Area Forecast Discussion
Issued by NWS Boston, MA

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862 FXUS61 KBOX 291142 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 742 AM EDT Sat Apr 29 2017 .SYNOPSIS... Mild start for today with showers and isolated thunderstorms moving through a portion of the area in the morning. Warm conditions follow once again for the daytime. After a cold frontal passage tonight, high pressure builds over New England Sunday with dry but cooler air. A sprawling low pressure in the Plains will push a warm front through our area Monday, then swing a cold front through on Tuesday. Another storm moves toward southern New England late Thursday or Friday, and could linger nearby into Saturday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Made significant changes to the rainfall chances this morning. Large convective cluster staying over the southern coastal waters this morning, will prevent convection from developing farther north. looking as the high resolution guidance, the latest runs of the HRRR looked good, but a time-lagged ensemble looked even better. Keeping a chance of of showers along the south coast this morning, but lowering rainfall chances farther north, especially north of the Pike. Tweaked temperatures and sky cover to reflect observed trends. Previous discussion... Mild start this morning with many spots still in the 60s. Showers are beginning to move across western portion of the area. Main focus is on the overnight convection moving into NJ. This area of thunderstorms will most likely maintain itself and move across the south coast including the southern waters. This area of convection is in line with an area 850 mb divergence and upper level jet streaks and is moving along the 925 mb moisture line. Guidance and SPC meso analysis continues to show that there is still enough instability as well as steep mid-level lapse rates across the southern half of the region to maintain the convection. Will have to watch this area closely for the potential for a few strong storms with heavy downpours as PWATs increase to 1.5 inches. This afternoon... Cold front will sweep across the region this afternoon turning the flow to a more westerly direction. Still some lingering moisture in the mid-levels will result in just clouds moving through. Still need to keep an eye on the potential for a pop-up shower/t-storm ahead of the cold front. Best are for this to occur is across the south coast where guidance is hinting as some surface instability as well as dewpoints in the 60s before the front pushes through. Lapse rates are also somewhat steep. Aside from clouds as the potential for a rogue storms, westerly winds aloft will increase between passing shortwave in southern Canada and Bermuda high to the south. Anticipate gusty winds near 20 MPH during the afternoon. Cooler temps aloft will also lag behind the front, so another warm day will be in store across the region with highs back into the 80s.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... Tonight and Sunday... Stalled front to the south of the region with zonal flow aloft. Some of the models including the NAM are showing another decaying MCS/thunderstorms moving across PA into southern NY tonight. Believe that these storms will follow along the stalled boundary and should stay south of the region. High pressure to the north will begin to slide down into southern New England. This will switch the winds from the west to a more northerly direction and eventually east by Sunday. Dry air will spill in the area resulting in the potential for radiational cooling. Temps overnight will drop into the mid 40s to low 50s. During the day, large temperature spread is possible with high temps in the mid 60s for the CT river valley to low 50s along the MA east coast. Depending on cloud cover temps out west could be to optimistic. Approaching warm front south and west of the region may result in some overrunning precip during the later half of the day on Sunday. Low confidence if this will occur as model soundings are quite dry at the low levels. Heights aloft are also building which will lowers chances. However, if this were to occur then the wester half of the region has the best shot, but for now kept pops low on Sunday. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Overview... 29/00Z guidance remains in rather good agreement through Wednesday, then diverge into two distinct camps. Operational GFS is the odd model out, as both the CMC and ECMWF are in line with the GEFS mean. The energy that will become this low pressure late next week is still over the northern Pacific. Expecting to see variations in the details until this energy can be better sampled, which probably will not happen until Monday at the earliest. Favored a consensus approach through Wednesday, then a non-GFS solution toward the end of next week. Details... Sunday Night... High pressure ridge moves offshore, allowing a warm front to start its approach. Increasing clouds almost a certainty, but cannot rule some showers late. Monday-Tuesday... A low pressure in the western Great Lakes lifts northeast into Ontario, pulling a warm front north through New England Monday or early Monday night. Questions remain regarding timing of the warm front and associated wind shift. Some