Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 192032 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 432 PM EDT Wed Jul 19 2017 .SYNOPSIS... Mainly dry but hot and humid weather persists through Thursday afternoon. There is growing concern for a round of showers and thunderstorms Thursday night, a few of which might be strong to severe and produce heavy rainfall. Then dry and warm Fri and Sat but turning less humid. Unsettled weather in the form of showers and thunderstorms are possible at times Sunday into Tue. A drying trend likely develops middle of next week with mild days and cool nights. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... 4 pm update... With daytime heating and deep boundary layer mixing, have seen some locations with dewpoints dropping down into the 50s as temperatures have warmed into the low 90s limiting heat indices from reaching the mid 90s. However prior to, mid-90 heat indices were observed across N CT, RI and SE MA. In addition to the drier air, faster winds with gusts up to 20 mph have also been observed. As we go into evening, should see diurnally-forced cumulus dissipate as winds dissipate and dewpoints slowly rise. Tonight... Quiet overall. Weak mid-level trough sweeps the region towards morning resulting in some scattered to broken cloud decks, presently observed upstream across the E Great Lakes Region, as winds veer more W/NW. A slight chance of a shower across the N/W high terrain possible given convergent lift upon a moisture deprived environment. Otherwise partial clearing and light winds ahead of the wind shift allowing the return of higher dewpoint air, could see some patchy ground fog preceding, prior to W/NW winds scouring. W/NW flow will also aid in scouring low clouds and fog lingering across the S/SE coast of New England, pushing it out to sea during sunrise. A DENSE FOG ADVISORY is possible once again for Nantucket, perhaps even the Chatham area. Will highlight the area with a SPECIAL WEATHER STATEMENT for the time being. Lows overnight around the mid to upper 60s. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... Thursday... A near-repeat of Wednesday, heat returns. In wake of the overnight into morning mid level trough, with daytime heating will see the return of W flow across the interior and SW flow closer to the coast. Beneath a slightly warmer airmass with H85 temperatures around +17C, the level around which the boundary layer mixes out, will see the mix down of drier air and faster winds, with gusts up to 20 mph, which will again define the SW-NE dewpoint boundary. Convergent lift of sufficient low-level moisture, can`t rule out an isolated shower around the Hartford-Providence-Boston corridor. Otherwise scattered cumulus and temperatures warming up into the upper 80s to low 90s. For those areas around and S of the boundary in higher dewpoint air, heat indices can get back into the mid 90s. Higher confidence across CT, RI, into Southeast MA. Evaluating heat indices that have occurred today, aforementioned areas may need a HEAT ADVISORY. Thursday Night... The chance of showers and thunderstorms associated with a mesoscale convective system (MCS), that if it originates, has the potential of producing strong to damaging winds along with frequent lightning and heavy rain (as well as the possibility of flooding) during the evening into overnight period. Looking at the ingredients that would maintain a MCS, particularly winds and shear through the layer coupled with low level inflow of robust moisture in an environment of favorably steep lapse rates and subsequent availability of instability. Good model agreement on a region of strong synoptic forcing and ascent sweeping the region with strong, parallel mean wind and shear vectors which would aid in damaging wind potential with any strong to severe storms. However unclear is the measure of low-level moist inflow, notably around H85, as well as moisture availability within the H5-7 layer. While a majority of guidance suggests 3-8 km lapse rates around 6C/km, if the layer ends up drier, slightly warmer, then there`s less mid- level instability available to maintain a MCS as the 19.12z GFS would suggest. Having interrogated closely all available guidance, feel a stronger MCS will originate out of the U of Michigan prevailing S/E with longevity with the Corfidi vector flow towards better moist-inflow and instability, only dissipating further S as flow becomes more W away from a quasi-stationary low-level thermal boundary. That boundary is more in question across the NE CONUS, how effectively if it can focus SW low-level inflow into the NE CONUS or rather it remains more W with less focus. However, given the atypical magnitude of synoptics over the area with a slightly more amplified trough pattern not usually seen for mid-July that supports decent venting aloft above a model consensus mid-level unstable environment, though marginal, can`t rule out some shower and thunderstorm activity. Dependence falls on moisture in the column as well as the magnitude of low-level inflow. If the non- GFS camp is correct yielding potential instability above 1500 J/kg within a strong-sheared environment with parallel mean wind vectors, then there is a the possibility of evening into overnight strong to severe thunderstorms with the main threat being damaging winds with secondary threats of heavy rain and potential flooding along with frequent lightning. With this forecast, will go with chance PoPs for now and won`t go into any special wording. There`s a lot of mesoscale elements, including the upstream anticipated MCS, and a lot of variables that are involved such that confidence is not high enough to go with stronger wording at this time. Will continue to highlight the possibility in the HAZARDOUS WEATHER OUTLOOK. