Area Forecast Discussion
Issued by NWS Boston, MA

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468 FXUS61 KBOX 181753 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 153 PM EDT Fri May 18 2018 .SYNOPSIS... High pressure builds to the north today, bringing dry but cooler weather with northeast flow. Widespread rainfall, locally heavy at times, expected Saturday. Warm and muggy on Sunday with hit- or-miss showers/thunderstorms. Dry on Monday with passing showers during the mid-week. Dry weather returns on Thursday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1020 AM update... Dry airmass has advected into SNE as high pres builds to the NE. The column is quite dry with just some high level moisture bringing high clouds. Most sunshine will be to the north, while cloud cover will be thickest near the south coast. Gusty NE winds 20-30 mph near the coast will gradually diminish this afternoon. The NE flow will result in a cool day near the coast, with highs ranging from the mid 50s across east coastal MA to upper 60s in the CT Valley. Little temp change is expected east coastal MA. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... A stalled front to our south will return north tonight into Saturday as a low pressure moves into the Great Lakes. 18/00Z guidance continues to slow the onset time of rainfall reaching the ground. Thinking most of tonight will be dry across southern New England, at least though midnight. Timing remains one of the uncertain components of this forecast. Rainfall should overspread our region Saturday morning. With the primary low pressure in the Great Lakes, the main focus for the heaviest rainfall is also expected to be to our west. That said, the possibility of locally heavy rainfall is not out of the question. Still a large positive precipitable water anomaly, as well as high freezing levels. Ergo, as previously noted, efficient warm rain process should be at work. Our region could take about 2-3 inches of rain in 6 hours without widespread flooding. Still mostly concerned about poor-drainage flooding of the urban and poor-drainage areas. Low clouds and/or fog along with breezy S/SW winds likely to impact south coastal New England through this event. Higher dewpoint air over still cool waters, definite potential for dense fog. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Highlights... * Warm and muggy on Sunday with hit-or-miss showers/thunderstorms * Dry weather returns by Monday * Passing showers Tuesday and Wednesday with seasonable temps Details... Saturday Night into Sunday...Moderate confidence. Bermuda high over the Atlantic with approaching western trough will result in moist southwest flow Saturday night into Sunday period. Models are in agreement that the region will warm sector late Saturday into early Sunday, resulting in a non-diurnal temperature swing for the overnight. Fog and drizzle with scattered showers are possible during the overnight. Cannot rule out a rumble of thunder as LLJ increases and showalters drop below 0. Sunday will be warm and humid as the region warm sectors leading to highs into the upper 70s. Approaching shortwave/cold front from the west could result in scattered showers and thunderstorms during the late morning/early afternoon hours. Mid-level lapse rates near 5.5-6 C/km, showalters dropping below 0 and both surface and MU Cape could be enough to develop a strong storm or two. Still some uncertainty due to the timing of the cold front as models seem to have trend a bit faster, leading to a dry Sunday evening for the area. Main risk with any storm will be heavy rainfall as PWAT values reach above 1.75 inches which is 2 STD above normal. Monday and beyond...Moderate confidence. Conditions should be improving as upper level trough pushes cold front through the region Sunday night/early Monday. Surface high pressure from Canada will move into the region resulting in dry weather. Zonal flow aloft, will allow for northern stream upper level disturbance to pass through during the week. Could see sct showers sometime on Tuesday or Wednesday, still some uncertainty on timing. Temperatures will be more seasonable with highs in the 70s. Dry weather returns on Thursday. && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/...High confidence. Through 00z...VFR. Diminishing NE winds near the coast. Tonight...Mainly VFR with lowering cigs after midnight. MVFR/IFR stratus developing along the SE New Eng coast toward daybreak. Dry weather will last much of the night with rain developing 09-12z south of BDL-PVD line. Saturday...MVFR/IFR conditions in rain and patchy fog spreading NE in the morning and continuing through the afternoon. Saturday night...Widespread IFR/LIFR with areas of fog, locally dense. Occasional showers with an isold t-storm possible. KBOS Terminal...High confidence in TAF. KBDL Terminal..High confidence in TAF. Outlook /Sunday through Tuesday/...Moderate confidence. Sunday: Mainly VFR, with local IFR possible. Breezy. Chance SHRA, slight chance TSRA, patchy FG. Sunday Night through Monday: VFR. Monday Night through Tuesday: VFR. Chance SHRA.
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&& .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/...Moderate confidence. Surge of increasing NE winds developing early this morning behind a cold front, and continuing into today with strong pressure gradient in place. Gusts to 25-30 kt. Small Craft Advisories will continue. Seas gradually building through today. Winds diminish somewhat tonight, but still gusty over south coastal waters. Areas of fog will reduce vsbys early this morning, especially south coastal waters. Outlook /Saturday Night through Tuesday/...Moderate confidence. Saturday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Chance of rain showers, areas fog, isolated thunderstorms. Visibility 1 nm or less. Sunday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Chance of rain showers, slight chance of thunderstorms, patchy fog. Local visibility 1 to 3 nm. Sunday Night: Winds less than 25 kt. Local rough seas. Monday: Winds less than 25 kt. Areas of seas approaching 5 ft. Monday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers. Tuesday: Winds less than 25 kt. Chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 2 AM EDT Saturday for ANZ232>235- 237. Small Craft Advisory until 2 PM EDT this afternoon for ANZ230- 236. Small Craft Advisory until 5 PM EDT this afternoon for ANZ231- 251. Small Craft Advisory until 8 PM EDT this evening for ANZ250. Small Craft Advisory until 8 AM EDT Saturday for ANZ254>256. && $$ SYNOPSIS...Belk/Dunten NEAR TERM...KJC SHORT TERM...Belk LONG TERM...Dunten AVIATION...KJC/Dunten MARINE...Belk/Dunten

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