Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 212336 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 736 PM EDT Tue Mar 21 2017 .SYNOPSIS...
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An arctic cold front crosses the region early Wednesday morning, followed by dry, windy conditions as well as very cold temperatures into Thursday. Wind chills will drop to between 0 and 10 below zero Wednesday night into early Thursday morning. High pressure will build west of New England on Thursday, then push off the mid Atlantic coast Thursday night. Milder weather returns Friday with the chance for a period of light precipitation. A cold front crosses the region Saturday with a period or two of rain, ice and/or snow possible into early next week, but confidence is low.
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&& .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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715 PM Update... Area of clouds across northern New England tending to dissipate at its southern edge across S NH/VT, while another band of high clouds crossing N PA/S central NY may reach across the region by 02Z-03Z as seen on latest IR satellite imagery. Light/variable winds are starting to shift to W-NW across NW Mass into S VT and the mid Hudson valley of NY as seen on 23Z observations. Noting a weak high pres ridge that will cross the region this evening. Arctic front should push across the region between 08Z and 12Z Wed. NW winds will quickly increase not long after frontal passage. Wind chill values will drop quickly through the early morning hours, probably down to the single digits and teens across the higher inland terrain by 09Z-10Z, and in the 20s elsewhere by 12Z. Forecast pretty much on track. Have updated near term forecast to bring conditions current and incorporate trends into the remainder of the overnight forecast. Previous Discussion... Departing shortwave to our north will allow skies to become mostly clear early this evening. The result will be dry and tranquil weather across southern New England for the majority of the night. An arctic cold front will sweep through the region toward daybreak. The very cold air will just be entering our areas towards 12z Wednesday...so expect temps to range form the middle 20s in northwest MA and lower to middle 30s across southeast MA at that time. Not much moisture associated with the arctic front...so expect mainly a dry passage other than perhaps a few brief passing flurries generally along the east slopes of the Berkshires.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... ***Windy and very cold Wednesday with wind chills dropping to between 0 and 10 below zero Wednesday night*** Wednesday... An anomalously cold airmass for late March will invade the region. 850T will drop to between -16C and -20C by mid to late Wednesday afternoon. Plenty of strong March sunshine will be fighting the strong cold advection into mid afternoon...so expect temps through early afternoon to be in the 20s across the interior to the lower 30s across southeast New England. The strong cold advection will begin to win out by late afternoon and by early evening temps should mainly be in the teens to lower 20s. Wind chills will have dropped into the single digits by early Wednesday evening...so quite a change from today`s mild weather. The other concern will be the strong winds that are expected. Super adiabatic low level lapse rates combined with lots of late March sunshine will probably allow winds to over achieve on Wednesday. Northwest wind gusts of 35 to 45 mph are anticipated with a few gusts up to 50 mph possible. Will hoist a wind advisory for central and eastern MA as well as Rhode Island. This may need to be expanded across our western zones, but will let later shifts examine more closely. Wednesday night... Strong northwest wind gusts will continue to usher very cold air into southern New England under mostly clear skies. Winds should start to gradually subside by late evening, but it still will be gusty well into the overnight hours. Low temps will range from 5 to 15 above. This will result in wind chills of 0 to 10 below zero, which is quite impressive for late March standards. && .LONG TERM /THURSDAY THROUGH TUESDAY/...
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Highlights... * Cold and blustery Wednesday night and early Thursday, then winds diminish * Mixed snow and rain showers to start Friday, then temps rise with mainly scattered rain showers * Gusty SW winds late Friday, then cold front passes before stalling across the region Friday night * Low confidence next Sat-Mon with unsettled weather possible at times with some rain, ice and/or snow possible Details... Thursday-Thursday night...High confidence. Large high pressure will build SE out of NY state/PA during Thursday, settling off the mid Atlantic coast by Thursday night. After blustery and cold conditions during Thursday, with gusts up to 20-25 kt inland, the winds will diminish before backing to W-SW Thursday night as H5 ridging breaks down over the Great Lakes and northern Plains states. Cutoff H5 low pres works E across the southern stream, which will help to bring warm air advection pattern eastward during Friday. Good isentropic lift will push into western areas late Thu night. With an approaching front out of central Canada, will see some scattered snow showers develop toward daybreak across the E slopes of the Berkshires into portions of the CT valley. Core of cold air sits across the region Thu, so expect highs only in the lower-mid 30s (running up to 15 degrees below normal), with temps bottoming out in the upper teens and 20s Thu night. Friday-Friday night...Moderate confidence. Snow showers will continue to spread across the region Fri morning, then SW winds will quickly freshen. Noting a strong SW low level jet (on order of 40-50 kt from H925 through H85) and decent mixing mainly along the coast Fri afternoon. Will see wind gusts increase to 25-30 kt by midday Fri, highest along the S coast, Cape Cod and the islands. This will help temps rise quickly to the 40s by midday, which will still run around 5 degrees below seasonal normals. So, any leftover snow will quickly change to rain showers. May see a brief period of sleet and/or freezing rain well inland, mainly across the higher terrain in N central and NW Mass Fri morning, but even there it will change over to rain by midday. Will see cold front push SE out of central Canada during Fri night, so will continue to see scattered showers across the region. H5 ridging across the Great Lakes into the northern Plains will continue to de-amplify, flattening out the northern stream flow. So, the front looks to slow or even stall somewhere either across or just S of the region during Fri night. Kept chance POPs going through the night, but expect temps to remain above freezing through the night, at least for now. Lower confidence due to exact placement of the stalling front. Saturday into Monday...Low confidence. While guidance is in agreement synoptically, still a lot of details to work out for this portion of the forecast. Biggest uncertainties will be the placement of the upper level low and how quickly it will diffuse over the weekend. Also the location of the stalled front which will be the focus on where the precip will be orientated as well as the thermal profiles. Saturday afternoon...Temperatures will be in the upper 40s, with the southern coast reaching into the low 50s. Widespread rain showers are expected to continue through Saturday due to the above freezing temperatures, with a change of precipitation late that evening. As temperatures continue to drop below freezing early Sunday morning, rain will change over to snow as the arctic cold front moves from north to south. Snow will creep down into Rhode Island and Connecticut that morning. There is the possibility of snow changing over to freezing rain or sleet depending on the thermal profile as temperatures warm throughout the day Sunday. Afternoon temperatures Sunday will struggle to reach into the 40s, with the majority of the region remaining in the upper 30s. Despite temperatures reaching above freezing Sunday, the surging cold arctic air aloft will pose several problems for precip type throughout the day. Both the GFS and ECMWF suggest that mixed precipitation will continue through Sunday across the region, and even into Monday evening. The system will move offshore by 00z Tuesday with possible lingering snow or rain showers. With an approaching short wave, another round of mixed precipitation is possible Tuesday afternoon. Sites at higher elevations having the greatest chance of seeing freezing rain or snow as temperatures drop below freezing. Most areas south of the Mass Pike should remain above freezing overnight. Below average temperatures are expected to prevail through next week.
