Area Forecast Discussion
Issued by NWS Boston, MA
Issued by NWS Boston, MA
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113 FXUS61 KBOX 151507 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1107 AM EDT Mon Jul 15 2024 .SYNOPSIS... Dangerous heat and humidity is on tap for our region today through Wednesday, with the chance for mainly afternoon and evening thunderstorms each day, and especially later Wednesday into Wednesday night. A cold frontal passage late Wednesday into Thursday will give way to a much more comfortable airmass to end this work week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1105 AM Update: The heat and humidity, as well as the risk for scattered t-storms which could be strong, remain the two weather-related stories for today. On the heat/humidity front...no changes made with standing heat advisories in effect. There`s been some residual midlevel cloud cover around associated with last night`s storms which occurred to our south and west but these are rapidly eroding to mostly clear skies. Temps were already into the upper 70s to mid 80s with dewpoints in the upper 60s to mid 70s. We could have a couple locations in the CT and Merrimack Valleys see heat indices flirt with excessive heat warning-criteria heat index values but felt it wasn`t significantly widespread enough to warrant an upgrade in headlines. We continue to encourage everyone to practice heat-related precautions such as staying hydrated, staying indoors in an air conditioned room and to check in on more vulnerable populations sensitive to heat including the elderly and pets/animals. Regarding the risk for scattered strong t-storms...strong heating should push afternoon CAPE values to sizable numbers, with mixed-layer (e.g. temps/dewpoint in lowest 100-mb) CAPE values around 1500-2000 J/kg. It is a pulse-storm enviroment with shear values around 30 kt or less, so if storms develop, what comes up will come down fairly quickly. Torrential downpours and isolated instances of straight line wind damage from water-loaded downdrafts would be the main risks, probably in that order. The bigger issue is on storm coverage. Models are pretty scattershot on locations but there`s something of a loose majority depicting scattered storms within a few miles either side north or south of the Mass Pike to the eastern coast during the 18-00z timeframe along a weak/subtle convergence boundary (SSW winds south of the Pike, with WSW winds north of it), gradually sagging southward and weakening thereafter. There is also a far more apparent MCS that is taking shape over Lower Ontario, and while models are more robust with storms over NY today, its possible as this feature moves into NY later this afternoon it could give a little more support to storm coverage. In sum, the risk for afternoon storms appears to be increasing and if storms materialize, some storms could become strong with torrential rain capable of street flooding and produce instances of wind damage. With the timing being around the 2-9 PM window and centered in some of our larger urban areas, the risk for street flooding coinciding with the PM commute is a possibility. Previous discussion: * Dangerous Heat & Humidity Today * Isolated strong-severe t-storms possible later today Upper level ridge over the Atlantic coupled with shortwave energy in south Central Canada will bring dangerous heat and humidity to the region today. Westerly flow aloft and at the surface with 850T near +19C/+20C should allow highs to reach the lower to middle 90s away from the immediate south coast. This coupled with dewpoints in the upper 60s to the lower 70s will result in afternoon Heat Indices of 95 to 100+ degrees. We continued the Heat Advisory for most of the region and did add northern Worcester county as well. The other issue will be the potential for afternoon/early evening scattered showers and thunderstorms. Anomalous heat and humidity will result in MLCapes increasing to 1500 to 2500 J/KG. This will combined with a surface trough which should be enough to trigger scattered showers and t-storms. 0 to 6 km shear is rather limited generally only 20 to 25 knots...but low level lapse rates will become quite steep. The HREF shows some decent 2-5 KM Updraft Helicity swaths in the vicinity of the BDL-ORH- BVY area. Seems like the guidance is focused on that initial surface trough. The Nadocast and CSU Machine Learning probs do indicate some severe weather probs too. While we do not think this will be a widespread outbreak because of limited forcing/shear...there will be the risk of isolated strong to severe t-storms. Main threat will be isolated strong to damaging wind gusts. Pwats near 2 inches will support brief torrential rain with any thunderstorms. && .SHORT TERM /6 PM THIS EVENING THROUGH TUESDAY NIGHT/... Key Points... * Dangerous Heat & Humidity Continues Tue * A few strong to severe t-storms possible later Tue Details... Tonight... Once the diurnal convection diminishes early this evening...we will need to watch a more organized area of showers & t-storms tracking across NY State. This activity will be weakening as it moves east out running the better forcing and given the loss of diurnal heating with poor mid level lapse rates. It is possible the activity survives in a weakened form late this evening across western MA/CT...but not sure it will even make it. Otherwise...dry and muggy tonight with overnight lows only dropping into the lower 70s. Low clouds and patchy fog should mainly be confined to the south coast, Cape and Islands. Tuesday... The dangerous heat and humidity continues on Tue. 850T are a tad warmer than today and will be on the order of +20C/+21C. Given southwest flow thinking highs will mainly be in the middle 90s away from the immediate south coast, Cape and Islands. We may even see high temps reach the upper 90s in the lower CT River and Merrimack Valleys. Heat Indices in the 97 to 103 degree range are on tap for the vast majority of the region and Heat Advisories remain posted. It is even possible a few spots reach heat indices up to 105 degrees. The other issue will be again the threat for scattered afternoon and evening showers & t-storms. We once again will have Capes on the order of 1500 to 2500 J/KG as a shortwave moves through NY State. This may end up being a later show with the greatest risk for convection during the later afternoon and evening hours especially across the interior. Steep low level lapse rates along with approaching shortwave will increase effective shear to between 25 and 35 knots. A few strong to severe thunderstorms will be possible with the best chance across the interior where the activity arrives earlier. The CSU Machine Learning guidance indicates severe weather probs with the main risk being locally strong to damaging wind gusts. Torrential rainfall and a very localized flooding risk will also exist given Pwats near 2 inches. