Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 211458 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1058 AM EDT Fri Apr 21 2017 .SYNOPSIS... Low pressure exits the Great Lakes today and tracks into Quebec with its rain shield overspreading southern New England this morning. A steady rain this morning will transition to spotty light drizzle this afternoon. A Drying Saturday as low pressure south of Nantucket moves out to sea. Weak high pressure provides dry weather Sunday into Monday. A potential coastal low may bring more rain Monday night into Wednesday of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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10 am update... Radar showing main swath of light to moderate beneficial rain pushing thru and east of eastern MA and RI. A weak surface low will begin to pass just south of our region this afternoon, and continued low level onshore flow is expected thru today. However satellite water vapor imagery showing mid-level dry punch moving into our region from the west. So while additional scattered showers and areas of drizzle are expected today, quantity of precipitation will be limited. This is supported by the HRRR. Will go with chance to low likely pops for light rain/drizzle, with areas of fog, for the remainder of the day. Early Morning Discussion follows... 1032 mb High over Quebec will combine with Great lakes low for a period of warm advection rains across the region this morning. Best jet dynamics track northwest of our region across NY state into northern New England. Meanwhile deep layer moisture plume tracks farther south across MA/RI and CT. With forcing for ascent and deep layer moisture out of phase with each other not expecting much in the way of heavy rain across southern New England. More of a modest rainfall with rainfall totals ranging from 0.25 inch up to 0.75 inches across northwest MA where there is some overlap with best forcing and deep layer moisture along with some upslope contribution given low level easterly flow. As dry slot overspreads the region this afternoon steady rains will transition to areas of drizzle. Definitely a chilly/raw day given precip and onshore flow off the cool ocean. Highs only in the mid to upper 40s across eastern MA to the low 50s across CT river valley from Hartford to Springfield. It will become quite breezy too along the eastern MA coastline as low level southeast jet increases. Model soundings support gusts up to 30-35 mph along the eastern MA coastline.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... Tonight... Parent low over southern Quebec gives way to secondary low pressure S of the islands. Deep layer moisture is lacking however abundant low level moisture present and should support spotty drizzle. Temps very uniform with lows in the 40s. Saturday... Weak low pres south of Nantucket slowly moves out to sea with mid level dry air overspreading the region as K indices dropping into the negative numbers during the day. So expecting low clouds and any spotty light rain/drizzle early to give way to clouds lifting and possibly giving way to some brightening within the cloud cover. This will result in temps recovering to about 55 to 60...warmest readings across interior RI and CT along with I-91 corridor of western MA. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... */ Highlights... - High pressure and dry weather Saturday night into Sunday morning - Coastal low and wet weather trends into midweek */ Overview... Quite a headache in forecasting a potential midweek coastal storm as global models struggle with mid-latitude features, particularly with the morphology of energy rolling into the N/W CONUS off the Pacific. Starting off with a broad view, subtle hints of global models coming to consensus of a phase 7/8 MJO indicating ITCZ convection shifting into the Central Pacific. Believe there is a sub-tropical connection of warm-moist air ahead of deeper H5 troughing across the N Pacific lending to ridge amplification into the NW Pacific and subsequent downstream trough-ridge-trough pattern extending from the W CONUS into the N Atlantic with anomalies of 1-2 standard deviations. With an increasing presence of H5 ridging over the E CONUS, favor a pattern persevering a wedge of surface high pressure from the N/E and that energy forecast in the vicinity of the SE CONUS becomes cut- off from the mid-latitude flow for a time. This had been for a long time advertised via the operational GFS and Canadian, but no longer the case with the 21.0z runs now in agreement with the operational 20.12z EC lifting the aforementioned