813 FXUS61 KBOX 221112 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 712 AM EDT Thu Sep 22 2022 .SYNOPSIS... Cold front moves through the region this morning and afternoon with potentially strong thunderstorms. Cold and blustery conditions settle in behind the front for Friday and Friday night. Milder and more seasonable weather will return this weekend into early next week. A few showers will be possible from time to time in the Sunday night through Wednesday time frame, but a washout is not expected. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 710 AM Update... * Scattered strong to severe thunderstorms possible today * Locally damaging wind gusts & heavy rain/street flooding Showers and thunderstorms were impacting mainly extreme west and north central MA early this morning. This activity was mainly elevated, but enough MUCape of 1000+ J/KG was present. So hail is possible in the strongest cores. The main severe weather threat will roughly be from 9 am to 1 pm across western MA/CT and late morning through about 4 pm across eastern MA/RI. This is when the associated cold front and its associated strong forcing arrives. Quite the impressive dynamics will be present with effective shear of 45-55 knots and 500-1000 J/KG of Cape. These type of events are challenging to forecast because a small change in Cape can make the difference between numerous severe thunderstorms vs. not much of anything. Some of the latest machine learning probabilities have increased the potential for severe weather over the past 24 hours. In fact...the latest SPC SCRAM is very aggressive in showing high probs of a line of showers and thunderstorms with the potential for strong to damaging wind gusts. Given the strong dynamics and fully leafed trees there is concern for pockets of wind damage and power outages. While this potential exists across the entire region; greatest risk will be across eastern MA & RI where instability and effective shear will be maximized. The other concerns will be frequent cloud to ground lightning, heavy rain and brief street flooding. Enhanced wording has been included in the forecast. Temps in the upper 60s and 70s will fall sharply into the upper 50s and lower 60s immediately behind the cold front. Previous Discussion Below: Robust mid-level short-wave trough digs and deepens into the northeast this morning and afternoon. This will force a surface cold front through the region that will produce widespread heavy rainfall and perhaps a fine line of convection capable of producing damaging wind gusts. While there is strong deep layer wind shear on the order of 50 knots in the 0-6 km layer and very strong synoptic forcing for ascent, the limiting factor with respect to severe thunderstorm potential this afternoon will be instability. Most of the region will only have around 100 J/kg of CAPE to work with. However, greater instability develops over southeast MA and RI early this afternoon just prior to the arrival of what should be a fine line of convection. Enhanced instability and fairly steep low-level lapse rates over this region will put this area at the greatest risk for a strong to severe storm. The most likely hazard will be damaging wind gusts capable of taking down large tree limbs which are still fully leafed. Also can't rule out a strong to severe storm developing in the warm sector this morning ahead of the cold front. Steeper mid- level lapse rates ahead of the front may allow a few of the scattered warm sector storms to produce some small hail. As far as timing goes, we're already beginning to see some warm sector storms on radar just over the MA/NY border. A few of these scattered storms will be possible through this morning. By around mid-morning we should start to the see a line of showers and storms moving into western MA and CT. Currently expecting this to be a relatively fast moving line that will reach the Worcester area around 10-11 am and eventually the eastern areas by noon. The front is expected to exit off the southeast coast by mid to late afternoon. Wind shift to northwest behind the front. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... Tonight Skies clear behind the exiting cold front as deep northwest flow advects a continental polar air mass over the northeast. Strong cold air advection causes temperatures at 925 to plummet to temperatures just slightly above freezing. Currently expecting a cold and blustery night tonight as moderate northwest winds will be blowing from 10 to 20 knots. However, the strong winds will support mechanical mixing that will limit radiational cooling. Generally expecting widespread low temperatures in the low to mid 40s at the surface. Areas near the coast stay a bit more mild in the upper 40s to low 50s. Tomorrow More of a late fall feel tomorrow as strong cold air advection aloft continues to support below normal temperatures in southern New England. Steep low-level lapse rates will support efficient mixing and a windy afternoon. Northwest winds from 15 to 20 knots may gust as high as 25 to 30 knots across much of the region. A dry day for most, but northwest flow over the warm Cape Cod Bay waters may support a few ocean effect showers for those over the outer Cape. At the very least expect ocean effect clouds to hide the sunshine from this area. Elsewhere, the dry column will support mostly sunny skies. