Area Forecast Discussion
Issued by NWS Boston, MA

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390 FXUS61 KBOX 281350 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 950 AM 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/... 10 am update... Soupy conditions persist today ahead of later day showers / widespread rain and an isolated risk of thunderstorms. First, the soupy conditions. The back-bent stalled frontal boundary across our region N of which NE flow prevails in an overall moist airmass. Agree with the previous forecaster, it may struggle to lift N. The thick deck of clouds significantly limiting warming across the region. Dewpoints dropped overnight with the N flow, albeit light, allowing for locally dense fog to develop into morning. SPECIAL WEATHER STATEMENT for the hazardous weather till 11 am. Should see conditions improve with daytime heating, though the higher terrain will likely stay immersed in the low cloud decks with visibility of 1 to 2 miles much of today into the evening hours before clearing out behind a cold front overnight. Secondly, late day wet-weather and the isolated risk of thunderstorms. An area of low pressure presently across the Mid- Atlantic will push E today parent with a weak open-wave mid level impulse. Associated lift and ascent, especially frontogenetically along attendant frontal boundaries of a decent moist-instability axis, and weak cyclogenesis, will yield widespread rain with embedded thunderstorms. Some things worth noting: 1.) Anomalous precipitable waters up to +2 standard deviations and above the 90th percentile with respect to climatology sweeping N, overrunning the mostly stalled frontal boundary. Consensus forecast of values up around 1.1 inches per S fetch of the moist-instability axis N over S New England, albeit weak per evaluation of H925-85 flow. 2.) With the moist-instability axis some convective threats are met with respect to K-indices, negative showalters, and total-totals. However, consensus and ensemble mean guidance has a majority of the MUCAPE values along the S-coast and points S. The region of stronger instability and deeper moisture is more centered off the Mid-Atlantic, especially over the warmer Gulf stream. Some concern that perhaps both instability and better moisture are robbed N. 3.) However, decent upslope ahead of mid-level troughing and positive differential vorticity in a right entrance region of an upper level jet streak. Forecast consensus signaling strong omega throughout the column of the aforementioned warm-moist airmass, especially within the mid-levels. Within the conditionally unstable profile, this adds weighting to the isolated thunder chances. Just a slip of instability to the N, can`t rule out that rumble of thunder. 4.) Finally, there still is some uncertainty as noted above. The evolution of the low along with the parent low to mid level convergence zone. Some of the latest guidance really putting out some hefty precipitation amounts. Watching closely the activity out of Jersey and the Delmarva. It`ll be interesting if we see a decent deep-layer forcing setup over the S coast, however we could see a wobble N or S in this region and in addition there could be some complications S if convective activity emerges that robs the environment N. Just some things we have to keep in mind. So having my coffee, watching radar, thinking we could see a good dousing of rain for the afternoon into evening period. Going with high categorical PoPs with the expectation of New England seeing on average 0.50 to 0.75 inches of rain, some locations seeing locally higher amounts. Again, difficult to say exactly where with absolute specificity, however the consensus of forecast guidance is focused S/E where there`s the possibility of instability / thunder and that`s where we`ll keep a broad focus. Would help with the drought conditions at the very least. Keep it cool given the thick clouds and light N winds. Believe it`ll be a struggle for the stalled frontal boundary to lift N. Highs in the mid to upper 40s, maybe an isolated 50 degree reading. Warmest conditions S/W, cooler N/E. As noted by the previous forecaster, still a good cold air advection signature further N/E. && .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 /13Z 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/... 14z update... Through 18Z today...High confidence. Mainly IFR/LIFR with CIGS below 800ft. VSBYs slowly improving if they haven`t already. Light N/E flow. Late today into tonight...High confidence. Mix of IFR/LIFR. RA moving in around 19z CT and 22z MA. RA tapers off from W to E 03Z-09Z. Low risk TSRA. IFR/LIFR in lingers afterwords. Wednesday...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 and LIFR through much of the period. KBDL TAF...High Confidence. 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/... 10 am 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...WTB/Doody/Sipprell SHORT TERM...Doody LONG TERM...WTB AVIATION...WTB/Doody/Sipprell MARINE...WTB/Doody/Sipprell

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