Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 280218 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1018 PM EDT Thu Apr 27 2017 .SYNOPSIS... Another mild night with some more patchy dense fog overnight will lead to a mix of showers and isolated thunderstorms during the morning. Warmer highs Friday will give way to another mild night Friday night. A cold front crosses the region Saturday night, bringing much cooler weather for Sunday, especially along the coast. Another cold front will likely bring a period of showers sometime Monday into Tuesday. Another storm may approach New England late Thursday. && .NEAR TERM /UNTIL 3 AM FRIDAY MORNING/... 1020 PM Update... Overall trend in the forecast remains on track for this evening. Will have to closely watch the fog potential as low level RH is not as saturated as it was last night. Even with southerly and easterly flow, feel temp/dewpoint spread may still be a bit to large. Will keep the dense fog going for not but will reevaluate it at the next update. A lot of dry air aloft for when the weakening cold front advances tonight. Appears pop trends are on track but still cannot rule out a rumble of thunder. Previous discussion.... South flow across CT and Western Mass, as well as across RI and the Cape/Islands. Meanwhile east flow along the Eastern Mass coast north of Plymouth and extending to the Worcester Hills. Lots of clouds in place with patches of low stratus along the coast but two cloud layers with bases 1500-3000 feet. The weak flow and dew points in the low to mid 50s suggest that as temperatures dip back close to dew point we will see fog developing. No changes in forecast thinking at this time, with a trend maintained to lowering clouds and fog. Min temps still progged at low to mid 50s. Within the stratus, marine fog also possible once again, but this time likely limited more to the S coast initially. Will be hoisting another Dense Fog Advisory for S coastal locations which may need to be expanded depending on how efficiently it is able to move inland with the S-SW flow. Not as confident further inland, but overnight crew can monitor its progress. In any case some clearing of this fog is likely toward the AM commute time thanks to the influence of mechanical mixing with the approaching SHRA (more on this below). Therefore, am capping the end time of the current Dense Fog Advisory to 09Z (5AM local). && .SHORT TERM /3 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/... Early this morning and the daytime tomorrow... Robust shortwave and attendant front will be approaching from the W and crossing the region mainly between 09Z an 18Z, slowing as it begins to parallel the W flow aloft, and possible stalling somewhere near S coast CT/RI and MA by mid day. Lapse rates are already going to be near 6.0C/km by the time of the arrival of this front thanks to a mix of the low-lvl warming with return flow and cooling in response to the approaching shortwave. However initially, column is too dry. This is remedied after 06Z as PWATs begin to approach 1.25-1.50 inches or nearly 2 std deviations above normal. This will increase the elevated CAPE profile such that MU Cape values by the time of the approach are generally around 500j/kg and only increasing as the front pushes E. Therefore, will continue to highlight the risk for showers with embedded elevated TSRA between 08Z (4AM local) and 15Z (11AM local) until drier air and subsidence pushes into the W. Although 0-6km shear is marginal at around 40 kt, the lack of overall instability will limit the TSRA to generally isolated and non-severe. As the front settles near the S coast, it will be a race between the drier air moving in from the W and NW and the remnant low- lvl moisture which will become trapped beneath the encroaching subsidence inversion. Although the drier air aloft and some capping inversion development should limit the secondary redevelopment of TSRA through the afternoon, am a bit concerned with the front hanging nearby as a source for lift even though it will be washing out given that lapse rates and CAPE profiles remain somewhat elevated. Later shifts may want to re- evaluate this threat. It is a low risk given the drying/subsidence mentioned above, but it`s something to watch and models are not producing any QPF during the afternoon which is a typical red flag. Temps will be warmer tomorrow thanks in large part to clearing and better mixing with the subsidence inversion