Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 221553 AFDGYX Area Forecast Discussion National Weather Service Gray ME 1153 AM EDT FRI JUL 22 2016 .SYNOPSIS... Warm and humid air streams in today as a cold front approaches from the northwest. This will set off scattered thunderstorm this afternoon and evening. The cold front stalls over the region on Saturday before moving south Saturday night. High pressure will build in from the west Sunday. High pressure will shift offshore Sunday night. A cold front will approach from the west on Monday and will push east through the region Monday night. High pressure will build in from the west Tuesday. A weak cold front will slowly settle south through the region Wednesday through Thursday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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---1150am UPDATE--- Have backed off on pops just a bit for this afternoon as it looks increasingly like the convection will hold off until this evening across most of the region. Again...the exception to this is the mountains where convection could get started late this afternoon. High resolution short term models continue to be in reasonably good agreement in holding off convection until around or just before 00z. Otherwise...have modified temperatures a bit to account for current trends. ---9am UPDATE--- Although there is still the possibility for severe convection today... it is looking more and more likely that it will be an evening event for most of the forecast area. Water vapor imagery is clearly showing significant mid level drying upstream of the forecast area moving into northern New England. The feature that is likely to kick off our convection today is still just to the north of Lake Superior. As always though... there are a few things that could change that. As it stand right now... both the HRRR and the latest NAM are indicating that convection in our CWA will hold off until after 6pm tonight. However... once it does start there appears to be plenty of energy to work with as the short range models are consistent in showing more than 2000j/kg of Cape. The short range models are also showing enough instability to get things started a bit earlier right along the New Hampshire and Massachussetts border. If that were to materialize then we could see some convection fire in our area along an outflow boundary. As it stands though...we are expecting the convection to largely be confined to 6pm or later across most of the forecast area. The mountains may be the exception and could go a few hours earlier as they will be the first to be impacted by the disturbance coming from the Great Lakes. Midnight shift had already started this trend in the forecast package so there are not a ton of changes to the current package. As the cloud cover begins to greak up today we will also see significant temperature rises. This will be coupled with an increase in low level moisture to create very warm heat indices across most of the area. Expect to see heat indices in the lower to mid 90s across the coastal plain and southern New Hampshire. Lastly... with the warm and sunny conditions there could be an increase in activity at coastal beaches today. Long period onshore swell will likely generate a moderate risk for rip currents. /Previous Discussion.../ Initially looking at two 500 mb waves moving through, one early today, and the other, a little stronger, tonight. This had sort of split the energy for potential severe wx today, and will give us a period with limited trigger for storms through a good part of the day. First round of t-storms has developed over upstate NY and southern Quebec. The southern end of the line is expected to diminish as move into VT, but the northern part may cross the nrn tier of zones between 09Z and 13Z. Not expecting these to be severe. Behind this line of storms will see a little subsidence and some drying, and this will likely limit storm development through at least 18-19Z. The early storms may leave a boundary across the north, which could produce a few storms before then, but best chc for storms will be later in the afternoon into the evening. Otherwise it will be warm with increasing humidity this afternoon, as temps rise into the mid 80s to around 90 in many spots and into low 90s southern NH. Urban parts of southern NH will see heat index values rising into the mid 90s this afternoon, as TDs climb into the upper 60s. SW flow will keep the mid coast a little cooler, and more clouds clouds over the ME mountains will keep temps there a little lower as well. As for the storms, best mode for severe will be bowing segments with damaging winds, but cannot rule out a enough of a rotating updraft to produce some large hail. Could see a more organized line develop late, but confidence is low, and looks to be more of a scattered severe event.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... Beyond the threat of severe continuing into this evening, should see sfc front slow and stall late tonight, as the wave aloft passes it by. This will allow for a decrease in coverage of convection late this evening and overnight, but given the presence of the sfc boundary and plenty of low level moisture, the chance for a few showers or storms will be there thru the overnight, especially from central NH thru the ME mid-coast and pointsnorth. Overnight lows will be balmy, mostly in the low to mid 60s in the north and upper 60s to near 70 in the south. On Saturday, a third 500 mb wave will move across New England and this will push the front southward in the afternoon. Another round of showers and storms is expected, and once again we will see the threat for severe storms as height falls and mid-level lapse rates combine with decent sfc instability to produce some rotatingupdrafts. The best chance for the strong storms would be south of the front and this may preclude much of a severe threat in the northeastern zones. Winds shift more westerly which will allow for slight lower TDs and slightly less warm highs on Saturday. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... High pressure will build in from the west Saturday night bringing clearing skies overnight. Lows will bottom out in the upper 50s to lower 60s north and mid to upper 60s south. High pressure will crest over the region on Sunday producing mostly sunny skies and highs from the mid 70s to lower 80s north and mid to upper 80s south. High pressure will shift offshore Sunday night and Monday as a cold front approaches from the west. Timing of this boundary not looking favorable for convection on Monday as best instability remains west of the forecast area. Best chance for any thunderstorm activity would come late in the day in far northwest zones. Should see a very warm and humid day with highs ranging from the mid 80s to lower 90s. Cold front will cross the region Tuesday night with scattered showers and thunderstorms. Lows will range through the 60s to near 70. High pressure will build in from the west behind the front on Tuesday bringing somewhat cooler and less humid air back into the region on brisk west winds. Highs on Tuesday will range from the mid 70s to mid 80s. High pressure will hold over the region Tuesday night and Wednesday as a weak cold front slowly settles into southern Quebec. May see a late day shower or thunderstorm in far northern zones but not looking for any widespread activity. Highs on Wednesday will top out in the mid 70s to lower 80s north and mid to upper 80s south. Cold front will slowly sag south through the region Wednesday night and Thursday which may set off some afternoon convection but not looking for anything too widespread as forcing with this boundary rather limited. && .AVIATION /16Z FRIDAY THROUGH TUESDAY/... Short Term...Mainly VFR through today, except for some sct TSRA mid-late afternoon. Could see a period of IFR coastal stratus and fog late tonight, and perhaps some valley fog as well. Long Term...VFR Saturday night through Tuesday. && .MARINE... Short Term...Persistent SW flow allows for swell to build today and tonight in the open waters reaching 5-6 feet or so through tonight. Long Term...No problems noted. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...Small Craft Advisory for hazardous seas until 8 AM EDT Saturday for ANZ150-152-154. && $$ Pohl is the U.S. government's official web portal to all federal, state and local government web resources and services.