Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 280742 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 342 AM EDT Tue Mar 28 2017 .SYNOPSIS... Wet weather continues into the overnight hours 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/... Weak Nly flow continues to impact the region even as the lvls below about H7 remain fully saturated. This has led mainly to widespread low clouds with a mix of modest fog/DZ during the early morning hours. One issue of note, which will need to be monitored through sunrise is the risk for a few spots to dip near or below freezing mainly along and N of the RT 2 corridor of Mass. The Nly flow has allowed dwpts to continue to drop through the overnight hours and are just now reaching freezing. This trend may continue and may require a last minute SPS for isolated freezing spots on area roadways, which could remain wet from yesterday`s rains/damp conditions. The Nly flow also continues to keep vsbys in fog higher than dense fog advisory thresholds. With this forecast update, am concerned that the warm front currently draped from NY state across and to the S of New England may in fact struggle to fully lift N of the region today. The trapped moisture and low clouds will limit early AM warming, weak cyclogenesis will continue across the OH valley then offshore of NJ/Delmarva late today, and there is a lack of forcing in the form of a S LLJ. Therefore, will keep the warmest temps mainly W and S of the region. If the front is able to move into S New England in many capacity temps could easily run into the 50s, but for now have capped highs mainly in the mid 40s. There is a modest CAD signature in the mass fields that also supports this thinking with latest model updates. The damp conditions will remain as moisture mainly in the lowest 400mb or so remains, exacerbated by marine Ely marine flow developing with the gradient this morning. Dry air aloft suggests that within the weak overruning supplied by the warm front the continued risk for DZ and fog will linger until more widespread rain develops in proximity to the second developing frontal wave. Expecting DZ/SHRA to give way to widespread RA late this afternoon and evening as the frontal wave approaches from the SW and column PWATs increase to around 1.00 inches. Given the expectation that the warm front remains to the S and W, have focused highest QPF along and S of the Mass Pike and predominantly along the S coast into the early overnight. Widespread QPF is mainly 0.25 to 0.5 inches but some locations across the S and SE portions of the area may see 0.5-1.00 inches before it fully finishes. The risk for TS is limited somewhat but there are indications of some elevated instability in the form of TT near 50 and conditionally unstable lapse rates late today and this evening. Therefore, will continue to include this risk in the wx. && .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. Tomorrow... 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 /07Z 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/... Through 18Z today...High confidence. Mainly IFR/LIFR with CIGS below 800ft (except for some interior sites which are currently MVFR but will too be dropping this morning). The IFR/MVFR conditions continue with a mix of fog/drizzle and low vsbys into the early afternoon hours. Mainly E flow. Late today into tonight...High confidence. Mix of IFR/LIFR continues but with more of a mix of RA/fog rather than drizzle. Rain tapers off from W to E 03Z-09Z. After rain ends a period of IFR/LIFR in fog lingers afterwords. Wed...High confidence. Improving conditions through sunrise with VFR everywhere by 15Z. NNW winds. Gusts to 20-25 kt at times. KBOS TAF...High Confidence. IFR to LIFR through much of the period. KBDL TAF...High Confidence. MVFR gives way to IFR and LIFR through much of the period. Outlook /Wednesday night through Saturday/... Wednesday night-Thursday... VFR as high pressure builds over the region. Strong gusty North winds along the eastern MA coast including BOS early at night with gusts 30-35 knots. Winds diminishing overnight and on 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/... 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...Doody SHORT TERM...Doody LONG TERM...WTB AVIATION...WTB/Doody MARINE...WTB/Doody

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