across the W. H85 temps are near +12C, which should allow for spots of upper 70s to low 80s. One caveat may be S coastal areas if the front does indeed stall over land, as it could allow for the continuation of some clouds much later into the day, limiting the heating in spots along the coast of RI and SE MA. Current highs may be too warm if this occurs. Tomorrow night... In spite of the spots of clearing during the evening, skies will once again be increasing as yet another weak front approaches from the W and winds shift to the SW once again. This should limit mins overnight to the mid 50s widespread. Given the lingering subsidence inversion will be holding POPs back until after sunrise in W MA/CT for another round of SHRA. && .LONG TERM /SATURDAY THROUGH THURSDAY/... Big Picture... Longwave pattern shows ridges off each coast with a trough over the Midwest. Shortwave pattern shows three systems of interest in the long term. The first is a shortwave racing across Quebec Saturday as high pressure builds north from the ocean. The second is a broad closed low over the Plains this weekend that ejects through the Great Lakes Tuesday. Trailing shortwave energy from the North Pacific generates a second closed low over the Plains midweek. Model mass fields are similar through early next week with some differences by midweek. Details... Saturday... Shortwave crossing Eastern Canada driven by a 160 knot upper jet. Upper venting along this jet seems focused over Northern New England through the day although some signs of it shifting to the northern MA border by evening. Axis of the shortwave passes late aftn/evening with signs of a surface wind shift over Srn New England 18Z-24Z. Stability parameters show 500-1000 J/Kg and totals of 48- 50. Precip water values are forecast to reach 1.6 inches, well above normal for the end of April. With the best forcing to our north, we will mention scattered showers N/W of I-495/I-84 and include a risk of thunder. Could be a few heavier showers embedded in any wet weather. Mixing to 900 mb and possibly 850 mb, which could bring down wind gusts of 20-25 knots. Temps in the mixed layer support max sfc temps in the upper 70s and low 80s. Saturday night through Sunday night... High pressure builds south from Canada Saturday night bringing drier weather, but also a trend to an east wind on Sunday. This will keep temps cooler near the eastern coast while warmest temps will be in the CT Valley. Expect a lower mixed layer Sunday, but temps near 925 mb should support interior sfc temps reaching the mid and possibly upper 60s while the coast is in the mid 50s. Winds turn from the south Sunday night as warmer air moves north. This leading edge of warm air may generate low clouds and drizzle and possibly a few light showers. Monday-Tuesday... Broad closed low over the Plains ejects through the Great Lakes. This system will sweep a cold front through New England Tuesday morning. Strong low level southwest jets develop, one inland ahead of the main low...and the second along the coast. The coastal jet begins to affect our area early Monday with a 65 knot core moving up across the South Coast and Islands by Monday night. Expect showers, especially Monday night and Tuesday morning. Precip water values climb to 1.7 inches, quite high for early May, and may contribute a few locally heavy downpours. Stability parameters also suggest potential for a thunderstorm. Wednesday-Thursday... High pressure builds over the region with mainly dry weather. Upper shortwave and cold temps aloft may generate diurnal clouds Wednesday. Models show another Plains low spreading clouds and precip our way but with 18-24 hour differences in timing. For now we will show chance pops developing from the southwest Thursday evening and spreading across the region overnight. But this timing is low confidence and will need to be monitored. && .AVIATION /02Z FRIDAY THROUGH TUESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/... Tonight...Moderate confidence. Lingering break in the clouds over Boston and Northeast MA, but cigs 1500-3000 in the interior and cigs 200-400 feet in patches along the coast. Expect east winds in Northeast MA to trend from the south during the night but remain light. This should allow cigs to reform where absent and lower where present. Low stratus is most confident along the South Coast and Islands where the wind is directly off the water. This trend should bring another round of fog