Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 072356 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 756 PM EDT Tue Apr 7 2020 .SYNOPSIS... High pressure in place will give way to a weak area of low pressure moving in from the Great Lakes. This low pressure will spread a period of rains later tonight into early Wednesday as it moves into the waters south of Southern New England. Another system moves in Thursday, bringing more rain and scattered thunderstorms. Scattered showers are possible Thursday night through Friday. Drier weather for this weekend before our next wet weather system early next week. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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Made some minor tweaks to the forecast this evening using the latest high resolution guidance. Goal was to try to refine the timing and location details. Have higher confidence in showers and possible thunderstorms towards CT and southern RI. Minor tweaks to temperatures to bring them back in line with observed trends. Previous Discussion... The primary feature of interest for much of the night, however, is a NW to SE oriented and rather well-defined warm front associated with a low pressure area as of early this afternoon near northern Michigan. The low and its related warm front will progress east- southeast through the northern mid-Atlantic region into Southern New England later tonight, with increasing clouds through midnight and rains spreading east-northeastward especially for the overnight hours. Temperatures look to remain mild enough even across the terrain to stay as rain. However worth noting the thermal contrast through the warm front is fairly robust, and in addition to that, I did notice on SPC`s mesoanalysis page a plume of weak mid-level instability (700-500 mb lapse rates ~6-7 C/km) even on the north side of the warm front. Some guidance - most notably the NAM and the RAP - brings a sliver of this weak elevated instability into a part of northern CT into central/southern RI and the south coastal waters overnight tonight (roughly from a South Windsor to Putnam CT to Coventry RI line). Though rain should predominate, I did opt to include some isolated thunder (basically rumbles) along and south of this line. Rains should continue into the Wednesday morning for most. Model guidance QPF off today`s 12z guidance was a bit greater than previous runs. Opted for rain totals thru 12z Wednesday from a quarter to third of an inch for southern RI into the Hartford area, then tapering to a tenth to two-tenths of an inch further north and east. Lows tonight mainly in the mid-upper 30s to the lower-mid 40s.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... Wednesday: Weak low pressure and attendant warm frontal-related showers pass south of the coastal waters through the morning hours. As these features pass to our south, likely to see a modest uptick in NE wind speeds into the morning hrs and especially near the eastern MA coast (indicated by the NAM 975 mb winds which tick up to around 25-kt just offshore). Eventually these winds will lighten and shift to easterly moving into the afternoon. Some gradual improvement with rains coming to an end by late- morning/early afternoon (latest towards Cape Cod and the offshore waters), and skies trending mostly to partly cloudy. So the second half of the day is looking somewhat better than the first half, but with NE to E onshore flow and a shorter period of sun, I opted to keep temperatures on the cooler side of guidance, into the upper 40s to the mid 50s. Wednesday Night: Weak shortwave ridging/modest 500 mb height rises, ahead of a strong frontal system to affect our area into Thursday, should allow for dry conditions through the first part of the overnight. Model guidance shows moisture levels starting to rise on SE flow into the second half of the night, with increasing clouds spreading from west to east. In addition, 500 mb height falls then spread into western MA into northern CT into the early morning hrs. PoPs increasing to chance type levels for these locations though most should stay dry. Lows mainly into the upper 30s to mid 40s with modest SE breezes. && .LONG TERM /THURSDAY THROUGH TUESDAY/... * Widespread rain Thursday with scattered thunderstorms * Lighter rain, and high elevation snow possible, on Friday with a few rumbles of thunder also possible * Potential for minor coastal flooding Wednesday to Friday around high tide across the east and south coasts of MA and RI. See the Tides and Coastal Flooding section below for more details. * Temperatures running near to below normal through Saturday, then a warm up Thursday... This still appears to be the most active weather day during this portion of the forecast. Potent mid level low shifts over northern New England, with a low pressure passing through our region. Almost certainly looking at some rain for much of southern New England, with some wet snow possible across the higher terrain. Drier air rushes in behind the cold front of this system, drying our the forecast into Friday. As for thunderstorm potential, surface-based instability is not the best. However, mid level lapse rates are better than moist adiabatic with a strong low-level jet from the south. These factors are enough to warrant at least a mention of isolated thunderstorms. Still too early to say if any of these thunderstorms could be severe. Friday... Low pressure continues to rotate away from our region. Still in the cyclonic flow, so cannot dismiss the idea of a few showers during the day, rain for most with snow possible across the higher terrain. Saturday through Sunday... The weekend looks drier as a ridge of high pressure makes its way east across the Mid Atlantic states. This should mean a warming trend into Sunday. Monday into Tuesday... Another low pressure should move across the Great Lakes, leading to another round of showers sometime from Monday afternoon into Tuesday morning. Timing is not yet locked down. Looking warm enough for all rain. