Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 200225 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1025 PM EDT Sun Mar 19 2017 .SYNOPSIS...
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Departing ocean storm will result in some left over non-accumulating snow showers or even light rain showers across the Cape/Islands this evening but the bigger story will be the strong winds. High pressure building in from the west will result in partly to mostly sunny and milder weather on Monday continuing into Tuesday. An arctic front will bring near record cold temperatures Wednesday and Thursday along with bitterly cold wind chills. A warming trend then develops Friday and Saturday.
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&& .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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1030 pm update... Trowaling moisture rearward of the deepening ocean storm well to the E of the region still undergoing weak deformation and ascent lending to the developing of some scattered rain / snow showers as discerned from latest WSR-88D returns. H5-7 vort lobe energy adds to enhancement of some of the banding encroaching onto the Cape and Islands. Conditions mild with temperatures hovering around the mid-30s. Unless any of the banding can get going with a bit more intensity, expect any frozen precipitation to simply fall and melt on contact. Some reductions in visibility down to 2 miles will be possible. Simply put, nuisance weather that should conclude as late as the early morning hours Monday. Thinking mainly diminishing trends after midnight. Winds still kicking though diminishing. Have downgraded headlines to wind advisory out on Nantucket. As we go towards midnight there is still the possibility of an isolated gust or sustained wind across the Cape and Islands. However greater confidence and weight is given to Nantucket for potential impacts due to northerly winds. Gusts up to 50 mph possible for the island. Thinking just some downed limbs and isolated power outages possible. Headlines go till midnight. Winds continuing to diminish into Monday as the ocean storm continues to track off S/E out to sea. Good boundary layer mixing up to H9 as discerned from the 0z Chatham sounding. Cold air advection proceeding, should see the Cape and Islands fall down around the low 30s with locations N/W across the interior from the upper 20s to upper teens, coldest conditions over N/W MA.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/... Monday... Partly to mostly sunny skies and milder temps will usher in the first day of spring. 850T near 0C and good mixing should result in many locations seeing high temps recover into the upper 40s to the lower 50s Monday afternoon. The exception will be the Cape/Nantucket where gusty northerly winds blowing off the cold ocean will hold high temps in the lower 40s. Wind chills across the Cape/Nantucket will be in the upper 20s to middle 30s with the gusty winds so a typical early spring day in southern New England. && .LONG TERM /MONDAY NIGHT THROUGH SUNDAY/... Highlights... * Moderating temps Mon and Tue with 50 possible for some Tue! * Arctic blast with near record cold returns middle of next week * Warming trend Friday and Saturday with some precipitation likely Overview and model preferences... Noting enough agreement between deterministic guidance through at least Fri that a blend can be used as a baseline. Given uncertainty with a S stream cutoff developing Fri into the weekend will lean more heavily on ensemble means to account for the full envelope of possibilities. Modest meso ridging Mon night into Tue gives way to another arctic wave and attendant arctic front by late Tue into Wed, with below normal cold Wed and Thu. This gives way to warm ridging by Fri and Sat, but a meandering S stream cutoff could allow for precipitation in the form of an inside runner. Timing somewhat uncertain, but by leaning on ensemble means, it appears mainly the weekend will be wet. Details... Mon night into Tue... Relatively warm period thanks to modest ridging yielding H85 temps near +2C by 12Z Tue. Although mid lvl temps will gradually decrease through the day it`s still likely that with downslope flow in areas of little to no snowpack (SEMA and S RI) will see highs in the low 50s, but still holding in the 40s inland in spite of the snowpack. Mainly sunny skies, although some days- end clouds possible ahead of the approaching arctic front. Tue night through Thu... Arctic front will slide through Tue evening. Right now it appears there will be little fanfare but with the robust upper lvl cold advection, could see a few SHSN or even an isolated squall developing especially during the daylight hours on Wed when the low lvl lapse rates will be steepest. Moisture is somewhat lacking, hence the low confidence and limited areal coverage potential. Strong pres gradient Wed will enhance the squall threat, but at the very least lead to cold N winds. Otherwise, still monitoring an offshore frontal wave moving off the mid Atlantic and along the front stalled to the S. As previous forecaster noted, not seeing enough backing in the flow to suggest precip shield impacting S New England, however as noted yesterday, there continues to be a slight northerly shift each run of the peak F-gen/deformation that suggests maybe enough low-lvl trofing for some precip outside of the SHSN or isolated squalls. Still worth watching for Wed. Below normal temps are expected as H85 temps drop between -18C and -20C which may be record breaking for CHH sounding per SPC climatology. The late March sun angle will likely still allow highs into the low-mid 30s each day, but assuming a break in the pres gradient Wed night and Thu night, mins could drop into the single digits and teens, potentially approaching some records. Fri... Gradual warming thanks to a rebound in ridging with the arctic wave moving E. This does have some connection to Gulf of Mexico moisture/warmth as H85 temps warm rapidly back to an average of +2C by Fri afternoon. This suggests highs once again back into the 40s and potentially even near 50. Will also be watching a warm front approaching from the W, but looking at the associated LLJ and dry soundings in the the low lvls, so not expecting much in the way of precip at least during the daytime Fri given these mitigating factors. Sat and Sun... Modest baroclinic zone is stalled across or just N of S New England with a weak low pres traversing it sometime on Sat. This suggests increased risk for rainfall as ensemble average PWATs are around 1.00 inches. This gradual southward progression suggests a risk for rainfall both Sat and into Sun, although some improvement is possible as timing will be better worked out as we approach. One other note of uncertainty is the placement of an arctic high pres to the N, which as the precip shield shifts S may allow for some changeover to a wintry mix. This is highlighted in some ensembles and bears watching as we approach. && .AVIATION /02Z MONDAY THROUGH FRIDAY/...
