Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 180851 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 351 AM EST Sat Nov 18 2017 .SYNOPSIS...
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High pressure over New England this morning provides dry weather much of the day. A warm front approaches the region late in the day with rain overspreading the area toward sunset and into the evening. Strengthening low pressure moves across western NY state tonight with warm air, strong winds and showers impacting southern New England. The attending cold front sweeps across the area midday Sunday with strong winds ahead and behind the front. Dry and cold weather overspreads the region Sunday night and Monday. Mild and dry weather Tue ahead of a cold front which brings a risk for showers Wed followed by dry and colder weather Thanksgiving day.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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4 AM update... Cold ridge of high pressure over the region this morning with predawn temps in the 20s, 30s along the coast including Cape Cod, islands and city of Boston. Sunshine to begin the day but clouds quickly on the increase as mid level ridge axis moves offshore and flow becomes cyclonic with WAA pattern developing. Despite increasing clouds today low level WAA will provide a nice temp recovery from this morning/s cold with highs this afternoon 50-55 along the coastal plain, extending into the Boston-Providence- Hartford corridor. Farther inland shallow cool air will be more stubborn to modify and highs there rising into the mid and upper 40s. Area of rain showers early this morning over eastern OH will weaken and mainly pass northwest of our region as this lead short wave trough deamplifies as it runs into the ridge over New England. Thus expecting mainly dry weather today with any rain holding off until 21z-00z and mainly confined to CT and western-central MA. Therefore expecting dry weather to prevail today across RI and eastern MA.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/...
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4 AM update... *** Strong SSW Winds South Coastal MA and RI *** *** Possible Fine Line of low convection *** Tonight and Sunday Morning... Wind Threat... Strong low level jet overspreads the south coast tonight. Good agreement from models on magnitude of jet 55-60 kt at 925 mb. Model soundings initially show low level lapse rates favorable this evening for 35-40 kt transfer in gusts. But as the night progresses strong WAA steepens low level inversion with gusts becoming less of a factor and sustained winds becoming the main issue with speeds 25- 30 kt. This will be sufficient to support wind advisory for the entire south coast including Cape Cod and the Islands. Rain and Convective Threat... Periods of rain with warm front lifting north across the region. Impressive warm sector for mid to late Nov overspreads the area late tonight with dew pts surging to 55-60! Thus temps will rise overnight toward 60 in the coastal plain. This will set the stage for a potential fine line of low top convection toward daybreak Sunday from west to east. Short wave energy merges into a negative tilt trough approaching the region toward 12z Sunday. Also a strong upper level jet streak of 130 kt approaches from the southwest 12z Sunday with LFQ over southern New England, enhancing QG forcing. In addition very strong frontal convergence as low pres deepens to 982 mb over NY state. Thus very strong forcing for ascent in a high shear environment. With dew pts rising to near 60 a few hundred joules of CAPE advects across the area, anomalous for mid to late Nov. However limiting factor for convection is marginal low and mid level lapse rates. So question becomes will there be sufficient instability to yield a strong response at the surface in the form of fine low of convection. Unfortunately model timeframe here is 36-42 hrs, on edge of our mesoscale guid with HREFv2 only out to 36 hrs. However the NCAR ensembles, ARW and NMMB all hint at possible fine line in simulated reflectivity fields. For now will insert slight chance thunder with gusty winds and heavy downpours as PWATs climb to 1.5 inches (+2 standard deviations). Sunday Afternoon... *** Strong Post Frontal Winds likely *** 979 mb low traversing northern New England/St Lawrence River Valley will yield strong pres gradient, post frontal CAA and impressive pres rise-fall couplet for a period of strong WNW post frontal winds Sunday afternoon with speeds of 40 to 50 mph possible. Later shifts will have to evaluate model trends for possible issuance of wind advisories for Sunday afternoon. Drying trend behind the fropa along with temps falling from morning highs near 60.
