Area Forecast Discussion
Issued by NWS Boston, MA

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643 FXUS61 KBOX 192006 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 406 PM EDT Wed Jul 19 2017 .SYNOPSIS... * Updates to near- and short-term forecast portions only... * Long-term due out here shortly... 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. Unsettled weather is possible at times this weekend into early next week with near or below normal temperatures. && .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/... Highlights... * Increasing chance for thunderstorms Thursday night * Less humid on Friday but still very warm * Unsettled weather possible at times this weekend into early next week with high temps near or below normal Pattern Overview... 00z guidance remains in generally good agreement for the long term. High pressure over the upper Mississippi Vally will result in northwest zonal flow through late Saturday. Several waves will pass through the flow resulting unsettled weather. High pressure will shift back towards the southwest early next week resulting in troughing over the Northeast. A coastal low will develop impacting New England Monday/Tuesday. Overall trend for this forecast is the unsettled weather, however much of the region will remain dry. Temperatures will also trend to be below normal especially Sunday into next week. Details... Thursday night...Moderate confidence. Northwest flow aloft with passing shortwave will be the trigger for any convection to develop. In fact, a complex of thunderstorms across the Great Lakes may survive all the way to coastal plain overnight. Strong to perhaps severe storms are possible. Warm, moist airmass remains over the region with K values well above 32C and increasing 850mb LLJ. CAPE values are forecasted to be near 1000 J/KG with increasing 50 kts of 0-6km bulk shear and steep mid-level lapse rates of 6.5 or greater. 850 mb moisture transport will advect through the region riding along passing shortwave. All of these features are lining up to create a linearly-organized convection with damaging winds being the main threat. While the potential risk is increasing, still a lot of model guidance spread. The GFS appears to be the outlier with no precip overnight. The NAM pushes a cluster/MCS across the southern half of the region which lines up with the RPM. THE UKMET and EC have kept the cluster near and North of the Pike. So even through there is still a spread on location the risk is still there. Friday...High confidence. Any cluster that moves through will be offshore by Friday morning. Weak ridging and subsidence on the backside of this still will lead to a dry forecast during the day. Cannot rule out a stray shower during the afternoon. Temperatures will warm into the mid to upper 80s to perhaps a few sites reaching 90F. Although warm, it may feel a bit more pleasant as mixing during the afternoon will lower dewpoints into the low 60s. Some westerly wind gusts near 15-20 MPH are possible. This weekend...Low confidence. Classic pattern over the weekend putting southern New England more on the unsettled side. Strong zonal flow aloft will result in several shortwaves riding through the flow. Each wave could bring showers/thunderstorms through the region. Still some uncertainty in timing and strength of each system. Current thinking is Saturday may trend dry where as precip may impact the area Sat PM into Sunday. Temperatures on Saturday will remain near normal with highs reaching the mid to upper 80s thanks to westerly flow. A temperature swing on Sunday as cold front advances through. This front may stall somewhere across the region creating a temperatures spread. Highs may remain in the upper 70s to low 80s. Next week...Moderate confidence. Potent shortwave moving through the Great Lakes will develop a coastal low over southern New England. This will result in temperatures below normal and unsettled weather. If the system trends farther north then temperatures will be at or above normal. Still a lot of uncertainty so stay tune to the latest forecasts. && .AVIATION /20Z 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/... Thursday night...Moderate confidence. Mainly VFR. However a cluster of strong -TSRA may impact SNE terminals resulting in gusty winds and lowering conditions. Friday into Saturday...Moderate confidence. VFR through much of the period. A spot shower at times. Sunday...Moderate confidence. VFR with MVFR conditions within any SHRA/TSRA. May have to watch for low clouds and stratus along east coast. && .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/... Thursday night through Sunday...Moderate confidence. Expecting winds and seas to remain below Small Craft Advisory thresholds. However, some nearshore mid to late afternoon southerly wind gusts around 20 knots at times may result in a few hours of choppy seas in the typically vulnerable harbors and bays. Areas of fog may impact the southern waters at times, especially during the overnight/morning hours. && .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

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