Area Forecast Discussion
Issued by NWS Boston, MA

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796 FXUS61 KBOX 281734 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 134 PM EDT Tue Mar 28 2017 .SYNOPSIS... Wet weather continues through tonight. Some improvement expected on Wednesday. High pressure brings dry and cool weather Wednesday night and Thursday. Low pressure from the Ohio Valley passes south of New England over the weekend. This brings a mix of precipitation Friday and Saturday, followed by dry weather Sunday and Monday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1 pm update (updated previous discussion)... Low clouds persist ahead of later day showers / widespread rain and an isolated risk of thunderstorms. First, low clouds. Despite daytime heating, the kinked frontal boundary remains, unlikely to budge until the surface low off the Mid-Atlantic sweeps by. Plenty of moisture trapped beneath the H925 inversion within the shallow cool airmass, clouds not expected to go anywhere. Not much rise in temperatures, highs in the mid to upper 40s. N/E winds. Secondly, wet-weather and isolated risk of thunderstorms. Generally speaking, low pressure presently across the Mid-Atlantic pushes E. Parent with a weak open-wave mid level impulse, associated lift and ascent, frontogenetically along attendant surface frontal boundaries of a decent moist- instability axis, and weak cyclogenesis, deep layer forcing is expected. Moderate rain is expected with forcing of anomalous precipitable waters, widespread over S New England given the environment of strong omega forcing. Yet focus along the S coast of New England and adjacent S waters where instability converges. Already seeing activity emerging around the NYC Tri-State region where SPC Meso- analysis shows a nose of low instability (MUCAPE) creeping N with values of 100 j/kg. Several convective indices meeting or exceeding thresholds as well. Some uncertainty on the better convergence area. Consensus of near-term high-res guidance there is some subtle discrepancy N-S across the aforementioned S region. Will come down to near-term trends. Will be interesting to see what develops over VA / NC and anything offshore, whether better instability and moisture are robbed N. Again, however, widespread rain is expected with the heaviest more in the vicinity of the S coast. So overall, still thinking we`ll see a decent slug of rain going into this afternoon and evening. Amounts of 0.50 to 0.75 with the heavier amounts focused S/E. Some locations, especially along the S coast may see higher amounts where there is a greater risk for convection. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... Tonight... Frontal wave once again shifts E of the region during the overnight hours. This will once again allow a mix of cold advection and drier air to entrain from W-E especially from about 03Z to 09Z. Rainfall gradually dissipates, but it may take some time for the lowest lvl moisture to fully erode. Therefore, some fog and low clouds may linger beyond the precip ending times. May need to monitor for a few spots of patchy dense fog as a result. Otherwise, another mild/damp night for the most part, but cooling will occur within a few hours of sunrise such that min temps could drop back into the mid 30s. Wednesday... Finally a break from the prolonged damp conditions. Drier air will entrain through the entire column through the day allowing skies to clear and sunshine to poke through. Although some cold advection clouds may increase through the peak afternoon heating. H85 temps, although cooling are still near 0C by early afternoon. Therefore, highs could easily still reach the upper 40s and low 50s especially where enough sunshine is observed. Breezy with NNW flow increasing through the day, gusts could reach 25-30 mph at times. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Big Picture... A rather flat longwave pattern with individual shortwaves moving east through that flow. One shortwave moves off through the Maritimes Wednesday night. Another ejects out of the Southwest USA and crosses New England Friday-Saturday. Models continue to change run-to-run, leaving low confidence in the details. Details... Wednesday night-Thursday... Surface low pressure passes south of Nova Scotia as high pressure builds in from Canada. Strong pressure gradient between the systems will maintain north breezes along the eastern MA coastline with lighter winds farther inland. The resulting cold advection should be sufficient to maintain mixing from at least 950 mb where winds will be 30-35 knots. This will mean gusty winds in at least eastern Massachusetts early in the night, diminishing overnight as the pressure gradient diminishes. High pressure builds over New England Thursday, bringing fair skies and mixing to between 850 and 900 mb. Temps at these levels support max sfc temps in the 