Area Forecast Discussion
Issued by NWS Boston, MA

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340 FXUS61 KBOX 251710 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 110 PM EDT Tue Jul 25 2017 .SYNOPSIS... This afternoon will remain cooler than normal and cloudy with thinning clouds late in the afternoon. High pressure will bring a return to dry and milder weather for Wednesday and most of Thursday. Cold front with a surface low pressure wave impacts southern New England Thursday night and exits the area late Friday. Dry weather and seasonable temperatures expected next weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1 PM Update... No significant changes to the forecasts. Dropped POPs just a tad but still expect widely scattered showers to cross MA this afternoon. Temperatures have been running a little cooler than forecast in a number of spots but think the high temperatures still look reasonable especially with a wdge of thinning clouds making its way south along the north shore toward greater Boston. Still will be mostly cloudy but should begin to experience some thinning in the overcast as the afternoon progresses. No significant changes made to the winds. 10 AM Update... Short wave trough moving across southern New England is anticipated to result in widely scattered showers later this morning into mid afternoon. Short term high resolution guidance dries up the band of showers approaching Berkshire County at 10 AM, and it`s possible they will struggle to get over the Berkshires. Nevertheless, water vapor imagery depicts a rather sharp short wave trough so have opted to have a period of POPs in the 20 to 25 percent range through mid afternoon across MA and adjacent northern areas of CT and RI. Temperatures and winds appear to be on track at this time with few changes needed. 7am update... No significant changes to previous forecast. Weak wave just now pushing into NW zones and will gradually rotate through the region today, yielding a few showers and low clouds. Could begin to see breaks by late afternoon as drier air works its way back into the column and is allowed to mix with lower levels. Previous discussion follows... Weak surface wave moving off the Mid Atlc coast with ridging nosing down from the north will maintain a moist NE flow across SNE. Cross sections show considerable moisture in the lower levels with moisture deepening in the column as mid level trof axis and shortwave approaches from the NW. This will keep lots of clouds across the region along with a risk of a brief shower although most of the time it will be dry. While it will be milder than yesterday, temps will still be unseasonably cool with readings in the mid/upper 60s. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... Tonight... Mid level trof axis exits the region this evening followed by rising heights and high pres building in overnight. Models show good drying moving in from the N and NE which will lead to clearing skies overnight. Good radiational cooling with light winds will result in a chilly night with lows ranging through the 50s with potential for upper 40s in the colder locations in the CT valley and interior E MA. Anomalous temp regime for late July. Wednesday... High pres in control with dry air in the column will lead to a pleasant day with lots of sunshine. Temps will recover into the upper 70s and lower 80s but developing seabreezes will keep it a bit cooler along the immediate coast. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Highlights... * Dry most of the day Thu. * Showers and possible thunderstorms late Thu with showers likely lingering into Fri * Dry weather and seasonable temperatures likely for the weekend Overview and model preferences... Although weak ridging will begin this long-term period, it will be followed by an unseasonably strong shortwave which will begin to carve out long-wave trof across the northeast CONUS and much of the E seaboard late in the week, and into the weekend. Although the initial wave is likely to cutoff per latest ensemble and operational model runs, the damage will have largely been done, leaving the longwave trof in its wake as it shifts E as early as the late weekend. Given this, periods of unsettled wx can be expected, along with generally cooler than normal temperatures through much of the latter half of the week. Generally, will use a consensus blend of guidance for this update. However, will opt to lean away from 24.12Z ECMWF which generates an anomalously deep low pres crossing the region Thu night night into Fri. This seems a bit overdone and is a bit of an outlier. Details... Wed night into Thu... Weakening ridge will shift E through the period, but remains the dominant player for sensible wx across New England through most of the day on Thu. Cold front/low pres development associated with a robust wave will slow the overall progression of any precip/lift these features can generate. Therefore, will continue to suggest mainly NIL pops with only slight chance in the W/NW by 00Z Fri. Wed night mins comfortably cool for