Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 191409 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1009 AM 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. Unsettled weather is possible at times this weekend into early next week with near or below normal temperatures. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 10 am update... */ Highlights... - Low clouds burning off quickly this morning with the high- angle July sun - Isolated shower activity possible around midday around N RI and E/SE MA - Otherwise, hot and humid day, especially E/SE MA, RI and the Lower CT River Valley with highs in the low 90s and the potential of seeing heat index values into the mid 90s */ Discussion... Hot and humid today as low clouds erode this morning with the high- angle, mid-July sun. Mixing up to around H85 where temperatures of around +16-17C reside and much drier air (see the 19.12z Albany sounding), ahead of a very weak mid-level trough discerned within the mesoanalysis with attendant slight enhancement in SW flow, should see the mix down of faster winds and drier air to the surface, especially away from areas with better SW fetch off the ocean, and coupled with partly cloudy conditions, should yield highs around the upper 80s to low 90s. Gusts up to 20 mph, more so for E/SE coastal plain. Challenge today is forecasting heat indices. Roughly speaking, with daytime, boundary layer mixing, dewpoints are likely to drop across the N/W interior. Main question is where the dividing line will be with maintained low 70 dewpoints. Believe this boundary will be roughly SW to NE from the SW coastline of CT through E- Central CT and N RI to just off areas S of Boston such as Quincy. This boundary will become the focus of W/SW convergence and possible shower chances. More on that below. But with the mix down of drier air, this will limit heat indices. Focus on areas which will see highs in the low 90s with dewpoints around the mid to upper 60s to low 70s, namely E/SE MA, RI, and the Lower CT River Valley. It is these areas which will have the greatest chance of seeing heat indices in the mid 90s. Highest confidence in SE interior MA. As to the shower chances, focus is on any convergent boundaries in environments within moist low-levels. Particular focus across E/SE MA and N RI where W flow collides with SW onshore flow. Noting the 19.12z sounding out of Albany and interrogating mesoanalysis, the K-indices say it all. Well-below the 30 threshold, will keep out the thunder and go with isolated wording feeling that any collision will yield a quick low-topped shower. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/... Tonight... A rather mild and muggy night is in store for the region. Low temps will drop into the middle 60s to the lower 70s. Patchy fog should develop late in the typically prone locations with the high dewpoint air/light winds, but dry weather should prevail. Thursday... Another hot day is on tap for the region with partly to mostly sunny skies. High temps should range from near 90 to the lower 90s in most locations. Dewpoints will probably be a few degrees lower than today, so heat index values may end up falling just short of 95 in many locations. It still will be close and something will have to watch since two days in a row of heat index values of 95 now requires a Heat Advisory. Mainly dry weather will prevail again on Thursday given dry mid level air and lack of synoptic scale forcing. However, an isolated shower/thunderstorm can not be ruled out given some instability and a weak front/sea breeze interaction possible. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... 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 /14Z 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/...High confidence. Today... IFR burning off, becoming VFR. Increasing SW winds especially over E/SE MA and RI terminals, gusts up around 20 kts possible, more W across the terminals. Isolated SHRA possible, mainly from SFZ-OWD-BOS, otherwise dry. Tonight... VFR. Any IFR is expected to be confined to S/SE coast with highest confidence over Cape and Islands. Can`t rule out patchy shallow ground fog across 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. KBOS Terminal... VFR. Will continue to hint at a sea-breeze around 15z but confidence continues to wane. Deer Island offshore is reporting light E/SE winds as of 14z as is Winthrop MA. The sea-breeze could just stay immediately offshore around midday. Into afternoon W/SW winds should be more dominant. KBDL Terminal... Low clouds burning off this morning, should be VFR the remainder of the day with breezy SW winds. There is the possibility tonight of some patchy shallow ground fog, however confidence is low at this time. Outlook /Thursday Night through Sunday/... 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/... Today through Thursday...High confidence. Weak pressure gradient will keep winds/seas below small craft advisory thresholds through Thursday. Near shore southwest winds will gust to around 20 knots along the south coast, resulting in choppy seas across the typically prone Harbors and Bays for a few hours later today. Otherwise, the main concern will be areas of fog across the S/SE waters, which may be locally dense through mid to late morning before improvement. Fog may redevelop again tonight and persist into part of Thursday morning. Outlook /Thursday Night through Sunday/... 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...None. MARINE...None. && $$ SYNOPSIS...Frank/Dunten NEAR TERM...Frank/Sipprell SHORT TERM...Frank LONG TERM...Dunten AVIATION...Frank/Dunten MARINE...Frank/Dunten is the U.S. government's official web portal to all federal, state and local government web resources and services.