Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 282048 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 448 PM EDT Wed Jun 28 2017 .SYNOPSIS... High pressure brings dry and cool conditions tonight. A warm front moves through the region Thursday with a few showers and thunderstorms Thursday night as more humid air moves in. Summer heat and humidity arrives Fri and continues into the weekend. A cold front likely breaks the heat and humidity later Sunday and will be accompanied by thunderstorms. Not as warm or humid early next week including the fourth of July holiday. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... Sct showers developing across northern New Eng and moving SE. Marginal instability and KI increasing to near 30 along with approaching left exit region of upper jet will bring a risk of a few showers or an isold t-storm this afternoon. Dry weather tonight with high pres south of New Eng. Weak shortwave will spread some mid/high clouds into New Eng with clouds increasing later tonight. Another cool night with dry airmass in place and light winds. Lows in the 50s. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... Thursday... Warm front lifts north across SNE to near the MA/NH border late in the day. No instability across SNE and main focus for showers will be to the north assocd with deeper moisture and mid level omega north of the warm front along with the nose of the 850 mb jet. Can`t rule out a few showers spilling into northern MA in the afternoon but it should be a mostly dry day. Most sunshine will be found toward the south coast with most clouds north of the Pike. Rather gusty S/SW winds developing in the afternoon as the low level jet strengthens across SNE. Gusts to 25-35 mph expected, strongest in the coastal plain. Shallow mixing will limit high temps to the upper 70s and lower 80s. While dewpoints will be increasing, very humid air will remain to the west. Expect dewpoints increasing to 55-60. Thursday night... Warm sector airmass in place. Increasing KI and elevated instability as a piece of the low level jet shifts south into SNE will result in a few showers and isold t-storms moving through. Milder night with lows remaining in the 60s as higher theta-e air moves into the region with increasing dewpoints. This may lead to fog patches developing overnight. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Highlights... * Hot and humid conditions return Friday and continue into Sunday * Increasing risk for scattered showers/thunder Friday into Sunday * Cooler and less humid Monday and Tuesday Overview... 12Z model suite over the medium range period continues to feature nearly zonal northern stream mid level flow, which keeps systems moving along into this weekend. The big question continues to be the timing of the approaching cold front out of south central Canada and the Great Lakes, pushing far enough S to bring cooler, drier conditions. With each run, models continue to keep the front further N and W of the region. Could see enough instability for diurnal convection to develop across the interior Fri into Sat, while tending to be drier closer to the coast as some residual subsidence building NW across coastal areas from the E-W elongated high pressure from VA/NC to the western Atlantic. By late this weekend, models continue to dig H5 trough over the western Great Lakes/northern Plains states, with responding mid level flow becomes more SW across the northeast U.S. All short range models signaling a short wave in the H5 flow moving steadily SE Sun/Sun night, which will allow the pre-frontal trough to move slowly SE late Sat into Sun. Models in fair agreement on timing of this feature, pushing off the S coast Sunday morning. Trailing cold front follows Sun night-early Mon, then tends to stall along the NW periphery of the Bermuda high, while another H5 short wave moves E. Timing issues come into play with this feature, as well as placement of the stalled front and subsequent potential for precip during the 4th of July holiday into Wed. Leaned toward a blend of available model guidance as well as ensemble means through around 12Z Mon, then transitioned to ensemble means beyond this due to the timing and placement systems amongst the individual models. Details... Friday-Friday night... Expect very warm temps and high humidities during this timeframe. Dewpts rise to the mid-upper 60s with a few spots briefly touching 70 during the afternoon/evening hours. As approaching pre-frontal trough slows in the parallel SW wind flow aloft, will see scattered showers/thunderstorms mainly from Cape Ann across to central and W Mass into N central CT Fri afternoon/evening, then orienting W-E Fri night, running from about the Mass Pike northward. Could see some heavier downpours with some thunderstorms across the interior Fri afternoon/evening as signaled by both the GFS and GGEM. CAPEs are in the 1000-1200 j/kg range, along with K indices in the lower-mid 30s into Fri evening. However, may see some capping as H85 temps rise to +16C to +18C. H5 heights slowly rise during the day as well, up to 5820-5840 m by around 00Z Sat, which could also limit the potential for stronger storms. Expect Friday`s highs in the mid-upper 80s away from the S coast, a bit cooler along the S coast and the higher inland terrain. Can not rule out a few spots reaching 90. Saturday-Sunday... Best instability/lift associated with the pre-frontal trough moves across the region during this