Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 190226 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1026 PM EDT Thu May 18 2017 .SYNOPSIS... After today`s record heat, we may see strong thunderstorms tonight, especially in western New England. A cold front pushing through the region Friday may trigger an isolated shower or storm, mainly south of the Mass Pike. High pressure moves over the region during the weekend bringing dry weather and more seasonable temperatures. A weather system from the Midwest sweeps east Monday with showers. Drier air moves in Tuesday. Another weather system will bring showers on Wednesday and Thursday. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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1015 PM Update... Line of strong to severe thunderstorms progressing across central and northeast sections of southern New England. Surface temperatures were remaining in the low to mid 80s across much of eastern MA still at 10 PM, and mid level lapse rates across much of southern New England were running 6.5 to 7+ at 10 PM. Although the line has shown some fragmentation in the last 10 to 15 minutes, a risk for severe wind gusts still persists. The gust front with G30-35 KT has pushed ahead of the deep convection. Hence, we are considering Special Marine Warnings for the NE MA coast as far south as Boston Harbor. We anticipate the activity will begin to settle down by around midnight with a much lower risk of severe. May still have some persistent convection even into the early morning hours with some hint of an instability axis becoming established across CT. Will need to keep an eye on the area later. Have adjusted POPs in accordance with ongoing convection with HRRR guidance for next 3 to 6 hours. Have also made some minor adjustments in temperatures per current trends. 650 PM Update... * Few severe storms still possible tonight in western New England * Most of activity may stay to our north but not confident enough to remove threat in SNE Still seeing temps in 90s across much of SNE this evening after a record-setting day at BDL, BOS, PVD, and a tie at ORH (see below). Winds have become more SW and will begin to diminish around sunset. Our focus remains on the potential for severe storms tonight, especially in western New England. Activity has been focused along axis of higher K-index values from northern PA into central NY and northern New England. This will slowly shift toward our area tonight, as we are already seeing moisture increase on the latest precipitable water analysis. Dewpoints, which dropped into 40s and lower 50s due to mixing, will slowly increase as well. High-res models continue to insist that activity now along NY/PA border west of BGM will reach western New England around 10 pm, then weaken as it heads into eastern MA and RI closer to 2 am. Despite loss of daytime heating, it`s fairly cold aloft (-11C at 500 mb) to help sustain convection. Mid level lapse rates look favorable and 0-6km shear will be increasing over region this evening, so our thinking remains same - that is, we see lines or small clusters of storms tonight with wind damage being main threat. This does not look like a widespread event but one which features a number of strong storms (40 mph gusts) and perhaps a few severe storms capable of producing wind damage. The limiting factor is fact that best synoptic scale forcing associated with short wave seems to stay to our north across northern New England (where storms are lining up now). But we do have subtle height falls over southern New England which may be enough. Once any activity weakens overnight, expect clouds to linger with patchy fog near South Coast. Lows should only drop back into the 60s to around 70.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/... Cold front drops through region Fri. Enough lingering moisture, instability, and convergence ahead of it to possibly trigger a few showers or even a thunderstorm, especially south of Mass Pike from late morning into mid afternoon. Drier air arrives from NW during day, but many areas will still top out in 80s. Forecast soundings once again show deep mixing, so we should see 20-30 mph gusts at times Fri afternoon. Clear and cooler Fri night with lows falling back into 40s and lower 50s. && .LONG TERM /SATURDAY THROUGH THURSDAY/... Big Picture... Longwave pattern is in transition next week with flow flattening to zonal. A trough then digs over the Eastern USA while a ridge builds along the West Coast. Shortwave scale shows Eastern high pressure in control over the weekend. Upper low pressure over the Southwest USA ejects across the plains to the Great Lakes by Monday, then crosses New England Monday night/Tuesday. Another upper low dives south from Canada and sweeps across New England Wednesday night/Thursday. Models are in fair agreement through Tuesday. Thereafter there are differences in handling of the late week system. Confidence is high for the weekend and early week. Confidence is moderate overall for the late week storm, but with low confidence on details of timing. Details... Saturday-Sunday... High pressure from Canada builds south over New England Saturday with a northwest flow aloft. The high then moves overhead Saturday night and Sunday. Expect dry weather through the weekend. Mid and high level moisture increase during Sunday suggesting increasing cirrus cover and possibly some mid level clouds. Mixing levels look to reach 850 mb. Temps at 850 will be 5-7C both days, which would support max temps in the mid 60s to low 70s. The easterly flow off the water may buffer temps in Eastern Mass, thus expect high 50s and 60s there Saturday, then temps a few degrees warmer Sunday as the flow shifts. Monday... Shortwave over the Great Lakes moves east, driving a cold front through NY/PA during the day and into Western New England during the evening. Increasing south-southwest flow during the early morning as a 40-knot low level jet shifts east. This increases by Monday evening to a 50-knot jet. This flow supplies plenty of moisture with PW values of 1.5 to 1.7 inches. Cross sections show a deep moisture layer most of the day. Sufficient confidence for chance pops after midnight early Monday and likely pops during the day Monday. Convective parameters are marginal at best, but enough chance to mention scattered or widely scattered storms. Cold front crosses the region Monday night between 00Z and 06Z. Drier air moves in during the overnight and should bring clearing skies. Tuesday... Upper shortwave crosses early Tuesday with cooling temps at 500 mb during the day. This should generate diurnal cumulus, but insufficient for showers. Mixing to 800 mb with temps 4-6C...that should support max temps in the 70s. Wednesday-Thursday... The next shortwave sweeps east from the Midwest Wednesday with increasing upper divergence/venting during the afternoon and night. Low confidence on timing of the onset, but expect a chance of showers starting either Wednesday afternoon or at night. && .AVIATION /02Z FRIDAY THROUGH TUESDAY/...
