Area Forecast Discussion
Issued by NWS Boston, MA

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800 FXUS61 KBOX 170520 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 120 AM EDT Sun Jun 17 2018 .SYNOPSIS... Warm and dry weather continues Sunday. Dangerous heat and humidity impacts the interior Monday along with record high temperatures. A cold front may trigger a few thunderstorms with heavy rainfall late Monday and Monday night. Behind the departing cold front, cooler and less humid weather follows Tuesday afternoon through Friday. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 10pm update... Hourly temps are running a bit warmer than forecast, and dwpts have adjusted slightly upward thanks to slightly stronger S component to the flow. Therefore, will adjust hourly temps dwpts as well as overnight mins slightly warmer. Still a good setup for radiational cooling so several sites should still dip to the low-mid 50s just prior to sunrise mixing. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM MONDAY/... Tomorrow and tomorrow night... Upper level ridge will continue to build across the area resulting in building heights and dry weather. Warm front will move through the region resulting in mid-levels wind to switch to a more southwesterly direction. This WAA pattern will help warm surface temps up the low 90s for the interior with mid 80s near the coastline. Winds will take their time to increase resulting in light flow aloft, and with the surface temp spread compared to the waters expect sea breeze development on both coastlines. Frontal system up in southern Canada will slowly approach the region from the north. Still not a lot of moisture in the region so the overnight hours will be dry. Increasing southwesterly winds ahead of the front will result in a mild night as dewpoints begin to increase. Expect a some stratus and fog to develop esp along the south coast where dewpoints will be the highest, which could limit vsbys during the early morning hours. Overnight lows will be in the mid to upper 60s. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... Highlights... * Dangerous heat in the interior Monday * A few thunderstorms with heavy rain possible late Mon/Mon night * Cooler and less humid Tue afternoon thru Fri but temps above normal Monday and Monday night... SNE will be on northern periphery of subtropical ridge with anomalously warm airmass advecting into the region with westerly flow aloft. 850 mb temps 20-21C and 925 mb temps even more impressive at 26-27C across northern MA. Soundings show deep boundary layer mixing which supports highs in the mid/upper 90s interior, hottest across Merrimack Valley. Cooler temps toward the south coast due to SW flow trajectory which will hold temps mostly in the 80s. Expect dewpoints to climb into the upper 60s to around 70 in the afternoon which will result in heat indices of 100-104 degrees in the Merrimack valley and portions of the CT valley where heat advisories will eventually be needed. The heat and humidity will allow atmosphere to destabilize, but poor mid level lapse rates will likely limit magnitude of instability with SBCAPES 1000-1500 J/kg. Forcing is limited during Mon so only expecting widely sct late day t-storms across the interior. Better chance of showers and t-storms ahead of southward advancing cold front Mon night within low level theta-e ridge axis and KI spiking into the upper 30s. PWAT axis exceeds 2 inches so potential for locally heavy rainfall with any storms. Tuesday... Cold front will be moving south of the coast during the morning, and may see a few lingering showers/isolated t-storm mainly south of the Pike. Otherwise, excellent low level drying moves from north to south with clouds giving way to increasing sunshine and less humid air advecting south into the region. The afternoon will have a much drier feel than the morning with temps mostly in the 80s. Wednesday through Saturday... Series of northern stream shortwaves within NW flow will carve out mid level trough across SE Canada and the Northeast with NW flow over New Eng through the end of the week. Result will be mainly dry weather but will have to watch the end of the week as some of the guidance indicating some showers. GEFS and EPS ensemble mean 850 mb temps averaging 12-14C so highs will be in the 80s for much of the region but comfortable humidity. Mid level ridge will try to build back into New Eng next weekend with heat returning but confidence is lower at this time range, and GEFS/EPS ensemble mean 850 mb temps keep heat to the west. && .AVIATION /05Z SUNDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday/... Through this evening...High confidence. VFR. Sea breezes likely, beginning around 14-15Z. Light S-SW winds elsewhere. Early Monday morning through the remainder of Monday...High confidence. Mainly VFR, although some IFR CIGS/fog possible mainly along the Cape/Islands 09z-14Z, these may linger longer after 14Z mainly on the islands. W-SW winds through the day, increasing to 10-15 kt. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Monday Night through Thursday/... Monday: Mainly VFR, with local MVFR possible. Windy with local gusts to 30 kt. Slight chance SHRA, slight chance TSRA. Monday Night: Mainly MVFR, with areas VFR possible. Breezy. Chance SHRA, chance TSRA. Tuesday: Mainly VFR, with local MVFR possible. Slight chance SHRA. Tuesday Night through Thursday: VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday/... Through Sunday...Good boating weather thanks to light flow and diminished seas through the period. Coastal sea breezes during the day. Sunday night...Approaching system from the north will increase wind gusts and thus seas overnight. Still below SCA. Outlook /Monday Night through Thursday/... Monday: Winds less than 25 kt. Areas of seas approaching 5 ft. Monday Night: Winds less than 25 kt. Local rough seas. Chance of rain showers, slight chance of thunderstorms. Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers, isolated thunderstorms. Tuesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Wednesday through Thursday: Winds less than 25 kt. && .TIDES/COASTAL FLOODING... Early Sunday morning high tide in Boston is 11.9 ft. Agree with prev forecaster that with the offshore flow and higher pressure will result in very little if any additional surge. Thus additional coastal flooding is not anticipated. && .CLIMATE... Here are the record and forecast highs for Monday June 18... BOS ... 94 (1907 & 1929) ... Forecast High 94 BDL ... 95 (1957 & 1994) ... Forecast High 96 PVD ... 94 (1929) ... Forecast High 89 ORH ... 93 (1907 & 1929) ... Forecast High 90 && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Air Quality Alert from 11 AM this morning to 11 PM EDT this evening for MAZ020>024. RI...None. MARINE...None. && $$ SYNOPSIS...KJC/Dunten NEAR TERM...Doody SHORT TERM...Dunten LONG TERM...KJC AVIATION...KJC/Doody MARINE...KJC/Dunten TIDES/COASTAL FLOODING... CLIMATE... is the U.S. government's official web portal to all federal, state and local government web resources and services.