Area Forecast Discussion
Issued by NWS Boston, MA

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507 FXUS61 KBOX 200349 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1049 PM EST Fri Jan 19 2018 .SYNOPSIS... A warming trend this weekend with continued dry weather. A storm system will bring the potential for heavy rain and strong coastal winds late Monday night into Tuesday. There is, however, an additional risk of some wintry mix/ice across the interior late Monday into Tue morning. Blustery, dry and seasonably cold weather follows Wednesday through Friday. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... 1030 PM Update... Most of the lower clouds have pushed offshore per 03Z observations, with area of mid and high clouds moving across as seen on latest GOES-16 Nighttime Microphysics RGB composite satellite imagery. May see some breaks in the clouds, but more will push E from W NY state overnight. Noting some patchy fog developing across portions of the Berkshires with light W-SW winds in place. Appears that temps pretty much bottomed out around 02Z in the 20s as warm air advection starts to push in. H85 temps rising to +1C to +3C on 00Z NAM run and should continue to rise through the remainder of the night. Have updated T/Td to keep a slow warming trend overnight. Previous Discussion... Expecting low clouds to linger as temperatures drop towards midnight. Watching the clearing line out of Upstate NY as warmer, drier air begins to push in from the W/SW, increasing which is discussed in much further detail below. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... Through Saturday... 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. Saturday night... 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 THROUGH FRIDAY/... Highlights... * Dry and mild Sunday along with light winds * Risk of a wintry mix/ice western MA late Mon into Tue morning * A period of heavy rain and strong coastal winds likely late Mon night into Tue * Dry, Seasonably cold Wed/Thu & Friday Synoptic Overview... Winter break? It looks that way given the lack of high latitude blocking combined with subtropical ridge emerging over FL from time to time will result in a fast/progressive northern stream jet of modest amplitude across the CONUS. Thus temperatures likely running at or slightly above normal the next 7-10 days. In addition given the progressive nature of the northern stream and streams appearing to be unphased, excessive precip events appear unlikely. Sunday... Dry/quiet weather with confluent mid level flow over the region providing plenty of subsidence. Thus sunshine, light winds and a mild airmass will support highs in the mid to upper 40s. Leaned toward the milder EC MOS here. It will feel even milder given the light winds and full sunshine. Not too shabby for Jan 21st. Monday... Turning colder as 1038 mb high over southern James Bay advects into southern Quebec with cold air bleeding southward into New England. This will set the stage for a wintry precip event for interior southern New England as warm advection from an approaching closed low rides up and over a cold dome spreading southward. Monday will begin dry but the GFS and its ensembles (GEFS) have precip overspreading the region during the afternoon. New 12z EC now introduces some light qpf late in the day across CT, west-central MA. We will lean toward a model blend which brings chance pops across the area Mon aftn. EC more amplified/colder solution than GFS/GEFS with high probability of mixed precip and ice across the interior Mon ngt into Tue morning. EC ensembles support the operational run with 50% probability of 32 degs or colder Mon ngt all the way to the CT/MA border. Thus will hedge the forecast toward this colder scenario especially given the cold air damming (CAD) signature with 1038 mb high over Quebec. Could be some impact here given it doesn`t take much ice to cause problems. Tuesday... How quickly shallow cold air scours out will hinge on timing and location of triple pt low development. If triple pt low tracks directly over the region (along with pres falls) shallow cold air would likely linger across northwest MA much of the day per EC and its ensembles (ECENS). Thus will have to watch this evolution closely as could be an extended period of freezing rain into Tue across northwest MA. Otherwise warm sector likely overspreads RI and at least southeast MA with temps surging into the 50s. More importantly will be a subtropical plume of moisture advecting up the eastern seaboard ahead of an approaching cold front with PWATs +2SD. Also strong southerly wind anomalies of up to +2SD within this moisture plume providing periods of heavy windswept rain. Also could be some embedded elevated convective elements as mid level lapse rates near moist adiabatic. Deterministic guidance has qpf ranging from 0.75 to 1.5 inches which is supported by both GEFS and ECENS ensembles with modest probs of 1 inch potential. System remains progressive which will limit excessive rainfall potential. As previous shifts have mentioned strong low level southerly jet of 65-70 kt at 925 mb crosses the region Tue, yielding a wind swept rain along with the risk of stronger winds aloft reaching the surface especially in heavier shower/convective activity and in the warm sector (coastal plain). As for impacts to river ice with potential rainfall near an inch next Tue, given dew pts remain at or below 32 degs much of the time leading up to Tue and then only briefly warming to 45-50 Tue aftn followed by dew pts falling back into the teens and 20s after Tue rainfall, not anticipating much melting, thus little impact to river ice. Wednesday, Thursday and Friday... Behind the departing Tue storm system it turns colder but seasonably cool with temperature anomalies at 850 and 925 mb very close to normal for late Jan. Mainly dry weather expected as well mid to late next week. && .AVIATION /04Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/... Overnight and Saturday...High confidence. VFR. Low-end VFR CIGs N of the Mass Pike expected to thin out during the early morning hours. Of greater concern is the increasing W winds. Strongest winds will be along the S coast and over the Cape / Islands. Potential 40 kt gusts over ACK around 12-15z Saturday. Roughly 30-40 kts S/SE- coast, 20-30 kts elsewhere. Threat of LLWS roughly 09-15z with a 45-55 kt SW jet at and just above 2 kft AGL. Saturday night...Moderate confidence. W winds diminishing, however low clouds potentially building across the region. Will keep low- end VFR with this forecast but there is the potential of IFR CIGs. KBOS Terminal...High confidence. Low-end VFR CIGs around 4 kft agl through evening. Increasing W winds overnight into Saturday with gusts to around 25 kts likely, possibly as high as 30 kts for an hour or two. KBDL Terminal...High confidence. Due to the valley location, W winds will increase but likely remain at or below 25 kts. Outlook /Sunday through Wednesday/... Sunday: VFR. Breezy. Sunday Night: VFR. Monday: MVFR/IFR conditions possible. Chance RA, chance SN, slight chance FZRA. Monday Night: Mainly MVFR, with areas IFR possible. Breezy. RA. Tuesday: Mainly MVFR, with local IFR possible. Windy with areas gusts to 30 kt. RA. Tuesday Night through Wednesday: VFR. Windy with local 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 Saturday Night/...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 centered around the Saturday morning hours, diminishing gradually thereafter. Outlook /Sunday through Wednesday/... Sunday: Winds less than 25 kt. Seas locally approaching 5 ft. 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. Rain. Tuesday: Moderate risk for Small Craft Advisory winds with local gusts up to 30 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 10 ft. Wednesday: Moderate risk for Small Craft Advisory winds with gusts up to 30 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 to 4 PM EST Saturday for ANZ232>234. Small Craft Advisory from 7 AM to 4 PM EST Saturday for ANZ230. Small Craft Advisory until 7 PM EST Saturday for ANZ231-251. Small Craft Advisory from 3 AM to 4 PM EST Saturday for ANZ236. Gale Warning from 5 AM to 4 PM EST Saturday for ANZ235-237. Gale Warning from 7 AM to 1 PM EST Saturday for ANZ250. Gale Warning from 4 AM to 7 PM EST Saturday for ANZ254-255. Gale Warning from 4 AM to 5 PM EST Saturday for ANZ256. && $$ SYNOPSIS...Nocera/Sipprell NEAR TERM...Sipprell/EVT/Thompson SHORT TERM...Sipprell LONG TERM...Nocera AVIATION...Sipprell/EVT/Thompson MARINE...Sipprell/EVT HYDROLOGY...Staff

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