Area Forecast Discussion
Issued by NWS Boston, MA

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698 FXUS61 KBOX 281927 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 327 PM EDT THU JUL 28 2016 .SYNOPSIS... A cold front over northern New England will drop south into our area this evening and may bring a few showers or thunderstorms across the interior. Low pressure will track south of New England Friday and should bring heavy rain south of the Mass Pike. Drier weather returns for Saturday, followed by another risk for showers Sunday and Monday. && .NEAR TERM /THROUGH TONIGHT/... 325 PM Update... Showers/storms have fired across northern New England, and we may see some scattered activity into parts of western/northern MA before sunset as weak cold front drops south into our region. Satellite loop shows bank of low clouds/fog entrenched from RI Sound into Buzzards Bay where it is through late afternoon, before starting to expand over South Coast, Cape, and Islands as we approach sunset which will last through tonight. Next area of showers/storms over mid Atlantic states will head our way tonight as low pressure develops off NJ coast and tracks NE. 12z models have sped up timing of onset of showers overnight, starting in lower CT Valley toward daybreak. Warm/muggy night ahead with patchy fog due to light S flow. Lows will drop back into the 60s and lower 70s. && .SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/... * Potential for flash flooding Fri, especially south of Mass Pike * Marginal threat for severe weather Fri near South Coast Active day coming up Fri as low pressure tracks south of New England along stalled front. Situation looks like a classic setup for heavy rain and flash flooding south of Mass Pike, and also potential for isolated severe weather near South Coast. * Heavy Rain/Flash Flood Potential... Classic flash flood pattern taking shape as anomalous 850 mb southerly jet (+3-4 SD) brings tropical moisture (over 2" precipitable water with surface dewpoints in 70s) over frontal boundary sitting south of region. Forecast soundings show potential for efficient warm rain processes with deep layer of saturation up through high freezing level (near 15K ft). In addition, model cross sections show strong low level frontogenetic forcing (almost more typical of a wintertime scenario) with -EPV above. Very good dynamics for the middle of summer! While we are confident in excessive rainfall falling somewhere south of Mass Pike, question is exactly where? Keep in mind there is a lot of uncertainty as to where this stalled front ends up, which will determine where axis of heaviest rain will be. Trend in most of models has been to shift things south, more over coastal waters south of New England. Model fields (lift/moisture) suggest not totally jumping on board just yet and holding back, keeping heavy rain/severe threat closer to South Coast. Model QPF is one of the least reliable fields so it should be used with caution. We may not really know for sure until radar trends begin to tell story within 6 hours or so. One thing we need to be aware of is if convection over mid Atlantic ends up pushing south of region and essentially robs available moisture. That would mean significantly less rainfall in southern New England, and is one possible outcome. Our rainfall forecast gives more weight to higher-res models in order to better show sharper cutoff in heavier rainfall on northern edge (north of Pike). We expect max 2-4" near South Coast, Cape Cod, and Islands with about 1" as far north as Mass Pike and up to 0.50" near NH border. Flash Flood Watch has been issued for CT, RI, and SE MA where higher rainfall rates (more than 1-2"/hr) would produce rapid, significant urban flooding in some of the more flood prone locations. Even though we are in drought conditions, extremely dry soils can also lead to rapid runoff, which could result in rapid flooding of some small streams and rivers, again primarily due to urban runoff. * Severe Potential... In collaboration with SPC, the Marginal risk for severe weather was expanded onto South Coast for Fri. Again, there is uncertainty as to how everything will play out due to uncertainty with position of front. Models show plenty of instability (surface-based CAPE near 1000 j/kg) and strong 0-6km shear (more than 45kt) near front. Pattern is favorable for short-lived waterspouts or an isolated tornado on gradient between stable and more unstable airmass, which models indicate will most likely be south of the islands and east of Nantucket midday Fri into the afternoon. It is in this area where 0-500m helicity is forecast to exceed 90, low LCLs will be present, and VGP (vorticity generation parameter) exceeds 25, which past research has shown to be favorable. Rain should fall most of day but taper off from W to E during afternoon, with most of it done toward evening on E MA coast. Highs will top out in 70s to near 80. Everything clears out Fri night but lingering low level moisture should lead to patchy fog, especially near South Coast. