Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 142156 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 456 PM EST Wed Nov 14 2018 .SYNOPSIS... Cold high pressure builds across northern New England tonight yielding much colder than normal temperatures for southern New England. The high provides dry weather most of Thursday however snow begins to overspread CT and RI around or shortly after sunset, as low pressure tracks up the eastern seaboard. As this low tracks along the south coast of New England Thursday night and Friday, snow changes to rain along and near the coast with snow to ice inland. Behind the storm dry weather follows Friday night into Saturday. A weak system may bring scattered snow showers Sunday night into Monday. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... 4 PM update... Two decks of clouds to consider this afternoon/evening. The lower clouds are linked to lake-effect which has been persistent since early this morning. However, with core of +1035mb high pres about to crest early overnight, these should be cutoff within the next few hours. The upper lvl CI, on the periphery of a modest jet, will likely persist, given little change in the mid and upper lvl pattern. Even with the CI, the high pres crest will yield decent decoupling, in spite of the breezy conditions through the afternoon. Mixed afternoon dwpts mainly in the single digits suggest plenty of room for inversion development overnight. Doubtful that the CI will limit radiational cooling, so leaned most heavily on the colder guidance, defined mainly bet the latest MAV/MET guidance. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY/... 445 PM update... *** Winter Storm Watch for Snow & Ice Inland Thu night *** *** Winter Weather Advisory for a wintry mix Thu night *** *** Wind Advisory - strong winds Thu night/Fri Cape/Islands *** Synoptic Overview... Anomalous cold airmass over New England tonight into Thu will set the stage for a winter storm as a loaded southern stream system over the Gulf states induces cyclogenesis along the eastern seaboard. Models in fairly good agreement on the large scale features including track of mid level low and surface circulation along with trajectory of the dry slot. As usually the case with these winter storms there are model differences in thermal fields which is typical at this time range. Details... Thursday... Mid level ridging in concert with 1034 mb high over the region provides dry weather Thu. While it will be cold with highs only in the 30s, much less wind than today which will make tomorrow more tolerable. However moisture from southern stream system approaches from the southwest late in the day with snow possible overspreading the CT and RI between 4-7 pm. The column along with surface temps are cold enough to support snow across all of CT-RI. Strong 850 mb jet approaching the region with lift focused in the snow growth region may yield a descent front end dump of snow. HREF indicating 30-50% probability for hourly snowfall rates approaching 1" toward 00z across CT and RI. Thus could see some impact to the late day commute from HFD-PVD. Elsewhere snow likely holds off until after 7 pm. Ptype/Snowfall potential... Anomalous cold airmass over the region this evening with dew pts in the single digits! This 1030+ mb high over Maine and New Brunswick combined with pres falls approaching from the southwest will likely induce a coastal front to setup over eastern MA, shifting winds from E to NNE during Thu evening. This will maintain low level cold air and support all snow for at least a few hours Thu evening. However as the low level jet approaches, onshore flow will increase and the coastal front will likely retreat inland first to the RT-128 corridor then I-495 corridor. This will result in the rain-snow line moving onshore into eastern MA. Complicating the forecast is also warm air aloft surging northward with mid levels becoming isothermal suggesting snow mixing with sleet. However the forcing for ascent is fairly robust during the evening (00z-06z) which may be sufficient to cool this layer along with heavy snow melting in this layer providing additional cooling. NAM soundings hold onto this snow potential until about midnight or so for the Greater Boston area. If this verifies looking at potential 2-4" before a change over to sleet and finally rain. However if sleet mixes in sooner along with coastal front moving inland quicker, a coating to 2 inches is more likely for Boston area. Just too early to say which scenario will play out. Given the wintry mix of precipitation expected a Winter Weather Advisory has been issued. For RI and southeast MA brief period of snow, heavy at times from 7 pm to 10 pm then snow changing over to sleet and finally all rain. Snow accumulations of 1-2" possible with up to 3" possible northern RI. A winter weather has also been issued for this area. For CT and especially western - central MA 2-4" of snow is possible before dry slot overspreads the area, changing snow and sleet to freezing rain around or shortly after midnight. Concern here is for an extended period of freezing rain/drizzle into Fri morning as low level cold air will be difficult to scour out with pres falls tracking up the eastern seaboard. This will support good northerly drainage flow down the CT river valley supporting freezing rain/drizzle from about midnight into Fri morning. So given the potential for 2-4" of snow followed by significant icing decided to go with a Winter Storm Watch to highlight the potential for significant winter weather. This is for all of northern CT into western-central MA Thu evening into Fri morning. Other contributing factors for the potential for a period of heavy snow is strong forcing for ascent in the snow growth region (-10C to -18C) and isothermal layers aloft. Strong Wind Potential... We hoisted a wind advisory for Cape Cod and the islands Thu night into Fri as low level jet moves across this area. Model soundings suggest high probability for 40+ kt ENE gusts. