Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 261407 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1007 AM EDT Mon Sep 26 2016 .SYNOPSIS... High pressure will center over New England tonight and allow for efficient radiational cooling. A low pressure area and associated front will bring showers to the area late tonight into Tuesday morning. Forecast confidence remains low for the remainder of the week, though an east to northeast wind flow looks to persist into next weekend. There will also be the chance for showers as well as periodic drizzle and fog. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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10 AM update... High pres over New Eng will slide offshore this afternoon. Some high clouds will move in from the west, otherwise a sunny day. Coastal seabreezes expected. Previous Discussion... The light pres gradient will give allow for sea breezes to develop even as light return flow picks up later in the day with the crest moving of high pres moving offshore. This will lead to slight warm advection and increasing low lvl humidity throughout the day and in spite of mixing, which will hold back the sfc dwpts somewhat in spite of the full column moistening. Increasing H85 temps through the day to +6C by late day. Mixing should be able to tap this lvl in spite of the increasing moisture and clouds through the afternoon. Therefore, expecting more widespread low 70s and upper 60s, near normal for this time of year.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/... Tonight into tomorrow... Meandering cutoff will begin to settle across the Great Lakes region as strong shortwave energy rotates on the S and E periphery late tonight into tomorrow. This shortwave is associated with an upper lvl jet streak whose equatorward entrance region sets up across the NE during the late night/morning hours tonight into tomorrow. As it stacks with its attendant sfc low, expect occlusion process which will dray moisture out of the gulf region, yielding PWATs near 1.5 inches and K-vales between 30-35. This occlusion approaches late, so suspect rain band associated with the mid-lvl F-gen will not arrive in the W until around midnight local, sliding E into the daylight hours, with partial improvement Tue W-E from 12Z (8AM local) to 18Z (2PM local). As the front approaches the S coast it is expected to slow as it begins to parallel the flow aloft. Looking at QPF values to range between 0.3 inches to possibly as high as an inch on the S coast as the front slows. Once again, much needed rain, but not drought- busting. LLJ around 40 kt at H85 will be moving across the region during the early AM hours and go along with a narrow corridor of elevated instability. This will be maximized along the S Coast/Cape/Islands especially after sunrise. Will include a risk for TS with heavier rain. Temps overnight cool to the low 50s but hold thanks to upper lvl warming and increasing dwpts at the sfc. By day, especially with some clearing in the W, highs increase into the low 70s to mid 70s in the warmest locations. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Highlights... * Showers with a few thunderstorms linger Tuesday night, mainly across the coastal plain * May see scattered showers at times into next weekend, but low confidence mainly on timing * Temperatures near or slightly below normal with raw northeast winds Overview... Medium range models and ensembles continue to signal anomalous high amplitude Omega blocking pattern setting up across the central and northern U.S. during this timeframe. Timing with this type of set up lends to low confidence on timing. Big question is where the H5 cutoff low will eventually set up, though 00Z model suite have a fairly good handle in slowly shifting the low south through the Great Lakes through mid week, then to the southern Appalachians by late week. Some placement issues come into play, mainly with the ECMWF after Thursday. However, when it comes to the surface features, much less confidence due to rather wide variance in location and track over time. Another question will be QPF amounts due to timing of the precip and whether the mid level S-SW flow will allow some tapping of the moisture feed up the eastern seaboard. One thing that does have some certainty is the prolonged NE wind flow that will be in place once the front passes Tuesday night into Wednesday. This will set up a prolonged fetch off the ocean, which will also keep low level moisture in place. Patchy drizzle can not be ruled out at times along the coast with the onshore flow. With this complex pattern, low confidence on exact timing and trends continues especially beyond Wednesday. Details... Tuesday night-Wednesday... Cold front pushes from N-S across the region Tuesday night, but most of the showers should be mainly along the south coast at least through midnight. Some instability also in place, so can not rule out