Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 201121 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 721 AM EDT Tue Sep 20 2016 .SYNOPSIS... A stalled frontal boundary along the south coast of New England will be slow to exit today...yielding scattered showers from eastern CT to RI to southeast MA. By afternoon the front will begin moving offshore with clouds giving way to sunshine from northwest to southeast. It will remain warm and humid. High pressure builds into the area beginning tonight and continues into Thu...yielding warm but lowering humidity. A cold front sweeps south from Canada Friday with a risk of showers, followed by cooler weather for the weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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7 am update ... Stalled frontal boundary continued to provide focus for weak QG forcing enacting on a persistent plume of tropical air from the remnants of Julia. However, will start to note some improvement through the morning and early afternoon from the N-NW as drier air entrains through the column, and upper lvl vort max shifts E with time. S coastal locations will likely remain under the impact of a few shra much of the morning and early afternoon, until some of this drier air can begin to filter in. The tropical plume is also going to take some time to move on. Continued trends from previous forecaster for the most part as the forecast is on track. However, did increase temps/dwpts through the day today. Increased dwpts E while increasing temps mainly to the W where clearing skies should allow for highs to once again soar into the 80s. Previous discussion... Stalled frontal boundary across the area this morning coupled within a deep tropical moisture plume (2"+ PWATs) advecting up the eastern seaboard is yielding a corridor of showers from south of Long Island into eastern CT/RI and southeast MA. 6 hrs into 00z model runs ending at 06z/2am...GFS and Rgem qpf appears to be verifying best. EC and NAM verifying too far west. Given southwest flow aloft corridor of showers will be focused across eastern CT/RI and southeast MA this morning. Satellite imagery showing clouds cooling upstream over NJ southward to DC. Given cloud enhancement in a tropical environment a few heavy downpours are possible across RI and/or southeast MA this morning. A drying trend will develop later this morning as short wave trough over southeast Ontario and Quebec serves to kick moisture plume offshore. However it may take until late in the day for showers and clouds to exit Cape Cod and the Islands. Elsewhere mainly dry conditions will prevail across the remainder of CT into northern MA. Patchy dense fog across this area will slowly burn off to at least partial sunshine. By midday at least partial sunshine will be observed over much of the interior. It will become warm with highs in the 80s and remain humid with dew pts holding in the 60s. Keep in mind the normal high for 9/20 is the lower 10-15 degs warmer than normal today.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... Tonight ... weak high pressure builds into the region. However with dew pts slow to fall...light winds/mostly clear skies and dew pts higher than expected low temps...anticipate widespread fog. Remaining warm with lows 60-65...few upper 50s western MA. Wednesday ... drier/less humid airmass overspreads the region with dew pts falling into the 40s! Still warm with highs in the upper 70s low lower 80s. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Highlights... * Warm and dry through early Fri. * Showers and isolated thunderstorms late Fri into early Sat. * Cooler and drier late weekend into early next week. Overview and model preferences... Amplified flow pattern across the W CONUS will give way to more amplified flow across the NE by the latter half of the week. This is thanks to continued ridge building downstream of the trof across the W, allowing for a portion of the Hudson Bay vortex to slide into the NE and Maritime. Therefore, the above normal conditions experienced early in the week, will shift to near or even below normal by the weekend and early next week. While there is generally good agreement, timing differences are apparent with the frontal passage signifying pattern shift for Fri-Sat. Will add more weight to the ECMWF which is less progressive and shares timing with several ECENS and GEFS members. Given the parallel flow, it`s likely the operational GFS is too fast these last couple of runs. Details... Wed night into Fri night... Strong zonal flow develops mainly N of the region, leading to the S New England falling under the anticyclonic shear side of the mean jet. Stationary front develops near the Canadian border, but best QG forcing remains there as well. As such, dry wx continues. Dwpts remain relatively low thanks to W flow, keeping mins in the 40s and low 50s, meanwhile H85 temps near +15C, suggests continued mid-upper 80s for highs. Fri into early Sat... Continued ridge building upstream will begin to allow energy wrapped around the Hudson Bay vortex to shift toward the SE. This will force the stationary front to the SE as well as column PWATs to reach near 1.5 inches, along with K-values reaching near 30. Therefore, as the front approaches late Fri, could see some showers developing, but also dissipating as it continues to washout, parallel to the flow aloft. Therefore, yet another non- drought-busting frontal passage as QPF values range only between 0.10 and 0.30 inches. Still, modest lapse rates near 6.0C/km and a late day increase in SB CAPE values 500j/kg suggest a low risk for TS and localized heavier rains in spite of the widespread lower amounts expected. This is also supported by 0-3km shear near 30 kt, which may help the unsettled conditions linger into the overnight hours after loss of diabatic support. ECMWF supports this later timing, so POPs will reflect timing from Fri night into Sat, early afternoon. Temps mainly in the 70s for highs and 60s for lows as dwpts increase to near 60 ahead of the frontal passage. Sun into Mon... Airmass associated with a Hudson Bay vortex begins to slip across New England, potentially signifying a bit of a pattern change as longer range models beyond 7 days are slow to allow S stream anticyclone back into the region. H5 temps drop to almost 3 std deviations below normal, with temps near -20C at this lvl as the previous forecaster noted. H85 temps could dip to between +2C and +4C, but with drought conditions continuing, highs may be able to overachieve somewhat. For now, will suggest highs in the mid 60s to low 70s, close to exact mixing values. Overnight mins are likely headed for the 40s, with a strong enough pres gradient Sun night to limit radiational cooling. && .AVIATION /12Z TUESDAY THROUGH SATURDAY/...
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Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Today...Moderate confidence, mainly for timing uncertainty. IFR/LIFR conditions in fog/low clouds lift across the interior through about 14Z. VFR will then dominate these locations for the remainder of the day as skies slowly clear. Along SE MA and RI, MVFR conditions may linger with shra through early afternoon, possibly taking until evening to fully clear Nantucket. Tonight ... dry but patchy MVFR in areas of fog. Otherwise VFR. Wednesday ... VFR and dry. KBOS TAF...High confidence in trends. Weak sea breeze may impact the terminal beginning late morning. KBDL TAF...High confidence in trends, lower confidence in timing of exact details, especially lifting of fog. Outlook /Wednesday Night through Saturday/... Wed night into Fri...High confidence. Mainly VFR. Localized late night fog possible Thu night especially. Fri night into Sat...Moderate confidence. Periods of MVFR possible in shra and isolated tsra. Otherwise continued VFR.
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&& .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Today ... showers across the southern waters...local downpours possible. Fog will limit vsby at times. Light winds. Tonight ... west to southwest winds improve vsby. Any evening showers move offshore overnight. High pres builds over the waters. Wednesday ... High pres over the waters yielding tranquil boating weather. Outlook /Wednesday Night through Saturday/... Wed night through Fri...High confidence. Winds and seas generally remain below small craft advisory thresholds through the period as high pres slowly moves over the waters. Fri night into Sat...Moderate confidence. Low risk for building southerly swell by daytime Saturday. This will be accompanied by localized showers and thunderstorms. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Nocera/Doody NEAR TERM...Nocera/Doody SHORT TERM...Nocera LONG TERM...Doody AVIATION...Nocera/Doody MARINE...Nocera/Doody is the U.S. government's official web portal to all federal, state and local government web resources and services.