Area Forecast Discussion
Issued by NWS Boston, MA

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318 FXUS61 KBOX 260151 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 951 PM EDT Tue Apr 25 2017 .SYNOPSIS... Low pressure will slowly track up the east coast tonight into Wednesday morning bringing periods of rain along with a few embedded thunderstorms. Low clouds along with some drizzle and fog are expected to linger Wednesday night into Thursday especially along the coast. Very mild to warm afternoons should return Friday and Saturday with the low risk for a few showers/thunderstorms. Much cooler weather follows Sunday especially on the coast. Another cold front will likely bring more showers to the region sometime Monday into Tuesday. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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***Heaviest rain and greatest risk for isolated thunderstorms between midnight and 9 am Wednesday morning*** 1000 PM Update... Overall trend in the forecast remains on track for tonight. Precip continues to move northward as stacked low pressure system off the North Carolina coast begins to more northward towards the region. Increase southeasterly LLJ will continue to provide the lift in a very moist environment resulting in continuous showers this evening. Overnight with the low gets closer, approaching warm front combined with southerly flow will help enhance precipitation and qpf amounts. Will have to watch the convection potential that will help increase precip amounts overnight and into Wednesday morning. But overall thinking from prev forecast appears to be on track. This moisture profile will also result in low clouds and fog, especially once Warm front moves in. While dense fog seems unlikely, we will need to watch the morning commute for vsbys issues. Previous Discussion... Previous forecast is in good shape with widespread light to moderate rain showers covering southern New England early this evening. The heaviest rain and the risk for isolated thunderstorms will mainly occur from near midnight to 9 am Wed. This a result of low pressure moving up the mid Atlantic coast, allowing low level jet and forcing to increase along with some elevated instability. Widespread 0.75" to 1.50" of rain expected, but localized 2" plus amounts possible in any convection. Highest risk for this will be across eastern MA/RI where the stronger low level jet and higher Pwat access resides. There also may be some enhancement across RI/SE MA as a warm front lifts north towards 12z. In fact, portions of this region may see temps rise to near 60 by daybreak, while the rest of the region will be in the 40s to lower 50s. Main concern will be for typical brief nuisance poor drainage street flooding, especially if some of this heavy rain persists into the Wed morning rush hour. However, significant flooding is not expected given the system is fairly progressive.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/... Models show the low pressure center to our south slowly moving northward during the day, heading south of Long Island. The surface low continues to weaken/fill. With the deeper moisture plume begins to shift northeast of our area. May see an isolated rumble of thunder early in the morning, otherwise anticipating some lighter rain and perhaps drizzle for much of the day. Highest pops during the morning, but precip chances persist thru the afternoon. && .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/... Highlights... * Low clouds/spotty drizzle Wed night+Thu with cool temps on coast * Mild to warm afternoons Fri/Sat with a low risk for a few t-storms * Backdoor cold front brings much cooler temps by Sun * Period of showers which may be briefly heavy sometime Mon/Tue Details... Wednesday night and Thursday... Closed upper level low will linger off the southern New England coast Wed night into Thu. Model cross sections indicate a fair amount of low level moisture lingering below the subsidence inversion. Given overall setup and time of year, feel low clouds will linger Wed night into Thu afternoon even across much of the interior. There also should be areas of fog and drizzle, especially on the coast Wed night and Thu am along with the risk for a spot shower or two. With that said, guidance is probably too warm with forecasted high temps on Thu given expected low cloud cover. High probably remain in the 60s for most of the region and perhaps stuck in the mid to upper 50s along the coast. Now if more sun is realized than expected it still would be possible for portions of the interior to break 70, but hedging cooler for now. Friday and Saturday... Upper level ridge of high pressure building off the southeast coast will be pumping higher height fields into southern New England. Decent shot for high temps to reach the 70s to lower 80s away from any marine influence along the coastline, especially the south coast with south to southwest flow. Most of this time should feature dry weather, but a few shortwaves will result in the low risk for a couple of showers/t-storms. Greater chance of development would occur if energy moves through closer to the time of peak heating on Fri and Sat. Saturday night and Sunday... Despite abnormally high height fields, strong high pressure will be building across eastern Canada. This will likely send a backdoor cold front south of the region late Sat or Sat night. Much cooler temps are likely for Sunday especially along the coast where they should remain in the 50s. Perhaps a few spot showers, but high pressure may suppress most of the activity to our southwest. Monday and Tuesday... Timing uncertain, but an approaching cold front will likely bring a period of showers. GEFS anomalies signaling decent low level jet coupled with fairly high Pwats so the