Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 190801 AFDGYX Area Forecast Discussion National Weather Service Gray ME 401 AM EDT Mon Jun 19 2017 .SYNOPSIS...
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A warm and very humid airmass will remain across the region through Tuesday. A cold front will approach from the Great Lakes Region today and bring widespread shower and thunderstorm activity to the region. Some of the thunderstorms could produce damaging winds and torrential downpours, especially across inland areas. On Tuesday, the cold front will gradually weaken as it slides offshore. The next weak trough will move across the area on Wednesday producing a few showers mainly in the mountains and foothills. The next more significant system approaches late Thursday into Friday bringing a return to unsettled weather.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Focus of the near term is on the potential for flash flooding and severe thunderstorms. For starters the flash flood threat is a little more clear across the NW half of the forecast area. A deep moist air mass is in place across the region...with PWAT values approaching 2 inches. However you choose to view this...2 to 3 standard deviations above normal...or in excess of 200 percent of normal...there is a lot of moisture to work with. Deep warm cloud depths...greater than 10000 feet...will support very heavy rainfall in any convection this afternoon. In addition...storm motions are projected to be parallel to the approaching cold front. So not only will storms bring heavy rainfall...but they will likely track over the same areas as the front slowly creeps Ewd. The flash flood watch remains in effect as a result. More uncertain is the threat for severe thunderstorms. The uncertainty is mostly related to the amount of sunshine today. That deep...moist air mass responsible for the flash flood threat will also support moderate instability this afternoon with any modest amount of heating. Despite weak lapse rates aloft...dew points sit around 70...and CAPE could be as high as 1500 J/kg with a little break in the cloud cover. This is important because wind fields in the low to mid levels are forecast to increase thru the afternoon today. Some model forecasts show close to 50 kts at H8 by mid afternoon. In addition models keep surface winds mainly S to SSW today. That turning with height and increasing speed will support effective shear near 40 kts. This will favor supercellular structure...despite linear forcing along the front. Convection allowing models are also showing initiation of storms in the higher terrain early in the afternoon...perhaps related to the elevated heat source or terrain induced circulations. Any such storm would form in an environment more conducive for robust updrafts. The deep...moist air mass will mitigate a large hail threat...but any storm will carry a threat for damaging winds thanks to that near 50 kt SW LLJ. Severe wording and damaging winds will remain in the grids thru the early evening. Finally...we will have to watch for channeled flow areas...like the CT River Valley where Sly wind can hang on the longest. The enhanced helicity associated with those backed surface winds and low LCLs will lead to a low probability tornado threat. Though the threat is localized...strong winds in the low levels and turning in the lowest 1 km could lead to a brief tornado stronger than EF0.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...
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Front will slowly push thru the forecast area overnight. The loss of daytime heating should allow storm intensity to drop markedly from earlier in the day. Given the very moist environment...any convection will still be capable of heavy rainfall. Ahead of the front will also be another mild night...with lows struggling to get into the 60s. On Tue a weak s/wv trof will cross the area during the afternoon. Despite weak lapse rates remaining over the Srn parts of the forecast area...farther N will see slightly steeper lapse rates. Model forecasts show around 500 J/kg MUCAPE building by afternoon...so scattered thunderstorms are possible again. Though this time around instability and moisture will be less than we see this afternoon.
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&& .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
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The deterministic models are in decent agreement on the longwave pattern into the upcoming weekend. We begin the period with an upper trough centered across the Great Lakes and Mississippi Valley. This trough will weaken as it propagates eastward and off the northeast coast by early Thursday. Weak ridging follows briefly on Thursday before a warm front pushes into New England by Thursday night ahead of the next impulse racing eastward along the U.S. - Canadian border. The warm front presses northward across the area on Friday as low pressure races east across Quebec province with the trailing cold front to cross the region late Friday and Friday night. We`re left with residual troffiness to start the weekend with the prospect for some clouds and scattered convection mainly across the higher terrain. The models diverge for Sunday... with the preferred ECMWF/GFS blend yielding a mostly dry day before digging shortwave energy carves out a deep trough to our west...with a more significant storm system arriving in New England from the Great Lakes and mid Atlantic region for Sunday night and Monday.
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&& .AVIATION /08Z MONDAY THROUGH FRIDAY/...
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Short Term...Coastal LIFR/IFR stratus remains in place over the Midcoast area. RKD and AUG remain LIFR at this hour...but with flow direction not expected to change much thru morning I do not expect the stratus to move much farther W. Otherwise scattered MVFR CIGs are possible ahead of the cold front. Early this afternoon SHRA/TSRA will form along and ahead of the front. Local IFR conditions are possible in any convection...in addition to strong wind gusts. Most likely areas to see strong TSRA would include HIE...LEB...MHT...and CON. Strong TSRA threat is much more uncertain farther E. Cold front will cross the area tonight...clearing out low level stratus and TSRA. Afternoon SHRA are possible in the mtns again Tue. Long Term... Wed...Sct MVFR in -SHRA. Thu night - Fri...Areas of MVFR in SHRA/TSRA with lcl IFR in fog and drizzle.
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&& .MARINE...
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Short Term...SCA conditions will continue today in persistent Sly flow. There will be some gusts above 25 kts...but mainly seas will be above 5 ft outside of the bays. SCA conditions continue into Tue as the approaching cold front will be slow moving. Long Term... Tue night - Wed AM...Small Craft Conditions are possible outside the bays. Fri...Small Craft Conditions are possible outside the bays.
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&& .GYX WATCHES/WARNINGS/ADVISORIES...
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ME...Flash Flood Watch from 2 PM EDT this afternoon through Tuesday morning for MEZ007>009-012>014. NH...Flash Flood Watch from 2 PM EDT this afternoon through Tuesday morning for NHZ001>009-011-015. MARINE...Small Craft Advisory until 6 PM EDT Tuesday for ANZ150>154.
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&& $$ NEAR TERM...Legro SHORT TERM...Legro LONG TERM...Schwibs AVIATION...Legro/Schwibs MARINE...Legro/Schwibs

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