Area Forecast Discussion
Issued by NWS Boston, MA

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227 FXUS61 KBOX 191816 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 216 PM EDT Thu Apr 19 2018 .SYNOPSIS... Low pressure passes southeast of Nantucket this afternoon, and passes south of Nova Scotia tonight. This will bring a period of chilly rain today. The rain may mix with wet snow, especially in the higher elevations. Drier air moves in on Friday, but with isolated showers possible. Building high pressure will bring gradually warming temperatures and dry weather into early next week. Storm system expected for mid to late week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 2 PM update... Surface low pressure center was passing well southeast of Nantucket early this afternoon. Upper level trough and shortwaves moving thru it will bring rounds of showers to southern New England during this afternoon and evening. Main precip type this afternoon would be rain, except portions of the interior (including the higher terrain of the Worcester Hills and east slopes of the Berkshires) could see some light snow showers. An inch of snow accumulation is possible over the higher terrain, with up to a coating elsewhere over the interior where any light snow may fall. Temperatures not expected to climb much more this afternoon, with highs in the upper 30s to mid 40s. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... Tonight... Even as the surface low moves off to the east tonight, the upper trough will be swinging overhead. The axis is sharp, and carries a -24C to -26C cold core. This may be sufficient to maintain clouds and a chance of showers through the evening. The trough moves off after midnight, after which we may see some clearing especially in southern sections. Friday... Cyclonic flow around the upper low continues on Friday. The moisture aloft diminishes through the day, which should allow at least partly sunny skies. The diminishing moisture will reduce the development of showers, but we expect at least widely scattered showers during the afternoon. The cold advection and cold temperatures aloft should allow mixing to at least 850 mb. Winds in this layer should reach 25 knots, so we expect gusts of 25-30 mph. Temperatures in the layer support mid to upper 40s, and if mixing goes above 850 mb then highs in the low 50s are possible. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... */ Highlights... - Near-seasonable, dry conditions over the weekend - Warmer conditions, dry, Monday into Tuesday - Mid to late week storm system Overview and model preferences... A couple of slow meandering cutoffs to monitor through the long term. The first, is the one that will most effectively define sensible wx details through the weekend, formed in N New England and shifting into the Maritimes. The slow movement maintains below normal H5 height anomalies through early next week, yielding temps near or below normal in spite of the continued modification. As this first cutoff shifts E, riding will build in its wake thanks to warm advection from the W. This ridge will dominate the forecast through the weekend and into early next week. The second cutoff meandering through the S stream all weekend lead to a merger point for Pacific energy mid-week yielding another risk for wet and more unsettled wx. Given synoptics are in fair agreement between both operational and ensemble data, will continue to use a blend for this forecast update. Discussion... Weekend... Upper lvl cutoff and attendant trof remain close enough to S New England that upper lvl flow is cyclonically curved through the period. This may assist in diurnal cloud development. Combine this with H85/H92 temps a full std deviation below normal (H85 roughly -4C to -6C while H92 temps run near -2C), and expect both highs and lows to remain below seasonal normals. Mon and Tue... Moderating temperatures as ridging gains control. H85 temps shift above 0C by Mon afternoon, with H92 temps approaching +4 to +6C. This will allow highs to reach near or even above seasonal normals, limited in part by the strengthening subsidence inversion as high pres crests across the region. Also, coastal areas limited by sea breezes thanks to low lvl flow SSTs still in the 40s. Overnight lows cool thanks to radiational cooling setup. Mid-late next week... A potential merger of Pacific, S stream and even N stream energy will redevelop longwave trof across the E CONUS, agreed upon by most ensembles as mean height anomalies once again drop below normal. This will yield another round of wetter and more unsettled wx. Timing could be as early as Wed for a new round of wet wx, but exact timing/details will be better defined as we approach. && .AVIATION /18Z THURSDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/... This afternoon and evening...Moderate confidence. Areas of MVFR CIGs/localized reduced vsbys with -SHRA, except reduced vsbys in -SHSN/SHRA possible across higher terrain of Worcester Hills and east slopes of the Berkshires. CIGs/VSBYs improving to VFR late this afternoon and evening, though scattered SHRA/-SHSN remain possible and could bring brief MVFR conditions. Exception to this is along the Cape/Islands where IFR conditions expected to prevail into early this evening, then improving to MVFR. Winds shift to the N-NW through the day, peaking during the evening hours. Overnight...Moderate confidence. Conditions improving to VFR. Friday...High confidence. VFR. Northwest winds will gust to 25 knots during the day. Scattered showers possible along the east slopes of the Berkshires, this could briefly cause MVFR cigs/vsbys. Elsewhere patchy sprinkles possible. KBOS Terminal...Moderate confidence in TAF. Mainly rain and MVFR conditions thru this afternoon, then improvement to VFR during the evening hours. Gusty NW winds on Friday preclude sea breeze development. KBDL Terminal...Moderate confidence in TAF. MVFR CIGs this afternoon with scattered SHRA, improving to VFR this evening. Outlook /Friday Night through Monday/...Moderate confidence. Friday Night through Saturday: VFR. Breezy. Saturday Night through Monday: VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/...High confidence. Today... A coastal weather system passes near the benchmark mid afternoon, then moves south of Nova Scotia tonight. Rain will lower vsbys to around 2-3 miles today. Winds remain below 25 knots and seas below 5 feet. Tonight... Increasing northwest winds tonight as the offshore weather system moves off past Nova Scotia. Winds will reach 30 knots at times. Seas will build on the southern waters late at night, with five to seven foot heights overnight. Small craft advisory has been issued for all waters. Friday... Northwest winds gusting to 25 knots, with seas near 5 feet on the outer waters. Small Craft Advisory in effect. Outlook /Friday Night through Monday/...Moderate Friday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of seas approaching 5 ft. Saturday through Monday: Winds less than 25 kt. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 2 AM to 3 PM EDT Friday for ANZ231>234-251. Small Craft Advisory from 4 AM to 6 PM EDT Friday for ANZ230. Small Craft Advisory from 4 AM to 3 PM EDT Friday for ANZ236. Small Craft Advisory from 9 PM this evening to 3 PM EDT Friday for ANZ235-237. Small Craft Advisory from midnight tonight to 3 PM EDT Friday for ANZ250. Small Craft Advisory from 9 PM this evening to 8 PM EDT Friday for ANZ254>256. && $$ SYNOPSIS...WTB/Doody NEAR TERM...WTB/Doody/NMB SHORT TERM...WTB LONG TERM...Doody AVIATION...WTB/Doody/NMB MARINE...WTB/Doody is the U.S. government's official web portal to all federal, state and local government web resources and services.