Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 192323 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 723 PM EDT Mon Mar 19 2018 .SYNOPSIS... Dry but unseasonably cold weather will continue through Tuesday. Another Nor`easter tracks south of New England Wed into Thu but close enough for the potential of 6+ inches of heavy wet snow, strong northeast winds and coastal flooding. High pressure brings a drying trend Friday into Saturday but remaining colder than normal. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... 730 PM update... Only change was to lean toward the colder MOS guidance tonight given ridge of high pressure crest over the region, allowing winds to decouple in the next few hours especially outside the urban areas, high terrain and Cape Cod/Islands. This results in another night with lows in the teens regionwide to around 20 in the urban areas, low 20s Cape Cod and Islands. Otherwise remainder of the forecast is on track. Earlier discussion below. ================================================================== High pressure remains in control of our weather. Some increase in clouds, but otherwise rather quiet. Winds will gradually back through north to northeast and remain rather light. Expecting some radiational cooling, but not ideal. Min temperatures Tuesday morning expected to be about 10 degrees below normal. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... Not expecting much significant weather Tuesday, as a high pressure remains nearby. As a low pressure develops off the coast of VA/NC and moves northeast, there is a low risk for some precipitation to reach the islands late in the day Tuesday. Expecting precipitation to overspread southern New England from SE to NW, mainly after midnight Wednesday. This timing is subject to change, and may hold off until after daybreak Wednesday to begin in earnest. Temperatures will remain below normal, setting the stage for some light snowfall Tuesday night. Snow will be more likely to accumulate at night, but any accumulations should be an inch or less through Tuesday night. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Updated 405 PM... Highlights... * Another Winter Storm Wed into Thu with 6+ Snowfall possible * Heavy Wet Snow & Strong NE winds may result in isolated Power Outages * Minor to Moderate Coastal Flooding Possible Eastern MA Coastline Synoptic Overview & Model Preferences... 12z NAM/GFS/UKMET and ECMWF have all trended northward with sub 990 mb occluded/vertically stacked low crossing over or near the 40N/70W benchmark Wed night and then northeast across Georges Bank Thu. This northwest trend was original supported by the overnight 00z Euro ensembles. This more northward solution seems reasonable given the building upstream ridge over the western CONUS, giving way to downstream trough amplification off the eastern seaboard. Thus will follow this northwest trend in the 12z guidance and 00z ensembles. Snow Potential/model qpf... As we get deeper into the month March a concern regarding accumulating snow is the increasing length of daylight and stronger/higher sun angle. However airmass currently over New England today and for several days has resulted in unseasonably cold temps with overnight lows in the single digits and teens. Thus ground temps are colder than normal. In fact dew pts over the region remain below zero this afternoon, indicating a modified arctic airmass in place. The other way to overcome warm ground temps is elevation but more importantly heavy precip rates are most effective, providing dynamic and diabatic cooling processes. This system has that potential with multiple guidance sources offering over an inch of qpf for the event. Obvious ptype issues across southeast MA, possibly into RI with potential mid level warm layer offering risk of sleet/ice pellets and rain closer to the coast with marginal blyr. However by Wed night column begins to cool with system moving east, eroding any warm layers and changing rain and/or sleet over to all snow overnight Wed into Thu across southeast MA. Regarding the I-95 corridor and points westward into CT and central- western MA ptype looks to be mainly all snow given cold and dry airmass on the front end of this system. In fact model 850 temps Wed cool from -2C to -6C over the region as precip commences and evaporative cooling achieves wet bulb temps. NAM soundings have some mid level warming initially but this should erode as mid level low track near the benchmark, essentially cutting off the northern progression of mid level warm air. In fact favorable track for mainly all snow with occluded/closed low tracking near the benchmark combined with cold confluent flow aloft over New Brunswick, maintaining