Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 301528 AFDGYX Area Forecast Discussion National Weather Service Gray ME 1128 AM EDT TUE AUG 30 2016 .SYNOPSIS... High pressure will be over the region today with seasonably mild temperatures and continued dry conditions. Temperatures will warm on Wednesday ahead of an approaching cold front with showers and thunderstorms possible. This cold front will slowly move offshore through Thursday with additional showers and a turn to cooler temperatures. Outside of an isolated mountain shower Friday... seasonably cool and dry conditions are expected before a dry and seasonably mild holiday weekend as a slow moving high pressure system moves into the northeastern United States. && 15z update... Skies continue to clear across southern and coastal Maine late this morning and this trend will continue into early afternoon as high clouds push east. Clouds associated with weak over-running across southern Canada will keep skies in northern zones mostly cloudy through mid afternoon. Have adjusted temps/td`s/sky grids based on current obs. No other changes planned attm. Previous discussion... .NEAR TERM /UNTIL 6 PM THIS EVENING/... Update...Minor changes to adjust for latest observational trends in temps and dewpoints. Previous discussion...High pressure is settling into the region this morning. Closer to the ridge center...valley fog will be more common across NH this morning. It will be patchy dense at times across the CT River Valley. The high pressure center will slowly shift E thru the day...allowing return flow to develop late. This should aid the sea breezes by the afternoon as well. Clouds will also spill into the area late as the upper ridge axis moves E too. A continued long period SW swell will continue today as well. Though the wave heights are around 2 to 3 ft...periods over 10 seconds will support a moderate risk for rip currents. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... A s/wv trof diving out of the Hudson Bay region will continue to push a cold front towards New England tonight. The impact of this trof looks to be a little blunted from a few days ago...thanks to the potpourri of tropical systems out in the Atlantic. With no room for the trof to amplify into...it just sort of sweeps thru the region and lifts out. This will be good enough for scattered showers and maybe an isolated thunderstorm. Initially there may be some WAA type showers...mainly for Nrn zones...but eventually convection along and ahead the front will move into the forecast area. The jet streak aloft strengthens late Wed...so coverage of precip should be highest later in the day versus the first half. Shear does increase as winds aloft do too...but instability remains rather meager...so the strong to severe threat seems rather limited. If we can produce better moisture return there could be a chance for a stronger storm or two however. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... High Impact Weather Potential: Some threat for onshore swell/rip current risk early next week. Otherwise...minimal. Pattern: A look at water vapor imagery early this morning across North America reveals the development/deepening of a broad longwave trough over the Gulf of Alaska with an even broader synoptic scale ridge dominating much of the eastern 2/3rds of the United States...with a relatively progressive flow along the northern edge of this feature near the U.S./Canadian border. For the remainder of this week...the primary change to the upper pattern will be downstream amplification in response to the Gulf of Alaska trough as it deepens further and moves ashore along the west coast of North America. A wave train will develop downstream of this feature with a rather potent trough arriving over the eastern United States Thursday and Friday /with a return to temperatures near seasonal norms/. However...with a lack of downstream blocking...the Omega block in place Friday morning will continue to slowly progress east through this weekend. While there are disparities in the model forecasts by Sunday-Monday...there is broad agreement that as the eastern trough exits our area...some energy /or a cutoff feature/ will remain over the southeastern states...yielding a Rex-block over the region as impressive heights for early September build into the northeastern United States/eastern Canada. Were this to occur...we could very well expect a return to much warmer than normal temperatures next week...with another stretch of dry days likely. Before this...eyes will be on frontal passage to start the period Wednesday night-Thursday as this presents the only real precipitation threat through the long term forecast. Tropics: The developing eastern trough late this week...and ongoing northwest flow behind this feature as it pushes east of the region this weekend favors keeping tropical activity well to our south through the long term forecast period. The GFS is an outlier in taking TD 9 and lingering it not all that far off the New England coast early next week as a weak system...and will continue to not buy into this idea. Can foresee...however...a period of enhanced swell early next