Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 201325 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 925 AM EDT Tue Sep 20 2016 .SYNOPSIS...
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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 but expect partial clearing northwest of I-95. 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.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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925 am update... Weak cold front was stalled along the south coast at mid morning. Enough forcing along with some weak elevated instability was allowing another round of scattered showers to develop across far southeast CT/RI and expect the activity eventually advance into southeast MA and the rest of RI through noon. PWATS still in excess of 1.5 inches, so brief very localized heavy rainfall still possible across portions of RI and SE MA especially near the south coast. Low risk of isolated thunderstorm, but opted to leave out of the forecast for now. Drier mid level air will work into the region by mid to late afternoon, so expect the activity to diminish over that time. Across the rest of the region, dry weather should prevail through the afternoon. However, very weak wind fields and trapped low level moisture will likely allow clouds to linger longer than some guidance suggests. While still expect partial sunshine to gradually emerge northwest of I-95 into the afternoon, this delay should hold highs mainly in the lower to middle 80s. Portions of the south coast/Cape/Islands will probably not reach 80. Despite this relatively high dewpoints and light winds will make it feel somewhat humid outside, especially for late September standards.
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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 /13Z 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. Rest of Today...Moderate confidence in TAFS but low confidence in timing. MVFR-IFR conditions should gradually improve late this morning and afternoon from northwest to southeast but specific timing is uncertain. MVFR CIGS and scattered showers may linger through mid afternoon across portions of southeast New England. Tonight ... dry but patchy MVFR in areas of fog. Otherwise VFR. Wednesday ... VFR and dry. KBOS TAF...Moderate confidence in TAF but low confidence in timing. Improvement to VFR expected by early afternoon but timing uncertain. KBDL TAF...Moderate confidence in TAF but low confidence in timing. Improvement to VFR expected by early afternoon but timing uncertain. 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...Frank/Nocera/Doody NEAR TERM...Frank SHORT TERM...Nocera LONG TERM...Doody AVIATION...Frank/Nocera/Doody MARINE...Nocera/Doody

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