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Tonight: High confidence in trends, moderate on timing. Increasing/lowering clouds associated with a warm front which will eventually spread rains across the TAFs. Initial VFR with deterioration to MVFR ceilings (possible IFR ceilings south of BDL to PVD to the Cape airports) after 08/05-07Z with VFR-MVFR vsby rains spreading ESE through 08/08-12Z. Due to the dry air around, timing is a bit uncertain but could be an hour or two sooner than indicated in the TAFs. Through midnight, mainly light/variable winds in the interior, SE 5-8 kt towards the Cape and at BOS. After midnight through 08/12Z, winds to shift to NE around 5-10 kt as front slides S of the coastal waters. Wednesday: High confidence in trends, moderate on timing. Initial MVFR, possible IFR with periods of rain to generally taper off through the morning. Gradual improvement from widespread MVFR to MVFR/VFR levels toward the afternoon. NE/ENE winds 4-8 kt in the interior, around 7-10 kt with occasional gusts to 20 kt coastal MA and the Cape. Wednesday Night: Moderate to high confidence. Increasing moisture levels on SE flow will result in VFR-MVFR ceilings lowering to MVFR-IFR levels in the interior and across the higher terrain. For the coastal plain into the Cape, initial VFR to trend VFR/MVFR into the pre-dawn hrs. Rains should hold off until the pre-dawn hrs in the CT Valley with dry conditions elsewhere. SE winds 6-10 kt. KBOS Terminal...High confidence in the TAF. KBDL Terminal...High confidence in the TAF. Outlook /Thursday through Sunday/... Thursday: MVFR/IFR conditions possible. Windy with gusts up to 30 kt. Chance SHRA, isolated TSRA, patchy FG. Thursday Night: VFR. Windy with gusts up to 35 kt. Friday: VFR. Windy with gusts up to 30 kt. Slight chance SHRA. Friday Night through Saturday: VFR. Breezy. Saturday Night: VFR. Sunday: VFR. Slight chance SHRA.
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&& .MARINE...
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Winds and seas should remain below SCA thresholds through Wednesday night. A warm front will bring a period of steady rains for most of the waters late tonight into Wednesday morning, with an outside/low chance for a rumble or two of thunder across the southern offshore waters overnight. Could see vsbys between 3-5 SM in rains, likely lowest south coastal waters. Winds to become NE on Wed AM, and could briefly reach marginal-SCA gusts with seas 3-4` (near 5` per the SWAN guidance off the NAM) on the eastern MA/Cape Cod waters. Wasn`t confident enough on criteria being met to issue an SCA for these waters Wed AM, but may get close. Winds lighten to under 20 kt and shift east later Wednesday and then SE into Wednesday night. Seas mainly 2-3` Wednesday night, though build to 2-4` by daybreak. Outlook /Thursday through Sunday/... Thursday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Chance of rain showers, isolated thunderstorms, patchy fog. Local visibility 1 to 3 nm. Thursday Night: Low risk for gale force winds with gusts up to 35 kt. Rough seas up to 10 ft. Friday: Strong winds with gusts up to 30 kt. Rough seas up to 12 ft. Slight chance of rain showers. Friday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 10 ft. Saturday: Winds less than 25 kt. Local rough seas. Saturday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Sunday: Winds less than 25 kt.
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&& .TIDES/COASTAL FLOODING... 418 PM Update: Astronomical tides are on the rise over the next several days (across multiple tidal cycles) due to the Supermoon. While the overall threat is limited by poor wave action, due to the high astro tides a number of tidal gages are forecast to be near flood stage or into minor flooding categories per Stevens Institute guidance. Appears the greater potential for minor coastal flooding later tonight into Wednesday should exist for eastern coastal MA and Cape Cod/Nantucket, especially around the period of high tides. By Wednesday night into Thursday, stronger southerly winds ahead of a strong frontal system will raise the coastal flood threat across a larger area to include the South Coast and Narragansett Bay. Coastal Flood Advisories have been posted thru Friday following these general expectations. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...Coastal Flood Advisory from 8 AM Wednesday to 8 AM EDT Friday for MAZ019-022. Coastal Flood Advisory from 8 PM Wednesday to 8 AM EDT Friday for MAZ015-016. Coastal Flood Advisory until 8 AM EDT Friday for MAZ007. RI...Coastal Flood Advisory from 8 PM Wednesday to 8 AM EDT Friday for RIZ002-004>007. MARINE...Gale Watch from Thursday afternoon through Friday afternoon for ANZ231-232-250-251-254-255.
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&& $$ SYNOPSIS...Belk/Loconto NEAR TERM...Belk/Loconto SHORT TERM...Loconto LONG TERM...Belk AVIATION...Belk/Loconto MARINE...Belk/Loconto TIDES/COASTAL FLOODING...Loconto

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