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&& .AVIATION /23Z TUESDAY THROUGH SUNDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/... 2330Z update... Tonight...High confidence. VFR. May see a brief band of CIGS around 3-4Kft in association with an arctic front toward daybreak Wednesday. Wednesday...High confidence. A few brief marginal MVFR cigs possible into mid morning, but otherwise VFR. Main story will be northwest wind gusts of 30 to 40 knots. Wednesday night...High confidence. VFR with gradually diminishing winds although it will remain gusty into the overnight hours. KBOS TAF...High confidence in TAF. Sea breeze comes to an end early by this evening. KBDL TAF...High confidence in TAF. Outlook /Thursday through Sunday/... Thursday-Thursday night...High confidence. VFR conditions. May see local MVFR-IFR VSBYS in scattered -SHSN after 06Z Fri across E slopes of the Berkshires. Friday and Friday night...Low to moderate confidence. Mainly MVFR in scattered -SHSN early Fri, changing over to -SHRA. May see brief -PL/-FZRA across higher terrain Fri afternoon/evening. VFR CIGS lower to areas of MVFR from W-E during the day, continue into Fri night. Saturday and Sunday...Moderate confidence. Periods of rain or snow showers along with some MVFR cigs are possible at times.
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&& .MARINE...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/...High confidence. Tonight...Winds and seas will generally be below small craft advisory thresholds for most of the night. However...NW winds should rapidly increase towards 12z Wednesday with gusts up to 30 knots behind an arctic front. Wednesday and Wednesday night...Northwest wind gusts of 30 to 40 knots expected under very strong cold advection. Strongest of those wind gusts will occur through Wed evening. Gale warnings are posted for all our waters. In addition...the anomalously cold airmass has prompted the issuance of freezing spray advisories for many of our waters Wed night into mid morning Thursday. Outlook /Thursday through Sunday/... Thursday...Moderate to high confidence. NW winds gusting to 25-30 kt, highest over the outer waters, diminishing below small craft criteria Thu evening. Light freezing spray will continues through midday, then ends as winds drop off. Seas up to 5-6 ft on the outer waters. Thursday night...High confidence. NW winds continue to diminish, then back to SW after 06Z Fri. Seas subside be low 5 ft by around 05Z-06Z Fri. Friday-Friday night...Moderate confidence. SW winds increase quickly Fri morning, gusting up to 25-30 kt. Small crafts likely. Low risk of minimal gale force winds for a time Fri afternoon-evening on portions of the outer waters. Expect winds to diminish Fri night as cold front passes and shift to W. Seas up to 4-6 ft, highest over the southern outer waters Fri afternoon/night. Saturday...Low confidence. N-NE wind gusts up to 20 knots and seas under 5 feet. Conditions may vary significantly depending upon the timing of the frontal passage. Sunday...Low confidence. E-NE winds 20 kt or less. Seas 4 ft or less. Gusts to small craft levels possible.
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&& .CLIMATE... Record lows for Wednesday, March 22: Boston (BOS) 8/1885 Hartford (BDL) 12/1934 Providence (PVD) 15/1988 Worcester (ORH) 8/1988 Record cold highs for Wednesday, March 22: Boston (BOS) 24/1885 Hartford (BDL) 29/1960 Providence (PVD) 28/1914 Worcester (ORH) 25/2002 Record lows for Thursday, March 23: Boston (BOS) 6/1934 Hartford (BDL) 9/1934 Providence (PVD) 8/1934 Worcester (ORH) 4/1934 && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Wind Advisory from 8 AM to 8 PM EDT Wednesday for MAZ004>007- 012>024-026. RI...Wind Advisory from 8 AM to 8 PM EDT Wednesday for RIZ001>008. MARINE...Freezing Spray Advisory from 7 PM Wednesday to 9 AM EDT Thursday for ANZ231>235-250-251-254>256. Gale Warning from 8 AM Wednesday to midnight EDT Wednesday night for ANZ230>237. Gale Warning from 8 AM Wednesday to 2 AM EDT Thursday for ANZ250-251-254>256. && $$ SYNOPSIS...Frank/EVT/Correia NEAR TERM...Frank/EVT SHORT TERM...Frank LONG TERM...EVT/Correia AVIATION...Frank/EVT/Correia MARINE...Frank/EVT/Correia CLIMATE...Staff

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