Lexington, Massachusetts, USA
000 FXUS61 KBOX 211952 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 352 PM EDT Thu Mar 21 2024 .SYNOPSIS... Gusty winds diminish overnight leading to a dry/sunny day Friday. Though southern New England is located between two pressure systems and will experience strong west/northwest gusts through this evening, before diminishing on Friday. A more active weather pattern then evolves for Friday night into the weekend. A strong frontal system may bring the risk for minor accumulating snow in the interior late Friday night, but transition to widespread soaking rain Saturday into Saturday evening, capable of poor drainage flooding and rises on rivers and streams. Drying out Sunday into early next week but temperatures will average below normal with persistent onshore flow. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Tonight Gusty westerly winds from this afternoon remain elevated tonight, but weaken slightly in response to a slight weakening in the pressure gradient. We should still experience steady westerly winds from 10 to 20 mph with gusts up to 30 mph possible at times. Despite clear skies overnight, radiational cooling will be limited thanks to mechanical mixing. Nonetheless, overnight lows will still be substantially below normal and cooler than last night with most locations in the upper teens/low 20s just before sunrise. Coastal areas will be slightly more mild in the mid 20s. For reference, the average low temperature for Boston for this time of year is about 32 degrees. This is consistent with an anomaly of about 10 degrees below normal. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... Tomorrow Surface high pressure builds over southern New England tomorrow bringing an end to the period of gusty westerly winds. There still remains a modest pressure gradient in place however, so steady westerly winds are still forecast but with more modest speeds in the 5 to 10 mph range. This should be enough to support diurnal mixing during the afternoon. Mixing -5 Celsius temps at 925 hPa down to the surface would translate to surface high temps in the low to mid 40s F tomorrow afternoon. Overall a quiet/sunny day and very dry with single digit to negative single digit dewpoints. Despite excessive dry air, winds are not looking strong enough to raise fire weather concerns at this time. Tomorrow Night Cloud cover increases tomorrow night ahead of a robust low-pressure system that is forecast to bring significant precipitation to southern New England. Precipitation will begin after midnight tomorrow night as a surface warm front associated with the low- pressure center over western New York begins to lift north over the region. Antecedent conditions ahead of the warm frontal boundary will be favorable to support a brief period of snowfall or perhaps freezing rain at the onset of precipitation, which will be during the very early hours of Saturday. Icing would be limited to trace amounts at this time and be focused over the CT River Valley and The Berkshires. Temperature profile only looks supportive of snow for the areas along and north of I-90 where a dusting to an inch of accumulating snow may be possible before a quick changeover to rain. Winds will gradually strengthen out of the south reaching speeds of 10 to 20 mph Saturday morning. The bulk of the precipitation and duration of the storm will take place during the day Friday. See the long term section of the AFD for more details. && .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... Highlights: * Minor snow accum possible late Fri night into early Sat in the interior * Widespread rain, heavy at times Sat into Sat evening capable of minor urban and poor drainage flooding and some river flooding * Trending drier for Sun into early next week, but blustery with below normal temps Friday Night into Saturday Night... Potent northern stream trough moves into New Eng with enough interaction with southern branch to bring a plume of anomalous PWATs northward into SNE. Meanwhile southern stream low pres is forecast to lift northward along the mid Atlc coast Sat then track south and east of Nantucket Sat night. Inverted trough extending north from the low pres combined with coupled upper level jet structure will result in strong forcing for ascent, acting on anomalous moisture to bring heavy rainfall to SNE Sat into Sat evening. Initial warm advection and deepening moisture ahead of the approaching mid level trough will result in precip developing from west to east mainly after midnight Fri night. The column is cold enough to support a few hours of snow north and west of I-95 corridor where a coating to one half inch is possible before snow changes to rain. Snow will persist longer further in the interior, with 1-3 inches possible north of the Route 2 corridor in far northern MA, especially over the higher terrain. Snow will change to rain here Sat morning and there could also be a period of freezing rain during the snow to rain transition. The bigger impact from this storm will be the heavy rainfall during Sat into Sat evening which will be focused along and to the west of the surface boundary/inverted trough. The heaviest rain will initially be focused across the interior during Sat but shift to the SE New Eng late Sat and especially Sat evening as low pres tracks south of Nantucket. Deterministic and ensemble guidance suggesting potential for 1.5 to 3 inches of rainfall. However, where this axis of heaviest rainfall sets up remains uncertain. GEFS and CMC ensemble probs of 2+ inches favor areas south of the MA Pike while ECMWF ensembles have heaviest rainfall in the interior. Still need to iron out these details but the heavy rainfall may lead to areas of minor urban and poor drainage flooding. In addition, MMEFS probabilistic hydrographs based on the GEFS are indicating high probs (70-80 percent) of minor flood for the Pawtuxet and Wood River basins in RI, with 30-40 percent probs of moderate flood. Rain will be ending from west to east Sat night