Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 131810 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 110 PM EST Sat Jan 13 2018 .SYNOPSIS...
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A strong cold front exits offshore early this afternoon with temperatures crashing into the 20s this afternoon. Mainly dry but very cold weather follows tonight through Monday. Mainly dry but cold weather is expected Sunday and Monday. Low pressure diving south from the Canadian Arctic will bring a chance of snow Tuesday and Wednesday. Dry weather may return Thursday and Friday. We will be watching a potential coastal storm that could change that Thursday forecast.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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115 PM update... At 1 pm cold front was on the verge of moving across the outer Cape and Nantucket. Otherwise post frontal airmass has overspread the region with temps falling rapidly into the 20s and 30s regionwide on gusty WNW winds up to 35 mph. This is resulting in wind chills in the teens and 20s. The gusty WNW winds and rapid drying post frontal airmass has ended the flash freeze threat. Therefore have expired the Winter Weather Advisory and will let the SPS expire on its own at 19z. Hydro... Cancelled the flood watch given rainfall has ended. However many rivers/streams are in flood given ice jams and rising water levels. Please refer to our flooding warnings and statements for additional information.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... Tonight... Very cold air will be ushered in on north to northwest winds. Although we won/t be extremely cold at 850 mb this airmass will come down shallow and expect overnight low temps to mainly be in the single digits to lower teens in most locations. We may even flirt with wind chill advisory conditions along the east slopes of the Berks...but it was too marginal to issue at this time. Meanwhile on the outer-Cape...NNW winds may allow for some ocean effect snow showers to develop overnight but with accums of only a dusting to less than 1 inch are expected. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Highlights... - Cool and mainly dry except light snow across extreme SE MA. - Advisory or even winter storm snow possible Tue into early Thu depending on track. - Seasonable late week. Overview and model preferences... The formation of an upstream Rex-block, signaling a strong positive PNA, combined with blocking cutoff high pres at H5 near the Azores, allows meandering cutoff to settle near the Great Lakes early in the week. This is a PV lobe associated with a current Baffin Island vortex, so it will carry with it another round of arctic influenced air as it settles just upstream. The primary focus associated with this cutoff will be a clipper system initially. However, there is some indication of secondary development as the trof begins to transition toward a negative tilt along the coast. While the ECMWF and many of its ensembles lead the charge, there remains enough uncertainty that a blend of ensembles from both the ECENS and GEFS is preferred as it will take into account both the more progressive and more amplified solutions. As the trof shifts ENE, will maintain this blend into the weekend. Details... Sun through Mon... Arctic airmass once again settles just to the W of New England, but not without its impacts. H92 temps dip as low -13C by early Mon morning. This suggest cool highs each day, mainly in the 20s, although with high pres cresting wind chills should be limited. Overnight lows, however are primed for radiational cooling, but will be mitigated by the now lack of a snowpack. Still will likely see mins fall as low as the single digits in sheltered valleys to the teens elsewhere. Strong subsidence is implied in soundings, which suggests mainly dry conditions. H85 temps are warmer under this inversion than temps below. With NNW flow gradually shifting toward the NNE, the delta-T values from SST to H85 hovers around 15C, while to H9, it is closer to 20C. Therefore, although shallow will likely see light (given only moisture is at the top of the mixed layer) snow showers across mainly the outer portions of Cape Cod and possibly Plymouth county Sun night into Mon as the winds take on a slight E component. Accums should will remain light. Tue into Thu... As cutoff sets up near the great lakes and its attendant longwave trof continues to dig, the final depth and tilt will ultimately determine the possibility of secondary coastal development near the mid Atlantic, after a weakening clipper impacts the region. At this time, ensemble low locations for the potential coastal low range from non-existent, to an inside runner to close to Bermuda, with some clustering near the 40/70 benchmark. However, this much spread is likely to continue into Sun, until the PV anomaly currently wrapped up the arctic becomes better sampled. The clipper does ingest some moisture on Tue as it approaches the Atlantic. This does yield decent moisture in the DGZ, however overall lift is weak, so SLRs are unlikely to be maximized. So a more progressive solution favored by the GFS, with QPF ranging only between 0.1-0.25 inches could stay below advisory criteria. Meanwhile, the more robust solution would quickly follow the clipper with influence of coastal low pres Wed-Thu with better lift in the DGZ and overall more latent-heat energy. There is some precedent for this solution given some of the ensembles and overall amplification of the pattern, and should it verify, it has closer to 0.5-1.00 inches of QPF which may warrant winter storms. Too early to call on totals at this time, especially since the more amplified solution also introduces warmer air (H92 temps average +3C across SE MA). Elsewhere, with H92 temps generally between -2C and -6C, mainly SN is expected, along with highs closer to seasonal normals in the upper 20s to low 30s and lows in the teens to mid 20s. Something to watch especially late weekend into Mon. Fri into the weekend... Enough breakdown of the upstream Rex to signal a transition. As opening wave transitions E and moderates, temps look to return to more seasonable levels, along with mainly dry air. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/...
