Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 202346 AAB AFDGYX Area Forecast Discussion...UPDATED National Weather Service Gray ME 746 PM EDT SAT AUG 20 2016 .SYNOPSIS... High pressure will ensure mostly dry weather through Sunday along with seasonably mild temperatures. A strong cold front will move through the region Sunday night with a period of rain expected across the region. Behind this...much cooler and drier air will arrive for Monday and Tuesday as high pressure builds into the region. As this high shifts offshore by midweek... expect temperatures and humidity to increase. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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Update...Have adjusted wx grids for the night based on the fog forecast. Leaning on the NARRE-TL for timing and areal extent of fog...which brings in the majority after midnight and mainly N of PWM. Inland patchy valley fog will be possible. Main forecast issue tonight will be coastal fog/stratus. It was fairly prevalent around Casco Bay earlier today...but has since lifted. Most of the Gulf of ME is cloud free at the moment...so the thinking is that I may have to push back the onset of any coastal fog some. There is an area S of Nova Scotia noted on visible satellite imagery pushing Wwd...and NARRE-TL favors this to arrive for coastal ME after midnight as another round of fog/stratus. At this time model guidance is focused mainly PWM and points E up the coast...which seems reasonable at this time. Previous discussion...High pressure has moved offshore and has begun to produce a more humid southerly flow. Another quiet night expected except for the question of when and where fog will develop. The southerly flow has already begun to develop areas of fog along coastal waters and during the evening we are expecting more ocean low clouds and fog to develop and move to coastal zones. Over the interior more late night valley fog expected once again. Temperatures will mild tonight with more humidity to be noted as dewpoints will be slowly on the rise.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... The valley fog will once again burn off by mid morning but with high pressure anchored off the coast producing a moist southerly onshore flow, coastal areas may have low clouds and areas of fog through the day and into the night. Models show the cold front and associated showers approaching late Sunday and moving through the area Sunday night. Used the RFCQPF guidance of between .50 in southern areas to 2 inches in the mountains. Models in general agreement with RFC guidance. Used a blend of models for max/min temps and pops. && .LONG TERM /MONDAY THROUGH SATURDAY/... High Impact Weather Potential: Minimal. Pattern: Current teleconnection indices show an ongoing -NAO with ridging ongoing across Greenland and blocking south of this feature...albeit a bit far to the north to bottle up anything as far south of the CONUS...but rather keep ongoing longwave troughing anchored near Hudson Bay. The PNA has done more oscillating of late...neutral now trending negative at the moment as shortwave energy over the Gulf of Alaska/western Canada flattens the western ridge somewhat...carving out a mobile shortwave trough that will head east along the US/Canadian border this week. Behind this feature...there is good EC-ENS/NAEFS agreement that the PNA returns strongly positive for the second half of the long term forecast period with a rebuilding of the western ridge. Overall...this upper level pattern evolution to our west... combined with semi-permanent ridging over the southeastern US/western Atlantic suggest that as the troughing over the northeastern United States to start the period moves east...the longwave trough position will retrograde somewhat west as next upstream shortwave merges with Hudson Bay low. Thus...the transition from northwesterly to southwesterly flow will occur early in the period with moderating temperatures and gradually increasing levels of humidity expected in the broad southwesterly flow. The best widespread precipitation chances will be late in the period as troughing to our west approaches the Northeastern states. The pattern does not favor high impact weather with weak forcing given the dominance of the northern stream and thus generally weak forcing aloft...typical of this time of year. Model Trends/Confidence: Overall...model agreement is above average through the long term forecast period with only slight timing differences towards the second half of the week. Thus...confidence in the forecast is above average. The Details.... Monday/Monday Night: Deep moisture plume will be exiting stage right to open the long term forecast on Monday with PWATS falling towards 0.5" by 00Z Tuesday /-1-2 sigma/. Consensus