Area Forecast Discussion
Issued by NWS Boston, MA
Issued by NWS Boston, MA
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799 FXUS61 KBOX 121720 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 120 PM EDT Fri Jul 12 2024 .SYNOPSIS... A few isolated showers/thunderstorms today will be followed by more widespread showers and thunderstorms tonight into Saturday. Hot and humid weather returns for Sunday into the middle of next week. Afternoon Heat Indices in the 95-100+ degree range are likely...which will eventually require Heat Headlines. While Sunday is dry, the risk for scattered showers and thunderstorms will exist in the Monday through Wednesday time frame...but plenty of dry weather expected over this time period too. Appears the heat and humidity finally will break by Thursday and Friday.&& .NEAR TERM /UNTIL 6 PM THIS EVENING/... 10AM Update The stalled front is slowly retrograding back inland, bringing the showers further north. No lightning with these showers yet, however, as diurnal heating increases instability to around 1000 J/kg this afternoon, there could be a few thunderstorms later today. With less then 25 knots sfc-6km shear, and mid level lapse rates less then 6C/km, not expecting any severe weather with thunderstorms today. 345AM Update Today Offshore stalled frontal boundary moves back onshore this morning/afternoon with a plume of 2+ inch PWATs. A modest low-level jet may trigger a few isolated showers/thunderstorms this afternoon, but with little no shear in the atmosphere any thunderstorms would be short-lived and sub-severe. Otherwise most areas should experience another hot/humid day with a mix of sun and clouds. Once again cloud cover will be more persistent over the south coast, Cape, and Islands. High temps generally in the upper 80s across the region. With dewpoints rising back into the mid 70s this will translate to heat indices in the low 90s. Steady southwest winds around 10 mph. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... Tonight and Tomorrow Short-wave energy approaching from The Great Lakes region will introduce a heavy rainfall threat to southern New England tonight into tomorrow. The atmosphere will be ripe for heavy rainfall with PWATs up to 2.2 inches which is nearly 3 standard deviations above normal. Warm cloud layer depths will be greater than 10000 feet which is usually a good indicator for more efficient rainfall processes. Even during the overnight hours with a lack of heating, there will be a sufficient amount of instability to work with with MLCAPE values exceeding 200 to 300 J/kg during the overnight hours, so rumbles of thunder will be likely as well. As far as timing for heavy rainfall goes, the latest suite of hi-res model guidance suggests heavy downpours may begin to trickle into western MA and CT a few hours before sunrise Saturday morning. Expect showers and thunderstorms to continue east into eastern MA and RI during the mid to late morning hours. Signals for excessive rainfall are rather robust from the 00Z HREF with some 10 to 20 percent probabilities of 6 hour rainfall exceeding 100 year average return intervals (ARIs). Furthermore, the HREF localized probability matched mean QPF product is highlighting the potential for as much as 5 inches of rain over localized areas. There is considerable uncertainty with respect to which locations will experience the heaviest rainfall, but generally all of southern New England will be at risk, especially the urban areas. Given the strong signals for heavy rainfall, we`ve raised a flood watch for all of southern New England minus The Cape/Islands from 2AM Saturday to 8PM Saturday. Activity should begin to wane by mid-afternoon on Saturday and eventually clear our by Saturday evening. The atmosphere will remain warm and muggy during this time, but with extensive cloud cover for much of the day on Saturday, high temperatures should be confined to the low to mid 80s. