Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 260754 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 354 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. Considerable forecast uncertainty exists for the rest of the week. An easterly flow may persist for the rest of the week with a chance of showers or areas of drizzle from time to time, especially near the south coast. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Noting several locations on MesoWest already down to the mid 30s and with another couple of hours of cooling to go, could see a few spots drop right to the freezing mark before it`s all said and done. Spatially, it fits the ongoing frost advisory so no last minute changes are planned. 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. && .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... * Widespread showers Tue, especially coastal plain. Improving conditions west * Unsettled weather possible through end of week but low confidence * Temperatures near or slightly below normal Overview... Rather complex high amplitude pattern across the CONUS and Canada in the extended period will lend itself to low predictability. All revolves around the evolution of a cutoff low as it digs south from the Great Lakes with strong upstream and downstream in place. GFS has trended toward Ecmwf and there is agreement among the guidance that cutoff will set up to the SW of New Eng but exact location of the cutoff along with area of confluence to the north is uncertain. And spread increases significantly next weekend. Additionally, there is considerable spread with location/track of sfc lows to the south which will impact sensible weather. Given the complexity of the pattern it is certainly a low confidence forecast as predictability of this pattern at extended time ranges is quite low. Tuesday... Cold front will be moving across SNE Tue morning. Strong signal for a band of widespread showers along and ahead of the front. Good coupling between low level jet and right entrance region of upper jet will result in deep forcing for ascent acting on deep moisture axis with anomalous PWAT plume around 1.75". All suggests widespread showers and can`t rule out a few t-storms near the coast where elevated instability parameters are favorable. Still some uncertainty with timing of the front and showers but focus Tue morning appears to be across the coastal plain with improving conditions in western New England. In fact, expect developing sunshine in the interior during the afternoon. Front expected to get hung up near the coast as flow aloft becomes parallel to the front and this may keep showers going near the south coast and Cape/Islands much of the day. It will be somewhat humid with dewpoints climbing into the low/mid 60s and breezy near the coast in the morning as low level jet swings through with gusts to 30 mph possible. Tuesday night... There is uncertainty with how far south drying gets as front stalls south of New Eng. GFS and Ecmwf show drying making it to the south coast by evening. However, as upstream trof/upper low digs south from the Gt Lakes flow aloft over New Eng backs and this may bring moisture back north and keep threat of showers along the south coast. ECMWF and UKMET have showers near the coast and this is how we will play it. Wednesday through Sunday...Low Confidence. As cutoff low digs south of the Gt Lakes backing upper flow will bring back deeper moisture into SNE, but the extent of moisture is unknown as moisture will be battling upper confluence and high pressure to the north. Also, location and track of low pres to the south will be dependent on exact positioning of the cutoff low. Potential for a period of unsettled weather with showers from time to time, but it could end of dry for a good portion of the period if there is greater influence from upper confluence and high pres to the north. Lots of uncertainty so we have just low pops in the forecast. With persistent NE flow likely, temps expected to be near or a bit below normal. && .AVIATION /07Z 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/... Through today...High confidence. VFR. Winds generally light, although shifting toward the SW through the day. Sea breezed 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...Moderate confidence. Area of showers with MVFR/IFR likely moving across SNE with main focus in the coastal plain. Improving conditions to VFR in the west. South wind gusts to 25-30 kt possible in the morning near the south coast. Wednesday through Friday...Low confidence. Areas of MVFR with showers possible at times, but VFR conditions if it is dry. && .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 over 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...High confidence. Low level jet ahead of cold front will result in a period of pre-frontal southerly gusts to 25-30 kt in the morning, then diminishing winds int he afternoon. Vsbys reduced in showers and fog. Low risk for a few t-storms across the south coastal waters. Wednesday through Friday...High confidence in an extended period of E/NE winds with speeds mostly below SCA although occasional gusts to 25 kt possible. Vsbys may be reduced at times in showers but this is low confidence. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...Frost Advisory until 8 AM EDT this morning for MAZ002>004- 008>013-017-018-026. 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...Doody SHORT TERM...Doody LONG TERM...EVT AVIATION...Doody/EVT MARINE...Doody/EVT is the U.S. government's official web portal to all federal, state and local government web resources and services.