location of the front suggests that rain may not have completely cleared my Maine zones by daybreak...so will allow for chance PoPs for a few hours in the early morning....with PoP focus shifting to upslope regime in far northern NH/northwest ME as the day unfolds given residual cyclonic flow and shallow upslope component beneath strengthening subsidence inversion. Expect clearing south and east of the mountains in developing subsident regime. Winds look moderate given good cold advection but little isallobaric help and only a modest gradient. This will likely yield winds in the 10-15g20-25kt range. T8s beginning the day near +10 C...but fall 1-3 degrees C during the day... reaching -1 to -2 sigma values by 00Z Tuesday...with the core of the cool air /+6-7 C/ settling overhead Monday night. Temps aloft support highs in the 70s south and east of the mountains with clouds and a few showers likely keeping spots from the mountains north in the 60s. This will set up a chilly night Monday night with core of the cool/dry airmass pretty much overhead and high pressure ridge axis nosing into the region. 1000 mb geostrophic winds around 20kts at 00Z Tuesday subside to 10-15kts overnight...suggesting that our deeper valley locations will decouple...but the ridges and hilltops will not. For these former locations...thus...expect temperatures to fall into the 40s...with our coldest spots near the Canadian border right around the 40F mark. Given the setup...will go below the guidance consensus on temps. Expect plenty of valley fog and fog over the inland lakes given this setup. Tuesday - Thursday: Very quiet period of weather in store through the midweek as flow aloft backs westerly and eventually southwesterly with departing trough...with surface high moving from a location over the Mid Atlantic early Tuesday to just southeast of the New England coast Wednesday to south and east of the Canadian maritimes by Thursday. Only possible chance for precipitation comes Tuesday night as warm front north of the Great Lakes rides east with precipitation along the leading edge of the deeper moisture /and associated low level jet/ moving just north of our region overnight with more gradual warming/moistening aloft across New England. There is good consensus for this precip to remain just north of the region...but worth keeping an eye on for far northern areas. Otherwise...partly to mostly sunny skies look to be in order with T8s rising from around +9 C Tuesday to +15 C Wednesday to +16-17 C on Thursday which is +1-2 sigma by these latter periods...not terribly abnormal...but likely able to allow locations in southern NH and far southwestern Maine to again make a run into the upper 80s to around 90 after highs only reach the 70s to around 80 on Tuesday. Overnight lows warm rapidly...upper 40s/lower 50s Tuesday morning with most locations at or above 60 by Thursday night. Friday - Saturday: There is above average deterministic/ensemble agreement in a frontal passage on Friday associated with weakening shortwave moving east along the US/Canadian border...with a relatively thin moisture plume ahead of this feature. Nothing particularly exciting from a dynamics perspective...but certainly a warm/humid airmass ahead of this boundary...with thunderstorms possible depending on frontal timing. While a widespread precipitation event is possible...there are no signs that this will result in very significant rains. With the potential precipitation will keep highs a bit lower on Friday...but still well above seasonal norms. Behind this feature...expect temperatures to drop somewhat to end the forecast period with dry weather Saturday...but with continued southwesterly flow aloft...expect the period to end warmer than normal. && .AVIATION /00Z SUNDAY THROUGH THURSDAY/... Short Term...VFR conditions will be variable to IFR/LIFR in developing areas of coastal stratus and fog as well as some interior late night valley fog. Valley fog burns off by 15z Sunday but coastal low clouds may remain. By Sunday night showers associated with the cold front will produce widespread MVFR/IFR conditions. Long Term...VFR conditions look to dominate flying conditions through Thursday...with restrictions limited to overnight fog potential each night HIE/LEB. && .MARINE... Short Term...Light south winds tonight and Sunday will increase some ahead of the approaching front Sunday night but will remain below SCA levels. Seas will also remain below 5 feet. Long Term...Winds and waves to remain below sca levels with northwesterly winds turning southwesterly for Wednesday and Thursday. Southwesterly winds may reach SCA levels on Thursday night and Friday. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$ NEAR TERM...Legro

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