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&& .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
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*/ Highlights... - Widespread strong to damaging W/NW winds Sunday into Sunday night - Winds will linger into Monday with perhaps less of an impact - Ebb and flow pattern of S warm air surges followed by N cold air continues */ Overview... Ebb and flow continues. Without greater amplification and subsequent transfer between the equator and the poles, the flow remains mainly zonal. The MJO largely subdued as the present -WPO/-EPO/-PNA all trend positive into December per ensemble means, counter-productive, lending seemingly to a signal of persistent W to E flow. The only consistencies it seems from analysis of potential vorticity and isentropic surfaces is a persistence of poleward transfer in the N Pacific and the NE Atlantic that lends to S shearing of energy into the Gulf of Alaska and S across Europe, respectively. Otherwise it`s zonal in-between. Any energy across the CONUS is quick moving with little residency such that forces acting to invoke greater curvature and storm development reach their full potential E of our region across E/SE Canada into the NW Atlantic. With racing storm systems over our region, warm-surge in advance followed by cool shots, both with attributes of gusty winds, a chance of wet-weather in-between along associated frontal boundaries, cold fronts with more emphasis. Attendant threats and impacts highlighted in the discussion below. */ Discussion... Sunday night into Monday... Winds shifting NW ushering colder air, remaining blustery. Consensus of forecast guidance, cold front sweeping 7-10a Sunday, undercutting colder air, lapse rates mixing out to H85, dry adiabatic, the top of the mix layer averaging W/NW 45 to 50 mph (40 to 45 kts). Widespread WIND ADVISORY likely, more likely daytime Sunday with boundary layer heating and contributions from quick pressure rises. Ensemble means, strongest winds likely across the high terrain and along the coast adjacent to 50-degree ocean waters, also N/E closer to the storm center, so N/E portions of MA at greatest threat. CIPS analogs with a high probability of at least 35 mph widespread gusts, lower for 40 to 45 mph, however considering local climatology, believe there is the potential to see 40+ mph gusts. Headlines may drop overnight with the potential of a shallow surface inversion, however model guidance consensus signals continued cold air advection right into Monday with additional energy rotating through the cyclonic flow across the region lending to an enhancement of the wind profile. Something will have to watch closely. Will highlight the wind threat in the HAZARDOUS WEATHER OUTLOOK. No headlines at this time associated with W/NW winds. Otherwise, drier air surging in aloft behind the quick sweeping cold front, will see an end to shower activity rather abruptly during the morning hours. With the blustery W/NW winds and cold airmass pouring in, across the warmer Great Lakes, the lake effect machine will be ongoing and could see some of that energy reach far enough E with the mean flow to impact N/W portions of MA and CT with some snow and/or rain shower activity, 2m temperature dependent. If snow, some light accumulations are possible, mainly for the high terrain. All threat diminish into Monday night, and it is during this period that winds may become light enough beneath clear conditions to allow for radiational cooling and night-time lows to drop well down into the 20s, possibly teens. Hard to say right now. If W/NW winds remain brisk, then the mechanical mixing in the environment would limit the radiational cooling potential. Will hold it conservative with wide- spread lows in the 20s. Highs Sunday into the upper 40s to low 50s right before the cold front punches through in the morning, upper 30s to low 40s for Monday. Tuesday... Quiet. High pressure. Return S flow. Turning mild. Still believe that warmest temperatures will be over interior SE New England, that an onshore will develop in response to an approaching storm system for the overnight period into Wednesday. Highs in the upper 40s to low 50s. Tuesday night into Wednesday... Possible showers. Keeping with slight chance PoPs, the EC on again off again on the N and S stream energy beginning to interact with a draw of S-moisture ahead of the N sweeping cold front before both systems push offshore quickly and phase downstream. The GFS keeps the dry-streak going. Given the emphasis on the cold front agreed upon within all available guidance, would expect that the associated lift on available moisture should get something going, especially considering the warmer ocean waters that line the shore. Thanksgiving onward... Thanksgiving looks quiet and seasonable, while into the weekend it appears another quick moving storm system sweeps the region. The ebb and flow pattern continues with milder shots and SW flow ahead of any disturbance followed by colder air and NW winds as the system departs.
