Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 211545 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 1045 AM EST Mon Jan 21 2019 .SYNOPSIS... Arctic air pours into our region today behind the departing storm, bringing dangerously cold wind chills. Moderating temperatures Tuesday into Wednesday, then another storm brings a soaking rain late Wednesday through Thursday night. Return of cold and mainly dry conditions Friday into this weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1030 AM update... Mid level vortex over VT and western MA this morning circulating an arctic airmass across the region with 10 am temps from about 5 above to 5 below zero. This frigid temps combined with NW winds 15-25G35 mph yielding dangerously cold wind chills as cold as -30 at Worcester! Speaking of Worcester, they tied a record low temp this morning of -6F, previously set back in 1970. So wind chill warnings and advisories will remain posted today given core of arctic airmass continues to overspread the region with 1000-500 thickness falling below 500 dam! A mix of sun and clouds this afternoon but also some flurries/very light snow showers at times given cyclonic flow aloft from mid level vortex combined with some upslope flow from NW winds streaming across the high terrain. Ocean effect snow showers continue for the outer Cape given the NW trajectory. Will be reviewing new model guid to see if extent of accumulations if wind turns more to the NNW increasing the fetch of cold air across the relatively warm waters. Speaking of fetch, you know it`s an extremely cold airmass when you have ocean effect snow showers at Nantucket with an extremely short fetch across the sound. Also coastal web cams capturing arctic sea smoke from extreme evaporation saturating the layer immediately above the ocean surface. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/... Overnight and Tuesday mid level heights are on the rise as a high pressure ridge builds into the northeast. This will relax our pressure gradient and bring down the winds by Tuesday as the surface low moves over head. Overnight, skies clear rapidly from the west as dry air moves in and plenty of sun is in store for Tuesday. Don`t let the sunshine fool you, though, since we`ll remain locked in a very cold pattern until Wednesday. Lows Tuesday morning in the single digits to below zero will rebound to the mid 20s by Tuesday. It`s not warm but compared to the single digit highs of Monday, plus the lack of wind, Tuesday will feel practically balmy. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... */ Highlights... - Soaking rains late Wednesday through Thursday night - Return cold air Friday into the weekend - Watching a following early week storm system */ Discussion... 21/00Z guidance remains in rather good overall agreement through Wednesday, then differences start to amplify through early next week. Will prefer a consensus approach to this forecast to smooth over the less predictable details through Wednesday, and to minimize the timing and amplitude differences for the second half of this forecast period. In general, broad mid level ridge over the eastern USA shifts over the North Atlantic Wednesday, while a trough dominates much of the central USA. This configuration should result in a potent SW flow. This will not only track a low pressure our way toward mid week, but also raise temperatures where mainly rainfall is expected. Decent signal for low level temperatures to be about 1 standard deviation above normal. Also looking like another moisture-rich system, with precipitable water values about 3 standard deviations above normal. Have high confidence in a period of wet weather sometime Wednesday into Thursday. Depending upon timing, looking like snow to start with a transition to all rain by late Wednesday afternoon. Small risk for precipitation to end as snow towards NW MA Thursday afternoon. Once this low pressure departs our region, we should have another period of very cold weather heading into this weekend. This could set the stage for another winter storm early next week. Still lots of moving parts with that storm, so not much confidence in the details at this time. && .AVIATION /15Z MONDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/...High confidence 1030 AM update... Mainly VFR but occasionally marginal MVFR in scattered flurries/snow showers. Also MVFR just offshore including Cape Cod and Islands with scattered snow showers. Previous discussion below. ================================================================= Today... Mainly VFR cigs, but areas of MVFR across eastern MA and Cape / Islands. SCT flurries or snow showers possible. NW gusts to 30 to 35 kt with isolated 40 kts across SE coastal terminals. Tonight and Tuesday...VFR. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF Outlook /Tuesday Night through Friday/... Tuesday Night: VFR. Breezy. Wednesday: Mainly VFR, with local IFR possible. Windy with local gusts to 30 kt. RA or FZRA, with a chance of SN or PL. Wednesday Night: Mainly VFR, with areas IFR possible. Windy with gusts to 30 kt. RA or FZRA. Thursday: Mainly VFR, with local IFR possible. Strong winds with areas gusts to 40 kt. Chance RA. Thursday Night: VFR. Windy with gusts to 35 kt. Slight chance RA. Friday: VFR. Breezy. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/... 1030 AM update... Expanded Heavy Freezing Spray for all eastern MA waters. Otherwise no changes. Previous discussion below. =========================================================== NW gales persist through today in the strong cold advection. With the arctic air pouring into the region and rough seas, widespread moderate freezing spray will develop on all open waters, with pockets heavy freezing spray. Tuesday NW winds come down through the day, gusting 20-25 kts by early afternoon. Seas start 5-7 ft with 9-11 ft on the outer waters. By evening 2-4 ft for coastal waters and 5-8 ft outer. Outlook /Tuesday Night through Friday/... Tuesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Wednesday: Moderate risk for Small Craft Advisory winds with areas of gusts up to 30 kt. Areas of rough seas. Chance of rain. Wednesday Night: Low risk for gale force winds with gusts up to 30 kt. Areas of rough seas. Rain. Thursday: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 14 ft. Rain. Thursday Night: Low risk for gale force winds with gusts up to 40 kt. Rough seas up to 13 ft. Chance of rain. Friday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 11 ft. && .TIDES/COASTAL FLOODING... 1030 AM update... Still running about a 0.3 ft surge in response to leftover easterly swells precluding water from draining seaward. Given high astro tide of 11.9 ft at Boston total water levels will top off just over 12 ft. So most prone areas will see very minor water inundation along the eastern MA coastline. Previous discussion below. ================================================================== Monday morning astronomical high tides are a few tenths higher along the east coast. While both seas and surge will have subsided, there is a lingering easterly swell and wave action offshore to lead to an additional 0.1-0.3 foot surge. With an astronomical tide around 11.9 at Boston at 11AM, there is the risk for some spotty minor coastal flooding. Overall, the flooding risk is lower, and will likely be limited to the typically prone locations. A Coastal Flood Statement has been issued. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Wind Chill Advisory until 8 AM EST Tuesday for CTZ002>004. MA...Wind Chill Advisory until 8 AM EST Tuesday for MAZ005>007- 010>021. Wind Chill Warning until 5 AM EST Tuesday for MAZ002-008-009. Wind Chill Warning until 8 AM EST Tuesday for MAZ003-004-026. RI...Wind Chill Advisory until 8 AM EST Tuesday for RIZ001>007. MARINE...Heavy Freezing Spray Warning until 6 AM EST Tuesday for ANZ231- 232-250-251-254. Gale Warning until 7 AM EST Tuesday for ANZ232>234. Freezing Spray Advisory until 11 AM EST Tuesday for ANZ233>235- 237-255-256. Gale Warning until 9 PM EST this evening for ANZ230. Gale Warning until 6 AM EST Tuesday for ANZ231-251. Gale Warning until 10 PM EST this evening for ANZ236. Gale Warning until 5 AM EST Tuesday for ANZ235-237. Gale Warning until 10 AM EST Tuesday for ANZ250-254>256. && $$ SYNOPSIS...Belk/Sipprell/BW NEAR TERM...Belk/Nocera/BW SHORT TERM...BW LONG TERM...Belk AVIATION...Belk/Nocera/BW MARINE...Belk/Nocera/BW TIDES/COASTAL FLOODING...Nocera

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