Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 261122 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 722 AM EDT Wed Jul 26 2017 .SYNOPSIS... High pressure will bring dry and warmer weather today but the risk of showers and t-storms returns Thursday. A coastal storm will emerge off the Mid Atlc coast Friday and is expected to track south of New England Friday night into Saturday possibly bringing a chilly rain to portions of the region. Dry and seasonable conditions return late next weekend into early next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 7AM update... Early AM vis satellite imagery showing slow dissipation of valley fog this morning, however with good mixing expected and no mid-high clouds to obscure it should be burning off completely within the next hour or so. Few pockets where it is locally dense, but these too are already lifting. Otherwise, noticed temps this morning are generally running a few degrees cooler than guidance, so adjusted hourly/high temps somewhat as a result. Otherwise, forecast on track. Previous discussion follows... High pres in control today with dry airmass yielding lots of sunshine with some afternoon cu developing. More seasonable temps today in the upper 70s to around 80 which will feel nice after the recent unseasonable cool weather, but these temps are still below normal for late July. Somewhat cooler temps along the immediate coast where seabreezes will develop. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/... Tonight... Approaching mid level shortwave from the NW will result in increasing mid/high clouds. Mainly dry conditions expected as main focus for late night showers will be to the north. Lows will range from the upper 50s to lower 60s. Thursday... Another mid level shortwave approaches from the north with increasing deep layer moisture and instability as K indices increase into the 30s and CAPES 500-1000 J/kg. This will bring the potential for sct showers and t-storms, especially interior where instability is greatest. 0-6km shear 30-35 kt so can`t rule out a few strong storms but instability will likely be a limiting factor. Some sunshine is expected, especially near the coast but generally looking at more clouds than sun with highs mid/upper 70s. Becoming a bit more humid as dewpoints climb into the mid 60s. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... Highlights... * Showers and occasional thunderstorms Thu evening especially * Risk for late Fri - Sat being cool/rainy, but this may be suppressed to the south * Dry and seasonable weather likely Sunday into Monday Overview and model preferences... While 26.12Z model guidance is beginning to settle toward a solution for the late week strong shortwave and gradual longwave trof production it is somewhat different from previous runs, particularly with the placement of the frontal boundary and the development of a frontal wave. Note that while there does seem to be a trend occurring here, there remain significant differences in the development of low pres along the stalled boundary mainly in the Fri-Sat time frame. So for now, will begin to shift toward a more suppressed solution Thu night into Fri, and a more unsettled/wet solution for late Fri into the early weekend. This does not favor any particular model or ensemble and gives room for better model convergence down the road. At odds appears to be the development of a weak wave associated with convection firing in the Canadian prairies and Great Lakes region late tonight into tomorrow. In spite of this initial uncertainty, there does continue to be reasonably good agreement thereafter that as the longwave trof and cutoff associated with the initial wave slide E, drier air and some ridging will bring about a quieter stretch to follow early next week. Details... Thu night... Ongoing convective risk thanks as shortwave and attendant weak frontal boundary slide across the region late Thu into early Fri night. Could see this activity actually continue a bit later than typical late afternoon/evening timing thanks in part to a slight increase in mid lvl lapse rates above 6.0C/km (keeping MU CAPE values elevated near 1000j/kg) and modest shear at about 30-35 kt. Therefore, can`t rule out a risk for SHRA/isolated TSRA most of the overnight hours, although given the typical diurnal minimum, risk drops especially during the early morning hours. Gradual POP reduction. Mins remain in the mid 60s thanks to a rise in dwpts through the day on Thu in response to deepening moisture. Fri into Sat... As mentioned above, there remains enough uncertainty here that anything described below is subject to change as we approach as there will be a tricky interaction between a dry high pres attempting to nose in from the NW