week with renewed rip current threat certainly possible for Labor Day...which given the warm temps will be worth keeping an eye on. Model Trends/Confidence: Strong deterministic/ensemble agreement exists through Saturday...with some divergence Sunday/Monday regarding how quickly/fully the northeastern trough exits the region and where/if a cutoff low forms over the Mid Atlantic region. Given that we have multiple tropical systems over the southeast to start the period...have to be a bit cautious in just how confident we are this weekend/early next week as...depending on any deepening of these two depressions...they will have some potential impact on the large scale flow. The Details.... Wednesday Night: Pre-frontal trough and upstream cold front continue to push south and east Wednesday night with height falls spreading overhead ahead of impressive mid level shortwave. While instability will be lacking...plenty of moisture /PWATs around 1.5 inch...+1-2 sigma/ will be available...and with good dynamic support for ascent expect at least scattered showers to continue through the night. With ample cloudiness...expect overnight temperatures to remain elevated...upper 50s north to upper 60s south...a bit above the guidance consensus. Thursday: With remnants of TD 8 well offshore...cold front will slow it/s southeastward progress...bisecting the area in the morning...but not clearing the coast until mid afternoon. Thus...continued shower potential /and thus chance PoPs/ looks good over southeastern areas. Can see some showers further north as well in the cold advection regime as temps aloft fall /below -15C/ near the core of the trough at H5...with a collocated spoke of available mid level moisture. Add to this daytime heating and some upslope help and isolated/scattered showers /rumble of thunder/ appear reasonable here as well. T8s straddling +10C suggest highs near 70 north...and upper 70s to around 80 south...in line with the guidance consensus. Thursday Night - Friday: Shower activity will come to an end Thursday evening with the loss of daytime heating and arriving high pressure from the northwest. A much cooler night expected Thursday night given the cooler airmass overhead and at least partial clearing. Can/t rule out a couple of mountain showers on Friday with some limited instability and upslope assistance beneath a developing subsidence inversion. Cu rule suggests we/ll have decent CU growth even south of the mountains...but in the downslope regime do not expect precipitation. T8s around +8C will result in highs a bit lower than Thursday...with upper 60s north to mid 70s south looking on target...right in line with guidance consensus. Friday Night - Sunday. A beautiful /except for the continued drought conditions/ weekend is in store as high pressure crests over New England with light winds and T8s warming from around +7C Saturday to +10C Sunday. Dry wedge overhead suggests little/no cloudiness...so full daytime heating should allow highs to range through the 70s Saturday with some 70s and lower 80s on Sunday. Comfortable sleeping weather in store both nights with dry airmass in place...with lows in the 40s and 50s from north to south. Expect we/ll drop a few degrees below guidance consensus each night. Labor Day: The forecast period looks to end under a dry regime with high pressure still anchored overhead. Already a decent signal in the NAEFS/EC-ENS for heights well above climatological norms...which argues for lots of sun...and temperatures above seasonal norms under light winds. && .AVIATION /16Z TUESDAY THROUGH SATURDAY/... Short Term...VFR conditions will dominate as high pressure slowly shifts E today. CT River Valley fog will produce local IFR or lower conditions at HIE and eventually LEB thru sunrise before lifting. A cold front will slowly approach the area from the NW tonight. Scattered SHRA and maybe an isolated TSRA will be possible along and ahead of the front...with MVFR or lower conditions in the heavier convection. Precip will be most likely late tonight into Wed. Long Term...Scattered showers Wednesday night and Thursday will bring the threat of isolated/scattered brief MVFR/IFR restrictions with VFR conditions dominating Friday through the coming weekend outside of morning valley fog HIE/LEB. && .MARINE... Short Term...Winds and seas look to remain below SCA thresholds thru Wed. Wave models do want to bring seas above 5 ft into the coastal waters today. Given that these models are already running a foot or so above observations...I knocked the guidance down a foot thru Wed. This better matches the idea of a very long period swell that the buoys are picking up. Long Term...Southwesterly flow will shift northwesterly on Thursday as a cold front moves through the waters...with winds turning light for the coming weekend as high pressure move into the northeastern United States. Greatest threat of SCA conditions will come to start the period with marginal SCA wave heights over the outer waters. Beyond this...wind/waves look to remain below SCA levels through the weekend. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$

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