Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 182258 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 658 PM EDT Tue Apr 18 2017 .SYNOPSIS... Dry and cool weather continues tonight into much of Wednesday afternoon, but a fast moving disturbance will bring a period of scattered showers Wednesday night. A Cold front slips south through Southern New England Thursday. Low pressure from the Plains races northeast to the Eastern Great Lakes, then redevelops along the New England coast Friday. This brings us a period of wet weather Thursday and Friday. Weak high pressure builds drier weather for Saturday and early Sunday. Another low pressure area coming out of the Southern Plains may bring more rain to our area late Sunday and Monday. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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7 pm update... An area of clouds which formed over Southern RI during the afternoon has been moving northwest in the onshore flow and will cross from Northeast CT into Western MA during the early night. High pressure over the Maritimes extends along the coast and will keep fair weather and light winds over our area tonight. Dew points range from the lower 20s northwest to the mid 30s southeast. Expect temperatures to fall for at least the first half of the night, then slow after midnight. Current forecast in the upper 20s to mid 30s looks reasonable...if it misses it will be just a few degrees on the cooler side. Cold front entering the Great Lakes has a cloud shield extending to Toronto and Buffalo. Movement on these mid and high clouds would bring them into Western New England around 2-3 am and to Eastern New England 3-5 am.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... Wednesday and Wednesday night... High pressure moving east of the Canadian Maritimes will allow for a return of southerly winds on Wednesday. Skies will start off partly sunny, but should see an increase in cloud cover during the afternoon in the warm air advection pattern. Given marine influence on southerly flow and an increase in clouds during the afternoon, high temps will mainly be in the 50s. Portions of the immediate coast may not even reach 50 given stronger marine influence in those locations. Dry weather should prevail through mid to possibly late Wed afternoon. However, approaching shortwave and increasing low level jet will result in a period of scattered showers Wed evening. Forcing is not that impressive, so rainfall amounts should be rather light. Bulk of the activity should focus itself south of the MA Turnpike and particularly near the south coast after midnight where better moisture resides. Low temps will mainly be in the lower to middle 40s. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Big Picture... Longwave flow is relatively flat late week but trending to a trough- northeast/ridge-west pattern over the weekend and early next week. Shortwave scale shows a weak ridge overhead Thursday, a shortwave crossing New England Friday night and Saturday, and a deeper shortwave crossing the Eastern USA Sunday night and Monday. A southern stream closed low lingers off the Carolinas on Tuesday. Model mass fields are similar in handling the mean flow through the weekend, then develop significant differences early next week. Confidence is moderate-high early in the long term and diminishes to low confidence Sunday through Tuesday. Contour heights are around or above normal through the period. This suggests normal to above normal temperatures for much of the period. However it is important to note that the upper flow is unlikely to detect the cooling effects of low level easterly flow into Southern New England. Details... Thursday...Moderate confidence. Model differences in details. The GFS shows a weak cold front settling south through our area which eventually stalls south of us Thursday night, while the GGEM shows a stalled warm front in place south of New England. While there is some dry air high up, the column below 700 mb remains moist through the day. Low level wind fields show some low level south flow at 1000 mb over the advancing cooler air with the front, but most flow above that is west-to-east and generates no lift. We will maintain chance pops through the day but with light scattered precipitation. Thursday night and Friday...Moderate confidence. Low level flow remains weak through midnight, then turns from the south late at night. Upper jet with the shortwave starts the night feeding over the Great Lakes, but shift east to Southern New England by 12Z with the nose of the jet over our area. This will mean increasing low-level lift and upper-venting especially after midnight. Thus, an increasing likelihood of rain overnight and into Friday. Precipitable water values will be 1.0 to 1.25 inches, which is well above normal for April 21. This means potential for heavy rain. Stability parameters show support for scattered thunder. Totals climb to the upper 40s and low 