM...Weak sfc low passing well S of the CWA may back some light SHRA NE into SW NH, and maybe along NH/MA border to the coast late tonight. Forcing is weak on the NE edge, and airmass is dry, so may not much at all. Air mass is dry enough to see maybe some snow/sleet mix in at the start, but this is unlikely outside of the higher terrain. Rad cooling N outside of srn NH and SW ME should be strong thru midnight or later so temps drop off quickly, with lows in the low to mid 20s in the mtns, and 25-30 inland areas away from the coast. In srn NH the clouds limit the lows to the mid to upper 30s. Previously... High Impact Weather Potential: None. Pattern: Pattern across Canada early this afternoon shows an omega block breaking down as western trough member slips under a cutoff high over northern Hudson Bay which will bring a growing northern stream influence over the flow across across much of the northeastern half of the lower 48 as we move through the remainder of the week. At the moment northern New England finds itself under dry northwesterly flow with a frontal boundary separating this region from significantly more moisture over much of the Mid- Atlantic and Ohio river valleys. A wave of low pressure along this boundary will work south and east through tonight...with clouds and precipitation chances over our southern areas being the primary forecast concerns tonight. Through this evening: Little in the way of sensible weather concerns through the evening with high clouds beginning to arrive over southwestern areas. 8pm temperatures will rapidly be falling into the 30s in our cooler mountain valleys...but remain in the mid 40s to around 50 for the coast and foothills. Tonight: Weak shortwave embedded in the northwest flow aloft moves south and east into the region along with an associated 80kt H5 jet streak with southern New England in it/s right entrance region. Surface low coaxes a modest 20-30kt southwesterly LLJ at H8 advecting 1" PWATs north and east to a position south of New England before stalling. Airmass well north of the boundary will remain very dry with PWATs below 0.25". There is a pretty solid guidance consensus that precipitation associated with the surface low will reach into far southern NH after midnight through daybreak. HRRR/NAM/GFS are all agree with wet bulb zero heights 2500-3000 feet...suggesting that outside of a few possible wet snowflakes over the hilltops...precipitation should take the form of rain. Very sharp cutoff /likely no precip south of a LEB-SFM line/ with no more than partly cloudy skies should allow lows into the 20s and 30s...with upper 30s likely under the cloud cover and precipitation over southern NH. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... High Impact Weather Potential: None. Pattern: Weak shortwave ridge axis builds over the region as high pressure noses in from the north. This will allow dry air to make another push with PWATs dropping to near -1 sigma. The arriving shortwave ridge axis will slow as it moves overhead with the pattern amplifying ahead of a mobile shortwave trough carving out over the Great Lakes region Wednesday night. The result will be one last quiet weather day with not significant impacts expected. Wednesday: Low pressure quickly passes south and east of the region with dry air returning to the entire area under nearly neutral temperature advection as a surface high noses in from the north. This dry push should lead to another mostly sunny day across the region. With onshore flow developing in the afternoon...expect coastal areas to be cooler than today...likely reaching no higher than 50. 40s will also be common in the mountains...with highs in between over southern NH and into the foothills of western Maine reaching into the 50s again given similar temperatures aloft. The somewhat cooler temperatures and increased llevel dewpoints compared to 24 hours previous coupled with light winds should allow for somewhat diminished fire weather concerns although afternoon RHs could still fall to near 30 percent in western Maine away from the coast. Wednesday Night: Flow throughout the column backs in response to shortwave digging across the Great Lakes. Primary forecast concerns center around precipitation potential as dynamic forcing for ascent increases atop strengthening southeasterly boundary layer flow. With dry wedge in place ahead of developing warm advection pattern...tend to believe that the 07.12Z GFS is overdone in it/s extent of upslope precipitation...with all high res guidance maintaining a more substantial wedge of dry air in the 5-10kft region. Tend to like the HREF placement...showing some light rain/snow showers along the southeastern slopes of the Whites towards daybreak Thursday. So...expect increasing clouds with some potential for a rain or snow shower along the southeastern slopes of the terrain with lows in the 20s in the mountains and 30s elsewhere. && .LONG TERM /THURSDAY THROUGH TUESDAY/... *** Rapidly intensifying coastal low will bring rain, snow, and coastal flooding Thursday into Friday*** The focus of the long term forecast will be on a rapidly intensifying coastal storm impacting the region on Thursday.This system can be traced back to the merging of two different waves, one from the southern stream over the four corners and one from the northern stream over British Columbia. These two jet streaks combine over the southern Ohio valley/Appalachians with the resulting 160kt jet streak triggering rapidly intensifying cyclone in the left exit region over southern New England. Confidence continues to grow on both the timing track of the storm with ensembles showing a tight cluster of low centers just offshore in the Gulf of Maine by 00Z Friday. Thus have fairly high confidence in the timing of the precipitation. What remains a tougher call is the precipitation type. Along the immediate coastline rain will dominant as the warm air aloft wraps into the center of the low and surface temperatures remain above freezing. As we move inland and up in elevation snow becomes increasingly likely with elevations above 2000ft seeing all snow. The transition zone remains highly uncertain and foothill locations may go back and forth between rain and snow multiple