Area Forecast Discussion
Issued by NWS Boston, MA
Issued by NWS Boston, MA
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317 FXUS61 KBOX 171405 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1005 AM EDT Wed Jul 17 2024 .SYNOPSIS... Dangerous heat and humidity will continue today. Scattered showers and thunderstorms will move into the region this evening before weakening. A cold front will bring much more comfortable temperatures and dewpoints to the region starting Thursday afternoon with dry conditions expected through the weekend. && .NEAR TERM /THROUGH THURSDAY/... 10 AM update... Not much change from previous forecast thinking. Expecting another round of showers and t-storms with potential for severe weather late afternoon and evening. Storms will likely fire along a developing pre-frontal trough, with updraft helicity fields and machine learning guidance highlighting areas north and west of I-95 corridor for best chance of severe weather. Moderate instability and shear with CAPES 1000-1500 J/kg and effective shear 30-35 kt support organized storms and CAMs indicating storm clusters and line segments with damaging wind the primary threat, although there is a localized flash flood threat as well given high PWATs. Timing wise, looking like storms move into western MA/CT 4-6 pm then move eastward this evening, but activity should weaken as they approach RI and SE MA given more stable marine layer. No change to heat headlines. Expect highs 90-95, hottest across CT valley and portions of eastern MA, with mid-upper 80s along the immediate south coast. High dewpoints 70-75 will bring peak heat indices to 95-100. Previous discussion... Key Points * Dangerous heat and humidity continue today with heat indicies of 95-100F * Another round of strong to severe thunderstorms between 4pm - 10pm. One more day of dangerous heat and humidity with high temps in the low 90s and dewpoints in the low 70s. This will bring heat indices up to 95F to 100F again this afternoon. The high heat and humidity will once again prime the atmosphere for another round of strong to severe thunderstorms. A potent shortwave drops out of the Great Lakes late today with a surface cold front out ahead of it. Moderate amount of instability will be available ahead of the cold front with HREF mean SBCAPE values around 1200 J/kg, and HREF max around 2000J/kg. Modest 0-6km shear of 25-30 knots will be sufficient enough for organized convection. Once again, mid level lapse rates are not favorable only around 5C/km. With strong forcing from the cold front and unidirectional shear, the primary storm mode will likely be a linear cluster of storms moving from west to east. The primary severe threat will be strong to damaging wind gusts with low level lapse raters around 10C/km and DCAPE values between 600-800 J/kg. Heavy rain is the only secondary threat with PWATS around 2.0 inches and warm cloud depths around 11kft. Not anticipating any flash flooding with the fast moving nature of thunderstorms. Hail and tornadoes do not appear to be a threat today due to weak mid level lapse rates and meager low level shear. Hi-res guidance from the HREF is in pretty good agreement of timing and strength of storms this afternoon/evening. The best chance for seeing strong to severe winds gusts is across western MA and CT, where better instability will be available. Storms will weaken as they move east into eastern MA and RI as they begin losing the support of daytime heating and run into more stable marine air. Storms look to enter western MA and CT around 4-6pm and weaken and move offshore between 8-10pm && .SHORT TERM /THURSDAY NIGHT/... Key Point * Still warm and muggy overnight Showers and possibly a rumble or two of thunder could linger near the coastal waters through about 2am, otherwise drying conditions overnight. Even with the passage of the cold front, the cooler temps and lower dewpoints lag far behind the front not arriving until Thursday afternoon. Overnight temperatures stay in the upper 60s to low 70s with dewpoints remaining near 70F. Low status and fog will fill in for the southcoast, Cape, and Islands one last time before drier air will kick it out Thursday afternoon. && .LONG TERM /FRIDAY THROUGH TUESDAY/... Key Point * 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. Drier air and cooler temps arrive Thursday afternoon with PWATS falling below 1.0 inches and dewpoints falling into the low 60s to even upper 50s, and staying there through the weekend. Temperatures will 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, even some upper 50s, 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 Tuesday or Wednesday next week as a shortwave drops into the southern plains before lifting north into the Northeast. && .AVIATION /14Z WEDNESDAY THROUGH SUNDAY/... Forecast Confidence Levels: Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Today...Moderate to high confidence. VRF inland. For the Cape and Islands, VFR dropping back to IFR in the evening. Moderate to high confidence that thunderstorms will move into western MA and CT around 22-00z, and move east through 02-04z. As storms approach the east coast, they will likely fizzle into light rain showers. SW winds at 10-15 knots gusting 20-25 knots. Tonight...Moderate confidence Showers could linger through midnight. VFR for western terminals overnight. VFR/high end MVFR for northern terminals. low end MVFR/IFR for southern terminals including the Cape and Islands. SW winds turning west at 5-10 knots, gusting 20-25 knots near the waters. Tomorrow and tomorrow night...High confidence. Gradual clearing becoming VFR for all terminals in the afternoon. West winds at 10 to 15 knots. KBOS TAF...Moderate confidence in TAF. VFR today with SW winds at 10-15 knots. Line of thunderstorms should weaken into light rain showers as they approach the terminals between 01-04z. KBDL TAF...High confidence in TAF. VFR with SW winds at 10-15 knots. Line of thunderstorms likely reaching the terminal sometime between 23z-02z. Outlook /Friday through Sunday/... VFR. && .MARINE... Forecaster Confidence Levels: Low - less than 30 percent. Medium - 30 to 60 percent. High - greater than 60 percent. Today through tomorrow: Moderate confidence. Gusty SSW winds continue today into tonight up to 25 knots. Seas climb to 4-6 feet tonight into Thursday. Winds turn west Thursday as the surface cold front moves through. The cold front will also help to kick out the low status and fog that has been stuck over the southern waters for the past few days. Thunderstorms will weaken as they approach the waters tomorrow evening, but could still bring brief gusty winds above 30 knots with occasional lightning. Tides and Rip Currents. A rip current statement is in effect today due to the moderate to high risk for rip currents. Strong southerly winds of 20-30mph and waves between 4-6 feet will likely cause strong rip currents to occur for south facing beaches. Another rip current statement will likely be needed tomorrow with waves increasing to 5-7 feet. Outlook /Friday through Sunday/... Friday: Winds less than 25 kt. Seas locally approaching 5 ft. Friday Night through Sunday: Winds less than 25 kt. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Heat Advisory until 8 PM EDT this evening for CTZ002>004. MA...Heat Advisory until 8 PM EDT this evening for MAZ003>007- 010>021-026. High Risk for Rip Currents until 8 PM EDT this evening for MAZ020>024. RI...Heat Advisory until 8 PM EDT this evening for RIZ001>007. High Risk for Rip Currents until 8 PM EDT this evening for RIZ006>008. MARINE...Small Craft Advisory until 8 AM EDT Thursday for ANZ232. Small Craft Advisory from 2 PM this afternoon to 8 PM EDT Thursday for ANZ235-237. Small Craft Advisory until 8 PM EDT Thursday for ANZ254>256. && $$ SYNOPSIS...KP NEAR TERM...KJC/KP SHORT TERM...KP LONG TERM...KP AVIATION...BW/KP MARINE...KP