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Key Points... * Hot & Humid Sun through Wed with Heat Indices 95-100+ * A round or two of scattered showers/t-storms sometime Mon/Tue/Wed * Relief from the heat & humidity likely Thu & Fri Details... Upper level ridging in the Atlantic will continue to result in above normal height fields across southern New England for the first half of next week. In addition...shortwave energy dropping into south central Canada will generate modest westerly flow aloft. The result will be above normal high temperatures in the upper 80s to the middle 90s in the Sun through Wed time frame...at least away from any localized marine influences near portions of the immediate coast. This coupled with humidity should result in afternoon Heat Indices of 95-100+ degrees...so Heat Advisories will be needed for much of this time. Surface ridging should generally result in mainly dry weather on Sunday. However...there will be the potential for a round or two of scattered showers and thunderstorms in the Mon through Wed time frame. Fast zonal flow makes this tough to time...but appears the greatest risks for more widespread convection will be Mon afternoon/evening and Wed afternoon/evening but timing certainly subject to change. This will be typical summertime activity...where much of this period features dry weather...but the risk for some showers and t-storms especially during the afternoon and evening hours will be possible. Appears that we may also have some sort of remnant EML around too...so there will be a threat for severe weather depending on timing and if winds are strong enough aloft. The CSU Machine Learning probs do indicate some modest severe weather probs as well for this time range. So certainly something will have to monitor in the coming days. Guidance is in fairly good agreement that amplifying shortwave energy in eastern Canada will result in lowering height fields along with relief from the heat and humidity by Thu and Fri. && .AVIATION /17Z FRIDAY THROUGH TUESDAY/... Forecast Confidence Levels: Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Rest of Today...Moderate Confidence VFR with periods MVFR/IFR as showers move north. SSW at 5-10 knots may gust up to 25 knots this afternoon. Friday Night...Moderate Confidence VFR in the evening, becoming MVFR/IFR overnight. Rain begins to move in from the south around daybreak. Saturday...Moderate Confidence Moderate to heavy rain and thunderstorms moves south to north, but the extent north remains uncertain. Best chance for thunderstorms looks to be between 11z-15z tomorrow morning, but could linger into the early afternoon. Another round of thunderstorms is possible in the late afternoon mainly across the western interior. Starting the day IFR/LIFR esspically during periods of heavy rain. As rain moves off shore to the SE, northern terminals will become VFR, while terminals near the south coast with remain MVFR/IFR for the afternoon. Saturday night...High Confidence VFR expect over the Cape and Islands, where pockets of MVFR/IFR will still remain. KBOS TAF...Moderate confidence in TAF. VFR this afternoon with light showers likely. IFR/LIFR early Saturday morning with periods of heavy rain and thunderstorms possible. VFR Saturday afternoon. KBDL TAF...Moderate confidence in TAF. VFR with light showers and possibly a rumble of thunder this afternoon. MVFR/IFR by Saturday morning with periods of moderate to heavy rain and thunderstorms possible. VFR Saturday afternoon Outlook /Saturday Night through Tuesday/... Saturday Night: VFR. Slight chance SHRA. Sunday through Sunday Night: VFR. Monday: Mainly VFR, with areas MVFR possible. Breezy. Chance SHRA, chance TSRA. Monday Night: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA, slight chance TSRA. Tuesday: Mainly VFR, with areas MVFR possible. Breezy. Chance SHRA, chance TSRA. && .MARINE... Forecaster Confidence Levels: Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent. Marine Tonight though Tomorrow Night Winds remain gusty out of the SSW tonight up to 25 knots through tomorrow afternoon. Waves begin decreasing through tomorrow night back to 4-5 feet for the outer waters, and 2-3 feet for the near coastal waters. With decreasing waves, will allow the high surf advisory to expire tonight and will not reissue for tomorrow. Outlook /Saturday Night through Tuesday/... Saturday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers. Local visibility 1 to 3 nm. Sunday through Sunday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Monday: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers, slight chance of thunderstorms. Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers. Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers, slight chance of thunderstorms. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Flood Watch from late tonight through Saturday evening for CTZ002>004. MA...Flood Watch from late tonight through Saturday evening for MAZ002>021-026. RI...Flood Watch from late tonight through Saturday evening for RIZ001>007. MARINE...Small Craft Advisory until 9 PM EDT this evening for ANZ254- 255. Small Craft Advisory until 8 AM EDT Saturday for ANZ256. && $$ SYNOPSIS...Frank/RM NEAR TERM...RM/KP SHORT TERM...RM LONG TERM...Frank/RM AVIATION...KP MARINE...Frank/RM