40s. Friday-Saturday... Questions continue with end-of-week system. General model agreement on surface low moving up the Ohio Valley and then jumping to the Mid Atlantic coast Friday night, passing south of New England Saturday or Saturday night. Projected low level winds show 25-30 knot southeast jet from NJ through Eastern PA and Central NY at 12Z Friday. Light south flow into Western CT and Western MA. Meanwhile model QPF shows measurable pcpn almost all the way to Boston. We passed on a straight importing of the model values in favor of limiting morning POPs and QPF to areas Worcester and west...then spreading east to the rest of our area during the afternoon/evening. Timing and track of the surface low and resulting northeast winds supports likely pops across all of Southern New England. Precipitable water values reach .75 to 1.00 inches over our area. Temperatures and resulting precip type are also a lingering question. High pressure departing through the Maritimes provides a small cold air damming signal, both in the pressure pattern and a 25 knot low level ageostropic flow. Surface winds off the Gulf of Maine should keep coastal areas above freezing, and daytime temps may nudge above freezing each day. But interior locations may at least have a rain/snow mix and may see a period of sleet or freezing rain especially Friday night and Saturday morning. Sunday-Monday... High pressure builds in with dry weather. Building heights and warming temps aloft suggest Monday will be several degrees milder than Sunday. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/... 18z update... Today into tonight..High confidence. Hold with IFR/LIFR with CIGS below 1900ft. VSBYs remain mostly VFR. Lowering MVFR-IFR with -RA/RA, low risk TSRA. N/E flow. Improving towards morning W to E. VFR into the CT River Valley however remaining S New England likely to remain beneath IFR-LIFR. Wednesday into Wednesday night...High confidence. Improving conditions through morning with VFR everywhere by 18Z, lingering longest over the Cape and Islands. NNW winds. Gusts to 20-25 kt at times. KBOS TAF...Will hold with IFR through 12z Wednesday. Cigs 800 feet or less. -RA/RA moving through 20z-6z. Some variability in visibility during this time frame, though a lowering trends prior to N/E winds shifting out of the W. KBDL TAF...Will hold with IFR through roughly 9z Wednesday with cigs 800 feet or less. -RA moving in presently becoming RA late afternoon into evening, improving shortly before midnight. VSBYs variable though a downward trend overnight with N/E flow before winds become W. Outlook /Wednesday Night through Saturday/... Thursday... VFR. Winds diminishing through Thursday. Friday-Saturday... MVFR lowering to IFR in the afternoon, then IFR/LIFR cigs/vsbys Friday night and Saturday. Rain expected but with a period of sleet and freezing rain possible inland Friday night-Saturday. East- southeast winds Friday become Northeast by Saturday. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/... 1 pm update...No major changes to the forecast. Today and tonight...High confidence. Mainly quiet boating weather as winds and seas remain below small craft advisory thresholds. These winds will shift, mainly E today, then veer around to the NW by early tomorrow morning. Fog/drizzle along with showers may lead to low visibilities at times. Low risk for a modest thunderstorm on the waters late today through the early overnight hours, especially on the southern waters. Wednesday...High confidence. NW flow increases with gusts 25-30 kt at times through the day and seas on the E waters increasing to 5-7ft by late Wed afternoon. Small Craft Advisories will be needed. Outlook /Wednesday Night through Saturday/... Wednesday night...Low-moderate confidence. Low confidence for low-end north gales along the eastern waters. Otherwise moderate confidence for north winds gusting 25-30 knots, diminishing overnight. Seas 5 to 8 feet, mainly on the eastern and southeast outer waters. A Small Craft Advisory will be needed. Thursday...High confidence. Diminishing north wind with speeds near 25 knots during the morning. Seas 5 to 8 feet on the eastern outer waters diminishing through the day. Lingering small craft advisory will be needed in these areas. Friday-Saturday...Moderate-high confidence. Midwest weather system moves to the Mid Atlantic coast Friday and passes south of New England Saturday. Increasing southeast winds during Friday but speeds remain below 25 knots until Friday night. Winds turn from the northeast Saturday and from north Saturday night with speeds 25-30 knots. Seas build Friday night and Saturday with heights 5 to 7 feet. Small craft advisory will be needed. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/Doody NEAR TERM...Sipprell SHORT TERM...Doody LONG TERM...WTB AVIATION...Sipprell MARINE...WTB/Doody/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.