energy N across SE New England. The 21.0z UKMET stands alone in suppressing activity to the S. The 21.0z Canadian is the strong outlier with the surface low and will ignore. A low confidence forecast beginning Monday. At least one consistent signal via consensus of forecast guidance that being a surface area of low pressure off the SE CONUS into midweek weakening beneath the presence of an increasing ridging pattern aloft. Follow closely to ensemble means especially with regards to precipitation output. Not absolutely confident we`ll see rain yet however can`t say we`ll stay completely dry. Will forego a more detailed discussion with this long-term forecast. In brevity, will keep it dry Saturday night through Monday beneath high pressure. Light winds, likely sea-breezes, the steering flow off the waters, could be some marine stratus issues along the cooler E coast. Kick up PoP chances into midweek as moisture fetch round the expected low off the SE CONUS ascends N over high pressure into S New England with particular focus Tuesday into Tuesday night. Will keep rain amounts on the light side of the envelope. Thinking with onshore flow and cloud cover would keep temperatures near- or below- average. As a final note have to closely monitor tides towards Tuesday and Wednesday (11.5 ft tide in Boston at 11:20p Tue and 11.9 ft 1207 am). && .AVIATION /15Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/... Today... Overall moderate confidence. Will see areas of -RA/-DZ lingering thru this afternoon. Confident on mainly IFR cigs with MVFR/IFR mix of visibility. Difficulty is nailing down the timing of visibility. E winds 15 to 25 kts along the coast with a low risk of 30 kt gusts continue. Otherwise light E flow. Tonight... Moderate confidence. Spotty -RA/DZ with widespread LIFR-IFR but trending upward to MVFR toward 12z Sat. Winds decrease. Saturday... MVFR early with low risk of spotty light rain...then trending upward to VFR during the afternoon along with dry weather likely. KBOS TAF...Continued high confidence in trends, but moderate confidence on exact details including timing, especially in regards to visibility. Steady -RA transitioning to light DZ with patchy -RA after 16Z. For much of today into tonight, IFR-LIFR prevails. KBDL TAF...Moderate confidence in timing and trends. MVFR cigs with improved vsbys probable thru 18Z, then should see a return to IFR conditions later in the day. Outlook /Saturday Night through Tuesday/... Saturday night through Sunday night...Moderate confidence. Ceilings clearing as winds initially shift out of the W. Lingering low-end VFR / MVFR. Winds becoming light into Sunday. Sea-breezes expected along the shores and expect some MVFR / low-end VFR SCT-BKN ceilings at times at times, higher confidence towards the shores. Monday through Tuesday night...Low confidence. Lowering cigs towards MVFR/IFR with increasing chances of -RA. Visibility impacts possible. Increasing E flow with potential for gusts around 20 kts. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/...High confidence. 7 am update...No major changes to the forecast. Today... Low pres tracks across southern Quebec with increasing low level jet over the eastern MA waters. Expect easterly winds of 20-30 kt across this region. Strongest winds late morning and early afternoon. Vsby below a mile at times in areas of rain and fog. Tonight... Weak low pressure forms south of New England. Winds slacken along with rain ending but could be spotty light drizzle and areas of fog. Saturday... Low pres south of Nantucket at 12z exits seaward with improving trend during the day. Outlook /Saturday Night through Tuesday/... Saturday night through Sunday night...Moderate confidence. High pressure in control. Light winds overall across the waters, onshore sea-breezes along the near-shore if flow is not already easterly. Monday through Tuesday night...Low confidence. Coastal low tracking N along the E coast, however both timing and trends remain uncertain. However, growing confidence that we`ll see both increasing areal extent of rain along with increasing E flow. Gusts up around 20 kts possible resulting in seas building up around 8 to 10 feet. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 10 PM EDT this evening for ANZ231- 232. Small Craft Advisory until 8 AM EDT Saturday for ANZ250-251- 254-255. Small Craft Advisory until 2 AM EDT Saturday for ANZ256. && $$ SYNOPSIS...Nocera/Sipprell NEAR TERM...Nocera/Sipprell/NMB SHORT TERM...Nocera LONG TERM...Sipprell AVIATION...Nocera/Sipprell/NMB MARINE...Nocera/Sipprell

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