Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 190720 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 320 AM EDT Sat Aug 19 2017 .SYNOPSIS...
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Widespread rain with embedded heavier showers and thunderstorms overnight. Main threat of localized flooding. Drying out into Saturday ahead of a cold front which moves offshore Saturday night. High pressure brings fair seasonable weather Monday and Tuesday. Another cold front sweeps through New England Wednesday, followed by more dry weather Thursday and Friday.
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&& .NEAR TERM /UNTIL 6 AM THIS MORNING/... 10 pm update... Upstream convection, predominantly across the mid Atlantic earlier this evening usurped a lot of the available moisture/unstable air within the warm sector, as warm front continues to slowly pivot offshore. Still with high PWATs/K-indices, enough moisture still left to work with LLJ which is increasing mainly across SE MA. This is likely to be the focus for the remainder of the night except some pop-up SHRA/TSRA along the advancing cold front which is moving into the Hudson valley at the time of this update. This will be slow, and likely not even offshore by 12Z, hence the continued risk for SHRA/TSRA through the overnight hours. However, the dwindling instability with its typical diurnal trend, combined with the core LLJ shifting E should allow this risk to wane through the morning. POP update reflects this. Otherwise, very humid tonight, as temps/dwpts will remain in the upper 60s and low 70s. Previous discussion follows... */ Overview (4p update)... Ahead of a deep low center over the N Great Lakes Region, tropical moisture is surging N, converging and over-running along a warm frontal boundary presently hung up along the E-waters immediately offshore of New England as discerned via H925-85 SPC mesoanalysis. Meanwhile the atmosphere has destabilized ahead of a pre-frontal trough where partial clearing has occurred in an area of higher surface dewpoints, deeper tropical moisture, and modest shear. All of this activity is forecast to converge over S/E New England and adjacent waters this evening and overnight out ahead of a surface cold front and attendant H5-7 mid-level dry punch. The H925-85 low level jet intensifying per isallobaric response out ahead of the mid- upper level ridge over the NW Atlantic, maintaining convection and heavy rain threats N/E through Saturday morning prior to cold frontal passage. Drying out NW to SE as winds turn westerly. Any dense fog that develops during the overnight period as it is expected with the very humid, muggy, moist airmass, eroding. Lows around the low 70s. */ Discussion (4p update)... Straight-forward, continued forecast of widespread rain with embedded heavier downpours and thunderstorms. Two things of continued concern: 1.) localized flooding, and 2.) strong to severe storms. Near-term high-res guidance such as the HRRR is preferred. However, 18.12z NAM and WRF models are only partially considered given poor initialization. Localized flooding... While all of S New England remains under threat, higher confidence of potential impacts is across S/E MA and CT and all of RI. Expect thunderstorms over SW PA into NJ to advect N/E with the mean wind, maintained and fueled by an inflow of low-level H925-85 tropical moisture. A measure of instability running up against the warm front still lingering along the immediate E waters as discerned via H925-85 SPC mesoanalysis, and out ahead of a sweeping cold front and mid-level dry punch, a convergence of moisture leading to heavy rain is progged. This along with high freezing level heights and H85 dewpoints well in excess of +12C (up to +17C), efficient warm-rain processes signaled .As we have seen already with prior heavy showers / storms, rainfall rates of around 1-2 inches per hours are easily possible. Quick dousing amounts up to 3 inches within 2 hours is not out of the question. The propensity of flash flooding is there but thinking isolated, localized rather than widespread. Will forego any headlines at this point and focus on short-fused products. Strong to severe storms... Watching closely as to whether we need to coordinate with SPC on a SEVERE THUNDERSTORM WATCH for portions of S New England. Per SPC mesoanalysis, instability has manifested across S/W MA and CT. However marginal, the better environment still resides well S/W where partial clearing has allowed for better low-level lapse rates and subsequent destabilization which has resulted in present, ongoing convection. Still convinced that ongoing activity presently is going to rob the environment to the N. However, not ignoring the low LCLs, high shear, and measure of instability across the region. While the greater threat is S/W, still need to maintain a watch for portions of our area. Somewhat banking on the convection over SE PA and NJ as it advects N/E across SE areas of New England to have some strong, possibly severe elements. Heavy rain and frequent lightning the main threats, but gusty winds also possible. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 AM SUNDAY/... Not a washout. Widespread rain with embedded heavier showers, possibly a thunderstorms, lingers over SE MA during the morning period, eroding with the influx of mid-level H5-7 drier air despite the cold front lagged. This is easily evident within the K-indices, contrary to other convective parameters such as CAPE. Should see gradual clearing across the region for a brief period before comma-wrap moisture sweeps through the region behind the low filling back in the dry punch. Some scattered cumulus through the day, however the cyclonic flow and some favorable mid level lapse rates with a weak cold pool aloft, could see some shower activity over the N/W MA and CT overnight by which point the surface cold front will have pushed offshore, winds becoming W and turning light as high pressure builds into the region of the OH River Valley behind the low. Lower surface dewpoints during the day but still out ahead of the cold front, should turn a bit more comfortable with highs into the mid to upper 80s. Clouds lingering overnight with the comma-low, may hamper what might be a favorable night of radiational cooling given light winds. Lows back down into the 60s. && .LONG TERM /SUNDAY THROUGH FRIDAY/...
