Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 171924 AFDGYX Area Forecast Discussion National Weather Service Gray ME 324 PM EDT Tue Apr 17 2018 .SYNOPSIS...
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A broad area of weakening low pressure will continue to spin itself out over the region into Wednesday. Cloudy, cool, and damp conditions will be slow to leave the area. Showers are most likely in the mountains, but Wednesday afternoon another round of showers for New Hampshire and western Maine is possible before the low pressure lifts out. Any relief will be short lived as another area of low pressure will approach from the west late Wednesday night. This will likely bring a round of rain and snow, especially for higher elevations. The low departs Friday to be replaced by high pressure and maybe some near normal temperatures finally.
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&& .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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Upper trof is slowly rotating across the Northeast thru tonight. Along and to the E of its axis onshore flow and deeper moisture are leading areas of light rain and snow mainly across Nrn NH and Wrn ME. Temps are slowly cooling aloft as the center of the upper low approaches...so more snow may begin to mix in...especially after sunset. Farther S across Srn NH that cooling aloft is allowing for some instability showers to develop. Gradual drying and loss of daytime heating will help those showers dissipate...eventually including farther NE into Wrn ME. Snow showers in the mtns will continue overnight...especially along the Wrn slopes where orographic lift will be strongest.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
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The forecast area will remain on the cyclonic flow side of the upper low Wed. Upslope showers in the morning will gradually get more intense/numerous into the afternoon...as a S/WV trof rotates around the backside of upper low. Hi-res model guidance indicates a broken line of showers developing on the leading edge of this S/WV Wed afternoon. I tried a best I could to include some chance PoP tracking from W to E across the forecast area in the afternoon hours. While showers should mainly be snow in the mtns...warm air should be deep enough near the coast for rain showers. Wed night upslope showers will continue...but more scattered as deeper moisture starts to pull E and flow becomes more WSW. Very late Wed night the next low pressure will approach from the W. Isentropic flow along the warm front will support precip breaking out from SW to NE around daybreak. Given the air mass in place this may be mostly in the form of snowfall.
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&& .LONG TERM /THURSDAY THROUGH TUESDAY/...
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Models continue to show a breakdown of the blocky high wave number pattern at 500 MB over the long range period, which would be some good news, as we would settle into a drier and near normal temperature pattern. Unfortunately, I`ve seen it before in the last few weeks, so I can`t say I have much confidence in it yet, but will go with the trend anyway, and after a low passes offshore late this week, we can hope for drier and warmer conditions this weekend and into early next week. Thursday into Thursday night will see two wave try to phase just to our west, once coming around the departing closed currently overhead, and the other moving in from the west. The best dynamics for this system will be to our south, but it will allow sfc low to develop south of Cape Cod and track ENE toward Nova Scotia, bring precip to the region Thu into Thu night. P-type remains an issue, but I think during the day Thu, boundary layer should be warm enough for just rain on the coastal plain and in southern NH. The foothills and mountain valleys may see some snow in the air, but will likely not see any accums. Probably only higher elevations, will see some accums during the day Thu, but precip there will be lighter than closer to the coast. Highs will be in the upper 30s in the N to the low to mid 40s in the south. Thu night the low will shift into Nova Scotia and intensify a bit, and this will keep precip back into nrn New England. Colder air does move in a loft, and could see some light snow accums in the mountain valleys and foothills, as precip turns to snow. The coastal plain and srn NH could see a brief period of snow late, but it will likely not accumulate as lows will be in the mid to upper 30s here. Lows drop to around 30 in the mtns. The upper level low continues to deepen as it moves slowly across the maritimes on Friday, so expect lots of clouds and some rain or snow showers, especially in the mtns. Highs will mainly be in the 40s. The closed low gradually shifts NE through the maritimes and across Newfoundland over the weekend, allowing a persistent NW flow aloft, which should allow ridging to build at the sfc. IT will still be below normal, but will be dry with a fair amount of sun. Lows both night will be in the mid 20s to low 30s, with highs Saturday in the 45-50 range Saturday and in the upper 40s to mid 50s on Sunday. Ridging builds in aloft and Mon/Tue look dry and sunny with highs running right around normal, generally in the 50s to around 60 in srn NH.
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&& .AVIATION /19Z TUESDAY THROUGH SUNDAY/...
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Short Term...Widespread MVFR conditions are gradually giving way to VFR. On the E side of trof swinging thru the area RA and some SN mixed in will work thru PWM and AUG...and possibly RKD later this afternoon and evening. Areas of MVFR will likely linger thru the overnight...but improvement is eventually expected to VFR. Wed afternoon another trof will swing thru the area and a band of SHRA/SHSN may track across central parts of the forecast area. This will most likely occur after 18z Wed...and local IFR conditions are possible with that precip. Thru Wed night upslope flow N of the mtns will keep HIE in MVFR conditions...with occasional SHRA/SHSN depending on near surface temps. Long Term...Expect conditions to deteriorate to IFR or lower Thu afternoon into Thu night. Will likely see some improvement, to MVFR at least on Friday, and perhaps some VFR at coastal terminals in NW flow. All terminal should see VFR by Friday night which will last thru the weekend.
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&& .MARINE...
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Short Term...SCA in effect outside the bays mainly for seas into tonight...but 25 kt gusts are expected to pick up again late tonight into Wed morning. Seas are expected to remain above 5 ft thru Wed night. Long Term...May need SCA late Thu and Thu night, but otherwise conds will stay below SCA criteria.
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&& .HYDROLOGY...
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Suncook River at North Chichester is cresting and will continue to fall thru Wed. Flooding is expected to continue thru Wed afternoon. Otherwise area rivers are near crest or starting to fall.
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&& .TIDES/COASTAL FLOODING...
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Tide anomalies are still running in excess of 1 ft...despite winds turning SWly. No surge guidance is really handling that well...but gradually thru tonight I expect the storm surge values to drop. That will likely still leave us with a small surge for the 102 AM high tide at Portland Harbor. With the astronomical tide forecast for 10.8 ft...even a small surge will push water levels towards flood stage and a statement may be necessary.
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&& .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...Small Craft Advisory until 8 AM EDT Wednesday for ANZ150-152- 154. && $$ NEAR TERM...Legro SHORT TERM...Legro LONG TERM...Cempa AVIATION...Cempa/Legro MARINE...Cempa/Legro HYDROLOGY...Legro TIDES/COASTAL FLOODING...Legro

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