and the front as it stalls from near the mid-Atlantic toward Nantucket. By daytime Fri, a second diurnal round of convection can be expected, which may remain mostly S of New England given the current mean for frontal placement. However, this second round of convection will also be the focus for a low pres wave development which would then traverse the front late Fri night into Sat. Noting definite convective feedback issues here, which will need to be refined to better tell how close of a passage this low pres will take late Fri night into Sat, which will have major implications for the axis of rain/clouds. The northern solution of this pattern though, with low pres passing near the 40/70 benchmark would be another round of damp/cool conditions beginning late Fri and continuing into much of the day on Sat. Brisk E flow given negative-u component of the LLJ is nearly 2-3 std deviations above normal would bring in cool marine air off of the Gulf of Maine. A secondary spike in PWATS to near 2.00 inches along with cool temps aloft could bring about another period where mod- heavy rain is possible. All-in-all, not a good solution for Saturday should the front remain close enough to New England. Temperatures Fri still near to below normal as some breaks of sunshine are possible (highs mainly in the upper 70s). Sat would depend on whether the wave impacts S New England or remains well S. If impacts are felt, temps will struggle to reach into the mid 60s, warmer if the latter occurs. Sun through Tue... One good result of the stronger solution implied most recently by operational guidance and some ensembles, is that it is the catalyst in shifting the longwave trof axis and cutoff in the Maritimes somewhat E. This allows drier air, and more anticyclonic motion into New England, yielding a 1020+ high pres. Therefore, will continue to trend toward a drier pattern for the late weekend, early next-week timeframe. In spite of the longwave trof, mid lvl temps will moderate such that temps could be close to seasonal normals. Mid week... Secondary shortwave approaches, finally phasing the mean jet with the longwave trof hanging to the S. && .AVIATION /12Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/... Today and Tonight...High confidence. VFR. Coastal sea breezes developing by mid-late morning. Thursday...Moderate confidence. Mainly VFR cigs with patchy MVFR. Scattered showers and t-storms developing, especially interior with brief lower conditions. KBOS Terminal...High confidence. Sea breeze expected by 15z. KBDL Terminal...High confidence. Outlook /Thursday Night through Sunday/... Thu night into Fri...High confidence. A few showers and thundershowers, mainly Thu evening. Then mainly near the S coast on Fri. Otherwise, predominantly VFR outside of some localized ground fog. Fri night into Sat night...Moderate confidence. Potential for a mix of IFR/MVFR in E-NE flow which could gust 25-35 kt mainly near the E coast on Sat. Rain, especially along S coast may be heavy at times, but all of this is contingent on how close low pres passes S of New England, which is somewhat uncertain. Sun...High confidence. Improvement, mainly VFR. && .MARINE... FORECASTER CONFIDENCE LEVELS... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday/...High confidence. Today...High pres over the waters will result in quiet boating weather with light winds and seas and nearshore seabreezes developing. Tonight...Winds turn southerly as high moves offshore. Light wind speeds, but may approach 20 kt over NE MA waters. Thursday...S/SW flow with gusts to 20 kt. Seas below SCA thresholds. Outlook /Thursday Night through Sunday/... Thu night into Fri...High confidence. Mainly quiet boating weather, SW winds will shift from SW to to the E-NE on Fri, but gusts should remain below 25 kt until Fri night. Seas generally at or below 4 ft. Fri night into Sat night...Moderate confidence. Strengthening low pres will pass S of New England. How close it ultimately passes remains somewhat uncertain and will determine final outcomes. However, NE winds could gust close to gale force Sat, but at least high end small craft thresholds. Seas also could reach as high as 7-9 ft on the open ocean waters. Still some time to sort this out. Otherwise, rain/fog mainly across S waters. Sun...High confidence. Improvement expected, remnant small craft advisory conditions gradually give way to quiet boating weathers late. && .EQUIPMENT... The KBOX radar is currently down. It is uncertain when it will return to service at this time. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...KJC/Doody NEAR TERM...KJC/Doody SHORT TERM...KJC LONG TERM...Doody AVIATION...KJC/Doody MARINE...KJC/Doody EQUIPMENT...

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