Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 152114 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 352 PM EST Wed Nov 15 2017 .SYNOPSIS... Dry weather tonight will be followed by some wet weather on Thursday, especially during the afternoon when a brief period of heavy rain will impact the region. Blustery winds follow the departing storm system Friday. Another shot of disturbed weather Saturday into Sunday with mild and blustery conditions in advance followed by return colder air and continued breezy winds. Cooler than average conditions continue through the week. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... A scattered to broken deck of cloudiness this evening will trend towards overcast after midnight. This a result of low pressure across the Great Lakes lifting northeast into southern Canada. Upper level ridging should still maintain dry weather for most of the night. An increasing low level jet/forcing should allow for some showers to impact interior southern New England after 2 or 3 am. There is a low risk for a very brief period of light snow/ice at the onset across the highest terrain of northwest/north central MA, but even if this occurs should be very short-lived. Low temps should occur by midnight generally in the upper 20s to middle 30s, but readings should slowly rise thereafter as clouds continue to increase. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...
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***A 1 to 3 hour period of heavy rain between noon and 5 pm on Thursday across central/eastern MA and RI with the low risk for localized damaging wind gusts near the south coast*** Thursday... Scattered showers will be on going Thursday morning in the warm air advection pattern. Attention will then turn to secondary low pressure that will develop off the south coast and rapidly intensity as it lifts northeast. The greatest precipitation amounts look to occur in Maine given the storm is just starting its rapid intensification in our area. However, some of our high resolution guidance has this occurring a bit faster than other models which is certainly possible given the setup. If this occurs, it would bring a higher risk for brief heavy rainfall into our region. There are two things that will have to watch for on Thursday. The first is a 1 to 3 hour period of heavy rain with the second being the low risk for convection/localized damaging wind gusts near the southeast New England coast. A 1 to 3 hour heavy rain is expected between noon and 5 pm across central and eastern MA/RI. The heaviest rain may occur in two areas, the first being near and northwest of the I-95 corridor where very strong omega will be associated with the rapidly deepening low pressure system. The second area may remain offshore or it may clip the southeast New England coast in association with the warm sector. If it does get onto the southeast New England coast, rapid pressure falls coupled with steep mid level lapse rates and some instability may result in a few embedded thunderstorms. The pressure falls coupled with any convection may result in a low risk for localized damaging wind gusts near the southeast New England coast. In fact, given the amount of low level helicity and cold temps aloft there is even a very low risk of a waterspout off the south coast. All in all, odds favor this being the run of the mill nuisance event with a brief period of heavy rain and perhaps brief gusty winds along the southeast New England coast. However, given the dynamics/pressure falls associated with this system that low risk for localized damaging wind gusts associated with possible convection does exist on the southeast New England coast. Thursday night... The low pressure system will continue to intensify as it lifts into downeast Maine and away from the region. The result will be dry weather Thursday night other than perhaps a few rain/snow showers along the east slopes of the Berks. The main story will be the increasing pressure gradient which should result in northwest wind gusts of 25 to 35 mph with perhaps a few gusts up to 40 mph. Low temps will be mainly in the 30s, but it will feel colder given the gusty winds.
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&& .LONG TERM /FRIDAY THROUGH WEDNESDAY/... */ Highlights... - Persistence of cooler than average air - Wet weather forecast Saturday into Sunday - Periods of gusty winds, potentially up to 45 mph - Wind threats early Friday, and late Saturday through Monday */ Overview... Persistence of cooler than average air with shots of mild weather in- between. Individual zonal-flow disturbances over the CONUS this week bottle up over the N Atlantic by the weekend, increasing meridional flow, invoking a brief ridging-block across the Davis Straight as indicated by the -NAO. A digging storm system over the weekend with polar connections deepening into E Canada becomes trapped briefly, its remnant circulation a focus for additional energy out of the N/W to rotate through resulting in maintenance in addition to shots of Arctic air sweeping S into the NE CONUS. Meanwhile upstream, the N Pacific ridge maintains, downstream of which energy shears S gaining greater cyclonic curvature and deepening. Subsequent ridging over the W and Central CONUS only aids in the SE transport of colder air. While a return to zonal flow is entirely plausible going into the holiday weekend, the longer term ensemble mean trend suggests that troughing continues over the NW Pacific downstream of which the ridge- trough pattern follows out across an open Atlantic. Believe the cooler and active pattern will continue however without the allowance for deeper storm development into the NE CONUS. Mainly thinking continued quick moving disturbances. Will hit the details in the discussion below. A preference to the 15.12z EC and ensemble means (GEFS and ECens). */ Discussion... Friday... Potential WIND ADVISORY headlines. Behind the deepening departing system into SE Canada, will see a H925 50-55 mph NW jet especially over E/SE New England, the height of which will be bright and early in the morning, diminishing into afternoon. Boundary layer lapse rates up to H9 around 6 C/km should promote the mix down of faster momentum to the surface. Using just a rule of thumb of 70 percent of the top of the mixed layer nets 40 mph. However believe there is the potential for gusts up around 45 mph during the morning before high pressure builds in from the W. Scattered to broken cloud decks that rotate across the region, especially E/NE, clear out towards the overnight period, winds diminishing. Radiational cooling proceeding into Saturday morning, leaning towards the