Area Forecast Discussion
Issued by NWS Boston, MA

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937 FXUS61 KBOX 251110 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 710 AM EDT Sun Jun 25 2017 .SYNOPSIS... Daytime scattered showers and possible thunderstorms will be the story for today and again Monday. Another upper level disturbance may bring a few showers or an isolated thunderstorm Tuesday. Then a warming trend begins for the latter half of the week with a return to summer heat and humidity by Friday. A warm front may bring some showers Thursday with more showers and thunderstorms possible Friday and Saturday. A backdoor front may drop south into the region Saturday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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7 AM update... Sunshine to start the morning will mix with afternoon diurnal clouds. Rather robust shortwave across MI tracks east to the St Lawrence Valley late today with modest mid level cooling across western New Eng. Steepening low level lapse rates and moistening in the 850-700 mb layer may trigger a few afternoon showers in the interior, especially northern and western MA. Also can`t rule out an isold t-storm but soundings suggest this is a low probability. Better chance will be across northern NY and VT closest to the core of the cold pool aloft. Minor adjustments to PoPs, otherwise forecast unchanged. Previous discussion... Scattered showers and possible thunderstorms with breezy SW winds. Cyclonic flow and attendant elongating vortmax energy through the mid-latitude long-wave trough pattern yields ascent upon available moisture. Within an environment of favorable lapse rates and deep boundary layer mixing, the better of which is N/W beneath a more favorable H5 cold pool, diurnal forcing yields scattered to broken cumulus along with the wet-weather activity. Emphasis along the high terrain given the low-level wind profile and subsequent convergent forcing. With the deep boundary layer mixing, instability potential up to 500 j/kg fuels possible thunderstorms, while allowing the mix- down of faster momentum and drier air. Likely will over-achieve on highs and thus leaned warmest of guidance, around the low-mid 80s. Dewpoints likely to drop off into the upper 40s to low 50s across the interior netting less humid, more comfortable conditions. Will be breezy out of the SW with gusts up to 20 mph possible. Some hints per high-res guidance of the possibility of an immediate onshore sea- breeze, but feel the SW flow will be strong enough to keep it off- shore.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH MONDAY NIGHT/... Tonight... Turning quiet. Diurnal forcing with daytime heating concludes and so does cloud and shower activity. Should see the two dissipate towards midnight. Clearing out and winds becoming light out of the W, there is an opportunity for radiational cooling. Some question though as to whether clouds linger and act as a blanket. For now will lean towards cooler guidance with lows around the mid 50s given just how dry it will become earlier in the day. Monday and Monday night... Showers and thunderstorms possible again. Pretty much a repeat of today and tonight (see the discussion above). However, wet-weather activity may be delayed on the point that favorable ascent may be delayed till the later-half of the day. Acting upon a more favorable environment of steep lapse rates across S New England as the H5 cold pool pushes S/E. Another day of over- achieving highs with low humidity dewpoints. Wet-weather activity lingers into the evening hours from earlier in the day, gradually dissipating as conditions clear and winds become light. Leaning the warmest guidance during the day, coolest guidance at night given synoptic conditions. && .LONG TERM /TUESDAY THROUGH SATURDAY/... Highlights... * A few showers/isold t-storm possible Tue/Wed * Warming trend toward end of the week with return to summer heat/humidity by Fri * A few showers possible Thu with sct showers/t-storms Fri/Sat * Backdoor front possible Sat but low confidence Overview... Mid level trof will begin to exit the northeast Wed, followed by building heights as subtropical ridge builds over the western Atlantic. Polar jet will be in the vicinity of New Eng late in the week into the weekend which will leave us susceptible to weak shortwave passages and bouts of showers and t-storms. Temps starting out slightly below normal Tue then warming through the end of the week. Will have to watch a backdoor front for next Sat. Details... Tuesday and Wednesday... Fairly robust shortwave lifts NE from the Gt Lakes Tue with mid level trof moving into New Eng by Tue evening before exiting on Wed. Increasing dynamic forcing and 500 mb temps cooling to -18 to -20C will bring sct showers and isold t-storms to SNE Tue into Tue evening with best chance in the interior. ECMWF is showing a more amplified trof with a second shortwave rotating around and inducing a coastal low south of New Eng Tue night with area of rain for SE New Eng. This does not