Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 120804 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 404 AM EDT Thu Oct 12 2017 .SYNOPSIS...
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Strong high pressure over Quebec this morning builds southward into New England today through Friday, providing a drying trend and seasonably cool weather. Warmer and more humid conditions arrive over the weekend as the high moves offshore and a cold front approaches from the west. This setup will also result in scattered showers at times especially Saturday and again Sunday night. The front moves offshore sometime Monday followed by drier, less humid and seasonably cooler weather early next week.
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&& .NEAR TERM /UNTIL 6 AM THIS MORNING/... 1045 PM Update... Light NE winds have been backing to N across interior Mass and N CT, bringing drier air into the region. Still noting higher dewpoints across E coastal Mass at 02Z. Expect dewpoints to continue to fall overnight, but will remain higher across SE Mass/RI into NE CT through around 06Z before dropping. Noting two areas of precip across the region, one over central Mass and the second along the S coast, Cape Cod and the islands. Still seeing reports of a few sprinkles at some locations through the evening as well. SPC mesoanalysis showing lowering H925 and H85 dewpoints from the N as southern periphery of the high pressure area over northern New England into Quebec shifts S-SE. Should continue to see precip erode from NE-SW especially from around 06Z onward through the early morning hours. Have updated near term forecast to incorporate current trends. Previous Discussion... Remainder of the light precip will continue to shunt S-SW as high pressure builds to the N, drying out towards Thursday morning. Echoing the previous forecaster, the NCAR ensembles capture this well with respect to the probability of 0.01 inches in an hour. Prevailing with light rain and forecasting rainfall amounts less than a tenth of an inch. Clearing conditions into morning, not thinking any fog given breezy conditions subsequent of isallobaric / gradient flow and mid level clouds lingering. Cold air advection proceeding, temperatures drop to lows in the 40s, around 50 along the shores. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM FRIDAY/... Thursday into Thursday night... Seasonable with abundant sunshine. Continuing to clear as high pressure slips E just to our N. Bright sunshine beneath a continued H925-85 airmass around +3-6C along with undercutting N/E flow from cooler, drier source regions associated with the 1030+ high now shifting into SE Quebec / Nova Scotia region. Winds diminishing late with the lessening of pressure rises, surface ridging building across the NE CONUS, winds relaxing. Given the clear conditions ongoing, light winds, a radiational cooling opportunity. Lows dropping into the 30s away from the coast. Considerable frost potential that may require headlines but for now will highlight in the HAZARDOUS WEATHER OUTLOOK. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
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Highlights... * Warmer and more humid this weekend * Risk of scattered showers this weekend, mainly Sunday night * Trend toward drier, less humid and cooler weather early next week Friday night... Tranquil weather especially during the evening hours with surface ridge across the area. Dry airmass still over the region with dew pts in the upper 40s and low 50s. This combined with light winds will promote temps to fall quickly with sunset. However return flow/ WAA pattern develops later at night with clouds overspreading the region. Thus temps should level off second half of the night. Fairly large spread among the models regarding strength of low level jet and accompanying moisture/instability burst as warm sector approaches. NAM and GEFS more robust and support shower/rain threat late Fri night from south to north in response to large jump in PWATs and K indices. Meanwhile remainder of guidance not as impressive and hold off any precip until sometime Sat. Given the uncertainty will follow a model blend which offers slight chance pops Fri night increasing to chance toward daybreak Sat. Saturday... NAM and GFS both have lots of clouds across the area as the warm sector overspreads the region. BUFKIT time sections show this moisture below 700 mb. However forcing for ascent is weak and instability is limited (SB and aloft). Thus any shower activity should be isolated/widely scattered. So chance pops from a model blend seem reasonable. A washout is not expected with many hours of dry weather anticipated. Model consensus is for 925 mb temps rising to +14C/+15C. Given lots of clouds are expected full sun temps will likely not be realized. Nonetheless highs should still manage to climb into the low 70s given low level WAA. It will be humid as well with dew pts rising into the low and mid 60s. Sunday... Core of warm sector overspreads the area with 925 mb temps rising to +16C to +17C along with dew pts climbing into the mid 60s, impressive for mid to late Oct. Still lots of clouds around in the warm sector but given magnitude of low level WAA highs of 75-80 seem plausible. For what it`s worth the EC ensembles show low prob of 80+ Sun afternoon across interior SNE. By late Sunday and especially Sunday night short wave trough and attending cold front approach the region from the west. Both EC and GFS show good moisture pooling ahead of the front Sunday evening and overnight with K indices rising into the 30s and PWATs +2 standard deviations above climo. This combined with modest synoptic and frontal scale forcing should result in a period of scattered showers late Sunday into Sunday night from west to east. However short wave trough is deamplifying with time as it climbs over the eastern seaboard ridge, resulting in weakening forcing for ascent. This combined with limited instability aloft and at the surface will likely result in showers diminishing in areal coverage from west to east. Thus higher pops west, lowering east. Other issue Sunday will be increasing southwest winds as low level jet develops over the area. GFS bufkit soundings indicate gusts up to 40 kt possible given warm sector airmass over the area providing steep low level lapse rates. Given trees remain fully leaved there is a low risk if wind speeds approach or exceed 40-45 mph some minor wind/tree damage is possible. Early Next Week... Cold front moves offshore early Mon with drier, less humid and cooler air overspreading the region. Coolest air arrives Mon ngt into Tue behind trailing short wave trough and attending secondary cold front. Both GEFS and ECENS ensembles lower 850 temps down to 0C over southern New England 12z Tue. However temps moderate by middle of next week. Thus brief touch of fall weather Mon night into Tue.
