Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 211923 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 323 PM EDT THU JUL 21 2016 .SYNOPSIS... Heat and humidity return Friday along with the risk for scattered showers and thunderstorms during the afternoon and evening, some of which may be severe. There may be a few leftover showers Saturday, otherwise the weather will be rain-free. Hot weather is expected to continue this weekend. An approaching cold front will bring another round of scattered showers and thunderstorms Monday night and Tuesday. More warm weather follows midweek. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1 pm update...no major changes to the forecast. Just some mid to high level clouds filtering across the region within the overall anticyclonic flow, otherwise mostly clear. Abundant sunshine with a boundary layer becoming well-mixed up to H85 where temperatures will warm to +18C, looking at highs today around the mid to upper 80s with likely a few locations coming in at or above 90 degrees (looking at the lower CT River and Merrimack River Valleys given low-lying downsloping wind locations). Light S/SW flow with the possibility of a sea- breeze along the immediate E-coast. Dewpoints begin to rise back into the 60s. && .SHORT TERM /6 PM THIS EVENING THROUGH FRIDAY NIGHT/... Tonight... Another quiet night wx wise thanks to last minute influence of high pres to the E. However in this case, in spite of the clear skies and weak pres gradient, rising low-lvl moisture suggests a more mild night across the region. Expect temps mainly in the low to mid 60s. Some patchy fog possible too thanks to this increase in moisture, but likely only in the typically prone spots. Fri... The primary issue for Fri remains the risk for late-day severe wx. Although heat/humidity will be a factor. H85 temps will be approaching +18C while sfc dwpts increase into the mid 60s to near 70, which given enough sunshine is enough for highs reaching the upper 80s to low 90s and a low risk for heat advisory criteria. Note that most of the following discussion will actually be moot for the Friday (daylight) time frame if the the continued slow- down in the guidance continues, suggesting the pre-frontal trof doesn`t arrive until late evening, potential limiting sfc based instability as we experience diurnal heat loss. Also, early Fri may be marred by upstream convective cloud debris limiting the destabilization. Still plenty of factors pointing in the right direction, remnant EML (peaks overnight Fri) with 6.0-7.0C/km lapse rates arrives in time for the return flow and moistening low levels. Yielding ML CAPE values in the afternoon near 1500j/kg (per ensemble probs). Profiles are relatively wide, with hail-CAPE values +300j/kg. Shear reaches 30kt at 0-3 and 0-6km with a nearly 40 kt H9 LLJ. Therefore, should be enough for organization of any storms that form, with hail an issue and some rotation in updrafts. Winds still look to be the primary issue, thanks to inverted-V profiles and LLJ, and as such will be upgrading wording to strong winds with this morning`s wx update. While winds/hail remain the primary risks, especially since shear/CAPE profiles support MCS organization which would peak overnight (at the point of EML peaking), there is a low end tornado risk given the lowest lvl shear values and EHI between 1.0-2.0. However, not a pure S New England tornado break case per internal study and the very dry mixed layer (high LCLs). For now given the combination of timing uncertainty and mitigating factors mentioned above, will be slowing timing and upgrading gusty winds to strong winds, especially with the recent slow-down and factors suggesting an MCS could be the primary convective mode. Will also be adding T+ (severe type) wording given continued agreement with SPC SWOs, which continues to highlight most of S New England away from the ocean. Stay tuned to updates as timing/mesoscale issues remain somewhat uncertain. && .LONG TERM /SATURDAY THROUGH THURSDAY/... Big Picture... High pressure covers the Southern and Central USA while troughs dig over the Pacific Northwest and the Maritimes. The high pressure weakens a little and shifts south, opening up a zonal flow across the northern tier of the USA. The Maritime trough directs a shortwave through New England Saturday which then kicks the trough off to the east. The Pacific NW trough ejects into the zonal flow and reaches New England Monday night-Tuesday. During mid-week, the zonal flow remains open to additional shortwaves, but timing consensus among the models is poor. We used a blend of the long range guidance. Forecast contour height fields remain higher/warmer than normal levels, so expect temperatures to run warmer than normal through the period. Details... Saturday-Sunday... Shortwave