Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 242306 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 706 PM EDT Mon Sep 24 2018 .SYNOPSIS... An approaching warm front will bring a soaking/heavy rain to the region Tuesday with some poor drainage street flooding issues possible. The steady rain should wind down Tuesday evening...but scattered showers and a few thunderstorms will be possible through the night with locally heavy downpours. Warmer and humid weather returns Wednesday with scattered showers/ thunderstorms in the afternoon and early night. High pressure brings cooler less humid weather for Thursday through Saturday. A cold front moves through on Sunday with a chance of showers. Dry weather returns Monday. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... 7 PM update... Warm air aloft is moving up the Appalachians, with increasing lift generating an area of rain over PA which is moving northeast. Trends from both AWIPS and manual means bring the leading pcpn to the CT Valley 08Z- 09Z. This is within 1-2 hours of the existing forecast. We will maintain the same timing with this update. Bottom line is the same... rain in the west developing late tonight and expanding east. One change in the forecast has been to expand the overcast across most areas this evening, then across far NE Mass and Cape Cod during the night. Based on 7 PM temps, expect most low temps to be in the low to mid 50s, but still possible that far northern MA touches the upper 40s. Previous discussion... Large high pressure over Quebec will shift towards the Maritimes tonight. This will result in a continued onshore flow...but winds should shift from more of the NE to the E. Model cross sections indicate that some temporary partial clearing may occur for a time tonight. The greatest chance for this to happen is across eastern New England. Either way...should see skies become cloudy again as a warm front approaches the region. Rain should arrive across western MA and CT after 4 or 5 am. Low temperatures in most locations should be mainly in the upper 40s to the lower 50s. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... Highlights... * Heavy Rain Tue with poor drainage street flooding issues possible * Low risk for a strong t-storm late Tue night near the south coast Details... Tuesday... An approaching warm front will team up with Pwats rising to 2+ SD above normal...while the southerly LLJ reaches 2-3+ standard deviations above normal. This will set the stage for a widespread soaking/heavy rainfall on Tuesday as the cool dome at the surface will provide a good over running situation for the high pwat plume/strong low level jet. Rainfall amounts of 1 to 2 inches anticipated for much of the region...except lower amounts across far southeast New England. There may be some localized 3"+ amounts across interior southern New England...where strongest 850 mb low level jet is positioned. The last two model runs of the ECMWF and a few of the HREF members show this potential. Overall...this heavy rain should result in typical nuisance poor drainage street flooding issues. While we can not rule out localized more significant flooding if those 3"+ inch amounts occur...appears to localized and confidence is not high enough to issue a Flood Watch at this time. Nonetheless...this needs to be watched. As for temperatures...it should remain rather cool through lunch time with readings mainly in the 50s to the lower 60s. The mildest of the readings will be near the south coast. Temperatures will probably rise into the 60s in most locations by evening...except the CT River Valley. Tuesday night... The steady/heavy rain should exit the eastern New England coast during the evening as the strongest forcing exits the region. However...Pwats will remain between 1.75 and 2 inches along with a modest low level jet. While precipitation will become more scattered in nature...showers with localized downpours will remain possible. In fact...there might be a rumble or two of thunder. The one thing will have to watch is an area near the south coast towards daybreak Wednesday. A 30 to 40 knot low level jet with dewpoints approaching 70 is always a concern this time of year. Some of the high resolution guidance indicates the low risk for a few strong thunderstorms near the south coast between 6z and 12z. Certainly plausible given strength of the LLJ and climatology. The NAM is the most concerning given it has 1000+ J/KG of Cape...while other guidance is less. Temperatures should rise Tue night across much of the region and actually may be near 70 in many locations by 12z Wednesday. However...most high resolution guidance continues to show this being a slower process across the CT River Valley which seems reasonable given climatology. Where low level cool air is trapped for a time...will have to watch for locally dense fog. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Big Picture... Ridges off the Pacific and Atlantic coasts with arctic low pressure digging over Hudsons Bay and generating a trough over the Plains. The Pacific high actually evolves into a Rex Block, diverting the airflow up to Alaska and the Arctic coast before turning south and bringing colder air into the Northern Plains. The Rex block eventually breaks over the weekend. Meanwhile the Atlantic high shifts west over the Southern USA over the weekend and early next week bringing warmer air. Shortwaves will move through the flow around the Central USA trough. One moves up the St Lawrence Valley Wednesday, another shifts north away from us and through Quebec Friday night-Saturday, and a third moves through Northern Maine on Sunday. Colder air moving south from Canada...warmer air moving over the Southern USA. This suggests a zone of strong temperature contrast developing over the Northern USA. Concerns... Wednesday... Shortwave moves through the St Lawrence Valley with a supporting upper jet of 140-150 kts. Our area is in or close