Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 211102 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 702 AM EDT SUN AUG 21 2016 .SYNOPSIS... Mainly dry weather will prevail for most of today other than a spot shower or two across the interior, but humidity will be on the increase. A cold front will bring showers and embedded thunderstorms with localized torrential rainfall and the potential for brief localized street flooding tonight. The cold front moves offshore early Monday followed by a stretch of dry weather much of next week. Warm days and cool nights Mon and Tue will be replaced by warmer and more humid weather Wed through Fri along with the risk of showers and thunderstorms by weeks end. Dry weather may return next weekend. && .NEAR TERM /UNTIL 7 PM THIS EVENING/...
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7 AM update ... Very tranquil weather across the region this morning with temps mainly in the 60s along with dry weather. At 7 am a band of showers were located south of Long Island extending north into NYC area and western CT is associated with a surge of higher moisture/PWATs in the return flow. This moisture axis is progged by models to pivot northeast thru the region today...however the mid level trough currently over the area weakens as it lifts northeast. Thus forcing for ascent will be diminishing with time. Therefore showers over western CT and MA this morning will likely weaken/erode as they try to move eastward today. This would translate to mainly dry weather for RI and eastern MA...with any showers remaining focused across western MA and CT. Previous forecast captures this nicely so no major changes with this update. Earlier discussion below. ================================================================== Areas of low clouds have once again developed, mainly across western/central MA and northern CT where deeper moisture resides. Should see these low clouds burn off by mid to late morning, allowing most of the region to have partly sunny skies for a time. High temperatures should reach into the lower to middle 80s in most locations. It will be more humid then yesterday, as low level moisture continues to increase out ahead of a cold front. The majority of today will remain dry in most locations given best forcing/dynamics to our west. However, weak warm advection along with upslope flow should generate some isolated to scattered showers in our far western zones mainly focused along the east slopes of the Berkshires.
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&& .SHORT TERM /7 PM THIS EVENING THROUGH 6 AM MONDAY/...
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*** Band of showers and embedded thunderstorms with localized torrential rainfall may result in some street flooding tonight *** 1) Heavy Rain and Brief Localized Street Flooding Tonight: A cold front will cross the region from west to east tonight. High Pwats in excess of 2 inches will combined with strong forcing along the front. The result will be a band of showers and embedded thunderstorms with localized torrential rainfall. Highest risk for the heaviest rain will be across western MA, where stronger jet dynamics exist. NCAR ensembles showing fairly high probabilities of localized 1+ inch per hour rainfall rates in western MA/northern CT, so some street flooding is a concern. The activity may then weaken for a time, but probably intensify again across Rhode Island and eastern MA after midnight. This a result of a developing low level jet and increasing instability with surface capes near 500 j/kg and perhaps a bit higher near the south coast. The result may be a period of localized torrential rainfall affecting the I-95 corridor bringing brief urban street flooding. The timing of the heaviest rain is expected roughly between 9 pm and 1 am across western MA/northern CT, and then from 1 am to 6 am across eastern MA/RI. 2) Low Risk For a Few Strong To Severe Thunderstorms: The main idea is that the overall severe weather threat across the region is low overnight. However, there is a low risk for a few strong to severe thunderstorms across eastern CT/RI and eastern MA. More times than not, we escape with little if any severe weather in these setups. However, given a decent low level jet with 70+ dewpoints and 500 J/KG of cape, a severe storm or two can not be ruled out with isolated wind damage. Highest risk for this will be across the south coast, where highest instability resides. If a secondary mesoscale Low develops in this region, there even would be a small risk for a brief tornado or waterspout given very low LCL/s and SST now in the middle 70s on the south coast. Again though, this remains a low probability but something will have to watch closely overnight.
