Area Forecast Discussion
Issued by NWS Portland, ME

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649 FXUS61 KGYX 282100 AFDGYX Area Forecast Discussion...UPDATED National Weather Service Gray ME 500 PM EDT Tue Mar 28 2017 .SYNOPSIS... Low pressure will pass to our south tonight. Behind this system, a northwesterly flow will introduce cooler and drier air into the region for the second half of the work week. High pressure will build into the region for Thursday. Low pressure will approach the region for late Friday into Saturday with snow and mixed precipitation likely for the region. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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21Z Update... Have adjusted PoPs and QPF and corresponding snow amounts based on the latest few runs of the HRRR which has a good handle on the convection. One area of lift associated with short wave and a warm front was producing showers over southern sections of NH and Maine. Elsewhere upslope snow showers were affecting the mountains and foothills. Precipitation chances were adjusted so categorical PoPs were not broad-brushed and instead focused on these two areas. A few other minor changes were made but nothing to change the forecast wholesale. Previous discussion... A weak shortwave impulse on GOES water vapor imagery was crossing the mid Atlantic region with a 1009 millibar surface reflection over the DELMARVA region and a weak surface boundary extending northward into western New England. Widespread low clouds remained across the region at moment. NWS Doppler radar mosaic showed an expanding area of precipitation across the mid Atlantic to southwestern New England that will cross southern New Hampshire and adjacent southwest coastal Maine during the late afternoon and early evening hours. Elsewhere...only scattered precipitation is likely as a cold front over Quebec drops southward across the region overnight. The activity should taper quickly after midnight as the surface low and shortwave impulse race offshore.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... On Wednesday...a freshening northerly breeze in the wake of the departing coastal low with a few clouds and upslope snow or rain showers across the higher terrain. Highs will range from near 40 in the mountains to near 50 over southeast New Hampshire. Some lingering upslope clouds and snow showers Wednesday night in the mountains...otherwise mostly clear elsewhere with a diminishing northerly breeze and the surface high begins building into eastern New England. Lows will range from the mid 20s to around 30. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Confidence is gradually increasing for a fairly significant late season snow and/or mixed precipitation event starting Friday and lasting into Saturday. 12z deterministic guidance is in general agreement today on track of short wave troughs and attendant SFC low pressure along with subsequent heavy snowfall for at least the southern half of the CWA. The ECMWF ensemble probabilities for 6 or more inches of snow have steadily increased over the last 24 hours, now offering a 50-70 percent chance of 6"+ to a fairly good chunk of the CWA. However, with that said, we are still at least 3 days out and even tight ensemble consensus can still go awry at this time range. There are also questions about boundary layer temperatures and perhaps even some warm mid level temps creeping into southernmost zones. The track of the overall system could still push further south also. These issues can certainly be the cause of a lighter snowfall. So being a bit conservative in the forecast temperature/qpf/snow grids is prudent at this time. However, we did lower SFC temps Friday afternoon and night as most raw model guidance slams SFC temps down to 31-33 degrees as strong UVVs move into the area and allow for cooling via lift, evaporation, and melting. At this time I would expect at least interior southern zones to pick up a plowable snowfall starting Friday afternoon or evening and ending Saturday. However, there is low confidence at this time for a lot more over a wide area, although possible. Bears watch for sure. A heavier snowfall this time of year would probably cause power outage issues. && .AVIATION /21Z TUESDAY THROUGH SUNDAY/... Short Term...IFR to LIFR conds persist into tonight. Look for improvement to VFR during Wed morning, with NW winds gusting to btw 20 and 25 kt in the afternoon. Sct MVFR conditions will be psb in mtn -shra/-shsn. Long Term...IFR conditions possible in snow and mixed precipitation starting Friday and lasting into Saturday. Conditions likely improve Saturday night and Sunday. && .MARINE... Short Term...Winds will remain below small craft through Wednesday afternoon. We`ll likely see winds and seas reach Small Craft Advisory threshold by Wednesday evening outside the bays in strengthening northerly flow. Long Term...SCA conditions will subside Thursday. They will once again be possible over the weekend as low pressure passes to the south of the waters. && .TIDES/COASTAL FLOODING... We`re entering the period of high astronomical tides. The Wednesday and thursday night high 105 AM Thursday and 152 AM Friday respectfully are 11.1 ft MLLW (referenced to Portland Harbor with a 12.0 ft MLLW flood stage). Fortunately the flow will be offshore so no problems are anticipated with these particular tides. However, the Friday night high tide at 243 AM is an 11.0 ft MLLW. With potential for a strengthening onshore flow prior to this particular high tide cycle...we will need to monitor this time period for possible coastal flooding and beach erosion. && .GYX WATCHES/WARNINGS/ADVISORIES... ME...None. NH...None. MARINE...None. && $$ SYNOPSIS... NEAR TERM...Hanes SHORT TERM...Schwibs LONG TERM...Ekster AVIATION... MARINE... TIDES/COASTAL FLOODING... is the U.S. government's official web portal to all federal, state and local government web resources and services.