Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 200811 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 311 AM EST Sat Jan 20 2018 .SYNOPSIS... Mild and dry weather expected this weekend but gusty winds today. A storm system will bring the potential for heavy rain and strong coastal winds on Tuesday, but this will be preceded by a risk of some wintry mix/ice across the interior late Monday into Tue morning. Blustery, dry and colder weather follows Wednesday through Friday. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 150 AM Update... Skies have become mainly clear across most of the region as seen on 06Z observations, except for some lingering high clouds across eastern areas and the lower CT valley. Where the skies have cleared, winds have also diminished, setting up excellent radiational cooling. Dewpoints have dropped to the teens to lower 20s. All this has allow temps to nosedive as well, with readings in the upper teens to around 20 across N central and W Mass as well as some of the normally colder valley locations, ranging to the lower-mid 30s across some immediate coastal locations. Little if any mixing is going on at present, which is matching up well with the decreasing low level lapse rates from H9-H95 on the 00Z GFS guidance, even as low level jet up to 45-50 kt is passing across S coastal areas. However, gusts up to 25 kt at 06Z reported from KORH at 1000 ft AGL. Patchy fog has also develop across portions of W Mass mainly around the CT valley region. Of course the observations reporting this, but also see it on the GOES-16 Nighttime Microphysics RGB composite satellite imagery, at least across the valley itself. Also seeing light fog across the Berkshires up to E central NH. Temps will bottom out in the mid teens to around 20 across central western areas, expect a bit milder in the urban centers, ranging to the 20s further E. Pressure gradient will start to increase toward daybreak, which should allow W-SW winds to increase and temps rising to the upper 20s and 30s by around 12Z. May see SW wind gusts up to 20-25 kt by then along S coastal areas. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SUNDAY/... Today... Digging N-stream energy out of the Arctic reaches of Canada, capturing and stretching energy out of a sweeping H5 trof axis into the W CONUS, parent surface low continues to deepen across the N Great Lakes region into the St. Lawrence River Valley down to around 990 mb up against high pressure over the SE CONUS up to 1030 mb. A tightening pressure gradient emerges along with a subsequent jet streak, a gradient wind threat exists around the Saturday morning timeframe. H925 westerly winds 45 to 55 mph, the height of which is around Saturday morning. Accompanying stout inversion, strengthening, with pronounced warming through the H8-9 layer. Limitations on the potential mix down of faster momentum to the surface, however, if the boundary layer is able to mix up to H95 / 1200 feet agl, then there is the possibility of getting wind gusts up around 40 to 45 mph. However, considering the cold, icy ocean, and snow pack ground, expect outcomes to mainly be gradient driven with gusts on the order of +5 to the sustained wind. Sustained around 30 mph, gusts around 35 mph possible along the S-coast, Cape and Islands, but again, colder conditions right at the surface are likely to limit mixing. Warm air advection proceeding aloft along with abundant sunshine, temperatures warming towards the low 50s through the day as the height of the winds aloft pass through, it is only then that gusts 20 to 30 mph are possible across much of S New England aside from the faster winds along the S/SE coast, both of which diminish towards sundown. GALE WARNINGS for the waters but no WIND ADVISORY headlines given such a marginal event. If any one location has the best chance of seeing wind advisory criteria with sustained winds over 30 mph it would be Nantucket. Tonight... Main surface cold front pushing S into the region, eroding beneath high pressure. Light winds expected with filtering mid to high level clouds across the region within the zonal pressure gradient aloft. Stout inversion building across the region down to H95, with higher dewpoint air lingering along with the likely boundary layer mixing of snowmelt and now cold air advection proceeding, expecting to seeing low clouds develop towards Sunday morning. Keeping it mild with lows around the upper 20s to low 30s for the timeframe. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Highlights... * Risk of light wintry mix/ice interior MA late Mon into Tue AM * A period of heavy rain and strong coastal winds likely Tue * Dry and colder Wed-Fri Sunday night into Monday night... Subtle cold front will drop south across SNE late Sun night into Mon as strong high pres builds across SE Canada. Weak overrunning to the north of this stalled boundary and a gradual moistening of the column will result in light precip developing across SNE, mainly Mon afternoon into Mon night. Pronounced cold air damming signature noted with high pres over the Maritimes and sfc ridging nosing down across SNE will set the stage for some wintry precip in the interior. Thermal profiles initially support mixed precip with some light snow/sleet/freezing rain into the evening, with risk transitioning to areas of freezing rain during Mon night interior northern MA as warming aloft becomes more pronounced while low level cold air hangs tough. Some uncertainty with how long low cold air near the surface hangs on but some of the