cold air over southern New England. A model blend of 12z NAM/GFS/EC and UKMET offers up to 1 inch of qpf across most of the region. Typical spring snowstorm with SLR ratio around 10-1 but lower closer to the rain-snow line and slightly higher interior MA. Given model consensus of up to 1 inch of qpf, and fairly cold thermal profiles, greater than 50% risk for 6+ inches of snowfall from the Cape Cod canal northwest thru eastern MA/RI/CT and central MA. Thus have issued a Winter Storm Watch for much of the region. Left out extreme western MA as qpf tails off. As is the case with any spring snowstorm with potential of 6+ inches there is a risk of power outages. Keep in mind from previous storms the onset of snapping tree branches is about 3-4" of heavy wet snow. Strong Winds... All model guidance has low level northeast jet of 50-60 kt Wed and Wed night with core of the jet along the south coast then pivoting up the eastern MA coast. Thus will likely need at least wind advisories for the coastline including the Boston to Providence corridor. In addition, these strong winds could result in blowing and drifting snow if 6+ inches of accumulation comes to fruition. Friday through Sunday... Colder than normal continues as northeast trough reloads and further amplifies. Will have to watch next wave late in the weekend if it tracks close enough to the coast for any impact. && .AVIATION /23Z MONDAY 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. 00z Update... Not much change from previous TAFs, dry weather, diminishing winds and VFR tonight. Earlier discussion below. =============================================================== Tonight...VFR. Weaker NW winds. Tuesday...VFR through the day with increasing high and mid clouds. Winds N-NE. Tuesday Night...Mainly VFR, with local IFR possible. Windy with gusts to 35 kt. Slight chance SN towards the islands. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF. Outlook /Wednesday through Saturday/...Moderate Confidence. Wednesday: MVFR/IFR conditions possible. Strong winds with gusts to 45 kt. Snow possibly heavy at times. Wednesday Night: MVFR/IFR conditions possible. Strong winds with gusts to 45 kt. Snow possibly heavy at times especially in the evening. Thursday: Mainly VFR, with local MVFR possible. Strong winds with local gusts to 40 kt. Thursday Night: VFR. Breezy. Friday through Friday Night: VFR. Saturday: Mainly VFR, with local MVFR possible. && .MARINE... Short Term /through Tuesday Night/...High confidence. Quiet winds and seas are expected through Tuesday. Winds increase and seas begin to build Tuesday night ahead of intensifying surface low pressure east of the Demarva Peninsular. By daybreak Wednesday, NE wind gusts well into gale force will likely have worked their way into the outer southern coastal waters. Outlook /Wednesday through Saturday/...Moderate Confidence. Confidence...Moderate NE winds will ramp up Wednesday with gale force sustained winds or gale force gusts overtaking all waters by mid morning. Storm force gusts are likely late Wednesday morning through evening across the outer southern and eastern waters. Anomalously cold air will help with fairly efficient mixing through the boundary layer to bring the stronger wind gusts to the ocean surface. Have increased the SWANGFS wave height guidance field looks like a fairly good first cut and increased a relatively modest 5 to 10 percent. This results in waves of 15 to 20 feet forecast for east of Cape Ann, Stellwagen Bank, and Massachusetts Bay with a little over 20 feet projected for just east of Cape Cod and Nantucket. Winds back to the N Wednesday night and very gradually diminish overnight but still remaining as gale force through Thursday morning. Winds and seas gradually subside Thursday afternoon and night with seas projected to drop below 5 feet across the outer waters by Friday afternoon. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Winter Storm Watch from Wednesday morning through Thursday morning for CTZ002>004. MA...Coastal Flood Watch late Wednesday night for MAZ019-022>024. Winter Storm Watch from Wednesday morning through Thursday morning for MAZ003>007-010>021-026. RI...Winter Storm Watch from Wednesday morning through Thursday morning for RIZ001>007. MARINE...Gale Watch from late Tuesday night through Wednesday evening for ANZ231>235-237-250-251-254>256. Small Craft Advisory until 2 AM EDT Tuesday for ANZ250-254. Small Craft Advisory until 10 PM EDT this evening for ANZ251. && $$ SYNOPSIS...Belk/Nocera NEAR TERM...Belk/Nocera SHORT TERM...Belk LONG TERM...Nocera AVIATION...Belk/Nocera MARINE...Belk/Nocera is the U.S. government's official web portal to all federal, state and local government web resources and services.