Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 271817 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 217 PM EDT Thu Jul 27 2017 .SYNOPSIS... High pressure moves offshore today giving way to more humid conditions which will be accompanied by scattered showers and isolated thunderstorms which may persist into this evening. A drying trend likely develops late tonight into Fri as a cold front moves through the region and then stalls near the south coast Fri. A coastal storm will push off the mid Atlantic coast and track south of New England this weekend, which will bring showers and a few thunderstorms, along with gusty onshore winds with cool temperatures this weekend especially close to the south coast. The coastal storm may linger south of New England into early next week with cool and unsettled conditions persisting. && .NEAR TERM /UNTIL 8 PM THIS EVENING/... 2 PM update... Not much change from previous forecast. Short wave trough exiting the eastern Great Lakes and Ontario continues to generate scattered showers over NY state into southern New England at 2 pm. Warm sector airmass remains well to the southwest of our area and expecting it to get shunted to our south as mid level trough is unable to back the mid level flow. Thus not much if any surface based instability to work with. Subtle height falls are yielding mid level lapse rates around 6c/km. However given synoptic scale forcing is fairly weak not expecting much if any embedded thunder. With PWATs up to 1.7 inches and K index values in the mid 30s advecting into the region, a few heavy downpours are possible this afternoon and early evening. Otherwise mostly cloudy conditions prevail along with somewhat humid weather with dew pts rising into the mid 60s. Previous discussion follows... Approaching mid level shortwave from the NW will bring a modest instability burst into New Eng today as K indices spike into the low/mid 30s with deepening moisture and PWAT plume exceeding 1.5 inches. Global and Hi-res guidance signaling a cluster of showers and a few t-storms moving into SNE from the west this afternoon. Instability is marginal with SBCAPES 500-1000 J/kg and mid level lapse rates are poor so risk for strong to severe storms is low, but some of the hi-res guidance indicating potential for localized heavy rainfall. Greatest threat will be in the interior north and west of the I-95 corridor where better instability, but do expect some showers making it to the coastal plain later this afternoon. Cloud cover and shower potential will limit heating with highs mostly in the low to mid 70s and could see temps fall into the 60s in showers over the interior high terrain. Increasing humidity as dewpoints climb into the low and mid 60s. && .SHORT TERM /8 PM THIS EVENING THROUGH 6 PM FRIDAY/... Tonight... Mid level shortwave and attending cold front move into the region which may help to sustain convection into the evening before drier air pushes in from the north. Trend will be for drying conditions overnight as the shortwave exits and K indices drop off. It will remain humid as low level drier air lags to the north given slow movement of front across SNE so areas of low clouds and fog may develop. Mild night with lows in the 60s. Friday... Anomalous closed low digs SE across the Great Lakes with confluent flow across New England likely keeping deeper moisture to the south with mostly dry conditions. However, can`t rule out an isold afternoon shower/t-storm as marginal instability develops, mainly south of the Pike and particularly across RI and SE MA where stalled cold front may provide a focus for convection. Expect partly sunny skies developing after any morning stratus/fog burns off. 850 mb temps 12-14C support highs reaching into the 80s, except cooler along the coast as sea breezes develop. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Highlights... * Risk continues for showers and a few thunderstorms possible Fri night and Sat, best chance S coast * Gusty E-NE winds possible along coastal areas, highest along S coast, Cape Cod and the islands Sat * Dry conditions return early next week, but timing uncertain Overview and model preferences... Per latest 00Z upper air obs, it`s somewhat difficult to believe that what is currently a very innocuous wave along the Canadian border with North Dakota/Minnesota will be one of the main components to the weekend forecast. This weak wave, deepening in response to upstream convection, looks to phase with a Hudson valley wave rotating into Quebec. What occurs after the phasing is where most significant disagreement begins. The N cutoff shifts E allowing the the remnant longwave trof along the E seaboard to meander. GFS has this longwave cutting off completely, and hence is the slowest to shift the pattern. ECMWF/NAM are more progressive with ensembles falling on either side. For this update, given GFS seems to be an outlier, will lean more on the more progressive ECMWF solution with this update. All-in-all with stalled front to the S, a slow frontal low pres associated with this wave will pivot, mainly SE of the 40/70 benchmark Sat, and slow as the system continues to dig. This brings about a prolonged period of NE flow off the Gulf of Maine and how long it last is based on the depth of the longwave trof. Using the ECMWF timing, will release it from the throes of this feature by early in the work week, but this remains somewhat uncertain. Beyond this timing, a period of ridging is likely, as the longwave trof weakens and the pattern reloads. Details... Fri night into Sun... Convection firing along a stalled warm front draped from the mid Atlantic, although there remains the uncertainty of convective feedback, will assist in generating a low pres wave which will traverse the front slowly late Fri night into Sat. This allows for the