Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 211431 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1031 AM EDT Fri Oct 21 2016 .SYNOPSIS... Sub-tropical low pressure lifting north toward southern New England will bring numerous showers and isolated t-storms with locally heavy rainfall this afternoon and tonight. Additional showers are likely Saturday as the storm moves into northern New England along with strong winds developing late Saturday into Saturday night. Dry but cool and blustery conditions Sunday into early next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1030 am update... Warm sector continues to surge northward thru the coastal plain of CT/RI and southeast MA at 10 am. Some brief clearing in the warm sector has allowed temps to soar into the low 70s at 10 am. However this will be short lived as visible satellite imagery and radar show clouds and showers just south of the islands and streaming rapidly northward. Thus sunshine across eastern CT/RI and southeast MA will be short lived and replaced by clouds and showers midday or shortly thereafter. Warm sector will continue advecting northward but will likely stall somewhere across northern MA as pres falls and associated cyclogensis evolve south of New England later today. Thus tricky temp forecast across northern MA today. So expecting low clouds and fog to lift across the interior but then may redevelop later today as showers become more widespread. Bands of rain showers across the interior...mainly west of the I-95 corridor. These showers will exit northward into central New England. However a new area of showers just south of the islands will come onshore early this afternoon from south to north. More importantly is area of convection farther offshore south of New England...which appears to be expanding with time per latest satellite imagery. This area is associated with moisture streaming north of a subtropical low north of the bahamas. Complexity to our forecast is the exact track of this convection this afternoon. Consensus from the 00z global guidance including much of the mesoscale guid including the NMM/HRRR and RAP is for this convection to move across RI and eastern MA. However some of the mesoscale guidance including the 00z ARW and NCAR ensembles is to track offshore convection farther west into CT and central MA. Will need to evaluate 12z guidance and latest satellite and radar trends before we have a better handle. Thus no major changes to previous forecast. Earlier discussion below. =================================================================== Previous discussion... Amplifying mid level trof moving into the Great Lakes will back the flow across the NE which will slow the eastward progress of the frontal boundary in New York state, while allowing subtropical low to lift north toward New Eng. One area of heavy rainfall will persist to the west assocd with the slow moving front and right entrance region of the upper jet. A second area of heavier rainfall with the subtropical low and high PWAT plume is likely but location is still somewhat uncertain. Guidance is favoring RI and eastern MA during the afternoon as area of strong omega lifts north from the ocean with anomalous PWAT plume focused across SE New Eng, but it still could be further west per some of the hi-res guidance. The other factor to consider is the airmass will destabilize this afternoon with SBCAPES around 500 J/kg and strong deep layer shear which may lead to isold strong t-storms developing. The hi-res guidance is indicating potential for localized heavy rainfall of up to 3 inches but low confidence on location if this occurs. Warm front lifts to the north with SNE in the warm sector today. Temps will warm into the upper 60s to lower 70s and it will be quite humid with dewpoints in the mid 60s. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... Tonight... Localized heavy rainfall with thunder potential will linger into the evening as the sub-tropical low lifts north across the region, then models are signaling a dry slot moving into the region during the second half of the night. Expect precip to taper off with areas of drizzle and fog developing overnight. Very mild and humid into the overnight with temps holding into the 60s, then falling through the 50s western New Eng late tonight as cold front approaches from the west. Temps may remain in the 60s all night in eastern New Eng. Saturday... Potent neg tilt trof and mid level low over PA will lift NE into New Eng during Sat. Strong QG forcing and mid level frontogenesis with deep moisture moving into SNE will lead to sct to numerous showers on Sat, especially western New Eng. High temps will occur in the morning with 50s west and 60s east, then falling into the 40s and 50s in the afternoon as strong cold advection develops. Strong winds will develop in the afternoon as the low pres deepens across northern New Eng with strong pres gradient and strong cold advection developing. Potential for westerly gusts up to 40 mph by mid/late afternoon and up to 50 mph for Cape/Islands. Wind advisories will likely be needed for portions of SNE. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Highlights... * Some rain lingers int Saturday night, along with strong winds. * Cooler and dry by Sunday. * Seasonably cool early next week. Overview and model preferences... The reasonably good agreement that models have shown for much of the mid and extended terms continue again with the overnight 00Z update. Only toward the end of next week, with a deepening clipper, do they begin to show more extreme differences at the synoptic scale. Deep trof/cutoff slowly shifting into Atlantic Canada suggests seasonably cooler conditions to follow the rain/unsettled wx ending by Sat night. These conditions linger into mid week thanks to a persistent draw of CP air out of central Canada. Not entirely dry, as weak reinforcing shortwaves could deliver a few shra on Mon, but definitely more fall like until the latter portions of next week. Update will feature a blend of operational guidance as well as persistence. Details... Sat night... Low pres will continue to rapidly deepening as it shifts into N New England and QC, possibility dipping into the 970s. Very pronounced trowal/comma head setup wrapping well to the S and W of this low as the upper lvl cutoff lags the sfc low. Good lapse rates