Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 190159 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 959 PM EDT Fri Aug 18 2017 .SYNOPSIS... 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 returns with fair seasonable weather Sunday through Tuesday. Another cold front sweeps through New England Wednesday, followed by more dry weather Thursday into Friday. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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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.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... 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/... Much of this period will feature a mid level ridge across the southern USA. For our neck of the woods, nearly zonal flow should prevail through early next week. Expecting a potent mid level trough to get close to our region towards late next week. Latest guidance suite is in reasonable agreement with the overall pattern, with the typical detail differences. Favoring a consensus blend to smooth over the less predictable details. Thinking heat and humidity slowly builds from Sunday on, peaking Wednesday before a cold front moves through our region. Temperatures during this period should be near to above normal. Once this cold front passes Wednesday night, expecting near to below normal temperatures which much more comfortable humidity levels. Increasing risk for showers and possible thunderstorms Tuesday night into Wednesday, otherwise mainly dry much of next week. && .AVIATION /02Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/... Through tonight...Moderate confidence. Conditions lowering IFR-LIFR over much of the region as the threat of RA/+RA with becomes more widespread. TEMPO VSBY impacts with RA/+RA and expected dense fog, more likely over S/E coastal terminals. Clearing out towards morning. Blustery S/SW winds with a few gusts up to 30 kts, strongest over the S/SE coastal terminals. Despite, LLWS impacts possible especially over far SE MA with 40 kts SW at 2 kft AGL. Gradual improvement NW to SE across the interior towards Saturday morning. Saturday into Saturday night...Moderate confidence. Gradual improvement for S/E terminals, lowest IFR-LIFR conditions with -RA/RA lingering for SE coastal terminals. SW winds prevailing turning W towards evening. SCT 4-5 kft low-end VFR CIGs during the day, becoming BKN-OVC N/W overnight with the low risk of -RA. KBOS Terminal...Moderate confidence. MVFR-IFR mix through morning. Feel there is a threat of RA/+RA this evening around 0-6z. TEMPO IFR VSBY impacts possible. Low risk of TSRA, feel that may stay S of the terminal, so will continue VCTS mention. KBDL Terminal...Moderate confidence. MVFR-IFR through the overnight period with the threat of RA/+RA along with TSRA roughly 21-03z. Should see improvement after midnight into the Saturday morning hours with CIGs lifting. Outlook /Sunday through Wednesday/... Sunday through Tuesday...High confidence. VFR. Areas of IFR in early morning fog/low clouds, then improving to VFR by mid morning each day. Tuesday night and Wednesday...Moderate confidence. Mainly VFR. Scattered MVFR is SHRA/TSRA, especially Wednesday. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/...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 20 knots or less, and seas 4 feet or less. Tuesday night and Wednesday...Moderate confidence. Increasing southwest winds ahead of a cold front should lead to rough seas across the outer coastal waters, especially on the southern coastal waters. There may be a few gusts to 25 kt on Wednesday, too. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 9 AM EDT Saturday for ANZ231>234. Small Craft Advisory until 2 AM EDT Saturday for ANZ236. Small Craft Advisory until 11 AM EDT Saturday for ANZ235-237. Small Craft Advisory until 5 PM EDT Saturday for ANZ250-254. Small Craft Advisory until 8 PM EDT Saturday for ANZ255-256. && $$ SYNOPSIS...Belk/Sipprell NEAR TERM...Doody/Sipprell SHORT TERM...Sipprell LONG TERM...Belk AVIATION...Belk/Sipprell MARINE...Belk/Sipprell

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