Area Forecast Discussion
Issued by NWS Boston, MA

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291 FXUS61 KBOX 231759 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1259 PM EST Fri Feb 23 2018 .SYNOPSIS... 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. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Forecast remains on track with spotty rain showers moving into the region. Many of the showers are not reaching the ground due to the dry air aloft, but some areas are reporting light rain. We have also received reports of sleet along the eastern slopes of the Berkshires. *** A period of light icing likely later today and early this evening over higher elevations in central and western MA *** Spotty showers will overspread the region before the large swath of rain moves in from the west. Main time frame of the freezing rain looks to be between 1pm and 9pm. Area of heavy rain however will likely not enter the region until 3pm. A Winter Weather Advisory remains in effect for the higher terrain through this evening due to the potential for freezing rain. Sounding profiles show the best chance for freezing rain/sleet to be across the higher terrain given the warm front aloft with cooler temperatures at 925mb. There still stands a low potential for ice accumulations up to a quarter inch on the eastern slope of the Berkshires. Given a large area of PWATs just to our south and southwesterly winds by 21Z, this moisture will likely be advected over the region, bringing areas of heavy downpours into the evening. Most models are suggesting precip accumulations just over a quarter inch.
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&& .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 /18Z FRIDAY THROUGH WEDNESDAY/...
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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...MVFR/IFR conditions will overspread the region by mid afternoon, with rain, pockets of sleet and freezing rain developing across interior MA. Greatest risk of FZRA over higher elevations. Tonight...MVFR prevails with rain this evening, and pockets of FZRA at higher elevations in northern MA. RA/FZRA will end by 3z along the eastern coast. Fog will settle in behind the precip, keeping conditions MVFR in the valleys. Saturday...Fog may linger along the southern coast and islands past 12z. Conditions will improve to VFR across the region by early morning. KBOS Terminal...Moderate confidence. MVFR through this evening with onset of rain near 21z. Rain ending near 3z with improving conditions around 6z. KBDL Terminal...Moderate confidence. MVFR/IFR through this evening. Patchy fog in the AM, burning off by 9z. VFR conditions to follow. Outlook /Saturday Night through Tuesday/...Moderate confidence. Saturday: VFR. Breezy. 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.
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&& .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: Winds less than 25 kt. 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...
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CT...None. MA...Winter Weather Advisory until 9 PM EST this evening for MAZ002>004-008-009. RI...None. MARINE...None.
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&& $$ SYNOPSIS...KJC/Sipprell NEAR TERM...KJC/Nocera/Sipprell/Correia SHORT TERM...KJC/Nocera LONG TERM...Sipprell AVIATION...KJC/Nocera/Sipprell/Correia MARINE...KJC/Sipprell

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