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&& .AVIATION /08Z THURSDAY THROUGH MONDAY/...
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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 Friday/...High confidence. Overnight... Mainly low end VFR CIGS. Local MVFR in any patchy -RA, mainly across the lower CT valley (KBDL-KHFD) through 04Z-05Z, then should dissipate as CIGS lift. NE winds becoming breezy especially along E coastal terminals, possibly up to 20-25 kt after 05Z. Thursday into Thursday night... Clearing, becoming SKC. NE winds remain gusty along the coasts with gusts up to 25 kts along the S-coast, diminishing into the overnight period. Could contend with shallow IFR-LIFR FG. KBOS TAF...High confidence. VFR. Winds turning NE and becoming gusty up to 20 kts into evening. Will hold -RA out of the terminal and keep CIGs no lower than low- end VFR as we go into this evening. CIGs lifting after midnight, SKC into Thursday. KBDL TAF...High confidence. Hold CIGs low-end VFR as we go late today into this evening with -RA, however can`t rule out some patchy MVFR. Low risk of VSBY impacts. Will clear out after midnight, SKC into Thursday. Outlook /Friday Night through Monday/... Mainly MVFR-VFR with scattered showers, especially Sunday night. Southwest winds become gusty Sunday with speeds up to 35-40 kt possible. Cold frontal passage sometime early Mon with dry weather thereafter along with diminishing winds and VFR conditions.
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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 Friday/... High pressure building S out of Quebec as an area of low pressure shifts SE out of the Ohio River Valley. The subsequent increasing pressure gradient causes NE winds to increase overnight, then continue through Thursday. Gusts 25 to 30 kts pushing waves 6 to 7 feet especially on the S waters. SMALL CRAFT ADVISORIES continue for all waters, both for winds and wave action diminish through Thursday night. Expect good visibility through the period. Outlook /Friday Night through Monday/...Moderate confidence. High pressure Fri night moves offshore Sat with winds becoming SW then. Vsby may lower in patchy fog, spotty showers Fri night thru Sun with showers most numerous Sunday night. SW winds increase to 20- 30 kt Sunday with gusts up to 40 kt possible. Cold frontal passage early Mon followed by diminishing winds, dry weather and good vsby.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Frost Advisory from 3 AM to 8 AM EDT Friday for CTZ002>004. MA...Frost Advisory from 3 AM to 8 AM EDT Friday for MAZ002>006- 008>014-017-018-026. RI...None. MARINE...Small Craft Advisory until 4 AM EDT Friday for ANZ232. Small Craft Advisory until 6 PM EDT this evening for ANZ233- 234. Small Craft Advisory until 2 PM EDT this afternoon for ANZ231- 251. Small Craft Advisory until 6 AM EDT Friday for ANZ235-237. Small Craft Advisory until 8 PM EDT this evening for ANZ250. Small Craft Advisory until 6 PM EDT Friday for ANZ254>256.
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&& $$ SYNOPSIS...Nocera NEAR TERM...Dunten/EVT SHORT TERM...Sipprell LONG TERM...Nocera AVIATION...Nocera/Sipprell/EVT MARINE...Nocera/Sipprell/EVT

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