to the right entrance region of the jet from afternoon through Wednesday night. Precipitable Water values, after backing off during the morning, surge back to 2 inches from mid-afternoon through early night. Stability indices are favorable for convection, primarily from mid aftn to early night. Model consensus would place cold frontal passage between 00Z west to 06z east. Southwest winds at 950 mb ahead of the front show 25-30 knots, much of which may mix to the surface in gusts. Temperatures in the mixed layer support sfc temps of 78-82F. Overall...a warm humid day with gusty SW winds and a period of showers/scattered tstms. Thursday-Friday... The upper trough lingers over the Central USA and maintains a southwest upper flow over New England. As the cold front moves offshore it will become more parallel with the upper flow which will slow down its departure. As such there could be a lingering chance of early morning showers/tstms along or off the South Coast and Islands. All of this should move out to sea Thursday afternoon. Otherwise surface high pressure builds over the region Thursday with dry weather and cooler temperatures/lower humidity. The high moves off to the east Friday. Temps aloft equiv to 3-6C at 850 mb will support max sfc temps in the 60s Saturday-Sunday... Two more shortwaves move through the upper flow during the weekend. The first will move north through Quebec and away from our area, so expect its associated cold front to either stall or dissipate by Saturday morning, leaving us with dry weather. The second shortwave is closer to the border, but could generate a few showers. Monday... High pressure follows Monday with dry weather. At the end of the forecast window, a warm front slides east toward Northern New England and could generate a few showers in our area Monday night. && .AVIATION /00Z TUESDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday Night/... Tonight...Moderate to high confidence. Ceilings are around 3000-3500 feet, with a few spots 2500-2900 feet. A dry area over EC NY shows no ceilings. Lower ceilings are moving up from Central PA, and should move into our area overnight. Expect an uneven trend to 2000-3000 foot cigs duirng the night, possibly lower toward morning as rain moves in. Timing of the rain is 08z-09z in the CT Valley, then to Central MA and RI toward sunrise. Tuesday...High confidence. Widespread low end MVFR to IFR conditions as a widespread soaking rain overspreads most of the region by 15z...but perhaps a bit later across the Cape/Islands. The rain will be heavy at times along with patchy fog. ESE wind gusts of 20 to 30 knots develop on the coast...but inversion results in lighter winds further inland especially across the the CT River Valley. LLWS expected to develop during the afternoon. Tuesday night...Moderate confidence. Low end MVFR-IFR conditions persist with localized LIFR conditions in fog. Widespread rain diminishes during the evening...but scattered showers and a few t- storms possible overnight with localized downpours. Low risk for a strong storm or two near the south coast towards 12z. LLWS expected. KBOS Terminal...Moderate to high confidence in TAF. KBDL Terminal...Moderate to high confidence in TAF. Outlook /Wednesday through Saturday/...Moderate Confidence. Wednesday and Wednesday night... VFR with areas of MVFR cigs/vsbys in showers and thunderstorms, ending from west to east after midnight. Thursday through Saturday... VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday Night/... Tonight...High confidence. High pressure over Quebec shifts east towards the Canadian Maritimes. While NE winds will shift to more of an E direction...gusts of 20 to 30 knots are still expected over the open waters. Small craft headlines continue. Tuesday...Moderate to high confidence. Potent SSE low level jet at 925 mb of 40 to 50 knots sweeps across the region in association with a warm front. This will certainly bring strong small craft wind gusts to all waters and perhaps brief marginal Gales. In coordination with GYX...we opted to hoist a Gale Watch Tue afternoon/early evening across our northern waters where there greatest risk appears. Strong small craft headlines elsewhere. Widespread rain that will be heavy at times will also reduce vsbys for mariners. Seas build back to between 6 and 10 feet across our waters. Tuesday night...Moderate to high confidence. Strongest LLJ lifts northeast of the region as we enter the warm sector. Therefore...winds should diminish but still expect 20+ knot wind gusts to persist overnight with the LLJ and seas will remain above small craft thresholds across most open waters through 12z. Scattered showers and perhaps a few t-storms along with fog will also reduce vsbys for mariners. Outlook /Wednesday through Saturday/...Moderate Confidence. Wednesday and Wednesday night... Small Craft Advisory may be needed. Southwest winds gusting to 30 knots, diminishing after midnight. Seas 5 to 8 feet. Showers, chance of thunderstorms, patchy fog. Local visibility 1 nm or less. Thursday and Thursday night... Small Craft Advisory may be needed. North to Northeast wind gusting 20 to 25 kt early, diminishing during the afternoon. Seas 5 to 6 feet on the outer waters, diminishing Thursday night. Chance of a shower early. Friday and Saturday... Winds less than 20 knots. Seas less than 5 feet Friday and Saturday, except seas near 5 feet on the outer waters Saturday. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 PM EDT Tuesday for ANZ231>234. Small Craft Advisory from 6 AM to 5 PM EDT Tuesday for ANZ230- 236. Small Craft Advisory until 6 PM EDT Wednesday for ANZ235-237- 254>256. Gale Watch from Tuesday afternoon through Tuesday evening for ANZ250-251. Small Craft Advisory until noon EDT Tuesday for ANZ250-251. && $$ SYNOPSIS...WTB/Frank NEAR TERM...WTB/Frank SHORT TERM...Frank LONG TERM...WTB AVIATION...WTB/Frank MARINE...WTB/Frank

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