Area Forecast Discussion
Issued by NWS Boston, MA

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374 FXUS61 KBOX 230807 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 307 AM EST Fri Feb 23 2018 .SYNOPSIS...
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High pressure over Maine moves east today. A frontal system approaching from the west will bring another round of rain and mixed precipitation this afternoon and evening. Active weather pattern continues this weekend with more rain and interior mixed precipitation Saturday night and Sunday. A near- seasonable, quiet pattern for early next week.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/... *** A period of light icing likely later today and early this evening over higher elevations in central and western MA *** 1040+ mb high pres across Maine moves east of Nova Scotia later today with developing warm advection pattern this afternoon. Some morning sunshine is possible, but clouds will quickly overspread the region late morning through afternoon. Deepening moisture plume and modest forcing for ascent will result in precip overspreading region mid/late afternoon from west to east. Surface pressures are still rather high as precip moves in indicative of some low level cold air. 925/950 mb temps are initially subzero as precip begins to move in suggesting a rain/sleet mix across central/western MA with plain rain elsewhere. Areas of freezing rain likely over higher elevations in MA per hi-res guidance surface temps near or slightly below freezing. HREF showing highest probs of FZRA over the east slopes of the Berkshires where up to one tenth of an inch ice accretion is possible. Winter weather advisories will be issued for the Berkshires and will extend eastward to the Worcester hills for light icing which will result in hazardous travel. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... Tonight... Rain will be exiting in the evening from west to east as mid level drying moves in behind departing surface wave and winds shift to west. Temps rising through the 30s, and into the 40s in the coastal plain. Patchy low clouds and fog will develop tonight with some clearing possible toward daybreak. Saturday... Surface ridging builds into New Eng bringing dry weather. Sunshine will give way to increasing afternoon clouds in progressive pattern, with just a low risk for a few showers moving into SW New Eng by evening. A rather mild day expected in the coastal plain. 925 mb temps range from +2C in NW MA to 8C near the south coast. With NW flow, temps will reach well into the 50s in the coastal plain with a few locations in RI and SE MA possibly near 60. Cooler 40s over higher elevations NW MA. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... */ Highlights... - Wintry mix transitions to freezing rain Saturday night into Sunday - Quiet, dry, near-seasonable early week period - Stormy pattern emerging for the beginning of March */ Overview... Upstream Pacific flow flattening, SE CONUS sub-tropical H5 ridge weakening, SW-NE thermal wind axis and storm track along the better baroclinic zone shifts S/E along which mid-level impulses eject out of a preferred H5 trof pattern across the W CONUS. An active weather pattern with a series of disturbances delivering mixed precipitation events thru the weekend. Building up over the N Atlantic signaled by a strongly -NAO, initial omega-block transitions to a rex-block, retrograding W. The preferred region of storm development shifting, a quiet early week period looks to turn stormy for the beginning of March. Preference to high-res guidance through the weekend capturing 2m temperatures in more detail, with ensemble means thereafter. Hit on targets of opportunity below. */ Discussion... Saturday night into Sunday... An over-running precipitation event associated with a classic warm occlusion into S Canada. Transitioning precip-types as a warm nose around H8 becomes pronounced, deepening above a shallow surface cold airmass, maintained by interior S funneling ageostrophic flow aided by a secondary coastal surface low invoked by strong synoptic lift / forcing as cold surface high pressure is situated N/E. Whew. Overall a favorable synoptic setup for an initial onset of snow and/or sleet before changing over to freezing rain, mainly across the interior. Change-over if not already rain towards the coast. No surprise consensus forecast guidance edged colder, more high-res guidance becoming available. CIPS analogs a higher probability of a >6 hour freezing rain event, yet notably a large spread in the top 5 analogs. While leaning N/W MA holding below freezing throughout, and that sub-freezing wet bulb conditions could extend as far S as N CT and NW RI, hesitation given a decent slug of precip falling through an ever increasing warm layer above a shallow cold layer. Forecast trend accordingly, entrenching cold overnight but chipping away into Sunday morning with increasing warm air advection aloft through which precipitation intensity reaches its peak. A retreat of mainly freezing rain outcomes N/W over time, reiterating that areas in N/W MA are likely to see freezing rain throughout after a brief burst of snow and/or sleet. Ice accretion amounts upwards of 0.1 to 0.2 inches given less dry air intrusions and deeper lift / forcing. This an precipitable waters in the 0.75 to 1.00 inch range, looking at storm-total precipitation amounts that can be well up around 0.50 to 0.75 inches, locally higher along SW facing high terrain slopes and along the anticipated kinked surface warm front. Early next week... Continued preference to ensembles. N Atlantic traffic build up, an omega-block evolves lending to a strongly -NAO. Evolution towards a rex-block retrogressing as energy continually emerges from upstream out of a W CONUS H5 trof pattern, indications the initial region of preferred storm development off SE Canada alters W closer to the E CONUS. The slowed pattern allowing cyclones to mature and later occlude in our neighborhood, forecast model consensus trending with a deep, strong storm in our vicinity for the beginning of March. Way out in time, can hardly touch on details and specifics, continued ensemble member spread. However can`t ignore such robust signal when tides are reaching their peak (Boston around 11.5 feet). Until then will keep it dry and near-seasonable right on up through Thursday. Then all eyes are upon a potential storm system. && .AVIATION /08Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/...Moderate confidence. Through today...Patchy MVFR/IFR cigs/vsbys early this morning, otherwise mainly VFR into early afternoon. Lowering to MVFR/IFR mid to late afternoon with rain developing, but pockets of sleet and freezing rain interior MA. Greatest risk of FZRA over higher elevations. Tonight...Rain in the evening, with pockets of FZRA higher elevations in northern MA, then mainly dry weather. MVFR/IFR conditions improving to VFR late. Saturday...VFR. KBOS Terminal...Moderate confidence. Low risk for brief MVFR cigs this morning. KBDL Terminal...Moderate confidence. A few hours of MVFR cigs possible this morning. Outlook /Saturday Night through Tuesday/...Moderate confidence. Saturday Night: Mainly VFR, with local MVFR possible. Breezy. Chance RA, FZRA, chance SN. Sunday: Mainly IFR, with areas MVFR possible. Windy with local gusts to 30 kt. RA, FZRA, patchy BR. Sunday Night: Mainly VFR, with local IFR possible. Breezy. Patchy FG. Monday: VFR. Monday Night: VFR. Breezy. Tuesday: VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/...High confidence. Today...NE winds veering to SE in the afternoon. Gusts to 20 kt over eastern waters late in the day. Vsbys lowering in rain and fog late. Tonight...A brief period of southerly gusts to near 25 kt possible NE MA waters in the evening, otherwise winds shifting to west with gusts to 20 kt. Rain exits in the evening with improving vsbys, but patchy fog may develop over southern waters. Saturday...Diminishing west winds becoming northerly in the afternoon. Seas below SCA. Outlook /Saturday Night through Tuesday/... Saturday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas locally approaching 5 ft. Chance of rain. Sunday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Rain, patchy fog. Local visibility 1 nm or less. Sunday Night: Winds less than 25 kt. Areas of rough seas. Chance of rain, patchy fog. Local visibility 1 to 3 nm. Monday through Monday Night: Winds less than 25 kt. Seas up to 5 ft. Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Winter Weather Advisory from 1 PM this afternoon to 9 PM EST this evening for MAZ002>004-008-009. RI...None. MARINE...None. && $$ SYNOPSIS...KJC/Sipprell NEAR TERM...KJC SHORT TERM...KJC LONG TERM...Sipprell AVIATION...KJC/Sipprell MARINE...KJC/Sipprell

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