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Area Forecast Discussion
Issued by NWS Boston, MA
Issued by NWS Boston, MA
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567 FXUS61 KBOX 131601 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1201 PM EDT Sat Jul 13 2024 .SYNOPSIS... Showers, and some downpours, into early afternoon. Hot on Sunday, but slightly drier with sunny skies. Dangerous heat and humidity is expected Monday, Tuesday, and Wednesday with afternoon heat indices between 95 and 105 degrees. The greatest risk for scattered showers and thunderstorms will be Monday and Wednesday, but much of this time will still feature dry weather. Relief from the heat and humidity arrives by Thursday and Friday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Canceled part of the Flood Watch, as the heaviest rainfall has moved father southeast. Will continue to monitor the remaining Flood Watch. Current thinking is this can likely be canceled prior to its 8 PM expiration time. Will see how things evolve over the next couple of hours. Previous Discussion... Still a risk for embedded thunderstorms this morning, but losing confidence that we will see them. Have not seen active lightning during the past couple of hours. Latest SPC mesoanalysis showed a similar picture. Plenty of buoyancy, but poor mid level lapse rates. Shear was offset to the northwest, so not expecting severe weather. Updated rainfall chances using the latest runs of the HRRR, which seemed to have the best handle on the existing rainfall. Will reconsider the Flood Watch later today, as will likely be able to drop portions prior to the existing expiration of 8 PM. Minor tweaks to temperatures to reflect observed trends. 645 AM Update... * Showers with a few t-storms and localized torrential rainfall into mid afternoon with the greatest risk south of I-90 A cluster of showers and embedded t-storm quickly moved across areas near the south coast earlier this morning and were already exiting the Cape. However...an additional band of showers has set up in northeast axis in the vicinity of a Springfield to Fitchburg line. So far rainfall rates have been under control...but given Pwats of 2+ inches in place and dewpoints between 70 and 75 any of this activity will be capable of producing torrential rainfall and very localized flooding...especially if it occurs in a vulnerable urban center. The mesoscale processes involved though make it very difficult to locate where this may occur...but latest guidance continues to indicate areas south of I-90 most at risk. Thinking is that the flood threat will be localized...but worth continuing the Flood Watch given the tropical environment in place. Embedded thunder is also possible with any of the activity especially across RI/SE MA. Previous Discussion below. Short-wave energy approaching from the west and convergence along a stalled frontal boundary is expected to support bands of showers with heavy downpours and embedded thunder this morning. These showers will have the potential to produce excessive rainfall as the atmosphere will hold tropical like characteristics with 2+ inch PWATs, deep warm cloud layer depths exceeding 11000 feet, and moderate instability with SBCAPE values near 1000 J/kg. Given the weakly forced environment, there remains quite a bit of uncertainty with respect to where the heaviest rainfall will setup. Latest hi-res model guidance has trended toward southeastern MA, RI, The Cape and Islands being at the greatest risk for excessive rainfall. Nonetheless, several solutions still support bands of heavy precip across western MA and CT. Given the uncertainty, no changes have been made to the flood watch headlines currently in place. As previously forecast, most of the shower/thunderstorm activity will take place before noon time today. Thereafter, we should see some clearing from northwest to southeast as the 2+ inch PWAT plume regresses to the southeast. After noon time however,the equatorward entrance region of a 100 knot 250 hPa jet is expected to develop over the northern half of the forecast area. Some hi-res model guidance has been hinting at the potential for this feature to provide additional forcing to support redevelopment of a few thunderstorms across interior southern New England during the mid- afternoon/evening hours. Confidence in this outcome is low, but additional thunderstorms after this morning`s "main event" will be possible before sunset. Aside from the shower/thunderstorm chances, expect continued muggy conditions with highs in the mid to upper 80s. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... Tonight A surface frontal boundary moves through the region overnight with northwest flow and a drier airmass. PWATs fall to values close to an inch. This should result in a quiet evening with clearing skies from northwest to southeast. There will be some relief from the humidity as dewpoints fall into the mid to upper 60s across the region, but the urban areas will still struggle to fall below 70. Tomorrow A subtle mid-level ridge builds over southern New England tomorrow with weak high pressure at the surface. This will support a sunny/hot afternoon. Not expecting humidity to be as oppressive as with previous days as light west/northwest flow will allow dewpoints to drop into the low to mid 60s across much of interior southern New England. Humidity may linger over the southeastern areas with dewpoints still in the low 70s. Expect highs in the upper 80s to low 90s with some mid 90s possible in the CT River Valley. Coastal areas may be slightly cooler as the gradient wind may be weak enough to support sea-breezes along the coast. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Key Points... * Dangerous Heat & Humidity Mon/Tue/Wed...Heat Indices 95-105 degrees * Greatest risk for scattered showers & t-storms Mon and especially Wed * Relief from the Heat & Humidity by Thu and Fri Details... Mon/Tue/Wed... Excellent model agreement in strong upper level ridge in the Atlantic and shortwave energy in Canada dropping towards the Great Lakes. This will result in westerly flow aloft with above normal height fields and high confidence in dangerous heat and humidity. High temps Mon/Tue/Wed should be well into the 90s in most locations away from any potential localized marine influences near the immediate coast. Dewpoints in the lower to middle 70s will result in very humid conditions...bringing afternoon Heat Indices into the 95 to 105 degree range each day. Heat Headlines will certainly be needed. In fact...the worst of the heat appears to be Tue & Wed when we may even need some Excessive Heat Headlines for the potential of Heat Indices of 105+. Regardless of the specific temps...the main story will be dangerous heat and humidity in the Mon through Wed time frame. The other issue will be assessing the potential for scattered showers and thunderstorms. Westerly flow aloft with the mesoscale part of it this time of year makes that quite a challenge. For what its worth...most of the guidance indicates Mon and especially Wed with the greatest risk for a round of scattered showers and thunderstorms...but can not rule out a few on Tue either. That being said...this is typical summertime convection so much of this time will still feature dry weather. Given the anomalous hot and humid airmass in place...instability will likely be high for this part of the country. In addition...appears we may have some sort of remnant EML to work with as well. So if wind fields aloft end up strong enough there will be the potential for severe weather. The CSU Machine Learning probs also continue to indicate this potential in the Mon through Wed time frame...so will need to continue to monitor this potential. Thu/Fri... We may be dealing with some lingering showers for the first part of Thu...depending on the timing of the front. However...the main story will be High pressure building in from the midwest behind the cold front. The result will be relief from the Heat & Humidity by Thu and Fri with seasonable or even slightly below normal temperatures. Quite the welcomed change from the first half of the upcoming week! && .AVIATION /16Z SATURDAY THROUGH WEDNESDAY/... Forecast Confidence Levels: Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Today...Moderate Confidence Areas of showers, with some downpours, for most of southern New England through about 18z. There is uncertainty on the overall coverage of TSRA, but the greatest risk is south of the MA Turnpike. Conditions should begin to improve after 18Z with VFR conditions settling in, but there is a risk for some redevelopment of scattered showers and a few storms mainly across the interior in northern and western MA between about 18z and 21Z. Confidence not high enough to include mentions of thunder during this time period, but did leave VCSH in the TAF update through 00Z. As for ceilings, they are rather variable this morning, but expect a lot of MVFR conditions. Those should tend to improve to VFR this afternoon, except for areas near the south coast, where MVFR-IFR conditions will probably persist. Tonight...Moderate Confidence Any remaining scattered showers should diminish during the early evening. Otherwise...MVFR-IFR ceilings will pretty much be confined to areas near the south coast. However...light-calm winds/high dewpoints may allow for patchy ground fog to develop especially in the typically prone locations. Tomorrow...High Confidence VFR. Light northwest winds becoming more westerly during the afternoon. Winds may be weak enough to support sea-breezes along the coast. KBOS TAF...Moderate confidence in TAF. KBDL TAF...Moderate confidence in TAF. Outlook /Sunday Night through Wednesday/... Sunday Night: VFR. Monday: Mainly VFR, with areas MVFR possible. Breezy. Chance SHRA, chance TSRA. Monday Night: Mainly VFR, with areas MVFR possible. Slight chance SHRA, slight chance TSRA. Tuesday: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA, slight chance TSRA. Tuesday Night: Mainly VFR, with areas MVFR possible. Breezy. Slight chance SHRA, slight chance TSRA. Wednesday: 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 Today Showers with embedded thunder move over the coastal waters this morning. This will be followed by some clearing during the afternoon. Winds remain steady out of the southwest. Seas generally 3 to 5 feet over the southern marine zones and 1 to 3 feet over the eastern marine zones. Tonight and Tomorrow Weak high pressure builds over the coastal waters tonight into tomorrow. This will support diminishing winds and subsiding seas with relatively calm conditions over the marine zones. Outlook /Sunday Night through Wednesday/... Sunday Night: Winds less than 25 kt. Monday: Winds less than 25 kt. Slight chance of rain showers, slight chance of thunderstorms. Monday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers, slight chance of thunderstorms. Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft. Tuesday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers. Wednesday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers, slight chance of thunderstorms. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Flood Watch until 8 PM EDT this evening for MAZ013-015>021. RI...Flood Watch until 8 PM EDT this evening for RIZ001>007. MARINE...Small Craft Advisory until 9 PM EDT this evening for ANZ255- 256. && $$ SYNOPSIS...Frank/RM NEAR TERM...Belk/Frank/RM SHORT TERM...RM LONG TERM...Frank AVIATION...Frank/RM MARINE...Frank/RM