above developing inversion will enhance the precip efficiency in this region as well. Therefore, could see some fairly persistent wrap-around precip lingering into the overnight hours Sat. While the highest risk will be in the upslope regions of the Berkshires could see some of it spill across the CT valley and into the Worcester hills as well. Only eastern areas will begin to see an end to the precipitation thanks to good downslope component with W- NW flow. Dry air begins to entrain, so expect rapidly dropping dwpts into the early morning hours on Sun. Mins are likely to hold in the 40s and low 50s however, mainly due to delayed cold advection and remnant cloud cover lingering beneath dry inversion aloft. Also, may need to watch for strong W-NW flow as strong isallobaric couplet forms overnight. LLJ is near 45 kt at H92 to as well. Mixing overnight could see yield winds to 40 kt at least, therefore, may need to have a wind advisory into Sun morning. Sun and Sun night... Gradual improvement as the downsloping and drier air aloft is able to finally force some of the moisture in the low levels out allowing for some clearing. Winds remain a factor, although may drop below wind advisory criteria by late morning, expect mainly 25-35 mph out of the W through the daylight hours. H92 temps hover around +3C so expect highs in the 50s, while mins drop into the 40s (held back by the continued pres gradient). Mon and Mon night... Secondary shortwave rotates through the trof to the NE, reinforcing the cyclonic flow briefly. Noting enough moisture between the sfc and H7 for there to be a few shra during the day with this passage, enhanced by additional mid lvl cold advection. Flow gradually shifts from predominantly W, to the NW after this frontal passage. Similar temps to Sun as the strongest cold advection lags into the overnight hours. Tue into Wed night... Definite return to cool fall conditions. H92 temps drop as low as -1C by early Wed morning (H85 as low as -6C). Winds remain elevated Tue, but shift gradually to the N-NW with gusts 20-30 mph. The winds weaken Tue night into Wed as high pres crests over the region. Therefore, highs in the upper 40s and low 50s are likely each day, with overnight mins dropping to near or below freezing across much of the region thanks to radiational cooling. Thu into Fri... Clipper system approaches from the W, however there is uncertainty on the final track. GFS is to the S, leading to S New England lying on the climatologically dry N tier. The ECMWF is stronger and to the N, suggesting a more unsettled/wet passage. In any case, will be including some POPs for the tail end of the extended forecast. && .AVIATION /14Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 1030 am update... Brief improvement to VFR across RI and southeast MA...possibly into northeast MA but then likely trending back to MVFR and eventually IFR/LIFR after 21z-22z. Across CT and central-western MA upward trends will continue for this afternoon but then back down to IFR/LIFR this evening. Short Term /through Saturday/... Today...High confidence on trends but lower confidence on exact timing and details. LIFR/IFR will continue across most sites through 15-18Z with some areas lifting to higher IFR/MVFR through the afternoon. Mostly fog this morning, but higher risk for showers and even a few thunderstorms this afternoon, heavier/steadier rain arrives this evening especially for E MA and RI. Tonight...High confidence. More IFR/LIFR likely in showers/drizzle and fog. Isold t-storm possible. Saturday...Moderate confidence. IFR/LIFR improving to MVFR with pockets of VFR cigs possible in the afternoon. Improving vsbys. Sct to numerous showers, especially west. Winds becoming W with gusts 30-40 kt developing in the afternoon. KBOS TAF...High confidence in trends. KBDL TAF...High confidence in trends. Outlook /Saturday Night through Tuesday/... Sat night into early Sun...High confidence. Lingering IFR/MVFR conditions persist even as rainfall gradually recedes across the east but continues across the west. Lower CIGS will be the primary cause of the lower categories. Other issue is winds, gusts 30-40 kt possible at times out of the W, sustained 20- 30 kt. Winds off the deck remain high as well 40-50 kt at about 2000ft. Sun into Tue...High confidence. Fairly persistent weather for the period. W winds continue 15-20 kt sustained with gusts 25-35 kt at times, diminishing by late Tue. Mainly VFR. Although some low clouds during the day light hours may come close to the upper end of MVFR. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 1030 am update...not much change from previous forecast. Heavy showers with embedded thunder likely late this afternoon and evening across the eastern MA waters...possibly westward into RI. Short Term /through Saturday/...Moderate confidence. Today...Mainly SE gusts to 20 kt as warm front lifts north of the region. Low vsbys in areas of dense fog will improve somewhat toward midday. Potential for locally heavy rainfall, especially this afternoon. Tonight...As center of low pres moves across the waters easterly winds will diminish then become SW/W toward daybreak as the low moves to the north. Vsby reduced at times in areas of rain and fog. Saturday...Gale force SW/W winds developing in the afternoon with gusts 35-45 kt. Gale warnings issued. Outlook /Saturday Night through Tuesday/... Sun night into Mon night...High confidence. W wind gusts continue, reaching 35-45 kt while staying sustained 25- 35 kt. Gale warnings are likely to continue through this entire period. Seas reach 10-12 ft on the ocean waters. Mon into Tue...High confidence. W winds continue, although they will gradually shift to the NW late Mon into Tue. Gusts reach 25-35 kt. Small craft advisories will follow the Gales. Seas slowly diminish through the period, but should remain between 5-8ft through much of the period on the ocean waters. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Gale Warning from noon Saturday to 8 AM EDT Sunday for ANZ231>235-237-250-251-254>256. Gale Warning from 2 PM Saturday to 8 AM EDT Sunday for ANZ230- 236. && $$ SYNOPSIS...KJC/Doody NEAR TERM...KJC/Nocera/Doody SHORT TERM...KJC LONG TERM...Doody AVIATION...KJC/Nocera/Doody MARINE...KJC/Nocera/Doody is the U.S. government's official web portal to all federal, state and local government web resources and services.