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Big Picture... Subtropical high maintains sultry summer weather over the Southern USA. Zonal flow over the Northern USA and Canada early week amplfies during the mid and late week. Contour heights are normal to a little above normal early week, but fall below normal by late week. This supports warm summery weather early week, followed by cooler, drier air moving into New England late week. Model upper contour fields are similar through much of next week. Thermal fields and surface pressure fields show a similar distribution. This maintains confidence in the forecast data for our area. Details... Sunday through Tuesday... High pressure builds over the region during this period. Upper trough is directly overhead Sunday morning with lingering moisture below 800 mb but dry air above. Mid level air is at -10C or warmer, while the destabilizing part of the cold pool is near the Canadian border. Expect dry weather Sunday with some dirnal clouds but otherwise a fine day. Temps aloft at 13-14C should support max sfc temps in the low to mid 80s. Expect dry weather Sunday night to Tuesday, although increasing mid and high clouds during Tuesday. Light wind flow will allow sea breezes on Monday afternoon, but gradiant on Tuesday suggests a general southwest flow. Mixing should reach between 800 mb and 850 mb each day. Temps of 14-16C Monday and 16-18C on Tuesday suggest max sfc temps in the 80s Monday and 85-90 Tuesday...cooler along the coast. Slow increase in humidity with dew points in the 60s. Winds in the mixed layer Tuesday suggest southwest gusts 15-20 knots. Tuesday night-Wednesday... Closed low digs over Quebec and the Northeast USA with shortwave driving a cold front into/through New England. Expect showers/scatterd tstms with the front Tuesday night/Wednesday. Cold fropa Wednesday brings an end to the showers, but sufficient mixing to bring northwest gusts to 20 knots behind the cold front. Thursday-Friday... Surface high pressure builds over the region while upper trough moves overhead. Cold pool in the upper trough moves in with 500-mb temps forecast to reach -16C to -18C by Friday. Temps at 850 mb will be 8-10C. Moisture profiles show lots of dry air during that time but with a moist layer at 850 mb. Expect dry weather with potential for some diurnal clouds. Expect max temps in the 70s, possibly near 80 in spots.
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&& .AVIATION /07Z SATURDAY THROUGH WEDNESDAY/...
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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/... Through 15Z...Moderate confidence. Mix of IFR/MVFR with the continuation of low CIGS and areas of fog. After sunrise, conditions will gradually improve especially across W MA/CT, but this may take until mid morning to fully break out to more widespread VFR. SHRA/TSRA mainly Cape/Islands continue through 10Z this morning, then shift offshore. IFR remains through that area through 15Z. Winds gradually shift to W-SW and recede. After 15Z through tonight...Moderate confidence. Gradual improvement to VFR all terminals except for Nantucket, which may see IFR/MVFR conditions linger in a mix of low CIGS and fog, but then mainly just low CIGS late this afternoon and evening. Some improvement possible there, so leaned somewhat optimistic in TAF for ACK. Winds mainly W-SW, with sea breezes possible E coast. KBOS Terminal...Moderate confidence. Mix of IFR this morning will gradually give way to improvement to VFR especially after 12Z. Occasional LIFR until winds shift mainly to the W, through 10Z this morning. Sea breeze possible but with a late start. KBDL Terminal...Moderate confidence. MVFR-IFR through 10-12Z but improvement to VFR thereafter. Timing may be off a bit in TAF. Outlook /Sunday through Wednesday/... Sunday through Tuesday...High confidence. VFR. Areas of IFR in early morning fog/low clouds, then improving to VFR by each mid morning and continuing the remainder of each day. Tuesday night-Wednesday...Moderate confidence. VFR with areas of MVFR cigs/vsbys in showers and scattered thunderstorms. IFR cigs possible in early morning fog/low clouds especially in areas that have had some rain.
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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/...Moderate confidence. S winds will be on the increase, gusting up to 30 kts across the S/SE waters. Small Craft Advisory remain as winds will result in heightened seas up around 5 feet. Heavy rain with embedded thunderstorms sweeping SW to NE across the waters tonight. Will likely be some visibility restrictions. Conditions improving Saturday into Saturday night as a cold front sweeps the waters late in the period with winds turning W. Outlook /Sunday through Wednesday/... Sunday through Tuesday...High confidence. High pressure builds over the waters. Winds remain 15 knots or less, and seas 3 feet or less. Winds pick up a little on Tuesday, but remain 20 knots or less with seas 4 feet or less. Tuesday night-Wednesday...Moderate confidence. A cold front approaches from the Great Lakes, crossing the waters late Wednesday or Wednesday night. Southwest winds increase with gusts 20 to 25 knots during Wednesday. Winds shift from the northwest Wednesday night. Seas build Tuesday night and Wednesday reaching 5 to 6 foot heights on the outer waters. The southwest flow may also nudge those higher seas into RI Sound as well. A Small Craft Advisory may be needed on some of the waters during this time.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 9 AM EDT this morning for ANZ231>234. Small Craft Advisory until 11 AM EDT this morning for ANZ235- 237. Small Craft Advisory until 5 PM EDT this afternoon for ANZ250- 254. Small Craft Advisory until 8 PM EDT this evening for ANZ255- 256. && $$ SYNOPSIS...WTB/Sipprell NEAR TERM...Doody/Sipprell SHORT TERM...Sipprell LONG TERM...WTB AVIATION...WTB/Doody MARINE...WTB/Sipprell

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