Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 221200 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 700 AM EST Tue Jan 22 2019 .SYNOPSIS... Arctic airmass and dangerously cold wind chills will diminish this morning as high pressure builds over Southern New England. Temperatures moderate into the 20s today, then to the 40s Wednesday and Thursday. An approaching frontal system will bring occasional rain Wednesday afternoon into Wednesday night with a period of heavy rain possible on Thursday. Mainly dry and more seasonable temperatures Friday then turning much colder Saturday. Snow showers are possible Sunday as another front approaches. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 7AM update... Overall, forecast remains on track this morning. Wind chill values still running close to advisory criteria, so plan to let this ride through its set ending time of 8AM local. Temps/dwpts and winds were brought up to match current trends, otherwise, no significant changes are necessary. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... Tonight... High pressure overhead early, but then shifts offshore during the night. This will support light wind and clear skies, so radiational cooling should allow temps to fall through the first part of the night, then increasing clouds later at night may cause temperatures to level out. In any case, we went a couple of degrees colder than min temp guidance. Wednesday... Increasing S-SW flow during the day with deepening column of moisture. Precipitable water values climb to 0.75 inches late in the day, but with higher values upstream. The column is marginally cold enough for some snow/sleet at the onset across the interior, but this should be short lived as increasing SW flow rapidly warms the boundary layer. Temps moderate into the upper 30s to mid 40s during the afternoon. Low level jet of 50-60 kt moves off the Mid Atlantic coast toward evening, enough to bring winds of 30-35 kt toward evening. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Highlights... * Rain Wed night possible starting as snow/mixed precip interior * A period of heavy rain Thu, especially SE New Eng * Mainly dry and turning colder Fri into Sat * Snow showers possible next Sunday Overview and model preferences... Taking a broad look, most guidance is favoring a period of negative AO/NAO coupled with negative EPO and slightly positive PNA. Unsurprisingly, this favors a pattern that will allow for arctic influence across the central and E CONUS late week and into the weekend. Available guidance supports this, with a lobe of the polar vortex diving southward late week as well. Otherwise, even with the cold air entrenched across the region now, the very weak ridging suggests that sfc high pres passage is mainly to the S Wed. Warm advection floods across the NE as a result, so any mixed precip should change rapidly to rain. Noting reasonable agreement between long range guidance sources such that a consensus blend can be used as a baseline. Details... Wed night... Initial round of overrunning precip as warm front crosses the region. Soundings/low lvl thermal fields, even taking wet-bulb processes into account warm rapidly. Areas N of the Mass Pike may begin as a period of snow/sleet before a rapid change to rain across the entire region. Strongest LLJ and highest PWAT plumes approach by 12Z so precip should remain light/moderate most of the overnight hours. Non-diurnal temp trend expected, with increasing dwpts/temps during the overnight. Thu... Setup for heavy rain. Combination of approaching frontal wave, PWAT plume nearly 1.5 inches (3 std deviations above normal) and coupled 80-90 kt LLJ at both H92 and H85. Periods of heavy rain expected, focused across the SE especially as the core of the LLJ traverses SE. Also noting a period of steep upper lapse rates which hint at convective potential, as previous forecaster noted. Will need to monitor for localized urban and even some river flooding, given that some rivers continue to run high from the storm this past weekend. Ice/snow melt at the surface will exacerbate this potential. Regarding the wind risk, best chance for strong winds will be across the Cape/Islands based on the current LLJ track. However, soundings show robust inversion which will limit mixing somewhat. Agree however, just given the heavy rain/convective risk that a few gusts may be strong enough to require headlines. Timing of cold advection suggests a risk for a brief change to SN before ending, especially in the slower ECMWF/NAM solutions. Fri... Cold advection with breezy conditions expected, however the coldest air remains NW associated with a secondary arctic front. Seasonable temperatures feel colder mainly due to wind chills. Mainly dry otherwise. Sat... Arctic frontal passage Fri night into Sat. A few light SHSN/flurries possible given some moistening above the BL. Otherwise, arctic air mass introduces temperatures mainly below normal for Sat, Sat night. Best chance for any accums will be in the higher terrain. Sun... Arctic shortwave and attendant clipper rotates from the Great Lakes. Current low pres track remains to the NW, however inverted sfc trof could lead to some precip, mainly SN based on current thermal profiles. QPF is light and could be very localized, but something to watch. Early next week... High uncertainty regarding another lobe of the arctic vortex rotating south through the fully established longwave trof. Sfc low pres tracks range from inland runners to very strong low pres well to the S of New England from current ensembles and is based partially on a weaker southern stream wave. Neither of these features are being well sampled, so this too will just be something to watch as we approach. && .AVIATION /11Z TUESDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/...High confidence Today... VFR. W-NW winds strongest during the morning, then gradually dissipating into the afternoon and evening. Tonight... VFR. Clear skies early along with light winds. High pressure moves offshore overnight and leaves room for some high clouds overnight. Wednesday... VFR through at least mid afternoon. Conditions lower and thicken through the afternoon with MVFR cigs/vsbys possible late afternoon in developing rain...possibly brief sleet or freezing rain in northwest Massachusetts. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF Outlook /Wednesday Night through Saturday/... Wednesday Night and Thursday: Mainly IFR, with local MVFR possible. Strong winds with local gusts to 40 kt. RA, patchy BR. Thursday Night: Mainly VFR, with local MVFR lingering. Windy with local gusts to 30 kt. Slight chance RA. Friday through Friday Night: VFR. Breezy. Saturday: Mainly VFR, with local IFR possible. Breezy. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/... 12Z update... Extended timing of Gales and east coastal freezing spray through mid morning. Current conditions offshore still show several gusts reaching gales and buoys indicate freezing spray continues. Today... Northwest winds at gale force diminish this morning as high pressure builds over New England. Moderate to heavy freezing spray lingers this morning, but diminishes as the wind diminishes. Seas 5 to 10 feet this morning, subsides through the day. Tonight... High pressure over the waters will bring fair skies and light wind. Seas 5-6 feet on the eastern outer waters early, but subsiding. Wednesday... Increasing southwest winds with gusts reaching 30 to 35 kt in the afternoon/evening. Seas building to 5-6 feet on the outer waters and coastal RI waters. Chance of rain. Outlook /Wednesday Night through Saturday/... Wednesday Night and Thursday: High risk for gale force winds with gusts up to 45 kt. Rough seas up to 16 ft. Rain, patchy fog. Thursday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Rough seas up to 14 ft. Chance of rain. Friday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 11 ft. Friday Night and Saturday: Winds less than 25 kt. Areas of rough seas. Local visibility 1 to 3 nm. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Wind Chill Advisory until 8 AM EST this morning for CTZ002>004. MA...Wind Chill Advisory until 8 AM EST this morning for MAZ003>007- 010>021-026. Wind Chill Advisory until 11 AM EST this morning for MAZ002- 008-009. RI...Wind Chill Advisory until 8 AM EST this morning for RIZ001>007. MARINE...Gale Warning until 7 AM EST this morning for ANZ230>237-251. Freezing Spray Advisory until 11 AM EST this morning for ANZ233>235-237-255-256. Gale Warning until 2 PM EST this afternoon for ANZ250-254>256. && $$ SYNOPSIS...WTB/Doody NEAR TERM...WTB/Doody SHORT TERM...WTB LONG TERM...Doody AVIATION...WTB/Doody MARINE...WTB/Doody

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