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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 Night/... 03z TAF Update...Moderate confidence through 06Z due to convection and then high confidence thereafter. SW winds diminish this evening. VFR conditions should persist most areas with localized MVFR/IFR in scattered showers/tstms. The tstms should weaken as they reach RI and the eastern MA coast 04-06z. Main concern continues to be strong wind gusts (40kt) with the tstms, especially those moving across NE MA. Also expecting patchy fog near South Coast, Cape, and Islands with local MVFR/IFR visibilities, which rapidly improve after sunrise Fri. Winds shift to W and then NW during day Fri. VFR outside of isolated showers/storm mainly focused south of Mass Pike thru mid afternoon. VFR Fri night. KBOS TAF...High confidence, including timing of VCTS later tonight. KBDL TAF...High confidence, including timing of VCTS tonight. Outlook /Saturday through Tuesday/... Moderate confidence. Saturday-Sunday... VFR. Diminishing northeast gradient winds Saturday, becoming variable in the afternoon. Light wind Sunday with developing sea breezes along the coasts. Monday... General MVFR cigs/vsbys in rain and fog with areas of IFR possible. South-southwest winds with 2000-foot winds reaching 40-50 knots along the coastal plain...potential for low level wind shear. Tuesday... VFR with west winds gusting to 20 knots.
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&& .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/... High confidence. SW winds diminish this evening. Seas build on outer waters S and E of Cape Cod where we will issue a Small Craft Advisory through Fri. Cold front drops south across waters Fri, and may trigger an isolated shower or storm through mid afternoon. More importantly, winds shift to W during the morning and then NW in the afternoon. May see brief 20-25kt gusts but not confident enough to expand SCA. Outlook /Saturday through Tuesday/... Moderate confidence. Saturday-Sunday... High pressure moves over the waters Saturday and off to the southeast Sunday. Expect northeast winds Saturday trending to light later in the day and then become southerly on Sunday. Seas less than 5 feet. No headlines expected. Sunday night-Monday... Weather system sweeps east from the Great Lakes. South-southwest winds will increase during Monday. Winds at 2000 feet will increase with speeds of 40-50 knots by afternoon. This may bring a period of 20-25 knot speeds by Monday afternoon. The steady southwest wind may bump seas to around 5 feet on the outer waters and exposed waters such as Rhode Island Sound. Visibility will be low at times in rain and fog. A cold front moves through Monday night, bringing a wind shift from the west and lighter wind. Tuesday... West-northwest winds will bring drier air. Expect speeds less than 20 knots, with seas less than 5 feet. && .CLIMATE... Records were broken in Boston, Providence, and Hartford today and tied at Worcester. Refer to the latest Record Event Reports which were issued earlier. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Air Quality Alert until 11 PM EDT this evening for CTZ002>004. MA...Air Quality Alert until 11 PM EDT this evening for MAZ004>007- 010>023-026. RI...Air Quality Alert until 11 PM EDT this evening for RIZ001>008. MARINE...Small Craft Advisory until 6 PM EDT Friday for ANZ235-254>256.
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&& $$ SYNOPSIS...WTB/JWD NEAR TERM...JWD/Thompson SHORT TERM...JWD LONG TERM...WTB AVIATION...WTB/JWD/Thompson MARINE...WTB/JWD CLIMATE...

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