overhead Saturday moves east of us by Sunday. The supporting 120 knot jet overlays its left exit region over VT-NH and possibly into parts of Southern New England. Moisture fields show some post-frontal drying across our area, but a pool of moisture under the shortwave advects into at least Northern MA during Saturday. This suggests a period of midday-afternoon clouds, especially over Northern MA, and either scattered or widely scattered showers. Total-totals will be near 50 and LI sub-zero, so thunder also possible. Otherwise rain-free with high pressure building over the region especially on Sunday. With cooler temps aloft, mixing may reach deeper Saturday... but should climb to at least 850 mb each day. Temps in this layer suggest max sfc temps around 90, especially on Saturday. Monday-Tuesday... Pac NW shortwave races east across Canada with surface low swinging a cold front across New England Tuesday. GFS shows a two-part shortwave with the main area in Northern Quebec and a detached less- defined shortwave moving across New England. The ECMWF shows a single well-defined shortwave crossing New England Tuesday. Lean toward the ECMWF scenario. This means dry weather much of the day Monday. Scattered showers/thunder possible in the CT Valley and Bershire slopes late Monday, and all areas Monday night into Tuesday until fropa. Temps aloft around 18C and possibly 20C, so max sfc temps should be around 90 or in the lower 90s. Wednesday-Thursday... High pressure builds over the region. Looks like a quiet weather period. Temps at 850 mb remain 16-18C so surface max temps should be 85-90F. && .AVIATION /19Z THURSDAY 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/... 18z update... Today...High Confidence. VFR. Light W/SW flow. FEW to SCT mid-high cigs. Likelihood of sea-breezes along the immediate E-coast. Tonight...High Confidence. VFR. Increasing W/SW flow. Gusts up around 15 kts for S-coast terminals. SCT to BKN mid-high cigs dropping down from the N. Friday into Friday Night...Moderate Confidence. VCSH during the morning period with SCT-BKN cigs. Clearing later half of the day ahead of SHRA/TSRA for the late afternoon into evening period. With any TSRA, TEMPO MVFR-IFR conditions. Some TSRA may be severe with strong to damaging W/NW winds, hail, even an isolated tornado possible. Focus 22.20z to 23.04z. S Winds with the likelihood of gusts up to 25 kts especially for S-coastal terminals. KBOS TAF...Sea-breeze may hang just off the door-step of the terminal around 19-20z. W/SW winds prevailing. KBDL TAF...VFR prevailing. Outlook /Saturday through Tuesday/... Saturday...Moderate confidence. Mainly VFR conditions expected. Brief periods of MVFR/IFR conditions possible in SCT SHRA/TSRA over SE New England. Patchy fog possible during the early morning Saturday. Sunday...High confidence. VFR. Monday...Moderate confidence. Mainly VFR conditions expected. Brief periods of MVFR conditions in SCT SHRA/TSRA possible. && .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. 1 pm update...no major changes. Today and tonight... Winds shift mainly to W-SW. A few gusts around 20 kt possible through the afternoon, and a very low risk of a few gusts reaching 25 kt, but these will likely be few and far between. Otherwise quiet boating wx. Winds drop off tonight. Friday... Increasing wind gusts ahead of approaching cold front. SW gusts 25-30 kt at times expected, will be issuing small craft advisory. This will also lead to some wind driven seas near 5-6ft. Outlook /Saturday through Tuesday/... Increasing southwest winds ahead of a cold front will continue to generate rough seas, particularly across the southern coastal waters into Saturday. Winds and seas briefly diminish Sunday behind a cold front. Showers and thunderstorms are likely possible over the waters Friday evening and night, and again on Monday afternoon. Storms Friday night may become strong, producing dangerous boating conditions at times. In addition, areas of fog may develop on Friday night, limiting visibilities. && .FIRE WEATHER... Massachusetts state fire officials have advised that given the forecast warm temperatures into the upper 80s to low 90s, low humidity values, and lack of rainfall as of late that there is a heightened fire danger. Such conditions likely exist for other parts of Southern New England, namely CT and RI. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from noon to 8 PM EDT Friday for ANZ231>234-236. Small Craft Advisory from 11 AM to 8 PM EDT Friday for ANZ235- 237-255-256. Small Craft Advisory from 2 PM to 8 PM EDT Friday for ANZ250- 254. && $$ SYNOPSIS...WTB/Doody NEAR TERM...Belk/Doody/Sipprell SHORT TERM...Doody LONG TERM...WTB AVIATION...WTB/Doody/Sipprell MARINE...WTB/Doody FIRE WEATHER...

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