showers may be heavy for a brief period of time. Again, timing uncertain and not expecting both days to be total washouts. Temperatures also uncertain given timing/wind direction. && .AVIATION /02Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/... Tonight...High confidence. Mainly IFR conditions persist in widespread rain showers tonight. Heaviest rain and greatest risk for isolated t-storms will be from 4z through 12z Wednesday. LLWS will be a concern especially across the Cape and Islands. Wednesday...Moderate confidence. MVFR-IFR CIGS continue with areas of -RA/RA and patchy fog, especially across the eastern half of southern New England. Reduced VSBYS likely, down to IFR in spots. May see areas of LIFR CIGS across the higher inland terrain and along the coast. SE winds gusting up to around 25 kt across Cape Cod and the islands. Low risk of TSRA early in the day, especially the S coastal terminals. LLWS impacts for the Cape and islands, and possibly as far north as BOS, during the morning hours. KBOS TAF...Moderate to high confidence in TAF. KBDL TAF...Moderate to high confidence in TAF. Outlook /Wednesday Night through Sunday/... Wednesday night into Thursday...Moderate to high confidence. IFR to even LIFR conditions likely Wed night into part of Thu am along with some fog, drizzle and a spot shower. Lower conditions most likely on the coast. Conditions may improve to mainly MVFR by late Thu morning and afternoon. Thursday night and Friday...Moderate confidence. MVFR/IFR conditions likely in areas of low clouds and fog patches Thu night into early Fri am. Improvement to VFR likely by Fri afternoon with the low risk for a few showers and perhaps a t-storm. Saturday and Sunday...Moderate confidence. VFR conditions probably dominate. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/...High confidence. Small Craft Advisories remain in effect across all waters for tonight into Wednesday. Low level jet of 50 to 60 knots lifts across most of the waters tonight, but the mixing of those winds to the sea surface is tempered by a strong temperature inversion. Hence, we anticipate most wind gusts will top at around 30 knots. There is a broad SE to ESE fetch which due to its length and breadth may bring 8 to 10 foot seas into Rhode Island and Block Island Sounds by late evening even though the inversion will likely inhibit gusts much higher than 30 knots. Winds and seas will gradually subside Wednesday afternoon as the tighter gradient moves into the Gulf of Maine and the coastal low gradually weakens as it drifts north. Outlook /Wednesday Night through Sunday/... Wednesday night through Friday...Moderate to high confidence. Despite wind gusts mainly 20 knots or less, left over southeast swell will result in small craft seas persisting through Friday across many of our outer and southern waters. In addition, areas of fog may result in poor vsbys for mariners especially during the overnight and early morning hours. Saturday and Sunday...Moderate to high confidence. West to southwest wind gusts to 20 knots Saturday will shift to the northeast by late Sat or Sat night behind a backdoor cold front. Overall, expect winds and seas to remain below small craft advisory thresholds. && .TIDES/COASTAL FLOODING... * Isolated minor coastal flooding impacts during astronomically high tides tonight through Wednesday night along the east and south coasts We are entering a spring tide cycle with high tides that are astronomically high. This combined with an anticipated storm surge of around 1 foot may set the stage for isolated pockets of minor coastal flooding along both south and east coasts around the time of high tide this evening as well as possibly for the Wednesday day and night tides as well. Have opted to renew the Coastal Flood Statement issued early this morning. One area that we are watching a little more closely is the Charlestown and Westerly shorelines along the RI southwest coast where the storm surge may reach 1.5 feet by the time of the evening high tide near 8 PM. This reach of coastline can be vulnerable to an ESE fetch, although it looks like the highest surge and wave action will occur after the evening high tide. The combination of runoff from heavy rain and elevated water levels near high tide this evening could exacerbate drainage of fresh water in some coastal urban areas and near the mouths of streams and small rivers. We will need to continue to monitor the tides through at least Wednesday night as there is not a significant offshore flow or pressure rises to help evacuate a lingering surge. The astronomical high tides remain very elevated through the end of the week. Due to the breadth of the fetch, we may encounter enough wave action with lengthening periods on top of an elevated tide to cause pockets of erosion tonight through Wednesday night along the Martha`s Vineyard south coast, Nantucket south and east coasts, and the outer Cape ocean side from Truro to Chatham. Boston High Tides (flood stage at Boston 12.5 feet)... 11.47 feet / Tuesday 11:20 pm 11.15 feet / Wednesday 11:50 am 11.92 feet / Thursday 12:07 am Providence High Tides (flood stage at Providence 7 feet)... 5.98 feet / Tuesday 8:11 pm 5.53 feet / Wednesday 8:41 am 6.21 feet / Wednesday 9:00 pm && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 3 PM EDT Wednesday for ANZ231>234. Small Craft Advisory until 10 PM EDT this evening for ANZ230. Small Craft Advisory until 5 AM EDT Wednesday for ANZ236. Small Craft Advisory until 8 PM EDT Wednesday for ANZ235-237- 250-251-254>256.
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&& $$ SYNOPSIS...Frank/NMB NEAR TERM...Frank/Dunten/NMB SHORT TERM...NMB LONG TERM...Frank AVIATION...Frank/NMB MARINE...Frank/Thompson TIDES/COASTAL FLOODING...staff

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