Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 221438 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1038 AM EDT Sun Jul 22 2018 .SYNOPSIS... Scattered showers and a few embedded thunderstorms with locally heavy rainfall anticipated into Monday with the focus shifting back to interior southern New England. Bermuda high pressure then builds west toward southern New England next week bringing tropical humidity and warm temperatures. Scattered showers and thunderstorms will be around most of the week, although a little more difficult to come by on Tuesday. A cold front crosses New England Friday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1035 am update... * Low risk for an isolated severe t-storm/localized flash flood event or two into early this evening, but nothing widespread Band of widespread showers working across eastern MA/RI late this morning...but rainfall rates were under control. This probably the result of limited instability and stronger synoptic scale forcing. The question then turns to this afternoon and early evening. While we should see scattered showers and a few thunderstorms redevelop...their intensity and areal coverage is uncertain. A lot of clouds coupled with weak mid level lapse rates will limit the amount of instability we can generate. Satellite imagery was indicating a few spotty breaks in the clouds across portions of western MA/CT. Given dewpoints in the 70 to 75 degree range it will not take much to muster 1000 J/KG of Cape. Limiting factors will be poor mid level lapse rates and deep layer shear was not that impressive. Nonetheless...modest southerly LLJ at 925 mb will persist with high dewpoints in place. So while a low probability...an isolated severe t-storm or two with very localized damaging wind gusts can not be ruled out. There also would a low risk for a very localized urban flash flood event given Pwats over 2 inches. So in a nutshell...mainly looking at scattered showers and a few embedded thunderstorms with localized heavy rainfall into early this evening. However...the threat for a very localized severe thunderstorm/urban flash flood threat can not be ruled out. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/... Unsettled, tropical environment continues, pushed W per a building Atlantic H5 ridge. Rising heights, subsidence and drier air rotate clockwise round the ridge of high pressure into New England shunting the deep tropically moist and anomalous low-level wind axes W, again any nudge upon which should induce scattered shower / thunderstorm activity along with pockets of heavy rain. Downward trend indicative in model forecast K indices. Localized flash flood threat continues with concern for urban corridors. Otherwise muggy. Will continue to experience oppressive humidity with 70+ dewpoints as temperatures remain mild overnight, warming into the 80s Monday. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... Big Picture... Ridges over the West Atlantic and Western USA frame a deep trough over the Eastern USA. Upper flow is very meridional early to mid week. The northern jet stream then nudges south late week, bringing a more zonal component to the flow over the North-central and Northeast USA. This will eventually allow a cold front to sweep over New England toward the end of the week. Contour heights will be above normal through the period. Thermal fields will be above normal through the period. This suggests warm temperatures when there is any sunshine, with temperatures cooler but still near normal where skies are cloudy. Model mass fields are in good agreement through Tuesday, then the GGEM becomes an outlier while the GFS and ECMWF remain similar through Friday, and only slightly diverge on Saturday. Forecast confidence is high through most of the week and moderate-high for Saturday. Concerns... Drying period... Upper ridge builds west early in the week, reaching its farthest extent west on Tuesday. Surface flow remains southerly through this period, so no break in the sultry air. But the 2 inch-plus precipitable water values shift west across the Mid Atlantic and NY state while values over our area are around 1.5 inches...still moist. RH cross sections show dry air will move in above 850 mb, limiting the ability of convection to build deep. Lift will also be minimal, especially in RI/Eastern MA. This may allow for some sunshine, especially in eastern areas, and limit the development of showers. Best chance for any showers would be in the CT Valley and areas west of there. The ridge moves back to the east Wednesday, allowing the two-inch PW values and