lowest of guidance. Highs in the 40s during the days dropping down to lows in the 20s possibly as low as the teens into Saturday. Saturday into Monday... Potent storm system digging across the NE CONUS into E/SE Canada. A polar connection rearward as the system undergoes a negative tilt, obtaining greater cyclonic curvature, the thermal gradient tightens yielding a strong W/SW jet streak response in advance of the system at all levels Saturday night into Sunday. Looking at initial over- running rains as the warm front lifts N over the area late Saturday followed by the main slug out ahead of the primary vortmax and right- front-quadrant of the H3 jet max, deep-layer forcing of a warm-moist conveyor belt, Saturday night into Sunday morning. Clearing out into Sunday with the lake effect snow machine ongoing N/W. Could see some possible influences reaching N/W areas of MA and CT through Monday. But the bigger story during this period is the WINDS. As mentioned above, strong SW response in advance of the storm. Potential for boundary layer lapse rates up to H9 to exceed 6 C/km, as warm air advection proceeds beneath a progged H9-7 dry inversion, through which winds are forecast to exceed 60 mph. Some uncertainty on the crux of the winds, however it would appear SE New England is under greatest threat of headlines out ahead of the sweeping cold front boundary. Signals within CIPS analogs and ensemble mean anomalies. Will hit upon potential headlines in the HAZARDOUS WEATHER OUTLOOK. Behind the cold front, a strong isallobaric / gradient response continues along with cold air advection. Lapse rates nearly dry adiabatic through which NW winds of 45 to 50 mph are forecast, brought about by additional energy rotating through the region, a local enhancement to the flow. Again, the 70 percent rule of thumb, gusts up around 35 mph, but can`t rule out 40 to 45 mph gusts ever briefly through the day. Threat for both Sunday and Monday, will see winds relax Monday night. Again, signals apparent of wind threats within CIPS analogs and ensemble mean anomalies. Tuesday and through the Thanksgiving Holiday Weekend... Leaning with a persistence of cooler than average air across the region via NW flow. This maintained by the circulation over E/SE Canada persisting S of which a zonal flow continues. Per ensemble means, there is little indication of equator-pole transport until individual systems quickly zoom through the zonal flow emerge out over E Canada into the N Atlantic, having obtained maturation, pulling N and S streams together. Closely watch Tuesday night into Wednesday, but early indications of S- and N-stream energy phasing downstream out across Newfoundland behind which NW winds persist across the NE CONUS. && .AVIATION /21Z WEDNESDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/... Late this afternoon and evening...Moderate to high confidence. Mainly VFR, but some marginal brief MVFR CIGS may impact eastern New England at times. Late Tonight...Moderate to high confidence. MVFR conditions should overspread much of the region after midnight. Some rain showers should arrive toward daybreak across the interior. Thursday...Moderate to high confidence. Low end MVFR to IFR conditions will dominate. Scattered showers anticipated, with a brief period of heavy rain possible between 18z and 22z. Greatest risk for a short period of heavy rain will be across eastern MA/RI where a few embedded t-storms along with locally strong wind gusts possible near the south coast. Thursday night...Moderate to high confidence. CIGS improve to mainly VFR but some marginal MVFR conditions still possible. NW wind gusts of 25 to 30 knots develop during the evening with a few gusts up to 35 knots possible. KBOS Terminal...Moderate to high confidence in TAF. Mainly VFR into this evening but some marginal MVFR conditions can not be ruled out. Greater risk of MVFR conditions arrive toward daybreak. KBDL Terminal...Moderate to high confidence in TAF. VFR into this evening with MVFR conditions holding off until near or after midnight. Outlook /Friday through Monday/...High confidence. Friday: VFR. Windy with gusts to 35 kt. Friday Night: VFR. Windy with local gusts to 30 kt. Saturday: VFR. Windy with local gusts to 30 kt. Chance SHRA. Saturday Night: Mainly VFR, with local MVFR possible. Windy with gusts to 35 kt. SHRA. Sunday: VFR. Strong winds with local gusts to 40 kt. Chance SHRA. Sunday Night through Monday: VFR. Windy with gusts to 35 kt. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/... Tonight...High confidence. 3 to 5 foot seas will persist across the outer-waters tonight from leftover swell, but winds will remain well below small craft advisory thresholds. Thursday and Thursday night...Moderate to high confidence. A rapidly intensifying low pressure system near the south coast Thursday afternoon will quickly lift northeast into downeast Maine Thursday night. The result will be an increasing pressure gradient and gale force wind gusts are expected. This may first occur Thursday afternoon across our southern waters especially if any convection occurs. There is even a very low risk for a waterspout. Higher confidence for northwesterly wind gusts of 35 knots across most open waters Thursday night in the cold air advection pattern behind this storm. Therefore, have converted gale watches to warnings for all waters except Boston Harbor and Narragansett Bay. Outlook /Friday through Monday/... Friday: Low risk for gale force winds with gusts up to 35 kt. Rough seas up to 11 ft. Friday Night: Low risk for Small Craft Advisory winds with areas of gusts up to 30 kt. Local rough seas. Saturday: Low risk for Small Craft Advisory winds with areas of gusts up to 30 kt. Local rough seas. Chance of rain showers. Saturday Night: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 15 ft. Rain showers likely. Sunday: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 14 ft. Chance of rain showers. Sunday Night: Moderate risk for gale force winds with gusts up to 40 kt. Areas of rough seas. Slight chance of rain showers. Monday: Moderate risk for gale force winds with gusts up to 35 kt. Areas of rough seas. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Gale Warning from 11 AM Thursday to 4 PM EST Friday for ANZ231>235-237-254>256. Gale Watch from Thursday evening through Friday morning for ANZ230. Gale Watch from Thursday morning through Friday morning for ANZ236. Gale Warning from 7 PM Thursday to 4 PM EST Friday for ANZ250- 251. && $$ SYNOPSIS...Frank/Sipprell NEAR TERM...Frank SHORT TERM...Frank LONG TERM...Sipprell AVIATION...Frank/Sipprell MARINE...Frank/Sipprell

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