Area Forecast Discussion
Issued by NWS Boston, MA
Issued by NWS Boston, MA
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157 FXUS61 KBOX 212350 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 750 PM EDT Fri Jun 21 2024 .SYNOPSIS... Showers and thunderstorms into early this evening may become severe and also result in a localized flash flooding across interior Massachusetts, Connecticut and western Rhode Island. This activity should diminish not too long after sunset. However...warm and muggy this weekend with more rounds of showers and thunderstorms. Some of those may again become severe and result in a localized flash flood threat. A cold front bring a short window of reprieve early next week, though increasing heat and shower chances by midweek. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... 740PM Update... The last of the thunderstorms are decaying into light to moderate showers as instability continues to wane with the loss of diurnal heating. Expecting the last of the showers to exit offshore by 02-04z with drying conditions overnight. With weak onshore flow, will likely see low clouds and fog form across eastern MA and RI tonight. 420 PM Update... * Severe T-Storm Watch through for interior MA/CT/W RI through 8 PM * Localized flash flood threat into early this evening * Activity diminishes after sunset with areas of fog possible late Showers & t-storms continued to develop across interior MA/CT and RI late this afternoon. This was along and west of a backdoor cold front currently draped across central MA and into RI. SPC Meso- Analysis indicates MLCapes on the order of 1500 to 2000 J/KG across interior southern New England with effective shear of 20 to 30 knots. While instability is not quite as high as yesterday and mid level lapse rates are poor...the effective shear is a bit better and we have a backdoor cold front/sea breeze fronts as well. Many of the high resolution models take an area of more organized convection in eastern NY/Berks and move it across our region into early this evening. This seems reasonable based on shortwave energy working eastward and may interact with the low level boundaries across our region. And given the better organization of this activity...a severe t-storm watch has been issued through 8 pm this evening for interior MA/CT and western RI. Main threat would be locally damaging wind gusts with a secondary concern for hail. Further east...while we can not rule out a strong storm or two working over the shallow backdoor cold front...the overall severe weather threat is low across eastern MA. In addition...these backdoor cold fronts always need to be watched for a localized flash threat. Pwats of 2+ inches will support torrential rainfall with any storms and there is the potential for some training along the backdoor front. The CSU Machine learning probs do show the potential for excessive rainfall across interior MA/CT and western RI. In fact...the HREF indicates some low probs for the 6 hour QPF to exceed a 100 year rainfall event. That certainly does not mean that will happen...but is at least a signal for potential localized flash flooding. This is particularly true if the activity would train over a particularly vulnerable urban center. Given the mid level lapse rates are rather poor...expect the bulk of the shower and thunderstorm activity to wind down by 9-10 pm. Otherwise...main concern overnight will be for areas of low clouds and fog developing given cooling boundary layer and remnant low level moisture with light/calm winds. Overnight low temps mainly in the 60s to around 70. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... Key Points... * Showers & T-Storms are expected to develop again Sat PM * Severe Weather & Localized Flash Flood Risk Again * Heat Advisories Continued for CT/SW MA Details... Saturday and Saturday night... The backdoor front that washes out tonight...seems to want to setup across northeast MA on Saturday. Therefore...a quite warm and humid day is on tap with highs 85 to 90 in most locations with the hottest readings around 90 in the Lower CT River Valley. Given Heat Indices are expected to be in the mid to upper 90s...we have continued the Heat Advisory across southwest MA/northern CT through the weekend. It will be cooler though in northeast MA especially by Cape Ann where highs will likely remain below 80. The main concern Saturday afternoon and early evening will be for another round of showers & t-storms. A shortwave working across the region in west northwest flow will combined with diurnal heating and a remnant boundary. We expect Capes on the order of 1500 to 2500 J/KG to develop and not much of a mid-level Cap. This should trigger another round of showers & t-storms. While mid level lapse rates are poor...effective shear will be stronger than the past few days on the order of 30-40 knots. This should be enough for the potential of scattered severe thunderstorms...which is supported by much of the Machine Learning guidance as well as the HREF Updraft Helicity Swaths. Also...Pwats remain on the order of 2+ inches. Therefore...these storms will be capable of producing torrential rainfall and localized flash flooding. The CSU Machine Learning probs as well as the HREF indicated low probs of 3"+ of rain inside 3 hours highlight this concern. It is tough to pinpoint the area of greatest risk given the mesoscale nature of these potential events. That being said...the guidance is tending to indicate a higher risk in areas from northern CT into western/central northeast MA closer to the low level convergent zone. A few storms may linger a bit longer Sat night...but will have to wait and see how things unfold. Otherwise...more low clouds and fog are expected to develop Sat night with the cooling boundary layer. Overnight lows should drop into the 60s to near 70. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Highlights: * Hot, humid, and potential for strong to severe storms Sunday afternoon. * Cold front moves across the region Monday, bringing brief relief from hot and humid conditions. But signal for a return of the summer heat and humidity by mid next week. Sunday: Hot and humid conditions fuel storms during the afternoon, which could become locally strong to severe. A broad mid-level trough moves across the northeast with 850mb shortwave energy moving across the region with an associated warm front. WAA at 850mb continues to indicate temperatures 20C to 22C, thus expecting highs in the middle and upper 80s, 90F for Connecticut River Valley. With dew points expected to be in the low 70s the `Heat Index` to reach the middle and upper 90s at northern Connecticut and the southern Pioneer Valley of Massachusetts. In collaboration with neighboring WFOs we`ve extended the `Heat Advisory` through Sunday, for those areas mentioned. All this fuels the potential for convection during the afternoon as there`s sufficient CAPE, modest effective shear between 30 and 40 knots, and helicity greater than 150 m2/s2. While the greatest threat appears to be north, a few stronger to severe thunderstorms remain possible, even a low risk of a brief tornado. CSU machine learning show a chunk of southern New England between 15% and 30% probabilities for wind, and widespread 2% prob for a tornado across most of New England, with a 5% to 10% across southern Vermont to southern New Hampshire and along the northern Massachusetts border. Don`t get hung up on the placement, more of less gives us greater confidence in severe weather occurring. Lastly, in addition to any severe weather there is the threat for heavy rainfall as well given PWATs are in the ballpark of two inches. Next Week: Mid-level trough moves east with a cold front on Monday with another round of showers and storms, could be lingering diurnal showers on Tuesday with a mid-level low over the Gulf of Maine, though guidance is still split. Maintained `slight chance` POPs across northern MA. Beyond, midweek it looks unsettled, forecast confidence is low due to model uncertainty with the timing of any showers and/or storms. Have greater confidence with temperatures, a warmer than normal week is likely as ensemble situational awareness tables indicate surface temperatures are between the 90th and 99th percentile. Highs returning to the middle and upper 80s with near 90F in northern Connecticut and southwest Massachusetts. And nightly lows in the 60s. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/... Forecast Confidence Levels: Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 00z TAF Update... Tonight...Moderate to Low Confidence. Showers and lingering thunderstorms continue to wane and move east this evening. Much of the activity will be offshore by 02z-04z. Light onshore flow will bring IFR ceilings and possibly LIFR fog for much of the eastern terminals. Further west, MVFR with periods of IFR possible. Saturday & Saturday night...Moderate to Low Confidence. IFR-LIFR conditions should improve to VFR across the interior and mainly MVFR near the coastal plain by early afternoon. Otherwise...the main concern will revolve around another round of showers & t-storms impacting the region Sat afternoon and early evening. A few of these will be capable of producing locally strong wind gusts and torrential rainfall. The shower & t-storm activity should dissipate again not too long after sunset Saturday night. Low end MVFR-IFR conditions should re-develop with the cooling boundary layer with some LIFR thresholds being met as well. KBOS TAF... Moderate to low Confidence in TAF. Showers with embedded thunder will continue through about 02z. After 02z, expecting CIGS to slowly drop to IFR/LIFR overnight. Mainly IFR/MVFR tomorrow, but there may be pockets of clearing and VFR in the afternoon. Low chance for a shower or thunderstorm in the afternoon between 18z and 00z, but confidence is low at this time. KBDL TAF...Moderate Confidence in TAF. MVFR/IFR tonight improving back to MVFR/VFR tomorrow. Chance for thunderstorms again tomorrow afternoon anytime between 18z-00z, but higher confidence between 20z-22z. Outlook /Sunday through Wednesday/... Sunday: Mainly MVFR, with local IFR possible. Breezy. Chance SHRA, chance TSRA. Sunday Night: Mainly MVFR, with areas VFR possible. Breezy. SHRA likely, slight chance TSRA. Monday: Mainly MVFR, with areas VFR possible. Breezy. Chance SHRA, slight chance TSRA. Monday Night: Mainly VFR, with areas MVFR possible. Chance SHRA, isolated TSRA. Tuesday through Tuesday Night: VFR. Wednesday: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA. && .MARINE... Forecaster Confidence Levels: Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent. Tonight through Saturday night...High Confidence. The pressure gradient remains relatively weak as the backdoor cold front of today...gradually washes out and lifts northeastward through Sat night. Long continues southwest fetch may eventually build seas to 4-5 feet toward daybreak Sun across our southern most outer-waters...but otherwise winds/seas will remain below small craft advisory thresholds. Otherwise...the main concern will be areas of overnight and early morning fog. In addition...some afternoon and evening t-storms may impact some of our mainly nearshore waters at times. Outlook /Sunday through Wednesday/... Sunday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Chance of rain showers, isolated thunderstorms. Sunday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Rain showers likely, slight chance of thunderstorms. Monday: Winds less than 25 kt. Areas of rough seas. Chance of rain showers, slight chance of thunderstorms. Monday Night: Winds less than 25 kt. Local rough seas. Chance of rain showers, isolated thunderstorms. Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Tuesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Wednesday: Winds less than 25 kt. Areas of seas approaching 5 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Heat Advisory until 7 PM EDT Sunday for CTZ002>004. MA...Heat Advisory until 7 PM EDT Sunday for MAZ010-011. RI...None. MARINE...None. && $$ SYNOPSIS...Frank/Dooley NEAR TERM...Frank/KP SHORT TERM...Frank LONG TERM...Dooley AVIATION...Frank/Dooley/KP MARINE...Frank/Dooley