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&& .LONG TERM /MONDAY THROUGH SATURDAY/... Highlights... * A touch of fall-like temps Mon and Tue * Warmer and more humid Wed thru Fri * Dry much of next week except a risk of T-storms Fri Overview and model preferences ... Robust northern stream short wave trough and attending surface cold front exit New England Mon. In it`s wake a cooler than normal airmass overspreads the northeast with 850 temp anomalies about -1 SD cooler than normal. However this is very short lived with the subtropical ridge over the southeast states re-emerging and building northward along the eastern seaboard Wed thru Fri. This will result in a return to warmer and more humid weather for southern New England. By the end of the week both ensembles /GEFS and EPS/ and deterministic guidance in good agreement that next northern stream short wave trough will exit the northern plains into the Great Lakes and then thru the St. Lawrence River Valley. This feature will lower heights across New England and open the door for a surface cold front to traverse the region. Good agreement among all guid sources that heights continue to lower across New England into Sat...driving frontal boundary offshore and promoting high pres over the Great Lakes to build into New England. Thus at least avg forecast confidence on a trend toward drier...not as warm and less humid weather next weekend. Details ... Mon ... Cold front moves offshore Mon morning with rapid drying behind the front as noted by falling K indices and PWATs. Also models advertising very strong mid level subsidence across the region. This will yield abundant sunshine. In addition...rapid drying coupled with strong mid level subsidence and deepening offshore low will result in gusty NW winds up to 25 mph or so. This will also yield dew pts falling into the 40s during the afternoon! Quite refreshing with highs in the upper 70s and lower 80s...mid 70s high terrain. Monday night ... Post frontal airmass will be marked by 850 temps lowering to +8c by 12z Tue along with surface dew pts in the 40s. 1026 mb High centered over PA 12z Tue will preclude most locations from decoupling but where winds go light min temps will dip into the mid and upper 40s! Thus a touch of fall-like temps. Elsewhere most locations will see min temps in the 50s. Tuesday ... 1026 mb high advects over the region resulting in a weak pgrad and promoting afternoon seabreezes. Despite weak cyclonic flow aloft column is dry and will support mostly sunny conditions. Cool start to the day combined with limited blyr mixing given 1026 mb high overhead will cap highs mainly 75 to 80. Very comfortable with dew pts remaining in the 40s. Wed/Thu and Fri ... Subtropical ridge builds back up the Eastern Seaboard this period yielding a return to a warmer and more humid regime. Highs climbing back to 85 to 90 Thu and Fri away from the south coast. Dew pts also climbing thru the 60s to possible 70. Northern stream short wave and attending cold front Fri will result in a risk of T-storms. Saturday ... Good agreement among ensembles and deterministic guid that height falls continue over New England...eroding the subtropical ridge and pushing the frontal boundary offshore. This suggest a trend toward drier...not as warm and less humid weather. However it`s day7 so won`t get too deep into the details just yet. && .AVIATION /12Z SUNDAY THROUGH THURSDAY/...
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Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tonight/... 7 AM update ... Not much change from the 06z and 09z TAF issuances. Earlier discussion below. =================================================================== Today...Moderate to high confidence. MVFR to IFR Cigs across the interior early this morning should improve some by mid to late morning. From late morning and into the afternoon, expect VFR to continue along the coastal plain with marginal VFR-MVFR Cigs across the interior. Tonight...Moderate confidence. A band of showers and embedded thunderstorms will sweep across western MA/northern CT between 01z and 5z, then cross eastern MA/RI between 5z and 10z. Localized torrential rainfall may bring brief IFR to LIFR conditions, with MVFR conditions probably dominating the majority of the time. There also is a low risk that a few of the thunderstorms contain locally strong wind gusts, with the highest risk on south coast. KBOS Terminal...Moderate to high confidence in TAF. Main risk for heavy showers and embedded thunderstorms between 5z and 9z. KBDL Terminal...High confidence in TAF. Main risk for heavy showers and embedded thunderstorms between 1z and 5z. Outlook /Monday through Thursday/... High confidence in a stretch of mainly dry VFR weather. Sea breezes most likely mid-late week.
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&& .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tonight/...High confidence. South to southeast wind gusts of around 20 knots are expected today ahead of the approaching cold front. While some choppy seas may occur, expect winds/seas to remain below small craft advisory thresholds this afternoon. The main concern will be a band of showers and embedded thunderstorms will move through overnight with the cold front. A few strong thunderstorms are possible with even the low risk of a waterspout. Outlook /Monday through Thursday/...Moderate confidence. Monday ... Winds will be shifting to the W-NW behind an exiting front early. Gusts may reach close to small craft thresholds (25 kt), along with some building seas (which should remain below small craft levels). Rain/fog early eastern waters, then clearing rapidly mid to late morning. Tuesday through Thursday ... High confidence. Quiet boating weather as slow moving high pressure builds over the waters. By Thu SSW winds will likely become gusty near shore. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Frank/Nocera NEAR TERM...Frank/Nocera SHORT TERM...Frank LONG TERM...Nocera AVIATION...Frank/Nocera MARINE...Frank/Nocera

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