times. With precip moving in in the middle of the day expect an initial push of rain for all but the highest summits. As the sunsets the cooling temperatures will combine with the intensifying storm to push colder air down into the region from the northeast resulting in a transition to snow with accumulations possible as far south as Fryeburg to Lewiston to Augusta in Maine. Further west into New Hampshire, most of the state will hold as rain as the source of cold air over New Brunswick and eastern Quebec isn`t readily available to push south of the Whites. With the rapid intensification and strong upper level support expect to see some banding locally increasing snowfall as the dynamics within the band help to further the cooling and help to switch over to snow. Overall the area of highest threat for intense snow aligns well with the current Winter Storm Watch and have opted not to change headlines at this time. Precipitation is not the only threat with this storm, stronger wind gusts are also a concern. Forecast soundings show the best mixing on the backside of the system where northwesterly cold advection sets up and especially through the foothills and Lakes Region of NH where downsloping may enhance the flow. Gusts to 40mph are possible and a wind advisory may be needed for a portion of the area. Along the coast the extent of the strong winds will depend more on the low track. The greatest wind impacts will be if the low is able to stay offshore keeping the gusts into the coast. If the low tracks along the coast, the calmer conditions at the center will limit gusts. While a very robust low level jet looks likely ahead of the system, feel there will be enough of a surface inversion, albeit weak, to prevent this from mixing down and thus have concentrated the stronger winds on the backside of the system for Thursday night. The storm will move out into the Maritimes on Friday setting us up for northwesterly flow with some isolated showers in the mountains for Saturday. This flow will continue through the weekend before another coastal low looks to move up the eastern seaboard for the start of next week. && .AVIATION /23Z TUESDAY THROUGH SUNDAY/... Short Term... Summary: High pressure overhead will give way to a weak area of low pressure moving south of the region tonight before high pressure noses back into the region from the north on Wednesday. Restrictions: VFR conditions will dominate the period from now through Wednesday night. There is the potential for a period of MVFR CIGS in -SHRA from MHT to PSM in the 08-12Z period Wednesday morning. Winds: Northwest winds 10g16kts /with local seabreezes developing/ will diminish to calm/light-variable overnight before starting the day from the northwest <10kts Wednesday before shifting to the east and southeast Wednesday afternoon /5-10kts/. Winds will go calm ore remain light easterly overnight Wednesday night. LLWS: No LLWS is expected through Wednesday night. Long Term... A coastal low will bring precipitation into the region starting midday on Thursday, expect conditions to rapidly deteriorate to IFR with rain for all but the highest elevations. Thursday night rain will transition over to snow moving southwards into the foothills. LLWS will be a concern for the midcoast through Thursday and thursday night with a LLJ of up to 60kts expected to remain above the boundary layer. On the backside of the storm strong northwesterly winds will mix down to 35kts in the wake of the storm. Conditions will subside for the weekend with just isolated showers and MVFR cloud cover remaining in the mountains. && .MARINE... Short Term... Quiet on the waters through Wednesday night with onshore winds this afternoon shifting northeast tonight and strengthening through Wednesday before shifting southeast Wednesday night. Long Term... A coastal low will develop near Nantucket and rapidly intensify as it moves through the Gulf of Maine on Thursday night. Strong flow around the center of the low will likely push winds to Gale with a few gusts to storms possible offshore in the Gulf of Maine. With the low potentially tracking through the middle of the forecast area, the potential exists for Gale force conditions both ahead of and behind the system and so have opted to issue a Gale watch for the 24hr period encompassing both these scenarios, although there may be a lull in the middle. && .HYDROLOGY... A period of heavy rain is likely along the Maine coast Thursday evening...transitioning to snow over the interior. Given snow in the mountains and a mix into the foothills...do not expect significant river flood issues. May have some short-fused issues along the coast esp with water levels rising towards the ~1AM Friday high tide which may prevent efficient surface water drainage along the coast. && .TIDES/COASTAL FLOODING... Have issued an extended coastal flood advisory for high tides beginning tonight and continuing through early Friday. Astronomical tides are near minor flood levels without wind impacts already...and with surge levels expected to increase ahead of Thursday/s storm...minor impacts are likely. There is some potential that the Thursday night high tide could see moderate /warning level/ impacts depending on exact timing/location of storm development and how long winds remain onshore. Still time to examine this tide cycle more closely. && .EQUIPMENT... The last GYX upper air observation was March 25 at 12Z. Unfortunately, a disruption in gas supply has temporarily halted observations from GYX. The order has been placed but a date of delivery remains TBD. The Sugarloaf NWR transmitter remains off the air with an unknown restoration time. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...Coastal Flood Advisory until 3 AM EDT Friday for MEZ023>028. Winter Storm Watch from Thursday morning through Friday afternoon for MEZ007>009-012>014. NH...Coastal Flood Advisory until 3 AM EDT Friday for NHZ014. MARINE...Gale Watch from Thursday afternoon through Friday afternoon for ANZ150>154. && $$ SYNOPSIS...Arnott NEAR TERM...Arnott/Cempa SHORT TERM...Arnott LONG TERM...Curtis AVIATION...Arnott/Curtis MARINE...Arnott/Curtis HYDROLOGY... TIDES/COASTAL FLOODING... EQUIPMENT...

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