Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 222321 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 721 PM EDT Tue Aug 22 2017 .SYNOPSIS... A cold front will bring the potential for scattered showers and thunderstorms late tonight, which may persist into Wednesday morning along the coastal plain. High pressure brings dry weather with very comfortable humidity levels Thursday through early next week. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... * Severe Thunderstorm Watch remains in effect for western MA until midnight 7PM update... Convection across NY/PA attempting to form QLCS within a corridor of enhanced shear along a 45-50 kt SSW-NNE oriented LLJ and pre-frontal trof. Overall, trend has actually been for this convection to wane somewhat as it approaches, likely given it is entering an area where moisture is lacking (over us here in S New England) as K-indices remain between 26-28. However, the moisture plume is advecting E with the LLJ and could allow for the maintenance of at least some MU CAPE values near 1000j/kg during the late overnight hours. POPs were timed using a blend of WRF and HRRR data which are currently handling the situation relatively well from a timing standpoint. The lackluster mid lvl lapse rates will remain, and this could prove a convective mitigation factor to consider as the storms approach, but given the ongoing Severe Watch and very strong forcing/LLJ, even in a more stable environment storms could retain a risk for strong winds. Previous discussion follows... Tricky forecast tonight. The high heat and humidity resulted in very high CAPE values for our region. Most unstable CAPE values were between 2,500-3,000 J/kg, with mixed layer CAPE values between 1,500-2,000 J/kg. Despite this, our region remains fairly capped this afternoon due to poor mid level lapse rates and low shear. The complication in the forecast tonight will be how quickly the CAPE values drop after sunset, and how quickly stronger shear arrives. It will be delicate balance. At this time, thinking strong convection may arrive east of the Berkshires between 8 and 10 PM, then weaken as it moves east from there. Helicity values in the 0-1km AGL layer range between 150-300 m2/s2. This would mean at least a very low risk for a tornado as these storms move across our region overnight. The other risks from any strong thunderstorms tonight will be gusty winds and locally heavy rainfall. Thinking any flooding risk tonight will be very localized as storms should be moving rather quickly. Training storms also appear to be unlikely. Well above normal temperatures continue tonight. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... The threat for scattered showers and thunderstorms may persist into Wednesday morning across the coastal plain, and perhaps even into early afternoon across the Cape and islands. Much drier air will arrive behind a cold front, which should move offshore by late afternoon. Near normal temperatures expected through this period. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Overview and model preferences... 22.12Z model guidance suite continues the trend of a mainly dry forecast through early next week. Synoptically, there remains good agreement, and this is in spite of a fairly amplified pattern across the N CONUS and Canada, something long range guidance typically struggles with. Reinforcement of an Alaskan vortex is the catalyst for building a highly amplified ridge across central Canada, and although not a true block, leads to enough downstream trof digging to reinforce the trof upstream of New England. In spite of the cyclonic curvature, flow aloft is confluent predominantly, shifting the mean jet over New England rather than in the typical ageostrophic lift region of the mean jet. This coincides with an area of subsidence in development of reasonably strong high pres which will generally define the sensible wx across New England through early next week. Thermal profiles are supported by drier/cooler flow within the upstream trof out of central Canada, supporting the previous forecaster`s thinking that a break from the summer heat/humidity can be expected. Details... Thu... Front shifts further offshore allowing deeper column drying and cold advection aloft. Dewpoints drop into the 50s and H85 temps drop to +10C to +12C, which should keep highs mainly slightly below normal, or in the mid-upper 70s with a few spots around 80. Subsidence maintains a dry forecast outside of CU development. Fri... Vigorous shortwave will rotate through during the daytime Fri. Lapse rates are reasonably steep given the cold advection leading the wave aloft, nearing 6.0C/km across much of the region. A slight spike in moisture possible as well as PWATs shift closer to the 1.00 inch mark according to some soundings. Although some convective feedback QPF is noted, but likely not well progged, will feature diurnally timed increase in POPs to at least slight chance during the afternoon and early evening hours Fri with this wave passage. Shower, or even a very isolated thunderstorm not out of the question. Otherwise, highs once again slightly below normal, mainly in the mid-upper 70s. This weekend... High pres crests over the region through the period as the base of the longwave trof also shifts W-E across New England. Soundings continue to be mostly dry, suggesting the risk for even any showers remains quite low, especially given the lack of strong forcing. Will continue to feature a mainly dry forecast with NIL POPs. Highs in the 70s predominantly, with comfortable overnights dipping into the 50s thanks to dewpoints in the upper 40s and 50s as well. Early next week... Low lvl ridging is reinforced by the influence of the stronger synoptic ridge shifting E in New England as the longwave moves E of New England. In fact, already noting that high pres will approach 1030+ hPa. Dry weather looks likely to continue, along with comfortable dewpoints. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/... Tonight...Moderate confidence. SCT SHRA/TSRA should move east of the Berkshires between 01Z- 04Z. Storms are expected to weaken as they shift east with time, but a few stronger storms cannot be ruled out. This line weakens for a bit early in the morning, but then another round of showers and storms could impact mainly southeastern MA after 10Z. Mix of mainly VFR with occasional MVFR/IFR in showers/storms, low confidence in IFR/LIFR stratus mainly Cape Cod and the Islands. Winds gusting 20-30 kt at times, with LLWS given 40-45 kt at around 2kft. Wednesday...High confidence in trends, but lower confidence in overall timing. SCT SHRA/TSRA may persist near and especially southeast MA into early Wednesday afternoon. Improvement to VFR conditions expected from west to east, but timing somewhat uncertain. Wednesday Night...High Confidence. VFR. KBOS Terminal...High confidence in TAF trends. Moderate confidence in timing. KBDL Terminal...High confidence in TAF trends. Moderate confidence in timing. Outlook /Thursday through Sunday/... Mainly VFR. Mainly W-NW flow with sea breezes by day. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/...High confidence. Strong southwest low level jet moves across the waters tonight into Wednesday morning. Gusty southwest winds ahead of an approaching cold front. Some risk for thunderstorms, but showers more likely. Will continue Small Craft Advisory headlines, but did tweak times. Winds and seas diminish Wednesday afternoon, with more tranquil boating conditions starting Wednesday night. Outlook /Thursday through Sunday/... Generally quiet boating weather. NW winds dominate, with gusts peaking only around 15 kt at any given time. Seas/waves remain below 4 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until noon EDT Wednesday for ANZ231>234- 250. Small Craft Advisory until 2 AM EDT Wednesday for ANZ230-236. Small Craft Advisory until 2 PM EDT Wednesday for ANZ235-237. Small Craft Advisory until 8 AM EDT Wednesday for ANZ251. Small Craft Advisory until 4 PM EDT Wednesday for ANZ254>256. && $$ SYNOPSIS...Belk/Doody NEAR TERM...Belk/Doody SHORT TERM...Belk LONG TERM...Doody AVIATION...Belk/Doody MARINE...Belk/Doody is the U.S. government's official web portal to all federal, state and local government web resources and services.