tonight, but not as widespread as last night, and should mainly be focus around the S coast of MA/RI and possibly snaking up the CT Valley late. In any fog LIFR vsbys possible at times. Some showers and isolated TSRA possible mainly between 09Z and 14Z from W-E. This will help improve conditions to mainly IFR. Friday and Friday night...High confidence in trends, but lower confidence in exact timing in TAF. IFR/MVFR CIGS and fog, along with showers and isolated thunderstorms end between 12Z and 16Z in the morning, with fairly rapid improvement to VFR in clearing. Winds turn W, with gusts to 15 kt at times. VFR continues into the overnight thanks to W winds, although there is a low risk for showers, but mainly after daybreak Sat. KBOS TAF...Moderate confidence in TAF. VFR conditions early will trend to IFR overnight, but lower confidence in the LIFR risk. KBDL TAF...Moderate confidence in TAF. There is a low risk for LIFR fog after midnight and early Friday. Outlook /Saturday through Tuesday/... Moderate confidence. Saturday... Moderate-High confidence. Mainly VFR, but with brief MVFR cigs/vsbys in showers and isolated thunderstorms in the afternoon/evening. Sunday... High confidence. VFR. Sunday night to Tuesday... Moderate confidence. MVFR/IFR cigs/vsbys in low clouds/drizzle and light showers Sunday night. VFR and areas of MVFR in showers Monday and Tuesday. Possible low level wind shear especially over RI and SE Mass Monday night and Tuesday morning. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/...High confidence. Small Craft Advisories will remain in effect mainly over the offshore waters through Friday night as a lingering 5-7ft swell remains. Winds shifting from S tonight to W during the day Friday then back to the SW Friday night should remain below the 25 kt threshold through the period. Otherwise, another round of overnight dense fog across the waters tonight and again Friday night. A low risk for a few thundershowers moving offshore during Friday morning. Outlook /Saturday through Tuesday/... Moderate confidence. Saturday-Sunday... West winds remain less than 25 knots. Seas briefly near 5 feet on the southern outer waters Saturday. Monday-Tuesday... Cold front approaches Monday and crosses the waters Tuesday. This will bring showers and possibly a thunderstorm Monday night and Tuesday. Seas build to 5-8 feet on the outer waters and exposed southern waters. Expect increasing southwest winds with gusts to at least 25 knots Tuesday. There is low potential for gale force gusts to reach down to the waters Monday night and Tuesday. Absent the development of southwest gales, a Small Craft Advisory may be needed. && .TIDES/COASTAL FLOODING... * Isolated minor coastal flooding impacts during astronomically high tide tonight along the east coasts A bit of a southerly flow combined with a bit more water moving through the river will allow for a few sites along the south coast to get close and/or reach flood for a short period. High tide will begin to subside within the next half hour. The astronomical high tides remain very elevated through the end of the week. Given a lingering swell offshore and its remnant surge, with high tide values only a few tenths shy of typical minor flooding levels, a Coastal Flood Statement continues for the high tide near midnight tonight for east coastal locations. At this time we anticipate only minimal impacts at typically prone locations. Also, due to the breadth of the fetch, we may encounter enough wave action with lengthening periods on top of an elevated tide to cause pockets of erosion tonight along the Martha`s Vineyard south coast, Nantucket south and east coasts, and the outer Cape ocean side from Truro to Chatham. Boston High Tides (flood stage at Boston 12.5 feet)... 12.15 feet / Friday 12:15 am Providence High Tides (flood stage at Providence 7 feet)... 6.25 feet / Thursday 9:50 pm && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Dense Fog Advisory until 5 AM EDT Friday for MAZ020>024. RI...Dense Fog Advisory until 5 AM EDT Friday for RIZ005>008. MARINE...Small Craft Advisory until 8 PM EDT Friday for ANZ235-237. Small Craft Advisory until 11 PM EDT Friday for ANZ250. Small Craft Advisory until 2 AM EDT Saturday for ANZ254>256. && $$ SYNOPSIS...WTB/Doody NEAR TERM...WTB/Doody/Dunten SHORT TERM...Doody LONG TERM...WTB AVIATION...WTB/Doody MARINE...WTB/Doody TIDES/COASTAL FLOODING...Dunten

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