development of NE flow, drawing air across the Gulf of Maine where SSTs rest between the upper 50s and low 60s. The combination of cooling flow and modest increase in moisture should lead to increased cloud cover through the period, culminating in temps remaining below normal. PWATs, closely associated with the low pres hover near 2.00 inches and ensemble probabilities keep the core of this higher moisture mainly offshore. Combine this with the influence of high pres from the NW under anticyclonic flow, and expect a very tight gradient in QPF and therefore, POPs as well. Will be tightening the POP/QPF gradient somewhat with this update based on the ECMWF/NAM blending suggested above. However, along S coastal areas, some heavy rain is possible given the higher PWATs and conditionally unstable soundings. Low risk for 1.00+ inches of rain, but very uncertain on this axis as it could remain entirely offshore. Overall, a period of potentially gray/cool/damp weather, with the highest risk for heavy rain along the S coast. Temps as much as 5-10 degree below normal except for where there is a chance for breaks of sun, mainly NW MA. Raw 2m more supportive of this than MAV/MET data. Otherwise, risk for some gusty NE winds as LLJ is nearly 5 std deviations above normal, out of the E. Coastal areas show 40-50 kt E-NE LLJ. Mon and Tue... Gradual improvement as the longwave trof slides slowly E assuming no cutoff forms. The increased anticyclonic curvature and induction of drier air in the form of subsidence should allow for more breaks in clouds and a much lower risk in precipitation as the higher theta-e ridge/PWATs and strongest LLJ shifts further offshore. Filling low pres wave noted as well. Moderate NW flow as well, suggest a slight moderation in temps at least closer to (but still below) normal as the shallow cold air should give way to better mixing, reaching H85 where average temps run about +12C yielding low-mid 80s, however, given the uncertainty, this will be dependent on how quickly the low pres clears the SE waters. Wed into late week... As the longwave trof gives way to resurgence of upstream ridging, more zonally oriented flow should allow for a return of drier with temps near to potentially above normal given current timing. Still uncertain as front begins to parallel the mid- upper lvl flow, and this setup is strongly dependent on the timing of the phased wave system this weekend. && .AVIATION /18Z THURSDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 18Z Update... Through 00z...high forecast confidence. VFR conditions will trend to MVFR as the day progresses. Scattered showers will continue to impact the region. After 00z...high confidence on trends but lower on exact timing. Showers and isolated T-storm will shift southward with a drying trend developing from north to south as the night progresses. IFR- MVFR this evening in showers and fog will trend toward VFR late especially away from the south coast. Friday...moderate confidence. Any morning IFR-MVFR along the south coast should lift to VFR by midday. Then marginal MVFR-VFR develops in the afternoon regionwide with isolated shower possible especially near the south coast. Friday night...moderate confidence. Marginal VFR-MVFR in isolated shower threat during the evening will trend toward IFR late especially south coast as risk of rain increases. KBOS Terminal...high confidence on trends but some uncertainty on exact timing of lowering cigs this evening then timing improvement overnight. KBDL Terminal...high confidence on trends but lower confidence on exact timing of changing categories. Outlook /Saturday through Monday/... Sat night...Moderate confidence. Area of rain/fog and low clouds possible mainly close to the S coast. This could be coincident with periods of MVFR/IFR conditions. Meanwhile areas across NW MA and extreme W CT could stay mainly VFR. Otherwise, winds gusting 25-30 kt possible, especially across SE MA/Cape Islands and RI out of the NE. Sun and Mon...Moderate confidence. Some improvement possible as rain/fog and lower CIGS shift gradually E of the region, however exact timing uncertain. This improvement would see more widespread VFR conditions develop where not already observed. Otherwise, winds still gust 20-25 kt mainly across SE MA/RI. && .MARINE... FORECASTER CONFIDENCE LEVELS... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/...High confidence. Today...S/SW winds with gusts up to 20 kt and seas below SCA. Late afternoon shower/t-storm possible. Tonight...Diminishing SW winds shifting to west overnight. Light seas. Patchy fog may reduce vsbys. Evening shower/t-storm possible. Friday...Light winds becoming E/SE in the afternoon with sea breezes developing nearshore waters. Light seas. Outlook /Friday Night through Monday/... Fri night...Moderate confidence. E-NE winds begin to build, reaching 20-25 kt by the early Sat AM hours. This also allows seas to build to 3-4 ft. Some rain/fog possible, mainly during the late evening early morning. Sat into Sun...Moderate confidence. Low pres will slowly pass south of the waters, bringing a period of rain/fog and potential for occasional thunderstorms. NE winds, could gust to low end gales mainly Sat and Sat night with seas 7-9 ft offshore. Gale warnings may ultimately be needed, or at the very least, high end small craft advisories. Mon...Moderate confidence. N-NE winds continue, but gusts should generally stay around 25 kt with seas still 5-8 ft. Small craft advisories are likely to continue even as rainfall/fog improves slowly. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...KJC/Doody NEAR TERM...KJC/Nocera/Doody SHORT TERM...KJC LONG TERM...Doody AVIATION...KJC/Nocera/Doody MARINE...KJC/Doody is the U.S. government's official web portal to all federal, state and local government web resources and services.