Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 202335 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 635 PM EST Mon Nov 20 2017 .SYNOPSIS... High pressure moves south of the region Tue with milder temperatures as winds shift to the southwest. A cold front approaches the region Tuesday night, and will combine with moisture working up the eastern seaboard to bring a period of rain through Wednesday morning, especially SE New England. Dry and cold conditions expected Wednesday night through the end of the week. Milder with a few showers possible Saturday as a cold front approaches, then blustery and colder weather follows Sunday and especially Monday. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... 635 PM update... Area of low/mid level moisture assocd with developing warm advection resulting in some high based SC/AC moving across SNE. These clouds expected to lift to the north overnight followed by clearing skies. Winds still gusty in some locations but will diminish overnight as pressure gradient weakens and decoupling from the PBL occurs. This should allow for some radiational cooling during the overnight. The return flow does suggest a slight increase in sfc dwpts overnight, but this will be countered by the cooling. Therefore, will still see mins dipping into the 20s and low-mid 30s across the region. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... Tomorrow... Weak mid lvl ridge and attendant sfc high pres center will begin to pass to the E of the region during the morning tomorrow. This will act to enhance the S-SW flow through the day, leading to increased low lvl moisture and rising dwpts/low lvl temps. H95 temps are progged to reach near +5C by late afternoon, which should allow high temps to rebound nicely into the low-mid 50s with a few spots in the upper 50s to near 60 by mid afternoon. Column remains dry under the ridge`s influence, so expecting a quiet day in terms of precip. The only fly in the ointment will be another round of gusty winds, this time mainly S-SW. H92 LLJ hovers near 35-40 kt, and the limited mixing beneath the continued inversion should limit mixing to this height. Therefore, feel most gusts/sustained winds will remain below wind advisory thresholds, but it will be breezy nonetheless as mixing should still yield gusts 25-35mph. Tomorrow night... Area of weak low lvl convergence develops ahead of approaching cold front associated with sharpening shortwave approaching from the W during the overnight hours. Column begins dry, with PWATs generally at or below normal values through at least midnight local, so feel the development of any precip will be mainly in the morning hours, especially closest to daybreak as low pres develops near the mid atlantic. A slight increase in moisture, with PWATs then approaching 1 std deviation above normal occurs as the convergence develops in the form of an inverted trof linked to the coastal low well to the S. An area of rainfall will develop along this axis and shift into S New England from the S through sunrise. Note there is still some uncertainty on exactly where this axis sets up, but current data suggests a focus from Westerly to Boston and points SE, with lower risk to the NW. POP gradient reflects this thinking. Leaned away from GFS/NAM, which continue to be easterly outliers and mainly offshore and added more weight to the more amplified ECMWF which remains consistent. With the continued influence of low-mid lvl moisture manifesting as increasing clouds and rising dwpts overnight mins will be mild compared to previous nights, mainly in the low 40s E of a line from Hartford to Beverly, then mid thirties further inland, where clouds will take longest to influence. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Highlights... * Morning rain Wed, especially SE New Eng, then clearing afternoon * Mainly dry and cool Thanksgiving Day and Friday * Milder with a chance of showers Sat * Blustery and colder Sun/Mon Overview... Models are in good agreement on the overall pattern in the extended period. High latitude blocking persists over Greenland to near the North Pole which results in the southward displacement of the Polar Vortex over SE Canada. Series of northern stream shortwaves rotating around this vortex will swing through New Eng on Wed, Thu/Thu night and again Sun/Mon. Result will be an overall cool pattern with temps averaging below normal. The coldest period will be late Sun/Mon as deep trough settles over New Eng but a brief warm up to milder than normal temps Sat ahead of this deepening trough. The extended period looks mostly dry with exception of a period of rain Wed morning, mainly SE New Eng as southern stream moisture links up with mid level trough, with another round of showers possible Sat as amplifying trough and attending cold front approaches. Wednesday... Fairly