as colder air begins to move back into the region. If cold air comes in quicker it is possible rain could change to a period of snow before ending, mainly interior where some minor accum are possible. But this is a low confidence forecast at this time. Regarding potential for coastal flooding Sat, it is looking like a rather low risk at this time. Along the south coast, the tide in question is the Sat evening high tide. Providence would need a 2.5 ft surge to reach minor flood, and we are currently forecasting a 1- 2 ft surge. No concern along the E coast for the Sat and Sat night high tides. Sunday into Wednesday... Deterministic guidance continues to trend drier for Sun into early next week as ocean storm remains well offshore with high pres building to the north. However, ensemble guidance still showing low probs for measurable QPF across SE New Eng Sun-Mon so we continued low pops in the grids. Still a chance that a piece of moisture could wrap around the low pres and back in across SE New Eng sometime Mon or Tue so will have to watch this. The shower chances increase again toward midweek as a shortwave approaches from the west. Persistent onshore flow will keep temps below normal. Not expecting any coastal flooding issues for the east coast Sun into early next week as forecast surge is much less than what is needed to reach minor flood. && .AVIATION /20Z THURSDAY THROUGH TUESDAY/... Forecast Confidence Levels: Low - less than 30 percent. Moderate - 30 to 60 percent. High- greater than 60 percent. Through 00Z...High confidence VFR with strong W to WNW winds at 15-25 kts. Gusts of 30-40 kts gradually ease to 25-35 kts by this evening. Tonight...High confidence. VFR with winds out of the WNW to NW at 10-20 kts, with gusts of 20-30 kts for most of the night. Anticipate that the gusty winds cease by roughly 08-12Z. Friday... High confidence. VFR with NW winds at 5-10 kts shifting to the W by late morning. Should continue to shift to the SW by the afternoon. Increasing mid/high clouds during the afternoon. Friday Night...High confidence in trends, moderate in exact timing. VFR to start, but will have conditions deteriorate as precipitation spreads in. Should see MVFR to IFR after 06Z with light snow to start for most. Will stay as snow across the higher terrain, but transition over to all rain elsewhere fairly quickly. KBOS TAF... High confidence in TAF. VFR with WNW winds at 20-25 kts through the day and winds gusting 35 kts. Should ease tonight, but still will be gusty until the Fri AM push. Winds shift to the SW Fri afternoon. KBDL TAF...High confidence in TAF. VFR with WNW/NW winds 15-20 kts and gusts 30 kts through the day. Winds ease tonight, but still remain gusty until the Fri AM push. Winds shift to the W/SW Fri afternoon. Outlook /Friday Night through Tuesday/... Saturday: Mainly IFR, with areas MVFR possible. Windy with gusts up to 35 kt. RA. Saturday Night: Mainly VFR, with local IFR possible. Strong winds with local gusts up to 40 kt. Chance RA, slight chance SN. Sunday: VFR. Windy with gusts up to 35 kt. Sunday Night: VFR. Strong winds with local gusts up to 40 kt. Slight chance RA. Monday: VFR. Windy with gusts up to 35 kt. && .MARINE... Forecaster Confidence Levels: Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent. Tonight Moderate westerly winds continue to blow from 30 to 40 mph this evening and tonight but gradually diminish to 20 to 30 mph by tomorrow morning. A period of light freezing spray will be possible overnight over the easter marine zones where temperatures will be very cool in the low to mid 20s. Significant wave heights up to 8 feet over the outer marine zones should subside to 4 to 6 feet with the diminishing winds by day break. Near-shore marine zone wave heights subside to 1 to 3 feet. Tomorrow High pressure builds over the coastal waters tomorrow and allows winds and seas to continue to diminish and subside to more calm levels. Tomorrow Night Relatively quiet marine conditions continue overnight, but winds begin to strengthen out of the south/southeast a few hours before sunrise as a robust low-pressure system approaches the northeast. Increasing winds will also be accompanied by gradually rising seas. Despite increasing winds and seas, SCY criteria conditions should persist until at least late morning/early afternoon Saturday. Outlook /Friday Night through Tuesday/... Friday: Winds less than 25 kt. Seas locally approaching 5 ft. Friday Night: Winds less than 25 kt. Chance of rain. Saturday: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Rain. Local visibility 1 to 3 nm. Saturday Night: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 14 ft. Rain. Areas of visibility 1 to 3 nm. Sunday: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 14 ft. Slight chance of rain. Sunday Night through Monday: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 13 ft. Slight chance of rain. && .FIRE WEATHER... Elevated fire weather concerns today and also potentially into Friday. Today offers a combination of gusty NW winds between 35 and 50 mph and lowest RHs around 20-35 percent. In addition, rainfall received on Wednesday were less than 0.2 inches. In collaboration with our neighboring WFOs and state partners, a special weather statement for Elevated Fire Weather Concerns goes into effect from 11am to 8pm . For Friday, NW to W winds are lighter but RHs project to be lower in the upper teens to 30 percent range. Will reassess with incoming guidance before coordinating with our fire weather partners over the coming days. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Wind Advisory until 8 PM EDT this evening for MAZ002-008-009. RI...None. MARINE...Gale Warning until 8 PM EDT this evening for ANZ230. Gale Warning until 2 AM EDT Friday for ANZ231>235-237-250-251- 254>256. Small Craft Advisory until 2 AM EDT Friday for ANZ236. && $$ SYNOPSIS...KJC/Dooley NEAR TERM...RM SHORT TERM...RM LONG TERM...Belk AVIATION...Belk/BL MARINE...Belk/RM FIRE WEATHER...
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