have support of other global models so we have discounted for now but something to watch in later forecasts. On Wed, mid level trof begins to exit the region but 500 mb temps around -21C through 18z before warming toward evening. Moisture is limited but can`t rule out a few showers given the rather cold temps aloft. Temps mid/upper 70s Tue and a few degrees warmer on Wed. Thursday... Increasing warm advection pattern as warm front approaches from the SW will lead to increasing clouds. GFS and ECMWF differ on the positioning of the warm front which impacts sensible weather and temps. GFS lifts the front to the north with main focus for showers/t-storms to the north and west. ECMWF holds the front to the south with wetter and much cooler conditions. A low confidence forecast this far out so minimal changes were made to PoPs and temps and will have chc pops in the interior. Higher theta-e air and better instability remains to the west where best t-storm chances are. Gusty SW winds developing, especially in the coastal plain as low level jet develops. The risk for t-storms will increase Thu night as higher theta-e air moves in. Friday... Warm sector airmass with low level theta-e ridge axis across the region and decent instability so expect sct showers and t-storms developing. Difficult to pinpoint shear profiles this far out but it does appear the region will be on the southern edge of stronger deep layer shear so severe weather is possible. Temps should reach well into the 80s to near 90 degrees but cloud cover could have some impact. Very humid with dewpoints near 70. Saturday... Tricky forecast as we may be dealing with a backdoor front close to New Eng. Impossible to know where this front will be but there is potential for a sharp temp gradient across the region depending on where the front is located. Very warm/humid south of the front with temps well into the 80s but much cooler to the north of the boundary with 60s possible. It will all depend on where the front is. Continued risk for showers/t-storms, mainly south and west of the boundary. && .AVIATION /12Z SUNDAY THROUGH THURSDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/...High confidence. Today... VFR. SKC initially becoming SCT-BKN across the interior towards midday and continuing through afternoon. SCT SHRA mainly N/W of BDL-SFZ-BOS with a lower risk of TSRA. SW winds becoming breezy with gusts up to 20 kts. Low risk MVFR with -SHRA, higher confidence over high terrain terminals. Tonight... VFR. SCT-BKN CIGs dissipating along with -SHRA chances. W/SW winds diminishing. Monday into Monday night... VFR. Repeat of Sunday into Sunday night. Mostly SKC initially with SCT-BKN CIGs developing midday and continuing through afternoon with the threat of -SHRA and a lower threat of TSRA mainly N/W of BDL-SFZ-BOS. However mid to high level CIGs will be on the increase from the W late Monday night into Tuesday morn. KBOS Terminal... 12z update...A weak seabreeze may develop east of BOS around midday but there should be enough SW flow to keep it just offshore. For both Sunday and Monday, will keep most of the -SHRA activity and lower risk of TSRA N/W. Blustery SW winds during the daylight hours with gusts up around 20 kts. KBDL Terminal...Similar thinking as described above for KBOS. Outlook /Tuesday through Thursday/... Tuesday and Wednesday...High confidence. Mainly VFR. A few showers and isold t-storm possible Tue and still can`t rule out a shower on Wed. Thursday...Moderate confidence. Mainly VFR with cigs lowering from NW to SE. Afternoon showers possible in the interior with showers/t-storms possible all SNE Thu night. Gusty SW winds developing, especially coastal plain.
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&& .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/...High confidence. General trend of light winds out of the W during the overnight periods while becoming blustery out of the SW through the day with potential for gusts up around 20 kts. This as frontal waves emerge from the W associated with a broad area of low pressure over the Great Lakes while high pressure remains dominant over the NW Atlantic. Rough seas and swell linger over the S/SE waters through Sunday around 5 to 6 feet. SMALL CRAFT FOR HAZARDOUS SEAS accordingly. Outlook /Tuesday through Thursday/... Tuesday and Wednesday...High confidence. Quiet boating weather with winds and seas below SCA. Gusts to 20 kt possible nearshore waters. Thursday...Moderate confidence. Increasing SW winds with gusts to 25+ kt likely, especially over nearshore waters. Building seas may reach 8 ft over southern waters Thu night. Chance of showers and thunderstorms Thursday night. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory for hazardous seas until 2 PM EDT this afternoon for ANZ254>256. Small Craft Advisory for hazardous seas until 8 AM EDT this morning for ANZ235-237. && $$ SYNOPSIS...KJC/Sipprell NEAR TERM...KJC/Sipprell SHORT TERM...Sipprell LONG TERM...KJC AVIATION...KJC/Sipprell MARINE...KJC/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.