sleeping, mainly in the 50s to low 60s in the heat islands. Daytime highs will be dictated by speed of incoming cloud cover but with H85 temps rebounding to near +14C, could see a few spots in the upper 70s and low 80s. Thu night into Fri... Potentially most unsettled period of the forecast. Very robust shortwave, especially by July standards, will be carving out a trof across the NE. Exactly how this plays out at the sfc remains somewhat uncertain, as both ensembles and operational guidance suggest anywhere from a standard cold frontal passage, to as deep as a 992mb low pres passage directly over New England. The latter may be a result of convective feedback, however if true it would certainly suggest a period of moderate to heavy rainfall, as lift would be much more significant, and able to fully realize PWATS nearing 2.00 inches. In any case, destabilized soundings and modest shear will be in place, allowing for some convective elements to accompany the widespread SHRA/RA. Will continue to suggest some TS and continue the likely POPs put forth from the previous forecaster. Will need to watch this period, because if the more robust low pres is the ultimate answer, heavy rain could yield some localized urban/street flooding potentially more widespread than more localized convection. The increased cloud cover and cool air associated with the digging trof will keep highs mainly below normal, suggesting highs may once again struggle to reach the low-mid 70s, but may be cooler if the more amplified solution. Sat and Sun... Although continued digging of the longwave trof occurs, a cutoff forms in the Canadian Maritimes which allows for the more anticyclonic sheared (W half) of the trof to slide across New England. This should allow for drier NW flow, limiting any additional SHRA/RA and drying the column enough for mainly dry wx as well. Still a few clouds, particularly diurnally driven can`t be ruled out given the trof base is just downstream. Temps remain cooler than seasonal normals. H85 +10C to +11C would yield highs mainly in the mid 70s, potentially 5-8 degrees colder than normal. Early next week... Although longwave trof continues to dominate across the E, the weakening cutoff/vortex in the Maritimes will give way to brief ridging as secondary wave moves out of the Canadian prairies. Therefore, although cooler temperatures are likely to continue, exactly how this complex interaction of trof plays out could allow for some unsettled wet wx at times, but definitely not a washout, as it keeps the deep layer moisture well to the S. && .AVIATION /17Z TUESDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/... This afternoon...Moderate confidence. Mostly MVFR ceilings through mid afternoon with gradually lifting to VFR ceilings during the late afternoon into the evening. Tonight...High confidence. VFR with clearing skies with the exception of patchy overnight fog at interior airports. Wednesday...High confidence. VFR. KBOS Terminal...Moderate confidence. MVFR ceilings lifting to VFR by late afternoon or early evening. KBDL Terminal...Moderate confidence. MVFR cigs lifting to VFR during late afternoon. Outlook /Wednesday Night through Saturday/... Wednesday night through Thursday...High confidence. VFR. Mainly S to SW winds. Thursday evening through Friday...Moderate confidence. Mix of MVFR/IFR conditions with widespread SHRA and risk for occasional TSRA mixing with fog. Winds will be shifting through the period, but cannot rule out some gusts to around 20 kt as these shifts occur. Saturday...Moderate confidence. Mainly VFR especially during the day on Sat. && .MARINE... FORECASTER CONFIDENCE LEVELS... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/...High confidence. Through tonight...Diminishing NE winds with speeds less than 20 kt. Hazardous seas will result in small craft advisories over the outer waters today but seas gradually subsiding below 5 ft tonight. Wednesday...High pres over the waters will result in light easterly winds with seabreezes developing nearshore waters. Seas below SCA. Outlook /Wednesday Night through Saturday/... Conditions during most of the period will likely remain below Small Craft Advisory thresholds. However, this is a risk that winds and seas may approach 25 knots and 5 feet, respectively, in an increasing SW flow Thursday afternoon and a northerly flow Friday afternoon and night, depending upon the strength of a low pressure area that passes along the southern waters Friday. Rain and fog will lead to visibility restrictions especially Thu night into Fri. && .CLIMATE... Record low maximum temp of 66 degrees set at PVD yesterday. Previous record was 67 set in 1997. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory for hazardous seas until 4 AM EDT Wednesday for ANZ250-254>256. Small Craft Advisory for hazardous seas until 6 PM EDT this evening for ANZ251. && $$ SYNOPSIS...KJC/Doody/Thompson NEAR TERM...KJC/Doody/Thompson SHORT TERM...KJC LONG TERM...Doody AVIATION...KJC/Doody/Thompson MARINE...KJC/Doody CLIMATE...

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