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Updated 430 PM... Highlights... * Fri & Sat: ideal summer weather, dry, warm but less humid * Sun into Tue: unsettled with showers/T-storms at times & cooler * Midweek: trend toward drier weather along w/mild days-cool nights Pattern Overview... Vortex traverse across northern Quebec Fri and Sat providing dry WNW flow aloft along with slight height falls across New England, but very anomalous height and temps anomalies across southeast Quebec with -3 standard deviations from climo! This will yield dry, warm but less humid weather across the area Fri and Sat. Then the overall upper air pattern continues to amplify with the northern stream becoming suppressed and the westerlies straddling the U.S.- Canadian border. Very atypical pattern for mid to late July. This will yield a period of active weather here Sunday into early next week with several rounds of showers. Daily Details... Friday and Saturday... Dry WNW flow across the area this period. Convective complex may come close to the region Sat per past several runs of the EC. However WNW flow aloft appears to suppress the 850 mb Theta-E ridge south of New England. Thus any convective complexes should traverse southwest of the area. Regarding temps, airmass remains warm across the area Fri and Sat (esp Fri) with 850 temps still +15C/+16C, so highs well into the 80s (85-90 Fri, 80-85 Sat) both days, but less humid especially Sat with dew pts falling into the low and mid 60s Fri, then into the 50s Sat away from the south coast. Overall, two nice summer days ahead. Sunday thru Tuesday... Very anomalous pattern unfolding with suppressed jet/storm track this period along with a robust short wave trough ejecting out of the Great Lakes Sunday. This will set the stage for a series of weak lows to track over or south of the region. Yes, the calendar reads July but looks more like a Fall or spring pattern. So risk for periods of rain/showers Sun thru Tue along with cooler temps given cloud cover, precip and onshore flow. There will likely be some dry periods as well in between waves however too difficult to pinpoint at this time range. Midweek... Mid level mean trough axis likely moves offshore with a period of dry and cooler weather overspreading the region. However this may be short lived as both ensembles and deterministic guid agree that northeast trough reloads shortly thereafter. && .AVIATION /21Z WEDNESDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/...High confidence. Rest of today into this evening... VFR. Breezy W/SW winds with gusts up to 20 kts. Tonight... VFR. Any IFR CIGs and VSBYs are expected to be confined to S/SE coast with highest confidence over Cape and Islands. ACK could see VSBYs to 1/2SM. Can`t rule out patchy shallow ground fog over the interior as winds will be rather light and conditions near or at SKC. Thursday...High confidence. Some early morning MVFR-IFR VSBYs with patchy shallow ground fog across the interior, eroding quickly as winds will be becoming W/NW into morning. This should also aid in scouring any IFR CIGs across the Cape and Islands. Ahead of the W/NW shift, could see an isolated SHRA/TSRA with the greatest threat over SE MA, RI, and E CT terminals. Thursday Night...Moderate confidence. Chance SHRA/TSRA for the entire region. TSRA may contain strong to damaging winds along with +RA. TEMPO MVFR-IFR impacts. Majority of impacts evening and towards midnight, clearing out during the early morning hours with SW winds becoming W. KBOS Terminal... VFR. Could see a sea-breeze around 15z Thursday that holds into the afternoon hours before mixing out and returning W. KBDL Terminal... There is the possibility tonight of some patchy shallow ground fog, however confidence is low at this time. SHRA/TSRA impacts possible Thursday evening. Outlook /Friday through Monday/... Friday and Saturday...VFR and dry weather likely. High forecast confidence. Sunday and Monday...a mix of MVFR and IFR likely along with scattered showers. Moderate forecast confidence. && .MARINE... FORECASTER CONFIDENCE LEVELS... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/... Tonight through Thursday...High confidence. SW flow becoming W towards Thursday morning before returning SW. During daytime periods and along the S/SE near shore, gusts up to 20 kts are expected. Its also in this region that there is the potential for dense fog redeveloping overnight, burning off Thursday morning. Seas remaining below 4 feet. Thursday Night...Moderate confidence. Possibility of showers and thunderstorms sweeping the waters late ahead of which SW flow maintains with gusts up to 20 kts, slowly veering out of the W into Friday morning. Outlook /Friday through Monday/... Friday and Saturday...high forecast confidence. Nice summer boating weather with tranquil seas and winds along with mainly dry conditions. Sunday and Monday...moderate confidence. Unsettled weather with several weak lows tracking over or south of New England. Yes, atypical for mid to late July. One of these waves may yield modest to strong NE winds so we have to watch this closely. Vsby likely limited in showers and areas of fog. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...Air Quality Alert from 11 AM to 11 PM EDT Thursday for RIZ001>008. MARINE...None. && $$ SYNOPSIS...Nocera/Sipprell NEAR TERM...Sipprell SHORT TERM...Sipprell LONG TERM...Nocera AVIATION...Nocera/Sipprell MARINE...Nocera/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.