50s, while CAPE fields show 500+ Joules/kg reaching at least into Connecticut. Low level marine flow may work against this, or limit it to elevated convection, especially from the Worcester Hills and east. Temperatures will vary, with the increasing surface flow from the east coming off water temps in the mid 40s keeping east coastal areas no warmer than the low 50s while the CT Valley may reach 60. As the upper jet moves to the coast, the various models show a redevelopment of the low along the New England coast late Friday or Friday evening. Once this redevelopment shifts east of our area and winds turn from the Northwest, we would expect pcpn to taper off from west to east. Saturday...Moderate confidence. High pressure builds in and trends us to drier weather. Some lingering moisture in the column will support partial cloud cover. An approaching northern shortwave and cold advection aloft may destabilize the airmass and allow for isolated showers. Sunday-Monday-Tuesday...Low confidence. Southern stream shortwave moves through the Southern USA generating that races to the Carolina coast by Sunday. Models disagree on what happens next. The GFS brings the low up the coast with rain reaching our area late Sunday or Sunday night. The ECMWF keeps the storm offshore and carries it out to sea. The GGEM favors bringing the rain up the coast, but holds off on doing so until Tuesday. As two of the three models indicate a chance of rain sometime during this period, we will show chance pops. But will slow arrival time from our guidance by roughly 12 hours. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
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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 Wednesday Night/... Tonight...Moderate to high confidence. Mainly VFR conditions but there is a low risk that MVFR cigs may develop after midnight across western MA/northern CT. Wednesday and Wednesday night...Moderate to high confidence. Mainly VFR conditions expected for the bulk of the daylight hours on Wed, but some MVFR cigs are possible especially during the afternoon. More widespread MVFR conditions expected Wednesday night along with some scattered rain showers. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Thursday through Sunday/...Moderate confidence. Thursday... Generally MVFR cigs/vsbys in off/on rain showers and fog. Variable wind directions, initially from the south but turning from the east by Thursday evening. Thursday night-Friday... Conditions lowering to IFR in rain and fog after midnight and continuing Friday. Easterly winds 20 knots or less through Friday, turning from the north Friday night. Conditions improving to VFR late Friday night. Saturday-Sunday... VFR Saturday with potential for MVFR cigs along the east coast of Massachusetts. Ceilings lower to MVFR by Sunday afternoon with MVFR or IFR vsbys in rain and fog late Sunday and Sunday night.
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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 Wednesday Night/... Tonight through Wednesday night...High confidence. High pressure over the Maritimes will keep winds/seas below small craft thresholds tonight into Wed morning. As the high moves east of the Maritimes a return southerly flow of air is expected Wed afternoon and night. Enough mixing should result in some southerly 25 knot wind gusts across our northeast outer-waters. Long southerly fetch should also result in 3 to 6 foot seas across those waters, so have hoisted small craft headlines. Outlook /Thursday through Sunday/...Moderate confidence. Thursday... East winds less than 20 knots and seas less than 5 feet. Areas of poor visibility possible in drizzle and fog. Thursday night-Friday... East winds Thursday night and Friday turn from the north Friday night. Winds less than 20 knots Thursday night and Friday, increasing to 20-25 knots Friday night. Seas less than 5 feet through the period, although heights may approach 5 feet Friday night on the outer waters east of Massachusetts. Saturday-Sunday... Winds 20 to 25 knots east of Massachusetts Saturday, diminishing at night and Sunday. Seas near 5 feet on the outer waters east of Massachusetts Saturday and Sunday. && .FIRE WEATHER... Relative humidity values will be higher on Wednesday than recent days. However, a period of southerly wind gusts of 20 to 25 mph late Wed am into the afternoon may still result in some fire weather concerns given recent dry weather in pre-green up. Fire weather concerns will diminish further late in the week with risk of periods of wet weather and higher relative humidity values. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 2 PM Wednesday to 8 AM EDT Thursday for ANZ250. Small Craft Advisory from 2 PM Wednesday to 2 AM EDT Thursday for ANZ251. && $$ SYNOPSIS...WTB/Frank NEAR TERM...WTB/Frank SHORT TERM...Frank LONG TERM...WTB AVIATION...WTB/Frank MARINE...WTB/Frank FIRE WEATHER...

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