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... Highlights... * High pressure Saturday * Showers and storms possible Sunday and Monday * Drier and seasonable weather mid-late next week Overview and model preferences... Interesting weakly split flow regime defines the mid-term period. Weak ridge-over-trof split flow develops and gradually moves across NE CONUS, finally giving way to joined ridging toward the latter half of the forecast period. The trof is the primary wx-maker for late weekend and early next week, providing a focal point for continued shra/tstm activity. Timing disagreements are minimal with the 12Z updates and even show continuity with ECMWF EPS and GEFS means. Therefore, will use a consensus blend for this afternoon`s long-term update. Details... Sat... Confluence aloft following the initial convective low pres passage Fri will allow for a period of quiet wx under weak high pres. H85 temps hover around +15C, a bit cooler than much of the last week so expect mainly 80s for highs. Sea breezes likely given a weak sfc pres gradient, so expect cooler temps near shore. Sun and Mon... Tricky forecast period as diffuse open wave and modest sfc trof rest across much of the mid-Atlantic region and into S New England. This should provide a focal point for continued shra/tstm development especially to the W and S, where highest moisture lies. PWATs linger around 2.0 inches to the SE but then drop into New England. Gradient winds mainly E-NE which, off the cooler Gulf of Maine waters which is likely to limit convective (particularly sfc based). Therefore not confident in how much thunder activity S New England will experience, however leftover shra is possible. CIPS analogs support this thinking, focusing the highest risk for convective activity, SW of New England. Will need to watch a secondary low pres wave development which could provide better low lvl convergence and F-gen. ECMWF is closest to the S coast with this feature Sun night, but if it remains further offshore, less rain will impact S New England. Will need to monitor as we approach as it has similar look to the low pres which could bring heavy rain to the region tomorrow. Tue through Thu... In spite of what occurs earlier in the week, the split flow finally joins in the form of a ridge upstream which gradually shifts to the E. This should bring about a period of mainly quiet wx with sfc high pres in place. However, the orientation of high pres brings in some cooler/drier air with H85 temps really only hovering around +12C, so temps should remain much closer to seasonable levels than they have been for the last week or so. && .AVIATION /19Z THURSDAY THROUGH TUESDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Moderate confidence through Friday night. E MA sea breeze has developed and finally made it into BOS. Winds should turn more S along coast later this afternoon. Otherwise scattered showers/storms possible across western and north central MA after 21z and may bring brief MVFR/IFR vsbys. Fog bank from RI Sound into Buzzards Bay should begin to expand around sunset and should bring IFR/LIFR to much of South Coast, Cape Cod, and Islands tonight into Fri. Elsewhere conditions lower to MVFR or even IFR toward daybreak Fri and last through day in showers/fog, especially across CT, RI, and SE MA. Conditions improve Fri evening to VFR, but patchy fog may bring local MVFR/IFR Fri night. KBOS TAF...Moderate confidence. Sea breeze persists through late afternoon before winds turn more to S. KBDL TAF...High confidence. Outlook /Saturday through Tuesday/... Saturday...High confidence. VFR. Sea breezes likely. Sunday and Monday...Moderate confidence. Some showers and a few thunderstorms possible through the period. Occasional MVFR conditions, but VFR likely dominates. Tuesday...High confidence. Mainly VFR. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday Night/...High confidence. Main concern is potential for strong or even severe storms over south coastal waters Fri, mainly south of islands and east of Nantucket where brief waterspouts are possible Fri afternoon. Issuing a Marine Weather Statement to highlight this potential, especially for recreational boaters. Otherwise winds will remain below 25 knots and seas below 5 feet. Patchy dense fog should expand over most of south coastal waters from Buzzards Bay tonight into Fri, while rain and fog reduce visibility Fri into Fri evening. Seas could approach 5 ft on outer waters SE of Nantucket. Outlook /Saturday through Tuesday...High confidence. Mainly quiet boating weather as high pressure slowly builds over the waters. Winds and seas below small craft advisory thresholds. Only caveat could be an isolated shower or thunderstorm mainly Sunday or Monday. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Flash Flood Watch from late tonight through Friday evening for CTZ002>004. Air Quality Alert until 11 PM EDT this evening for CTZ002. MA...Flash Flood Watch from late tonight through Friday evening for MAZ017>024. RI...Flash Flood Watch from late tonight through Friday evening for RIZ001>008. MARINE...None. && $$ SYNOPSIS...Doody/JWD NEAR TERM...JWD SHORT TERM...JWD LONG TERM...Doody AVIATION...Doody/JWD MARINE...Doody/JWD is the U.S. government's official web portal to all federal, state and local government web resources and services.