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Highlights... * Blustery & Quite Chilly Fri night * Mainly Dry with moderating temps this weekend especially by Sun * A few showers at times Sun night through Wed, but not a washout Details... Friday night... A ridge of high pressure will combined with Hurricane Fiona passing well to our east and into the Canadian Maritimes Friday night. This will continue to result in blustery conditions with gusty NW winds and unseasonably chilly temperatures. Appears that there will be enough wind aloft to keep the region from decoupling. This should result in overnight low temps mainly in the upper 30s to the lower 40s. If a few of the typically most prone low-lying locations are able to briefly decouple; overnight lows would drop in the lower to middle 30s with frost. If this does happen it would be very localized; so no frost headlines anticipated at this time. This Weekend... Deep upper trough will lift into the Canadian Maritimes this weekend allowing for rising height fields. This should allow for high temps to recover well into the 60s in many locations Saturday. It still will be breezy on Saturday, but not as strong as on Friday. Temperatures will moderate further by Sunday as the next shortwave drops into the eastern Great Lakes. This should allow high temps to reach back into the 70 to 75 degree range in many locations. Generally expecting dry weather for most of the weekend. Approaching shortwave should result in some mid/high cloudiness gradually increasing as Sunday wears along. May even have a few showers entering our western zones toward evening. However, main threat for some showers will not be until Sunday night when warm advection/forcing increase ahead of an approaching shortwave. Monday through Wednesday... Forecast confidence diminishes in the Mon through Wed time frame. Deep upper trough will drop into the eastern Great Lakes Mon/Tue allowing for some shortwave energy to rotate into our region at times. However, timing and magnitude of the energy are uncertain. All we can say is there will be the risk for a few showers at times Mon and Tue, but not expecting a washout by any means. Seasonable temps expected with highs mainly in the upper 60s to the middle 70s. Appears the trough axis may shift to our east by Wed, resulting in cooler temps with highs mainly in the 60s. && .AVIATION /12Z THURSDAY THROUGH MONDAY/... Forecaster Confidence Levels: Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent. High confidence. 12Z TAF Update... Today...Moderate Confidence. Active day with numerous showers and thunderstorms with frequent lightning already working into western and north central MA. Main activity will be through 16z-17z across the interior and through about 19-20z across eastern MA/RI. Brief IFR-LIFR conditions are expected in the thunderstorms with torrential rainfall and locally strong wind gusts. Outside the thunderstorms, winds continue to increase out of the southwest gusting to 20-30 kt ahead of cold front, then shifting to NW behind the front during the afternoon. Again Stronger wind gusts possible in storms, especially in RI and southeastern MA. Tonight...High Confidence. Mainly VFR but with brief marginal MVFR cigs possible across the interior. NW winds 10-15 kts gusting 20-25 knots with a few gusts up to 30 knots possible. Friday...VFR outside some marginal ocean effect MVFR cigs at times on the Cape and Nantucket. Moderate northwest winds 15 to 20 knots with gusts of 25 to 30 knots with a few gusts up to 35 knots possible. KBOS Terminal...Moderate Confidence in TAF. A round of thunderstorms expected especially between 14z and 18z KBDL Terminal...Moderate Confidence in TAF. Thunderstorms expected through about 16z-17z. Outlook /Friday Night through Monday/... Friday through Saturday: VFR. Windy with gusts up to 35 kt. Saturday Night: VFR. Breezy. Sunday: Mainly VFR, with local MVFR possible. Breezy. Sunday Night through Monday: Mainly MVFR, with areas VFR possible. Breezy. Chance SHRA. && .MARINE... Forecaster Confidence Levels: Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent. Marine Today through Friday Rough conditions expected over the coastal waters over the next 36 hours. First we'll see increasing southwest winds and building seas ahead of an approaching cold front this morning and afternoon. Showers and potentially strong thunderstorms move over the coastal waters this afternoon. Winds shift to the northwest behind the front but remain fairly strong with 10 to 15 sustained and gusts greater than 20 knots. Marine conditions continue to worsen tomorrow night and Friday. Northwest winds rise to 15 to 20 knots tonight and 20 to 25 knots tomorrow. A gale watch is in effect for Friday afternoon into Friday night where gusts greater than 34 knots will be possible. Outlook /Friday Night through Monday/... Friday: Low risk for gale force winds with gusts up to 35 kt. Rough seas up to 10 ft. Chance of rain showers. Friday Night: Moderate risk for gale force winds with gusts up to 35 kt. Rough seas up to 10 ft. Saturday: Low risk for gale force winds with gusts up to 35 kt. Seas up to 5 ft. Saturday Night: Low risk for Small Craft Advisory winds with areas of gusts up to 30 kt. Seas up to 5 ft. Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft. Sunday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Chance of rain showers. Monday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...High Surf Advisory from 4 PM this afternoon to 6 PM EDT Friday for MAZ020>024. RI...High Surf Advisory from 4 PM this afternoon to 6 PM EDT Friday for RIZ006>008. MARINE...Small Craft Advisory until 8 AM EDT Saturday for ANZ230- 233>237-251-256. Small Craft Advisory until 2 PM EDT Friday for ANZ231-232-250- 254-255. Gale Watch from Friday afternoon through Saturday morning for ANZ231-232-250-254-255. && $$ SYNOPSIS...RM/Frank NEAR TERM...Frank/RM SHORT TERM...RM LONG TERM...Frank AVIATION...RM/Frank MARINE...RM/Frank