Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 240139 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 939 PM EDT TUE AUG 23 2016 .SYNOPSIS... High pressure will remain in control with warmer temperatures Wednesday but continued low humidity. More humid conditions will return Thursday and Friday which will lead to a risk for showers and thunderstorms Thursday night and Friday as a weak front passes. Another high will bring dry and less humid weather next weekend. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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No major changes to the forecast this evening. Brought the near- term forecast back in line with observed trends. Previous discussion... High pressure, light winds, clear conditions, all makes for a radiational cooling setup, especially in regions of sandy soils and/or devoid of substantial rainfall as of late. Evaluating dewpoints across the region, many readings in the upper 40s to low-50s during max-heating of the day. Going to be tough to get radiational fog to develop unless a particular location can readily lose heat or is close to a nearby body of water. Thus second the previous forecaster`s remarks of patchy fog towards early Wednesday morning, particularly in the CT river valley and perhaps interior SE Mass towards the coastlines of Narragansett Bay out towards the Cape. Lows around the mid-50s. Cool spots in low-lying locations or notably prone interior areas. Warmest across the high terrain, within urban centers, and along the shore.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... Wednesday... High pres shifts a bit further south of New England bringing warmer temps as sw flow develops. The column remains rather dry so another day with unlimited sunshine. 850 mb temps increase to near 16C supporting highs well into the 80s with a few locations possibly reaching 90 in the CT and Merrimack valleys, but a bit cooler along the south coast. Humidity levels will remain low as mixing of dry boundary layer keeps dewpoints down into the 50s. Wednesday night... Remaining dry and milder as the high begins to move offshore with increasing sw flow. Mainly clear skies but models show increasing moisture aloft which will result in some high clouds overnight. Lows will generally be in the upper 50s to lower 60s. && .LONG TERM /THURSDAY THROUGH TUESDAY/... */ Highlights... - Shower and thunderstorm chances for Friday - Cooler and drier weather for the Weekend - Another shot of wet weather Monday - Monitoring Invest 99L, low confidence forecast into next week */ Discussion... While eyes are on the tropics, still need to evaluate the broader pattern across the N Hemisphere. Most notably are the distinctive signals within atmospheric teleconnections. With maintenance of a Bering Straight / N Pacific low, ridging prevails across the NE Pacific and Gulf of Alaska yielding a strong negative East Pacific Oscillation. Downstream, troughing emerges over the Inter-Mountain West with any upstream disturbances through the flow, but forecast guidance struggling in handling individual features, thus the spread and waffling of the Pacific N American pattern. Nevertheless still favor ridging further E and thus the Atlantic High remains dominant with its presence across the SE CONUS / NW Atlantic. As such, any potential wet-weather outcomes across the NE CONUS do not look promising with respect to the ongoing drought situation. With ridging and anti-cyclonic flow maintained across the NE CONUS, and low pressure prevailing across the Hudson Bay region of Canada, a SW-NE confluent flow regime anchors across the region once again through which disturbances out of the W deamplify. A re-occurring theme, heat and humidity are captured by S flow ahead of weak areas of low pressure zipping through the Great Lakes Region into the St. Lawrence River Valley. Chances for showers and thunderstorms emerge across the NE CONUS ahead of a trailing cold front. An unsupportive environment of lift and forcing, with the bulk of dynamics to the N and quasi-parallel low to mid level flow to the sweeping cold front, the wet-weather chances are isolated to chance. Can`t bet on likely given this synoptic setup. Only promising aspect to the forecast is the follow-up cooler and drier airmasses behind cold frontal passage with high pressure building S out of Canada. It is perhaps with the influence of Invest 99L that the Atlantic ridge weakens lending lowering heights and an allowance of N-stream energy to dive S. A significant change to the pattern to which can have implications and subsequent interactions of energy associated with Invest 99L into the NE CONUS by late next week into Labor Day Weekend. Purely speculative at this time so far out in the forecast. Two areas of uncertainty, that from the broader pattern W, and from the tropics to the SE, makes for one giant headache in the long-term forecast. Thus an ensemble and probabilistic forecast approach, with a nod to the tropics with respect to storm tracks and accompanying strength forecasts, as well as the HWRF. Looking at two periods of wet weather, one around Friday and the other around Monday, with the likelihood of a warmer, more humid airmass initially, followed by chances of showers and thunderstorms for the midday into evening periods. Best chances look to be across N/W portions of S New England, though the outcomes on Monday could prove better given falling heights, cyclonic flow, and potentially higher instability. A more favorable environment of lift and forcing over Friday`s scenario with less parallel flow to the accompanying cold front. With each cold front passage follows refreshing periods of cooler and drier air. A low confidence forecast as we get into next week, approaching the end of September into Labor Day Weekend. Will need to keep an eye on Invest 99L and the remnants of Fiona. && .AVIATION /02Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/...High Confidence. Mainly VFR. Risk of MVFR-LIFR patchy dense fog within the CT River Valley, perhaps prone locations in SE Mass and along the S/SE coast. SKC for Wednesday and a possible repeat of patchy dense fog for late Wednesday night into Thursday morning. KBOS Terminal...Will keep winds out of the S/SW, strong enough to prevent any sea-breeze action for Wednesday. KBDL Terminal...Monitoring closely for patchy dense fog expected to develop in the CT River Valley into Wednesday morning. Possible risk but moderate confidence at this time, not enough to prevail in the TAF. Outlook /Thursday through Sunday/... Thursday...High Confidence. VFR. S winds, perhaps breezy with gusts up to 20 kts. Low confidence of fog issues across the S/SE coast into Friday morning. Friday...Moderate Confidence. SHRA/TSRA, mainly across N/W portions of S New England. Possible TEMPO MVFR-IFR impacts with any storms. Looking towards midday into evening. S winds backing out of the NW overnight. Saturday into Sunday...High Confidence. VFR. Light winds. Likely sea-breezes. Winds turning E/SE Sunday, potentially breezy late. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/...High confidence. 7 pm major changes. Tranquil boating conditions through the period as high pres builds over the waters. Mainly sw winds 15 kt or less with light seas and good vsby. Outlook /Thursday through Sunday/... Thursday...High Confidence. S winds, perhaps breezy with gusts up to 20 kts. Low confidence of fog issues across the S/SE coast into Friday morning. Waves below 5 feet. Friday...Moderate Confidence. S/SW winds, breezy up to 20 kts. Waves building above 5 feet across the S/SE waters. Winds back late into Saturday morning out of the W/NW. Saturday into Sunday...High Confidence. High pressure, light winds, gradually veering out of the S/SE into Sunday, becoming breezy late. Waves remaining below 5 feet. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...KJC/Sipprell NEAR TERM...Belk/KJC/Sipprell SHORT TERM...KJC LONG TERM...Sipprell AVIATION...Belk/KJC/Sipprell MARINE...KJC/Sipprell is the U.S. government's official web portal to all federal, state and local government web resources and services.