Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 252030 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 430 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 PM THIS EVENING/... 355 PM Update... Tonight...High confidence. A widespread soaking rain is expected to continue through the overnight hours. Vertically stacked low pressure has been lingering along the coast of the Carolinas today, with high pressure well to our northeast in the Canadian Maritimes. The low makes little northward progress tonight, filling some as it meanders into the coastal Mid Atlantic region. Southern New England has seen a steady feed of moisture from the south/southeast aloft, and east at the surface, which continues tonight. NAM/GFS show plume of 1.25 to 1.5 inch precipitable water feeding into our area during tonight, accompanied by a 50 kt H850 jet. A low level inversion is in place near the surface, could see some gusts 25 to 30 mph along the coastline. Models showing some marginal elevated instability tonight, with K indices in the low 30s, isolated rumbles of thunder are possible. Including lingering lighter precip on Wed, thinking event total QPF ranges between 1 to 1.5 inches for much of the area, with locally higher totals closer to 2 inches. While rivers and streams are expected to remain in their banks, we could see some minor urban and poor drainage flooding tonight into early Wednesday morning. Deeper moisture plume looks to move northeast of our area towards 12Z Wed, so precip quantity should be diminishing towards or after daybreak. With surface dewpoints in the 40s and low 50s, temps overnight will not drop much/remain nearly steady. && .SHORT TERM /6 PM THIS EVENING 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 /21Z TUESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/... Late this Afternoon and Tonight...High confidence in trends, moderate confidence in timing. For most of the area, conditions continue to deteriorate to IFR/LIFR during late this afternoon and evening with rain/showers and fog. In BDL/BAF vicinity, where MVFR conditions are expected into early evening, then deteriorating to IFR. E wind gusts 25 to 30 kt over Cape/Islands with winds veering to SE tonight with similar gusts. LLWS developing Cape/Islands tonight as SE/S low level jet at 50+ kt develops, especially after midnight. LLWS may extend as far north as BOS. Isolated TSRA possible tonight. 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 confidence in TAF. Potential for LLWS 08Z- 14Z with SE low level jet. KBDL TAF...Moderate 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... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 3 PM EDT Wednesday for ANZ231>234. Small Craft Advisory until 8 PM EDT this evening for ANZ230- 236. Small Craft Advisory until 8 PM EDT Wednesday for ANZ235-237- 250-251-254>256. && $$ SYNOPSIS...Frank/NMB NEAR TERM...NMB SHORT TERM...Belk LONG TERM...Frank AVIATION...Frank/NMB MARINE...Frank/Thompson TIDES/COASTAL FLOODING...Thompson is the U.S. government's official web portal to all federal, state and local government web resources and services.