a few thunderstorms through around 04Z-06Z before moving offshore. Noting the PWAT plume of 1.6 to 1.7 inches lingers along the south coast, but rather sharp drying line pushes in behind. High dewpts look to linger especially along the coast, mainly in the lower 60s overnight, so could also see patchy fog develop. Questions come into play during Wednesday, as models having trouble with area of showers moving westward mainly during the afternoon as low pressure passes offshore in the SW flow aloft. Drier air in place early Wednesday away from the coast, but PWATs increase late Wed/Wed night with onshore flow which will aid in scattered shower development with unsettled upper pattern in place. Highs on Wednesday will be in the mid 60s to lower 70s, mildest across the CT valley where it will be driest for most of the day. Thursday through Sunday...Low confidence. As H5 cutoff low pres slowly shifts S over time, the upper flow becomes mainly S-SW bringing deeper moisture feed up the coast. This will combine with the persistent NE low level flow to help develop spotty showers and possibly patchy drizzle at times mainly along the coast. Will also see some late night/early morning patchy fog. Question through the remainder of the week is the placement and timing of any showers. Big question for late this week into the upcoming weekend will be the position of high pressure trying to build south out of Quebec and eastern Ontario, and whether it brings drier air into the region. At this point, it looks unlikely as the models are tending to bring better organized moisture northward in the S-SW mid level flow. Overall, high uncertainty remains in play with this pattern. Have kept low POPs going at this point. With the persistent NE low level flow, expect temperatures to run near or a few degrees below seasonal normals. && .AVIATION /14Z MONDAY THROUGH FRIDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/... Today...High confidence. VFR. Light/variable winds early, shifting to SW through the day. Sea breezes expected on coasts. Tonight and Tuesday...Moderate confidence, mainly due to timing. VFR to start the overnight. Gradually expect lowering to MVFR/IFR from W-E especially 06Z through 12Z, first due to lowering CIGS, then due to mix of RA and BR with lower vsbys. Expect this to gradually lift from W-E 12Z-20Z, although these lower categories may linger longer across the S Coast/Cape/Islands into Tue evening. KBOS TAF...High confidence in TAF through 06Z Tue, including sea breezes. KBDL TAF...High confidence in TAF through 06Z Tue. Outlook /Tuesday Night through Friday/... Tuesday night...Moderate confidence. Showers linger with MVFR- IFR conditions likely, with the best chance across the coastal plain. Conditions should improve to mainly VFR across the interior, but could lower to MVFR-IFR in patchy fog in the normally prone inland areas. Wednesday through Friday...Low confidence. Scattered showers will linger from time to time with patchy MVFR conditions at times, but should see VFR conditions where it is dry. May see patchy late night/early morning fog each day with local MVFR-IFR CIGS/VSBYS. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/...High confidence. Today...High pres moves across the waters. Winds and seas remain below small craft advisory thresholds. Tonight into tomorrow... SW winds increase during the overnight hours as a front approaches the waters. The increase in winds reach 25-30 kt late evening, with seas building 5-6 ft by Tue morning. Small craft advisories will be issued mainly for the outer waters where seas/winds will be the strongest. Front moves over the waters tomorrow afternoon, with rainfall and possibly even an isolated thunderstorm. Outlook /Tuesday Night through Friday/... Tuesday night...High confidence. Light SW winds will shift to N-NE as cold front passes. Seas around 5 ft on the outer waters early, then subsiding. Reduced visibilities in showers and fog, mainly from Cape Cod southward. Low risk for a few thunderstorms on the southern waters to near Cape Cod and Nantucket. Wednesday through Friday...High confidence in an extended period of E-NE winds. Wind speeds mainly below small craft criteria, but may gust to around 25 kt Thursday and Thursday night on the outer waters, lingering into Friday south of Block Island. Moderate confidence of reduced visibilities at times in showers and patchy fog. May also see increasing seas due to prolonged NE fetch, especially across the eastern open waters. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 11 PM this evening to 2 PM EDT Tuesday for ANZ235-237. Small Craft Advisory from 11 PM this evening to 8 AM EDT Tuesday for ANZ250. Small Craft Advisory from 11 PM this evening to 5 PM EDT Tuesday for ANZ254>256. && $$ SYNOPSIS...Doody/EVT NEAR TERM...KJC/Doody SHORT TERM...Doody LONG TERM...EVT AVIATION...Doody/EVT MARINE...Doody/EVT

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