lingering showers during the afternoon are possible, especially north of the Mass Pike. But expect most of this to move off to the north. Cold front sweeps through our region Monday night. A 50-60 kt low level jet arriving late at night will provide some organization to the modest instability and convergence along this cold front. Expecting plenty of showers, with isolated thunderstorms, despite the nighttime passage. Locally heavy showers possible. Becoming drier behind this front Tuesday, but it will be gusty. Above normal temperatures with sunshine breaking out for the afternoon. Wednesday... Surface low over the Maritimes and cloud shield into Northern New England. Decent cold pool over Northern New England should be just close enough to generate some diurnal clouds, especially north of the Mass Pike. Lower max temperatures, but still above normal. Thursday-Friday... Signal the past several days has been for a storm to impact our region during this time. The details are much harder to come by with confidence. Non-GFS model consensus on timing would bring rain into our area Thursday, most likely mid to late afternoon. The greatest chance for rainfall still looks to be Thursday night and Friday. && .AVIATION /12Z 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 Sunday. Today...High confidence. VFR with a risk for some showers this morning. srn RI/far SE MA sites have the best chc for -TSRA prior 14Z from W-E. After these storms move through, expecting SCT-BKN VFR clouds through the day. Winds shift from SW to W this morning with a few gusts around 20 to 25 kts this afternoon. Tonight...High confidence. Westerly flow to start with VFR conditions. Tomorrow...High confidence. Flow will turn east with VFR conditions. Low prob of a passing showers during the later half of the day. KBOS TAF...High confidence in TAF, low prob of -TSRA around 12z. KBDL TAF...High confidence in TAF, low prob of -TSRA around 9z. Outlook /Sunday Night through Wednesday/...Moderate confidence. Sunday night to Tuesday... Ceilings lower to MVFR/IFR Sunday night and the start of Monday in low clouds and fog/drizzle. Brief improvement to VFR possible Monday, then a cold front moves through Monday night and early Tuesday with showers and scattered thunderstorms. Low level wind shear likely towards the south coast, especially the Cape and islands, ahead of this cold front with southwest winds of 50-60 kt at 2000 feet AGL. This cold front moves offshore Tuesday morning, followed by clearing skies and southwest winds gusting 25-30 kt. Wednesday...VFR.
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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 Sunday/...High confidence. Mariners heading for the far southern outer coastal waters will need to be wary for rough seas and thunderstorms this morning. Small Craft Advisories have been issued for the outer waters this afternoon as passing frontal system combined with lingering swell will push waters back to 5 feet. Also have issued a Small Craft Advisory for Boston Harbor and Narragansett Bay for wind gusts near 25 kts. There is a chance that this may be expanded for the other near shore waters, but confidence was to low to issue it at this time. High pressure from the north will slide across the region by Sunday. This will allow for both wind gusts and seas to relax below SCA. Outlook /Sunday Night through Wednesday/...Moderate confidence. Monday... Warm front moves north of the waters late Monday. East winds turn from the southeast and eventually from the south but remain less than 25 knots. Seas build Monday night on the exposed southern waters as southwest winds increase ahead of a cold front. Very strong winds at 2000 feet above the surface will increase to 50-60 knots, so potential for gusts to 30 knots Monday night. Showers and possibly thunderstorms Monday night. A Small Craft Advisory will probably be needed. Tuesday... Cold front moves across the waters early Tuesday. Winds will shift west after this front moves through. Showers and thunderstorms end after the wind shift. Southwest winds may gust 25-30 knots Tuesday. A Small Craft Advisory will probably be needed. Wednesday... West winds will be 20 knots or less. Roughs seas linger on the southern outer waters.
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&& .TIDES/COASTAL FLOODING... * Isolated minor coastal flooding impacts during astronomically high tide tonight and Sunday night along the east coast The astronomical high tides remain elevated this weekend. Will hold off on issuing a coastal flood statement for tonight`s high tide, as the wind direction is not favorable and will limit the risk for splashover. However, with the potential for onshore flow during the Sunday night high tide, a Coastal Flood Statement may be needed. Boston High Tides (flood stage at Boston 12.5 feet)... 11.88 feet / Sunday 2:39 am 11.45 feet / Monday 3:35 am && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from noon today to 2 AM EDT Sunday for ANZ254>256. Small Craft Advisory from 1 PM this afternoon to 8 PM EDT this evening for ANZ230-236. && $$ SYNOPSIS...Belk/Dunten NEAR TERM...Belk/Dunten SHORT TERM...Dunten LONG TERM...Belk AVIATION...Belk/Dunten MARINE...Belk/Dunten TIDES/COASTAL FLOODING...Staff

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