Area Forecast Discussion
Issued by NWS Portland, ME

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000 FXUS61 KGYX 151106 AFDGYX Area Forecast Discussion National Weather Service Gray ME 706 AM EDT Tue Jun 15 2021 .SYNOPSIS... Low pressure with showers and a few storms crosses the region today, then exits to the northeast tonight. High pressure builds into the region for the middle of the week with seasonable temperatures and dry northwesterly flow. Temperatures start to increase by the end of the week. A cold front crosses Saturday with showers and thunderstorms. && .NEAR TERM /TODAY/... 705 AM Update... Refreshed PoP according to latest radar analysis... mainly to tighten the gradient between a zone of convergence near the coast and isolated shower activity elsewhere. A few pockets of dense fog remain this morning, but are generally improving, especially toward the west where dry air is filtering in behind a subtle surface front. Also worked the HRRR, which best captures the precipitation coverage this morning, into the temperature forecast... to allow for a little stronger warming west and a little cooler temperatures east... but only a couple degrees difference ultimately. Rest of the forecast is well on track. Previously... Rain showers and a few thunderstorms are the story for today as two areas of low pressure cross New England. Looking aloft, a vorticity maxima rounds the base of deep troughing over eastern NOAM, which will spur coastal cyclogenesis south of New England... along a cold front extending south from Quebec, associated with the long wave trough aloft. This makes for somewhat of a complex surface scenario today with cloudy and humid conditions along the coast and over most of Maine ahead of this front and warmer, drier conditions behind the front over much of New Hampshire. Thus PoPs are highest within about 50 miles of the coast early in the day... but will bloom elsewhere during the afternoon as height falls and steepening lapse rates aloft allow showers to develop. As for convective chances... again, a complex scenario today. Better dynamics exist with the southern wave, but stable onshore flow undercuts instability east of Portland... especially as low pressure hugs the coast this afternoon and evening. Meanwhile the cold pool dipping down from Canada with the poleward longwave trough steepens lapse rates enough for a general slight chance elsewhere as 500-1000 J/kg of MLCAPE develop. The surface pressure trough pushes down from Quebec this afternoon and evening, providing a focus for convection while taking advantage of daytime mixing... enough for scattered thunderstorms across the north, but without much kinematic support. For temperatures... expect temperatures to be subdued, not to exceed 70 degrees ahead of the front with more clouds and humidity. Over New Hampshire however I expect at least 70 degree readings with mid-70s over the southern interior depending on breaks in clouds. && .SHORT TERM /TONIGHT THROUGH WEDNESDAY/... Dry air pouring in aloft will allow showers and storms to wane with the diurnal trend tonight, diminishing as the system as a whole shifts east and exits toward the maritimes. Dew points and temperatures will also come down nicely overnight with low temperatures in the 40s toward the north and 50s toward the south. Opted a little warmed than consensus... since strong mixing doesn`t look to be a factor until the pre-dawn. For tomorrow, mostly sunny skies prevail with a fresh northwest wind on the order of 5-10 kts. Given sun, mixing, and downsloping, again chose the warm side of guidance with highs in the upper-60s (north) to as warm as the mid/upper-70s for southern zones. The offshore breeze even allows coastal locations to warm into the 70s for the first time in several days. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Upper level pattern for midweek will feature a trough over eastern Quebec and Labrador which keeps a relatively cooler airmass over northern New England along with northwesterly flow. The result is dry conditions, with seasonable temperatures. Highs will be in the mid 70s to upper 60s. Have kept close to the model consensus for these few days without much disagreement. Only noteworthy change was to account for the seabreeze along the midcoast, keeping temps down slightly for that area. By Friday night the overall flow pattern has opened up which will usher in some short waves. Increasing model agreement over the past few runs for the next wave to approach the region Friday night and move through on Saturday. Expect showers and thunderstorms with this, although do not expect much in terms of widespread heavy precipitation. Forecast soundings continue to show CAPE across the area of around 500 j/Kg, which while modest in general is respectable for global models at this time scale and thus indicative of solid thunderstorm potential. The southern and western part of the CWA has the best upper level support as a 80kts jet streak co-located with the front approaches from Quebec. Will need to monitor the surface conditions through the week as the northward extent of the low level moisture in the warm sector will largely determine the extent of the thunderstorm threat. Southerly flow, which becomes onshore as you move east through Maine will likely keep SW Maine and NH as the main threat areas. Additionally will be the timing of the front, with a mid-day passage maximizing thunderstorm potential. Sunday and Monday shower chances decrease as we see a brief high pressure builds in ahead of the next wave bringing additional showers on Tuesday. For next week will need to keep an eye on both the northern stream bringing another front across the Great lakes as well as the Gulf of Mexico which could enhance the moisture into the front. && .AVIATION /12Z TUESDAY THROUGH SATURDAY/... Short Term...IFR/LIFR with fog, low ceilings, and occasional showers rule this morning for eastern and southern terminals while MVFR and VFR hang on toward the north and west. The system responsible will very gradually shift east through today, allowing low ceilings to lift and scatter from west to east through about mid-day. Some uncertainty exists with KPWM and KRKD... with potential for fog and low ceilings to hang on for several hours longer. With this exception, VFR is expected to prevail and winds will switch to the west... then northwest... tonight into tomorrow. Long Term... High pressure and VFR conditions through the end of the week with afternoon seabreezes. Friday increasing southerly flow ahead of a front could bring some advection fog to the midcoast. The front crosses the region on Saturday with scattered showers and thunderstorms. && .MARINE... Short Term...Low pressure crosses the waters today with showers and perhaps a stray thunderstorm with weak onshore flow. The system exits northeast to the Canadian maritimes tonight with winds turning offshore through Wednesday. Winds will stay below SCA thresholds, however incoming swells from Tropical Storm Bill may bring seas to near 5 ft off the Midcoast of Maine. Long Term... Quiet over the waters through the end of the week. Southerly flow picks up on Friday with a cold front crossing Saturday bringing showers and possibly thunderstorms. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$ NEAR TERM...Casey SHORT TERM...Casey LONG TERM...Curtis

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