robust mid level trough and attending cold front moves into New Eng. Southern stream moisture assocd with a sfc low to the south and inverted trof lifts north into SNE and interacts with mid level trough approaching from the west and right entrance region of the upper jet. Result is an area of rain which overspreads SNE, especially SE New Eng in the morning. There is some uncertainty with extent of rainfall and QPF amounts in the east. International models are much wetter than US models as ECMWF/GGEM and UKMET to a lesser extent indicate up to an inch of rain across SE New Eng while GFS/NAM less than 0.25". Favorable jet dynamics with right entrance region of 150 kt upper jet combined with modest low level jet across SE New Eng within deep moisture plume with PWATs nearing 1". EPS ensembles also support the deterministic guidance with high probs of 0.50" SE New Eng. As a result we blended the ECMWF with less robust UKMET. Expect a period of widespread rainfall Wed morning in the coastal plain, especially SE MA, locally heavy at times. A period of light rain may briefly get into the CT valley but lower confidence this far west. After fropa, rapid drying moves in from the west with clearing skies from west to east during the afternoon. Thursday through Saturday... High pres in control Thu/Fri with dry weather and below normal temps. Sunshine both days, but some ocean effect clouds may impact Cape Cod Thursday. Then increasing SW flow ahead of next amplifying trough will bring milder temps for Sat. The warm advection will result in clouds increasing and cant rule out a few showers as the cold front approaches. Sunday and Monday... Strong cold front moves through Sunday followed by blustery and colder conditions. The core of the cold air aloft settles into the region Monday which will be the coldest day of the extended period. Ensemble mean 850 mb temps -10 to -12C should keep temps in the 30s Mon. Mainly dry weather but can`t rule out a few flurries or snow showers, especially interior higher terrain as mid level trough moves overhead. && .AVIATION /00Z TUESDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday Night/...High confidence. Through tonight... VFR. SCT-BKN cigs 060-080 will lift to the north after midnight with clearing skies. Gusty winds diminishing. Tomorrow... VFR. Increasing SSW wind gusts 25-35 kt through the day. Tomorrow night... Mainly VFR although a few patchy near MVFR clouds are possible toward early Wed morning. Some light SHRA also possible mainly across W/Central MA/RI. SSW winds around 10 kt. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Wednesday through Saturday/... Wednesday: Moderate confidence. Areas of MVFR/IFR possible in rain, especially SE New Eng in the morning. Improving conditions in the afternoon. Wednesday Night: High confidence. VFR. Gusts to 20-25 kt Cape/Islands. Thanksgiving Day through Friday Night: High confidence. VFR. Saturday: Mainly VFR, with areas MVFR possible. Windy with local gusts to 30 kt. Chance SHRA. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday Night/...high forecast confidence. Through the overnight hours... One more impulse of stronger winds approaching Gales this evening, so have lengthened gales on the outer waters through the evening hours. Seas will continue to diminish, albeit slowly. Small craft advisories will need to follow Gales. Tomorrow and tomorrow night... Although seas on the east waters may drop below 5 ft by early morning, as winds shift to the S-SW gusts will once again increase, especially after sunrise to 25-30 kt, so small craft advisories will continue. There is a risk for Gales for a period during the late afternoon and evening hours, which may require another set of Gale warnings, but in any case, small craft advisories at least will linger into tomorrow night, especially as seas increase in response to the S-SW flow. Outlook /Wednesday through Saturday/...High confidence. Wednesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Vsbys reduced in rain during the morning, improving in the afternoon. Wednesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Thanksgiving Day: Winds less than 15 kt. Local rough seas. Thursday Night through Friday: Winds less than 25 kt. Seas up to 5 ft. Friday Night: Low risk for Small Craft Advisory winds with gusts up to 30 kt. Seas up to 5 ft. Saturday: Moderate risk for Small Craft Advisory winds with local gusts up to 30 kt. Areas of rough seas. Chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Gale Warning until 10 PM EST this evening for ANZ231>235-237- 250-251-254>256. Small Craft Advisory until 10 PM EST this evening for ANZ230- 236. && $$ SYNOPSIS...KJC/Doody NEAR TERM...KJC/Doody SHORT TERM...Doody LONG TERM...KJC AVIATION...KJC/Doody MARINE...KJC/Doody

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