guidance has it lingering into Tue morning for interior northern MA. Increasing risk for some travel impacts. Tuesday... Other than a risk of some freezing rain lingering across interior northern MA in the morning, ptype will be all rain. Period of heavy rain appears likely on Tue moving west to east across SNE as strong low level jet develops ahead of the cold front. PWAT anomalies +2SD with low level wind anomalies up to 2-3SD which is a good signal for heavy rainfall. In addition, marginal elevated instability develops supporting a risk of some convection and elevated thunder. Deterministic GFS/ECMWF/GGEM all show potential for QPF of 0.75 to 1.5 inches and ensemble guidance supports this with moderate to high probs of 1+ inches. This system is rather progressive which should prevent rainfall from exceeding these amounts. Other concern for Tue is potential for strong winds accompanying the 65-70 kt low level jet. Low level lapse rates are forecast to be near moist adiabatic which would allow for decent mixing, especially where temps get into the 50s across RI and eastern MA. Potential for wind advisory gusts, especially in any heavier showers/convective activity. Not expecting much impact to river ice as there is only a brief window of higher temps/dewpoints on Tue followed by colder conditions. Wednesday through Friday... Turning colder and blustery behind the departing storm in NW flow pattern. Temps near normal Wed trending below normal Thu/Fri. Coldest day expected to be Thu as GEFS and EPS showing ensemble mean 850 mb temps dropping to -12 to -14C across SNE. Temps on Thu likely remaining below freezing across much of SNE with 20s interior. Mainly dry weather expected during this period, although a brief flurry/snow shower can`t be ruled out Wed/Thu over higher terrain as a few shortwaves move through. && .AVIATION /08Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/... Through 12Z...High confidence. Mainly VFR conditions. Patchy fog with MVFR to local IFR VSBYS across portions of the mid CT valley W to the Berkshires as temps fell back close to dewpts and winds dropped off to light/variable or calm. VSBYS should improve by around 12Z. SW wind gusts up to 20-25 kt possible by daybreak across S coastal terminals. Today...Moderate confidence. Mainly VFR conditions, though may see low end VFR CIGS develop across E slopes of the Berkshires by midday through the afternoon. Expect W-SW winds to gust up to 25-30 kt across the higher inland terrain, ranging up to around 35 kt along the S coast, Cape Cod and the islands with low risk of 40 kt gusts during the afternoon. Threat of LLWS through 16Z-18Z as a 45-55 kt SW jet at and just above 2Kft AGL moves across, with best chance from KPVD-KPYM southward. Tonight...Moderate confidence. VFR. May see patchy MVFR CIGS across the E slopes of the Berkshires off and on through the night. W winds will diminish, but may see leftover gusts up to 25-30 kt along the S coast early. Sunday...High confidence. VFR. W-NW winds around 10 kt or less as pressure gradient relaxes. KBOS Terminal...High confidence in TAF. VFR conditions expected. W-SW winds gusting up to 20-25 kt, highest around midday into mid afternoon. KBDL Terminal...High confidence in TAF. Outlook /Sunday Night through Wednesday/... Sunday Night: VFR. Monday: MVFR/IFR conditions possible. Chance RA, PL, SN. Slight chance FZRA. Monday Night: Mainly MVFR, with areas IFR possible. RA likely, FZRA likely interior northern MA. Tuesday: Mainly MVFR, with local IFR possible. Windy with gusts to 30 kt. RA. Pockets of FZRA possible early interior northern MA. Tuesday Night: VFR. Windy with local gusts to 30 kt. Wednesday: VFR. Windy with areas gusts to 30 kt. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/...High confidence. Gradient W-SW winds will approach gale force toward daybreak across the southern waters. GALE WARNINGS posted. Waves build to 10 feet at the height of the winds through midday today, diminishing gradually thereafter. Outlook /Sunday Night through Wednesday/... Sunday Night: Winds less than 25 kt. Monday: Winds less than 25 kt. Slight chance of rain. Monday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain. Tuesday: Low risk for gale force winds with gusts up to 35 kt. Rough seas up to 12 ft. Rain. Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 11 ft. Wednesday: Low risk for gale force winds with gusts up to 35 kt. Rough seas up to 11 ft. && .HYDROLOGY... A Flood Warning remains in effect for the Connecticut River at Middle Haddam, where ice jams are causing river fluctuations. For details on specific area rivers, including observed and forecast river stages, please visit the Advanced Hydrologic Prediction Service (AHPS) graphs on our website. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Gale Warning from 7 AM this morning to 4 PM EST this afternoon for ANZ232>234. Small Craft Advisory from 7 AM this morning to 4 PM EST this afternoon for ANZ230. Small Craft Advisory until 7 PM EST this evening for ANZ231- 251. Small Craft Advisory until 4 PM EST this afternoon for ANZ236. Gale Warning until 4 PM EST this afternoon for ANZ235-237. Gale Warning from 7 AM this morning to 1 PM EST this afternoon for ANZ250. Gale Warning until 7 PM EST this evening for ANZ254-255. Gale Warning until 5 PM EST this afternoon for ANZ256. && $$ SYNOPSIS...KJC NEAR TERM...Sipprell/EVT/Thompson SHORT TERM...Sipprell LONG TERM...KJC AVIATION...KJC/Sipprell/EVT MARINE...KJC/Sipprell/EVT HYDROLOGY...

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