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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 Tonight/... 1815z update... Thru 00z...high confidence. VFR and dry weather. WNW winds up to 30 kt will shift to the NNW toward 00z and decrease in speed. After 00z...high confidence. VFR and dry weather with N winds 10-20 kt, highest winds over Cape Cod and Islands. Also this is where MVFR cigs will develop along with scattered ocean effect snow showers tonight. Sunday...high confidnece. VFR and dry weather except MVFR in scattered snow showers over Cape Cod and Nantucket. Modest north winds 10-20 kt. Sunday night...moderate confidence. Some uncertainty on timing and areal coverage of MVFR conditions. MVFR in scattered snow showers continues over Cape Cod and Nantucket. However as winds shift from N to NE these MVFR conditions spread westward into eastern MA toward daybreak Mon. KBOS Terminal...high confidence in TAF thru Sunday then some uncertainty if snow showers make it westward into Logan Sun ngt. KBDL Terminal...high confidence in TAF. Outlook /Monday through Wednesday/... Martin Luther King Jr Day: Mainly VFR, with local IFR possible along extreme southeast MA in some light SHSN. Breezy. Monday Night: Mainly VFR, with local IFR possible along extreme southeast MA in some light SHSN. Tuesday: MVFR/IFR conditions possible. Chance SN. Tuesday Night: Mainly IFR, with local MVFR possible. Chance SN. Wednesday: Mainly VFR, with areas IFR possible. Breezy. Chance SN.
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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 Tonight/... Today...strong cold front cross the waters later this morning and early afternoon with NW wind gusts around 25 knots. Tonight...High confidence. NNW wind gusts around 20 knots in the cold air advection pattern...but seas will remain above small craft thresholds even though swell will be on the downward trend. Outlook /Sunday through Wednesday/... Sunday: Winds less than 25 kt. Areas of rough seas. Local visibility 1 to 3 nm. Sunday Night: Winds less than 25 kt. Seas up to 5 ft. Slight chance of snow showers. Local visibility 1 to 3 nm. Martin Luther King Jr Day: Winds less than 25 kt. Seas up to 5 ft. Slight chance of snow showers. Areas of visibility 1 to 3 nm. Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Tuesday: Winds less than 25 kt. Local rough seas. Chance of snow, chance of rain. Visibility 1 to 3 nm. Tuesday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of snow. Visibility 1 to 3 nm. Wednesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of snow. Visibility 1 to 3 nm.
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&& .CLIMATE... Record high temps set yesterday: PVD 62 in 2018 BDL 60 in 2018 ORH 59 in 2018 && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 7 AM EST Sunday for ANZ231>235-237- 250-251-254>256. Small Craft Advisory until 7 PM EST this evening for ANZ230- 236.
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&& $$ SYNOPSIS...Frank/Doody NEAR TERM...Nocera SHORT TERM...Frank LONG TERM...Doody AVIATION...Nocera/Doody MARINE...Frank/Doody CLIMATE...

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