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Highlights... * Cooler than normal conditions persist. * Weak low pressure may bring scattered rain/snow showers late Sun into Mon Overview and model preferences... Zonal flow, mainly at the base of a persistent longwave trof, develops in the wake of late week storm and attendant shortwave. Given this is really on the elongated base of this trof, there is a continued conduit for Canadian CP air to filter into the region through the weekend, in spite of the slight SW-NE orientation of the mean jet aloft. The next feature to watch, is a lobe of arctic energy rotating around a cutoff low pres near James Bay. This should enact at least modest cyclogenesis along a stalled frontal boundary along or just S of S New England by late Sun into Mon. Whether the inland low pres, tied closely to the vort-max to the N is able to efficiently transfer energy to coastal low later Mon. Given there is some uncertainty, thermal profiles also retain uncertainty. These will be better resolved with time. Otherwise, with vort-max shifting E into the middle of next week, a return of drier but chilly weather is anticipated, given the longwave trof remains in play. Models are showing enough agreement with the 14.12Z guidance update, enough that a consensus blend can be used. Details... Fri night into Sat... Blustery W-NW flow continues following the low pres shifting into the Canadian Maritimes. Strong cold advection coincident with this flow. Lows mainly in the 20s with colder wind chills Fri night give way to highs mainly in the mid to upper 40s Sat. Sun... Weak W-E oriented front settles near S New England, but the area remains entrenched beneath cold airmass and influence of lingering inverted ridging. Therefore, will favor mainly dry conditions through the daylight hours with temps remaining in the low-mid 40s. Sun night into Mon... Inland low pres development, with potential coastal low development possible. At this time, airmass looks to cool enough in the low lvls Sun night to support predominantly SN across the region except mainly the immediate S coast where onshore component will draw across SSTs still around +10C. Whether the low pres develops further offshore Mon will determine whether colder air gets locked in. At this time, consensus blend supports a mainly wintry event and will continue with this. Limiting factor will be QPF, as the origins of this low pres across the CONUS. Therefore, looking at mainly an advisory lvl system if that. Into mid next week... High pres, associated with an arctic airmass will settle across the region into mid week. The pattern remains under influence of the longwave trof. So, expect the dry and cooler than normal conditions to persist. && .AVIATION /22Z WEDNESDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/...High confidence. 18z update... Tonight... VFR and dry weather with gusty NW winds diminishing with sunset. Thursday... VFR to start but MVFR 21z-00z across CT-RI and south coast of MA as snow overspreads this region. Snowfall hourly rates may approach 1 inch per hour toward 00z from BDL-PVD and points southward. Thursday night... MVFR to start but then lowering to IFR/LIFR after 03z. Snow heavy at times 00z-03z but then changing over to rain south coast. Snow and sleet changes over to freezing rain inland with all rain in the coastal plain. Strong NE winds develop south coast after 06z. Friday... IFR/LIFR in the morning with rain showers except freezing rain possible interior. Strong NE winds early Cape Cod. Improving to MVFR- VFR during the afternoon with wind shift to the west. KBOS Terminal...High confidence in TAF thru 00z Fri. Snow may hold off until after 00z Fri then briefly heavy until about 03z/04z. Coating upwards to 2 inches possible. Low prob of 3 to 4 inches but coating to 2 inches more likely as of now. KBDL Terminal...High confidence in TAF thru 18z Thu. Snow arrives Thu 21z-00z with hourly snowfall rates approaching 1 inch per hour possible toward 00z and thereafter until 03z before changing to sleet and freezing rain. Outlook /Friday Night through Monday/... Friday Night through Saturday Night: VFR. Breezy. Sunday: Mainly VFR, with local IFR possible late. Slight chance SN late. Sunday Night: Mainly IFR, with areas MVFR possible. Chance SN, chance RA. Monday: MVFR/IFR conditions possible. Breezy. Chance RA, chance SN. && .MARINE... *** NE Gales Likely Thu Night/Fri morning *** Tonight and Thursday...light winds with 1030+ mb high over Maine. Thursday night...NE gales with vsby reduced less than a mile initially in heavy snow then over to rain as low pressure tracks along the south coast. Friday...low pres near Cape Cod exits with improving weather during the afternoon. Outlook /Friday Night through Monday/... Friday Night: Moderate risk for Small Craft Advisory winds with areas of gusts up to 30 kt. Rough seas up to 13 ft. Saturday: Winds less than 25 kt. Areas of rough seas. Saturday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Sunday: Winds less than 25 kt. Seas locally approaching 5 ft. Sunday Night: Winds less than 25 kt. Chance of rain. Monday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain. && .HYDROLOGY... Renewed river flooding is possible across RI and eastern MA with 0.50 to 1.0 inches of rainfall possible Thu night into Fri after a few inches of snow first. Will have to watch this closely. && .TIDES/COASTAL FLOODING... Astronomical tides are very low (below 9 ft) in Boston. Thus with this upcoming storm worse case scenario would be isolated minor flooding possible. && .CLIMATE... Record low temps for Nov 15 BOS 17 in 1933 PVD 18 in 1905 BDL 17 in 1905 and 1933 ORH 14 in 1933 Record low max temps for Nov 15 BOS 30 in 1933 PVD 30 in 1933 BDL 30 in 1933 ORH 30 in 1972 and 1996 && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Winter Storm Watch from Thursday afternoon through Friday morning for CTZ002>004. MA...Wind Advisory from 1 AM to 10 AM EST Friday for MAZ022>024. Winter Weather Advisory from 4 PM Thursday to 1 AM EST Friday for MAZ017>021. Winter Weather Advisory from 7 PM Thursday to 1 AM EST Friday for MAZ007-013>016. Winter Storm Watch from Thursday evening through Friday morning for MAZ002>006-008>012-026. RI...Wind Advisory from 1 AM to 10 AM EST Friday for RIZ008. Winter Weather Advisory from 4 PM Thursday to 1 AM EST Friday for RIZ001>007. MARINE...Gale Warning from 10 PM Thursday to 4 PM EST Friday for ANZ232>235-237-254>256. Gale Warning from 7 AM to 4 PM EST Friday for ANZ230. Gale Warning from 1 AM to 4 PM EST Friday for ANZ231-250-251. Small Craft Advisory until 7 PM EST this evening for ANZ236. && $$ SYNOPSIS...Nocera/Doody NEAR TERM...Doody SHORT TERM...Nocera LONG TERM...Doody AVIATION...Nocera/Doody MARINE...Nocera/Doody HYDROLOGY... TIDES/COASTAL FLOODING... CLIMATE... is the U.S. government's official web portal to all federal, state and local government web resources and services.