Area Forecast Discussion
Issued by NWS Boston, MA

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442 FXUS61 KBOX 160851 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 351 AM EST Thu Nov 16 2017 .SYNOPSIS... Wet weather overspreads the area this morning from west to east, followed by a brief period of heavy rain and gusty winds from late morning into early afternoon. Blustery winds follow the departing storm system tonight and Friday. Another round of gusty showers impacts the area Saturday afternoon into early Sunday. A return to colder air and blustery winds arrive later Sunday/Sunday night. Cooler than average conditions continue into next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 330 AM update... *** A brief period of heavy rain & isolated Thunder between 10 am and 4 pm from west to east with the low risk for localized strong wind gusts near the south coast *** Potent short wave currently moving across MI early this morning will sweep thru southern New England late this morning and afternoon, bringing a round of heavy showers, isolated T-storm and strong gusty winds to portions of the area. Showers first develop 4 am to 7 am across western MA/CT, then the remainder of the area 7 am to 10 am. At 3 am temps are already above freezing across the entire region. Thus ptype will be all rain for the region. Parent low pressure moving across Ontario early this morning gives way to secondary/triple point low development south of New England later this morning. This is in response to strong forcing for ascent from approaching short wave trough. Mid level lapse rates steepen to about 7C/KM. This instability coupled with strong forcing will result in a period of heavy rain with embedded thunder from about 15z-21z from west to east across CT/MA/RI. Hi res guidance including the NCAR ensembles, ARW, NMMB and HREFv2 suggest rainfall totals of up to an inch is possible in a fairly short time period. This may result in minor poor drainage/street flooding, especially if elevated convection becomes more widespread. Other issue today will be the potential for strong winds. Secondary low/cyclogenesis is accompanied by an increasing low level SSE jet that clips the south coast including Cape Cod and the Islands from about 16z-20z. Greatest risk for strong winds will be in the warm sector and model guidance suggest this will be along the south coast including Cape Cod and the Islands. NAM most robust with strength of low level jet but also has some support from the remainder of the hi res guid. In addition, strong pressure rise-fall couplet across RI and southeast MA this afternoon will aid strong wind potential via enhancing isallobaric wind component. Furthermore heavy showers with embedded thunder and robust but shallow updrafts will further enhance wind threat across this region. Given these factors decided to hoist a wind advisory for the south coast. Day shift may need to consider expanding advisory into southern Bristol and Plymouth counties pending 12z model trends. As for convective threat, most of convection later this morning and afternoon appears elevated as core of warm sector and 50+ dew pt air remains offshore. So despite very high low level helicity values in vicinity of developing triple point low, SB instability likely gets shunted offshore. Nonetheless will have to monitor potential rotating storms near the south coast, but again likely remain offshore. As for temps, secondary/triple pt low development will trap low level cool air inland with highs only in the 40s. However farther south warm sector airmass clips south coast, Cape Cod and Islands where temps will climb into the 50s. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... Tonight... Low pres over Gulf of ME continues to intensifies along with increasing CAA in its wake. The combination will result in windy and colder conditions tonight. NW winds 10-20 mph with gusts up to 35 mph over Cape Cod and the Islands. Temps will fall into the 30s but gusty NW winds will provide wind chills in the 20s. Friday... 987 mb low over the maritimes continues to provide blustery NW winds across the region. Despite lots of sunshine highs will remain cooler than normal with temps 40-45 but wind chills in the 30s. && .LONG TERM /FRIDAY NIGHT 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... 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 /09Z THURSDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/...Moderate confidence. 06z update... Thru 12z... VFR cigs at 06z will trend down to MVFR toward 12z. Showers enter western MA/CT 09z-12z, dry weather elsewhere. High confidence. After 12z... MVFR with a period of IFR 15z-21z from west to east. Widespread showers with embedded downpours and isolated T-storm possible. Gusty SE winds up to 40 kt possible south coast including Cape Cod and Islands. A period of LLWS just to the north from PVD-BOS possible. High confidence on trends but some uncertainty on how much wind mixing down to the surface. Tonight... Any lingering MVFR in showers early quickly improves to VFR along with a wind shift from SW to NW with gusts of 25 to 30 knots. A few gusts up to 35 knots possible. High confidence. Friday...VFR, dry weather and NW winds 15-25 kt with a few gusts up to 30 kt possible. High confidence. KBOS Terminal...period of active weather 16z-20z with +SHRA and isolated -TSRA possible. LLWS likely south of Boston toward the Cape. KBDL weather 14z-17z with gusty +SHRA. Outlook /Friday Night through Monday/...High confidence. 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 Friday/...Moderate confidence. Today... Low pres develops south of New England this morning then intensifying and coming onshore into RI/southeast MA this afternoon then emerging over the Gulf of ME this evening. SE gales likely and then becoming SW late. Heavy showers and embedded thunder will limit vsby. Low probability of an isolated waterspout around the islands and offshore. Tonight and Friday... Low pressure over Gulf of ME tonight heads into the Maritimes Fri and provides near gale force NW winds. Dry weather and NW winds provide good vsby. Outlook /Friday Night through Monday/... 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...Wind Advisory from 10 AM this morning to 8 PM EST this evening for MAZ020>024. RI...Wind Advisory from 10 AM this morning to 8 PM EST this evening for RIZ006>008. MARINE...Gale Warning from 11 AM this morning to 4 PM EST Friday for ANZ231>235-237-254>256. Gale Watch from this evening through Friday morning for ANZ230. Gale Watch from 11 AM EST this morning through Friday morning for ANZ236. Gale Warning from 7 PM this evening to 4 PM EST Friday for ANZ250-251. && $$ SYNOPSIS...Nocera NEAR TERM...Nocera SHORT TERM...Nocera LONG TERM...Sipprell AVIATION...Nocera/Sipprell MARINE...Nocera/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.