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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 Monday/... 2z update...Some changes made to TAFs to adjust to trends. Otherwise no major changes. Overnight...High confidence. VFR inland. MVFR/IFR lingering on the Cape / Islands towards midnight. Strong N winds in addition sustained around 15 to 25 kts with gusts up as high as 40 kts, mainly Nantucket. Winds diminishing overnight. Monday...High confidence. VFR. N winds. Gusts 25 to 35 kts into mid afternoon across the Cape and Islands. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Monday Night through Friday/... Mon night through Tue...High confidence. Mainly VFR. Winds light out of the W. Wed and Thu...High confidence. Mainly VFR although isolated SHSN possible mainly Wed with brief lower categories. Winds gusting 20-25 kt on Wed out of the NW. Lighter Thu. Fri...Moderate confidence. Winds shift to the SW. Mainly VFR, but a MVFR CIGS may begin to filter in from the S late in the day.
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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 Monday/... 1030 pm update... Tonight...Moderate to high confidence. Continued pressure gradient between departing ocean storm and high pressure building into the mid Atlantic will result in strong winds and high seas overnight. However the ocean storm shifting S/E, both winds and gradient gradually relax. Already seeing winds come down allowing for headlines to adjust downward. Storm warnings adjusted to gales and some gales adjusted down to small craft advisories. Will see these headlines continue into the morning hours Monday. Roughly 10 to 15 foot seas expected across our easter outer- waters with the longer fetch. Monday...High confidence. Northerly Gale Force Wind gusts should diminish to between 25 and 30 knots by late morning and continue into the afternoon. Nonetheless...pressure gradient will remain strong enough that will need small craft headlines. Outlook /Monday Night through Friday/... Mon night into Tue...High confidence. Winds and seas gradually diminish Mon night dropping below small craft advisory thresholds by early Tue morning after starting around 25-30 kt and 5-7 ft respectively. Quiet boating weather follows for the daytime on Tue. Wed and Wed night...Moderate confidence. Winds shift to the NW with gusts potentially reaching low end gale force. Gale warnings may be needed, however at least small craft advisories will be needed. Seas peak late Wed around 8-10 ft on the ocean waters. These conditions linger into Wed night but any Gales may be turned over to small craft advisories by late Wed night. Risk for light freezing spray begins late day Wed and continues into early Thu. Thu...High confidence. Mainly quiet boating weather after winds and seas recede early. Fri...Low confidence. Winds shift to the SW. May reach low end Gale force by Fri afternoon, but more likely small craft advisories may be needed. Timing somewhat uncertain.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...Wind Advisory until midnight EDT tonight for MAZ024. RI...None. MARINE...Gale Warning until 8 AM EDT Monday for ANZ231-232-250. Small Craft Advisory until 5 PM EDT Monday for ANZ233>235-237. Small Craft Advisory until 8 PM EDT Monday for ANZ230. Small Craft Advisory until 2 PM EDT Monday for ANZ236. Small Craft Advisory until 11 AM EDT Tuesday for ANZ251. Gale Warning until 2 PM EDT Monday for ANZ254-255. Gale Warning until 2 AM EDT Monday for ANZ256.
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&& $$ SYNOPSIS...Frank/Doody NEAR TERM...Frank/Doody/Sipprell SHORT TERM...Frank LONG TERM...Doody AVIATION...Frank/Doody/Sipprell MARINE...Frank/Doody/Sipprell

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