deep moisture back over New England later Wednesday and Wednesday night. Tropical flow... Even with the Atlantic ridge building west early week, the low level flow remains out of the south and will maintain very humid conditions with dew points expected to linger in the low to mid 70s. Once the ridge shifts east, deep moisture returns...and the southwest flow ahead of the Canadian shortwave will continue to draw warm very humid air into our area for Thursday. Away from the suppressing effect of the ridge, and with CAPE of 1500 J/KG moving in from NY, expect broader area of showers and scattered thunder on Thursday. Cold front Friday... Canadian shortwave will move across the Great Lakes later Thursday/Thursday night sweeping a cold front into the Northeast on Friday. By Friday afternoon much of Southern New England is in the right entrance region of the supporting upper jet. Lift is favored in this airmass with CAPE of 1500-2000 J/Kg and mid-level lapse rates of 6-6.5 C/Km. This should favor showers/tstms along and ahead of the cold front. But even after the front moves through, warm humid weather will linger on Saturday. && .AVIATION /14Z 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/...Moderate confidence. Today...Mainly MVFR to IFR conditions in scattered showers and a few embedded thunderstorms through early evening. SE wind gusts of 15 to 25 knots anticipated. LLWS will be a concern across eastern New England. Tonight into Monday... SCT -RA/RA elements, possible embedded +RA/TSRA. Continued MVFR- LIFR threats with respect to CIGs and VSBYs. May see an improving trend E to W especially on Monday, conditions lifting to low-end VFR and -RA/RA concluding. Greater focus of -RA and lower conditions shifting towards the CT River Valley. KBOS Terminal...Moderate confidence in TAF. KBDL Terminal...Moderate confidence in TAF. Outlook /Monday Night through Thursday/... Monday: MVFR/IFR conditions possible. Breezy. Chance SHRA, isolated TSRA. Monday Night: Mainly IFR, with areas VFR possible. Chance SHRA, slight chance TSRA, patchy BR. Tuesday: Mainly VFR, with local IFR possible. Breezy. Chance SHRA. Tuesday Night: Mainly VFR, with areas IFR possible. Chance SHRA, patchy BR. Wednesday: Mainly VFR, with local MVFR possible. Breezy. Chance SHRA, slight chance TSRA. Wednesday Night: Mainly VFR, with local IFR possible. Breezy. Chance SHRA, slight chance TSRA, patchy BR. Thursday: Mainly VFR, with local MVFR possible. Breezy. Chance SHRA, slight chance TSRA. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday/...Moderate confidence. Today... Anomalous S low level jet will allow seas to build across our S waters to between 5 and 10 feet across our open waters today. This a result of E/SE gusts 20 to 30 kts. Already 7-8 foot seas being reported S of W Long Island with such gusty winds. Rain, heavy at times, with embedded thunderstorms through the morning hours, continuing along a S-N band into afternoon especially over the E-half of New England. Lower visibility in addition to fog over the waters. Sunday Night into Monday... Atlantic high building back W overnight into Monday, wind gusts should diminish slightly as drier weather builds in over the waters from the E. This will allow conditions in the inner- waters to subside, however seas will remain heightened over the outer waters resulting in small crafts to continue. Outlook /Monday Night through Thursday/... Monday: Winds less than 25 kt. Areas of rough seas. Areas fog, slight chance of rain showers, isolated thunderstorms. Monday Night: Winds less than 25 kt. Local rough seas. Slight chance of rain showers, isolated thunderstorms. Tuesday through Tuesday Night: Winds less than 25 kt. Seas up to 5 ft. Wednesday: Winds less than 25 kt. Seas up to 5 ft. Slight chance of rain showers. Wednesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Chance of rain showers, slight chance of thunderstorms. Thursday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Chance of rain showers, slight chance of thunderstorms. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...High Surf Advisory until 7 PM EDT this evening for MAZ020- 022>024. RI...High Surf Advisory until 7 PM EDT this evening for RIZ006>008. MARINE...Small Craft Advisory until 2 AM EDT Monday for ANZ230>234-251. Small Craft Advisory until 2 PM EDT Monday for ANZ236. Small Craft Advisory until 6 PM EDT Monday for ANZ235-237-250- 254>256. && $$ SYNOPSIS...WTB/Sipprell NEAR TERM...Frank SHORT TERM...Sipprell LONG TERM...WTB AVIATION...Frank/WTB/Sipprell MARINE...WTB/Sipprell

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