timeframe. Have likely POPs with this feature. Have mentioned scattered thunderstorms as well. Also noted SLIs below zero, K indices in the lower 30s and TQ values up to 20 as this feature passes. Can not rule out some gusty SW winds with some of the storms as well. Signal from the 12Z GGEM that there could be some heavier rainfall as the trough moves across late Sat into Sat night, especially across NE CT/RI/SE Mass. Noting PWATs up to 1.8 to 2 inches overnight Sat night, so could see some downpours that may cause local street and poor drainage flooding. Another aspect will be the very sultry conditions with dewpts in the 65-70 degree range. A few spots may see heat indices in the lower 90s during Sat afternoon. Sunday night-Monday... Trailing cold front moves across Sun night, moving near or just S of the south coast Mon morning. Any leftover showers/ thunderstorms should push offshore by around midnight or so Sun night, but could linger across the Cape and islands into early Mon morning. Will also see some patchy fog develop as higher dewpts linger there. Winds shift to light W Sun night, except SW at around 5-10 kt along the S coast. This will delay the drier air from moving in until Monday. Dewpts drop back to the mid-upper 50s across the interior Monday, but linger in the lower-mid 60s along the S coast. Skies will become partly cloudy from NW-SE Sun night, but clouds may linger across the outer Cape and Nantucket closer to the stalling front into Monday. Monday night through Wednesday...Low confidence. Flat H5 trough across the NE U.S. looks to lift into northern New England and Quebec. For now, have forecasted mainly dry conditions Mon night and Tue, even as front stalls close to or S of the south coast. A weak wave may move along this front sometime late Tue through Wed, but quite a bit of spread amongst the model guidance. Did mention chance POPs for this period, as suggested by blended guidance. && .AVIATION /21Z WEDNESDAY THROUGH MONDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/...High confidence. Through 00z...VFR with sct-bkn cigs 050-070. Low risk for an isold shower or t-storm across northern MA. Tonight...VFR. Light winds. Thursday...VFR. Low risk for a few afternoon showers north of the Mass Pike. Increasing SW winds with gusts to 20-30 kt developing, strongest in the coastal plain. Thursday night...Mainly VFR. But a few showers/t-storms expected to move through with brief lower conditions. Gusty SW winds coastal plain in the evening. KBOS Terminal...High confidence. KBDL Terminal...High confidence. Outlook /Friday through Monday/... Friday through Saturday...Moderate confidence. Patchy late night/early morning fog each day. Otherwise mainly VFR, except local IFR in scattered showers/thunderstorms, mainly during the afternoon and evening hours through the period. A few storms may be strong, with gusty winds. SW winds through the period with gusts around 20 kt, highest along the S coast. Sunday-Sunday night...Moderate confidence. Mainly VFR. Periods of MVFR-IFR conditions in scattered SHRA/TSRA Sunday into Sunday evening. Patchy late night and early morning fog with local IFR CIGS/VSBYS. Winds shift to S-SW Sunday to W Sunday night from N-S. Monday...Low to moderate confidence. Mainly VFR. May see isolated showers move into N central and W Mass Mon afternoon/evening. Winds back to SW again across RI/E Mass Mon afternoon. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Thursday Night/...High confidence. Tonight...Quiet boating weather with high pres south of New Eng yielding light winds and seas. Thursday...Increasing S/SW flow as low level jet develops. Enough mixing to support gusts to 25-30 kt in the afternoon over nearshore south coastal waters and particularly Narragansett Bay and Boston Harbor. Less wind over outer waters due to low level inversion. SCA issued for south coastal waters and Boston Harbor. Thursday night...Low level jet remains across the region resulting in SCA SW gusts and building seas. Outlook /Friday through Monday/... Friday...Moderate confidence. Small craft advisories will likely be needed. SW winds gusting up to 25-30 kt likely, especially across the waters E of Cape Cod and southern waters. Seas up to 6-8 feet, highest on the southern outer waters, but may start subsiding Fri night. Saturday and Sunday...Moderate confidence. SW winds continue, gusting up to 25-30 kt on the southern outer waters. Seas lingering at around 5 ft, but should subside during Sat night into Sun. Scattered showers and thunderstorms, some with reduced visibilities and strong gusty winds especially on Sunday. Patchy late night/early morning fog with reduced visibilities. Sunday night and Monday...Moderate confidence. Winds briefly shift to W Sunday night into Monday morning, but back to S-SW again by midday Monday. Seas up to 4 ft on the outer waters. Leftover showers/thunderstorms linger across the southern waters along with patchy late night/early morning fog reducing visibilities. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 1 PM Thursday to 8 AM EDT Friday for ANZ231>235-237-255-256. Small Craft Advisory from 3 PM Thursday to 2 AM EDT Friday for ANZ230. Small Craft Advisory from 1 PM Thursday to 4 AM EDT Friday for ANZ236. && $$ SYNOPSIS...KJC/EVT NEAR TERM...KJC SHORT TERM...KJC LONG TERM...EVT AVIATION...KJC/EVT MARINE...KJC/EVT is the U.S. government's official web portal to all federal, state and local government web resources and services.