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... Key Points... * One more day of dangerous Heat & Humidity Wed * Scattered showers & t-storms late Wed/Wed night * Relief from heat & humidity Thu/Fri/Sat with seasonable temps Details... Wednesday and Wednesday night... One more day of dangerous heat and humidity is on tap for Wednesday ahead of an approaching cold front. High temps should reach the lower to middle 90s away from the immediate south coast with Heat Indices between 97 and 103 degrees. The Heat Advisories continue for the vast majority of the region. We do expect scattered showers and thunderstorms to develop later Wed/Wed night as shortwave/cold front approaches. Depending on the timing...there will be the risk for scattered strong to severe thunderstorms as forcing & wind fields increase with the approaching cold front. The CSU Machine learning probs also indicate modest probs for the potential of severe weather. Some of this activity may persist well into the night given increasing effective shear. Torrential rainfall with any of the thunderstorms will again result in a localized flood threat. Thursday/Friday/Saturday... Some showers may linger into the first part of Thu depending on the timing of the cold front. Otherwise...high pressure building in from the midwest behind the cold front will result in significant relief from the heat and humidity. Highs will be well into the 80s with overnight low temps mainly in the upper 50s to the lower 60s...which we have not seen in quite a while. Humidity levels will also return to comfortable levels. Sunday... A weak cold front dropping south may trigger a few showers...but the vast majority of the day looks dry at this point. Highs look to be well into the 80s. && .AVIATION /14Z MONDAY THROUGH FRIDAY/... Forecast Confidence Levels: Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 15z TAF Update... Today: High confidence. Mainly VFR conditions today...but expect to see scattered showers & t-storms to develop this afternoon into this evening. Activity will be hit or miss...but any thunderstorm may contain locally strong wind gusts and brief torrential rainfall. Winds will increase from the SW at 10 to 15 knots by afternoon with some 20-25 knot gusts near the south coast. Tonight...Moderate Confidence. Any diurnally driven activity should diminish during the first half of the evening. We will have to watch for an MCS moving through NY State by late evening...but it should be weakening and not sure if it will be able to survive into western MA/CT. Otherwise...VFR conditions expected except for low clouds and fog that will impact areas near the south coast, Cape and Islands. SW winds 5-10 knots. Tuesday...Moderate Confidence. Mainly VFR conditions once any low clouds/fog patches burn off across areas near the immediate south coast. We will have to watch for another potential round of scattered showers & thunderstorms during the afternoon and evening. SW winds of 10 to 15 knots with some gusts of 20-25 knots by afternoon. KBOS TAF...Moderate Confidence in TAF. Main uncertainty is do we see a few thunderstorms in the vicinity of the terminal this afternoon and early evening. Thinking greatest risk is between 18z and 24z. KBDL TAF...Moderate Confidence in TAF. Main uncertainty is do we see a few thunderstorms in the vicinity of the terminal this afternoon and early evening. Thinking greatest risk is between 18z and 24z. Outlook /Wednesday through Friday/... Wednesday: Mainly VFR, with areas MVFR possible. Breezy. Chance SHRA, chance TSRA. Wednesday Night: Mainly MVFR, with areas VFR possible. Breezy. Chance SHRA, slight chance TSRA. Thursday: Mainly VFR, with areas MVFR possible. Chance SHRA. Thursday Night through Friday: VFR. && .MARINE... Forecaster Confidence Levels: Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent. Today through Tuesday...High Confidence. A ridge of high pressure off the Atlantic coast will result in persistent SW winds across the waters through Tue. Gusty nearshore SW winds up to 25 knots are expected in the vicinity of Buzzards and Cape Cod Bay as well as Nantucket sounds this afternoon. Therefore...we hoisted small craft headlines for these waters this afternoon. We should see the same situation develop Tue afternoon. In fact...the potential for 25 knot nearshore SW wind gusts may cover a larger area. An additional small craft advisory will likely be needed. The other issue will be overnight and early morning fog with the focus across the southern waters. Outlook /Wednesday through Friday/... Wednesday: Low risk for Small Craft Advisory winds with gusts up to 20 kt. Seas up to 5 ft. Chance of rain showers, chance of thunderstorms. Wednesday Night: Winds less than 25 kt. Local rough seas. Chance of rain showers, chance of thunderstorms. Thursday: Winds less than 25 kt. Local rough seas. Chance of rain showers, isolated thunderstorms. Thursday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers. Friday: Winds less than 25 kt. Seas locally approaching 5 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Heat Advisory until 8 PM EDT Wednesday for CTZ002>004. MA...Heat Advisory until 8 PM EDT Wednesday for MAZ013-016>019. Heat Advisory until 8 PM EDT Tuesday for MAZ003>007-010>012- 014-015-020-021-026. RI...Heat Advisory until 8 PM EDT Wednesday for RIZ001>005. Heat Advisory until 8 PM EDT Tuesday for RIZ006-007. MARINE...Small Craft Advisory until 7 PM EDT this evening for ANZ231>234-254. && $$ SYNOPSIS...Frank NEAR TERM...Frank/Loconto SHORT TERM...Frank LONG TERM...Frank AVIATION...Frank/Loconto MARINE...Frank/Loconto