Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 231114 AAA AFDGYX Area Forecast Discussion...UPDATED National Weather Service Gray ME 714 AM EDT Mon Apr 23 2018 .SYNOPSIS... High pressure will take control of the region today and bring us a short stretch of very nice weather. Sunny skies this afternoon will allow temperatures to climb into the lower 60s for much of the area. That being said, it is sea breeze season so best to keep a jacket handy if you are going to be near the water. Another cool night is expected under clear skies and light winds, as readings fall to near freezing. Southerly winds on Tuesday will bring in even warmer air, with afternoon highs expected to climb into the mid to upper 60s. As we head into Wednesday unsettled weather returns with the approach of our next storm system. Beyond that, another fast moving system may be in the cards for the end of the week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Update...Minor changes to reflect latest observational trends. Previous discussion...At the moment surface ridge axis is centered over the forecast area. That is allowing both clear skies and near calm winds...and temps have responded by falling thru the 30s in the typical cold spots. Despite the chilly start to the day...full sunshine and a very dry air mass will allow for deep mixing of the boundary layer. The 23.00z raob at GYX showed mixed layer extending to about 800 mb...well above model forecasts. That is not atypical for this time of year...and so I expect more of the same this afternoon. If we assume mixing to that level again today...we should see widespread lower 60s. The SWrn zones in NH may see readings a little warmer than that. Light pressure gradient will also support development of sea breezes this afternoon. So along the coast high temps will be a little earlier in the day...with temps falling as colder marine air works inland. I have blended in higher resolution guidance to show this temp trend...as well as the quick drop in readings behind the sea breeze front after sunset.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/... Expect another clear and chilly night under high pressure. As ridge axis slips Ewd the readings may not be as cold as we are currently seeing out there...but I still went well below most guidance. I preferred to use the current temp spread across the forecast area as a starting point and blended in MAV/MET guidance. Return flow will increase Tue and allow some warmth aloft to sneak Nwd into the forecast area. If we continue the deep mixing of the last couple of days and as is expected today...it should be another warm afternoon. Model guidance is in agreement of at least +5C at H8...with pockets of +7 showing up on the 23.00z GFS. That suggests widespread mid 60s away from the coast...with some locations making a run at 70. Near the coast...onshore flow will limit warming somewhat...especially N of PWM where Sly flow will knock readings down during the afternoon hours. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... For the most part, we will continue the slow parade of cut off lows across the southern part of North America. One such low, currently over the Midsouth, will make its way toward the East Coast on Wednesday, while a northern stream trough digs in behind it, keeping a cold flow of air out of Canada into the central and eastern parts of the United States. This trough may try to close off over the Southeast USA as well, but it will gradually be pushed eastward toward the East Coast this weekend while ridging builds through the Plains behind it and the next slow moving cut off low arrives in California. The general result for our area will be an initial round of rain centered on Wednesday evening and another possible in the Friday-Saturday time period, with cool temperatures expected from midweek on. Slow-moving cut off upper level low pressure makes its way through the southern Appalachians Tuesday night, with surface low pressure forming near the Carolina Coast. To the north of this low, over New England, expect winds to turn onshore southeasterly with increasing cloud cover. Through the day Wednesday expect an increasing chance of rain with temperatures quite cool. Went a few degrees below guidance for high temperatures on Wednesday east of the mountains, forecasting highs generally in the low 50s. With water temperatures this time of year in the low 40s and an onshore flow expected, we may be lucky to reach 50 on the coastal plain. Rain becomes steadier from southwest to northeast through the day Wednesday and into Wednesday evening, spreading across the region Wednesday night. It looks like mostly a 0.5 to 1 inch rainfall for the area, but some areas may get more than an inch. With some snow left in the mountains, we should see some higher flows on the rivers with a possibility of minor flooding in the headwaters. The surface low pulls north on Thursday, but we will likely see the upper low linger over our area which continues to cause showery weather, especially during the day when diurnally forced convection can get going. Models not in full agreement on whether to push a front through and bring some offshore winds for Thursday or not. If so, we will see temperatures a bit warmer, in the upper 50s to mid over the coastal plain and southern New Hampshire 60s, but if onshore southerly flow continues, much of Maine may stay in the low to mid 50s. Friday is our one off day in this extended forecast. Caught in between waves, we should see a period of drier conditions with temperatures peaking in the upper 50s to low 60s. The GFS model would be the one to spoil this outlook, though, as it develops a coastal low that moves north toward Cape Cod and may bring some rain Friday evening. ECMWF and CMC models wash this wave out as it moves into the Atlantic, keeping conditions dry in New England which is the preferred solution at this point. Large scale upper trough arrives on the East Coast on Saturday, with both the northern and southern stream troughs aligning. Still unclear which wave will become dominant and if the two troughs will fully merge. Since the GFS develops the stronger wave on Friday, it does not do much with the main trough axis moving through on Saturday. Meanwhile the ECMWF and CMC bring a more organized low pressure system through the region... either to the north or south... with rain arriving. Thus expect another period of rain, with the most likely time period being Saturday. Cool, showery weather likely continues into Sunday with the cold upper trough axis overhead. && .AVIATION /11Z MONDAY THROUGH FRIDAY/... Short Term...Widespread VFR expected under high pressure. Light pressure gradient should allow for sea breezes to form at PSM...PWM...and RKD this afternoon. Tue afternoon background flow becomes more Southerly...and sea breeze may extend all the way to AUG. Long Term...Expect increasing clouds and lowering ceilings Tuesday night into Wednesday as an onshore flow develops and rain moves in from south to north. Should see a prolonged period of IFR conditions lasting into Thursday morning, but possibly hanging on through Thursday afternoon across much of Maine. VFR returns Thursday night and Friday. && .MARINE... Short Term...Winds and seas will remain below SCA thresholds with high pressure in control. Afternoon sea breezes are expected to form this afternoon...and then a little stronger Tue afternoon as background flow becomes more Southerly. Long Term...Onshore southeast to east flow develops Tuesday night and Wednesday, and could occasionally gust to 25 KT or so with wave heights building into the 5 to 9 FT range by Thursday when the wind will finally shift to southwest or westerly. && .FIRE WEATHER... Another warm and very dry day is expected today. Minimum RH values are expected to drop into the upper teens. The good news is that winds are expected to be very light as well...as ridge axis will be nearly overhead. Sea breezes developing in the afternoon may allow for some stronger winds to push inland mainly after 2 PM. This could locally increase the fire danger. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$ NEAR TERM...Legro SHORT TERM...Legro LONG TERM...Kimble AVIATION...Kimble/Legro MARINE...Kimble/Legro FIRE WEATHER...Legro

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