Area Forecast Discussion
Issued by NWS Boston, MA
Issued by NWS Boston, MA
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749 FXUS61 KBOX 161334 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 934 AM EDT Tue Jul 16 2024 .SYNOPSIS... Dangerous heat and humidity will continue through Wednesday with little nighttime relief expected. Scattered showers and thunderstorms will be possible both Tuesday and Wednesday. A cold frontal passage late Wednesday into Thursday will give way to a much more comfortable airmass to end this work week. && .NEAR TERM /THROUGH TONIGHT/... 930 AM update... MCV moving through the Gt Lakes will pass to the north later this afternoon and evening. This will trigger storms across New York state this afternoon which will move into western New Eng toward evening. Timing wise it looks like 6-10 pm is the main window for a few storms moving across SNE, especially interior MA. Instability and shear are marginal this evening and so storms should be weakening as they move into SNE past peak heating, but can`t rule out a few severe storms. No change to heat headlines. 850 mb temps 20-21C support highs in the 90s away from immediate south coast, with mid-upper 90s across the CT valley and eastern MA. Dewpoints during peak heating should mix out into the upper 60s, with lower 70s across RI and SE MA. Max heat indices will be 98-103. Previous discussion... Heat and humidity continues today much like we saw yesterday but to a greater magnitude as the airmass warms slightly. 850 mb temps rise from around 19C to 20-21C and thus we`ll see high temperatures reaching a few degrees warmer today, into the upper 90s for many. This, with dewpoints in the 70s will lead to heat indices up to 102F which is why a Heat Advisory is in effect. The other weather story of the day is, also similar to Monday, a round of showers and thunderstorms that will be moving into southern New England this evening, weakening as they slide east. The driving force behind these storms is a shortwave rotating around the broader cyclonic flow over the Great Lakes and into New England. Ahead of the shortwave today`s heat and humidity will lead to decent instability (1000-1500 J/kg of CAPE), but with only marginal shear available and the late time of arrival of the line (6-7pm in western MA/CT most likely) the favorable window for any severe weather will be small. Timing of any storms would be in the evening (after 6 pm) with diminishing chances after sunset. The SPC has placed a Slight Risk of severe weather (level 2 of 5) over western MA/CT which is where we`ll have the best chance of seeing damaging winds and hail. Overnight fog and stratus clouds will return to the south coast and low temperatures will be very warm, in the mid 70s. && .SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/... Wednesday expect more of the same, but take solace in the fact that this will be our last day of the seemingly endless humidity we`ve been dealing with of late. While the Heat Advisory continues through Wednesday, a cold front will be knocking on our door heralding relief from the oppressive heat. Unfortunately that, along with a negatively tilted upper trough, will serve as a forcing mechanism for one final round of strong to severe storms. The SPC this morning upgraded western/central MA and CT to a Slight Risk (level 2 of 5) where an earlier arrival (compared to Tue) of the best forcing may bring a better chance of strong or severe storms. Confidence in timing of the arrival of the storms is only moderate at this point, but guidance would indicate storms arriving several hours earlier than Tuesday, as early as mid afternoon. The environment is more favorable on Wednesday as well thanks to increased instability (2000J/kg+ of CAPE) and 0-6km bulk shear (30-40 kts) which should allow for some sustained updrafts. Given this favorable shear storms may be more organized and any could produce damaging gusts along with heavy downpours and localized urban and poor drainage flooding. After sunset storms should weaken and eventually dissipate during the overnight hours. Expect one more night of lows in the 70s. && .LONG TERM /THURSDAY THROUGH MONDAY/... Key Points... * Dry and seasonal weather Thursday through this weekend After an extended period of hot, muggy, and unsettled weather, a major pattern change is store for SNE starting Thursday afternoon. A cold front will traverse the region late Wednesday into early Thursday. Showers and thunderstorms could linger into Thursday morning, otherwise drying out Thursday afternoon with PWATs dropping to 0.70 inches. Dewpoints will also be falling Thursday afternoon into the low 60s to even upper 50s and staying there through the weekend, even into early next week. Temperatures will also be much more comfortable Thursday and Friday with highs in the low to mid 80s, warming into the upper 80s for the weekend. Overnight lows will also stay more comfortable in the low to mid 60s through the weekend. As for precipitation, high pressure and rather dry airmass will drop down from Canada helping suppress rain and thunderstorm chances through the weekend. Next chance for precip looks to be early to mid next week as a weak shortwave exits the Great Lakes. && .AVIATION /13Z TUESDAY THROUGH SATURDAY/... Forecast Confidence Levels: Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 12Z TAF update... Today and tonight...Moderate confidence. Early this morning conditions are VFR for most terminals though IFR once again for the Cape and island airports. Fog and low stratus should clear the south coast by 14Z allowing for VFR the rest of the day. SW winds around 10 kt to start then increasing to 10-15kt with gusts 20-25kts in the afternoon. There is another risk for SHRA/TS again Tue evening, with the best chance from ORH westward. These will be weakening after sundown with generally VFR conditions, although stratus and fog at sub- VFR levels again expected for the Cape and Islands. SW winds around 10 kt. Wednesday and Wednesday night...Moderate confidence. VFR. SW winds 10-15 kts. A line of SHRA/TS will once again move into southern New England toward sundown, weakening as it moves east which may bring brief low vsby and strong winds. KBOS TAF...High confidence in TAF. Thinking that SHRA/TS dissipate before reaching BOS Tue evening. KBDL TAF...High confidence in TAF, moderate on storm timing. Possible SHRA/TS again on Tue evening. Outlook /Thursday through Saturday/... Thursday: VFR. Chance SHRA. Thursday Night through Saturday: VFR. && .MARINE... Forecaster Confidence Levels: Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent. Today through Wednesday night: High confidence. A ridge of high pressure off the Atlantic coast will continue to result in persistent SW winds across the waters through Wednesday night. There is a risk for thunderstorms on the northeastern waters through sundown but the better chance for thunderstorms making it to the waters will be on Wednesday evening and Wednesday night. We will continue to deal with morning fog across the southern waters Tuesday morning. Seas will be reaching 5 ft along the southern waters overnight into Wednesday. Outlook /Thursday through Saturday/... 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. Friday Night through Saturday: Winds less than 25 kt. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Heat Advisory until 8 PM EDT Wednesday for CTZ002>004. Air Quality Alert until 11 PM EDT this evening for CTZ002. MA...Heat Advisory until 8 PM EDT Wednesday for MAZ003>007-010>021- 026. RI...Heat Advisory until 8 PM EDT Wednesday for RIZ001>007. MARINE...Small Craft Advisory from 5 PM this afternoon to 8 AM EDT Thursday for ANZ254>256. && $$ SYNOPSIS...BW/KP NEAR TERM...KJC/BW SHORT TERM...BW LONG TERM...KP AVIATION...BW/KP MARINE...BW/KP