Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 011825 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 225 PM EDT FRI JUL 1 2016 .SYNOPSIS... An approaching cold front will bring numerous showers and scattered thunderstorms this evening, and a few of them may become strong to severe with locally heavy rainfall. High pressure builds in behind the front and dry and very warm weather through much of next week. && .NEAR TERM /THROUGH TONIGHT/... 2 PM update... TOR watch issued for CT Valley and Berkshires until 10 pm. Instability will be the wildcard as there is a notable increase in deep layer shear and 0-1km helicity this evening. CAPES will probably be limited to 500-1000 J/kg but mid level cooling will help to maintain instability into the evening for storms to hold together as they move into SNE. Main concern for any brief spin- ups will be in the CT Valley where s/se surface flow will serve to maximize helicity. Hi-res guidance shows potential for storms to organize into a line as they move into western New Eng after 5 pm as they tap into increasing shear. Before then, just expect widely scattered showers/t-storms this afternoon as instability is limited. SPC mesoanalysis shows MLCAPES around 500 J/kg across CT with better instability to the SW. && .SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/... ***High confidence in numerous showers and scattered thunderstorms with locally heavy rainfall roughly between 5 pm this evening and 2 am Saturday morning*** ***Low confidence on whether some storms become strong to severe*** Tonight... Shortwave and impressive height falls will sweep across New England this evening. Strong forcing noted as 0 to 6 km shear values increase to between 35 and 45 knots. Despite the loss of daytime heating, instability may actually increase during the evening as mid level lapse rates steepen with the height falls. Models show 500 to 1000 J/kg of CAPE persisting through 06z tonight. As a result we are fairly confident in numerous showers and scattered thunderstorms, with locally heavy rainfall this evening. Brief poor drainage street flooding is possible. In addition, dangerous cloud to ground lightning is a big concern given its the start of the holiday weekend. The biggest uncertainty is if some of these storms become strong to severe. As usual this is not clear cut since instability is marginal, but strong forcing will try to make up for it. It is a very delicate balance, but there certainly is some potential for some strong to severe thunderstorms. Highest risk is across western MA/northern CT where better instability is expected, but cannot be ruled out across the rest of the region given the strong dynamics. If any storms are able to become strong to severe, biggest threat will be for localized wind damage, but there is a secondary concern for hail given cooling aloft. Lastly, a low risk exists for an isolated tornado given SREF/NCAR ensembles showing pretty good probabilities for 0 to 1 km helicity of 150+ and LCL/s below 800 meters. So to summarize, high confidence in showers and scattered t-storms this evening with localized heavy rainfall and dangerous cloud to ground lightning. Confidence is low on whether some storms become strong to severe since instability is marginal, but it is certainly something to watch closely given strong dynamics. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Big Picture... Upper trough over the Northeast USA Sunday-Monday lifts out through the Maritimes. Subtropical high covers the southern half of the country and builds north by midweek. The GFS and ECMWF have different magnitudes for 500 mb contour heights midweek but both show the same trend with above normal values for July. The GGEM is more of an outlier. Details... Sunday-Monday... Upper trough and cold pool with Vertical Totals 26-27C, but cloud moisture limited to a layer either side of 800 mb. Could be diurnal clouds but no showers. Mixing reaches to at least 800 mb each day with temps at that level supporting max sfc temps in the Low to Mid 80s. Tuesday through Friday... Upper ridge builds. The broad area of subsidence will allow dry weather with plenty of sun each day. Mixing will reach above 850 mb, supporting max sfc temps in the upper 80s and lower 90s. An approaching cold front may bring a chance of showers Friday although a west-to-east flow may hamper southward progress of any cold front. && .AVIATION /19Z FRIDAY THROUGH WEDNESDAY/... FORECASTER CONFIDENCE LEVELS... LOW...LESS THAN 30 PERCENT MODERATE...30 PERCENT TO 60 PERCENT HIGH...GREATER THAN 60 PERCENT Short Term /through Saturday Night/... Through tonight...Moderate confidence. Cigs lowering to MVFR/IFR south of the Mass Pike later this afternoon into this evening. Patchy fog also developing tonight. Otherwise, widely sct showers/t-storms this afternoon, likely becoming more widespread this evening as a line of convection moves west to east across SNE. Some of the storms may be severe with damaging wind and hail, mainly across the interior. Improvement expected by 12z in most locations, except for patchy stratus/fog lingering on the outer cape/islands. Saturday...High confidence. Patchy MVFR/IFR lingering over cape/islands early, otherwise VFR. W/NW wind gusts to 20 kt. Saturday night...High confidence. VFR. KBOS TAF...Moderate confidence. Main thunderstorm risk 01-05z. KBDL TAF...Moderate confidence. Main thunderstorm risk 22-03z. Outlook /Sunday through Wednesday/... High confidence. VFR through the period. Patchy IFR in fog possible early each morning. && .MARINE... Low level jet will result in southerly wind gusts increasing to between 20 and 25 knots this afternoon and evening. We did issue a small craft advisory for our northeastern waters, where 4 to 5 foot seas should develop. Across the rest of the region, think wind gusts will be just below criteria but close enough to monitor closely. Biggest concern for mariners is the risk for a few strong thunderstorms this evening, with 40+ knot wind gusts possible. Outlook /Sunday through Wednesday/... High confidence. Winds remain below 25 knots and seas below 5 feet through the period. && .FIRE WEATHER... Hit or miss showers and thunderstorms are possible this afternoon and tonight. However, widespread precipitation looks to be nil over the next 7 days. High pressure and dry conditions appear to take hold of the region Saturday, lasting through the much of next week. Much of this time will be less humid with RH values dropping close to 30 percent for much of this time. Winds overall appear to be light during this period. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 4 AM EDT Saturday for ANZ250. && $$ SYNOPSIS...WTB/Frank NEAR TERM...KJC SHORT TERM...Frank LONG TERM...WTB AVIATION...WTB/KJC MARINE...WTB/Frank FIRE WEATHER... is the U.S. government's official web portal to all federal, state and local government web resources and services.