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&& .AVIATION /09Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/... 06z... Today...high confidence. VFR and dry weather prevails. The exception will be across western MA/CT where MVFR conditions in light rain will increase between 21z-00z. Light winds this morning increase sharply this afternoon from the SSW with gusts approaching 30 kts by sunset (22z). Elsewhere SSW winds increasing to 10-15 kt. Tonight...High confidence. VFR-MVFR to start in areas of light rain but trending toward IFR-MVFR after midnight. Strong SSW 20-25G45kt winds along the south coast including Cape Cod and Islands. Elsewhere winds not as strong and this results in LLWS across much of RI and eastern MA including BOS. Sunday...moderate confidence. Some uncertainty on probability of -TSRA and areal coverage along and ahead of strong cold front. MVFR-IFR with widespread showers, some with locally heavy downpours. Could be a fine line of low top showers with embedded thunder roughly 15z-18z. Gusty winds may accompany this convection. Strong SSW winds 20-25G45kt continue along the south coast including Cape Cod and Islands. Away from the south coast expect LLWS to continue thru the morning across RI and eastern MA. Then a strong cold front sweeps across the area late morning into the early afternoon, with improving conditions behind the front and a wind shift to the west. KBOS Terminal...High confidence in TAF thru 00z then some timing issues on arrival of lower cigs and rain. Then period of rough weather tonight into midday Sunday with LLWS along with heavy showers. Isolated thunder possible 15z-18z Sunday, then strong cold front sweeps across the area with improving conditions after 18z Sunday. KBDL Terminal...High confidence in TAF thru 21z then some timing issues on arrival of lower cigs and rain. Unsettled weather tonight into Sunday morning with heavy showers and possible isolated Thunder. Outlook /Sunday Night through Wednesday/... Sunday Night: VFR. Windy with gusts to 35 kt. Monday: VFR. Strong winds with local gusts to 40 kt. Monday Night: VFR. Windy with gusts to 35 kt. Tuesday: VFR. Breezy. Tuesday Night: VFR. Windy with local gusts to 30 kt. Slight chance SHRA, slight chance FZRA. Wednesday: VFR. Breezy. Slight chance SHRA. && .MARINE...
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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 Sunday/...High confidence. Today... Light winds this morning with high pressure overhead. However SSW winds increase this afternoon and approach Gale force by late in the day. Tonight... SSW gales along with poor vsby in showers and fog. Sunday... Strong cold front sweeps across the region late morning and early afternoon, with SSW prefrontal gales and WNW gales behind the front. Showers, locally heavy in the morning along with fog but improving with the wind shift. Outlook /Sunday Night through Wednesday/... Sunday Night: Strong winds with gusts up to 35 kt. Rough seas up to 14 ft. Monday: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 11 ft. Monday Night: Low risk for gale force winds with gusts up to 35 kt. Areas of rough seas. Tuesday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Slight chance of rain showers. Wednesday: Winds less than 25 kt. Local rough seas. Slight chance of rain showers.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...Wind Advisory from 10 PM this evening to 5 PM EST Sunday for MAZ020>024. RI...Wind Advisory from 10 PM this evening to 5 PM EST Sunday for RIZ005>008. MARINE...Gale Warning from 5 PM this afternoon to 6 AM EST Monday for ANZ232>234. Gale Warning from noon to 11 PM EST Sunday for ANZ230. Gale Warning from 1 AM Sunday to 6 AM EST Monday for ANZ231- 251. Gale Warning from 7 PM this evening to 7 PM EST Sunday for ANZ236. Gale Warning from 3 PM this afternoon to 6 AM EST Monday for ANZ235-237-255-256. Gale Warning from 7 PM this evening to 6 AM EST Monday for ANZ250-254.
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&& $$ SYNOPSIS...Nocera NEAR TERM...Belk/Nocera SHORT TERM...Nocera LONG TERM...Sipprell AVIATION...Nocera/Sipprell MARINE...Nocera/Sipprell

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