Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 191105 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 705 AM EDT FRI AUG 19 2016 .SYNOPSIS... Dry weather prevails today and Saturday. Humidity levels will increase Sunday with a low risk for a few showers or a thunderstorm across western areas. More widespread showers and thunderstorms with locally heavy rainfall will move through Sunday night into early Monday as a cold front sweeps across the region. High pressure brings dry and cooler weather Tuesday with a gradual warming trend Wednesday and Thursday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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7 AM update ... Very pleasant late summer morning across southern New England with temperatures in the upper 60s to low 70s along with modest dew pts in the 60s. Just some patchy mid level clouds streaming across the region. This moisture combined with daytime heating and weak cyclonic flow aloft may be sufficient to yield some modest convective buildups. However dry air above 700 mb will provide a weakly capped environment. Thus NOT expecting any spotty showers like yesterday afternoon. Hence a dry forecast prevails. Previous forecast captures this nicely so no major changes with this update. Earlier discussion below. ==================================================================== Upper lvl ridging regains control in spite of a weak shortwave passage this afternoon. Subsidence promoted from this building ridge yields an overall drier column with much lower K-values than yesterday, so in spite of this weak shortwave, not expecting the same amount of shra activity experienced yesterday. Went will NIL pops with this update. H85 temps a bit cooler, mainly +12C or so. This suggests mostly mid-upper 80s although with a slight downslope component CT valley could reach near 90F. Sea breezes likely, as overall pres gradient will be weak through the day.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... Tonight... Another good setup for radiational cooling as pres gradient remains weak clearing skies is expected with enhanced mid lvl ridging, and dwpts are drier by afternoon mixing. More widespread upper 50s and low 60s. This will also lead to increased fog potential across the interior as weak return flow develops aloft with moisture trapped beneath the subsidence inversion. Will need to monitor typically prone valleys. Sat... Continued height rises as high pres shifts to the E and allows for return flow to develop, although weak. The increased moisture suggests higher risk for interior cloud development, which combined with the cooler start should yield slightly cooler highs, mainly in the low-mid 80s across the region. In spite of the increase in low-lvl moisture, the building heights should limit shra development, so will continue with NIL pops. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Highlights... * Beneficial rains likely (possibly heavy) Sun ngt into early Mon * Dry and cooler later Mon into Wed Synoptic Overview & Model Preferences ... Both ensembles and deterministic guidance in very good agreement on the timing of next rainfall event...Sunday night into early Mon morning. Typical model differences at this time range with GFS a tad faster (about 6 hrs) than the ECMWF. Otherwise models are in good agreement. A new trend with this system is that the mid level trough deamplifies as it approaches New England and gets sheared out as it lifts northeast. Thus system may not be as robust as earlier thought. Nonetheless a period of widespread rain likely Sun ngt into early Mon. Temperatures ... Seasonable airmass ahead of approaching front with highs Sunday 80- 85. However it will turn humid with dew pts climbing to 65-70. Off to a warm start Monday morning but turning noticeably less humid during the afternoon behind the front. In addition fairly robust CAA with 850 temps falling to about +8C by Tue morning! Also dew pts tumble into the 40s! Thus a touch of early autumn in the air Tue morning with many locations dipping down at least into the 50s with a few upper 40s possible.. Then 1026 mb surface high provides seasonable weather Wed and Thu with temps slowly warming back thru the 80s. Precipitation ... Mainly dry Sat night into much of Sunday with chance of showers and isolated T-storm increasing late in the day across western sections as mid level trough and frontal boundary approach. A good signal for showers and t-storms with locally heavy rainfall Sunday night into early Mon as subtropical moisture surges north ahead of a strong cold front. PWATs briefly spike to 2+" which is 2SD above normal. Favorable jet dynamics in the right entrance region of the upper jet...however nose of low and mid level jet track across NY state into Northern New England. Thus best thermal and moisture advection occur north and west of our region. In addition...model guidance showing trends of mid level trough deamplifying as it approaches New England and becoming sheared. Therefore rainfall event may be more modest than widespread and drenching. Early indications suggest most areas should pick up 0.25-0.50" with locally up to 1 inch possible especially western and northern MA. Cold front with axis of showers/t-storms should be near eastern MA Mon morning with drier air and increasing sunshine and gusty NW winds moving in from the west during the day as the front moves offshore. 1026 mb High pres then becomes anchored over southern New England bringing dry weather to the region. && .AVIATION /12Z FRIDAY THROUGH TUESDAY/...
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Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/... 7 AM update ... VFR and light winds prevail today. Thus no major changes from earlier TAFs. ========================================================================= Through today...high confidence. VFR outside of some patchy morning fog at typically prone terminals. Winds light mainly SW will shift to the NW today but remain light enough for sea breezes on all coasts. Tonight and tomorrow...high confidence. Mainly VFR outside of some light fog in the NW valleys of MA early Fri Sat morning. Another round of sea breezes likely during the day Sat. KBOS TAF...High confidence. Sea breeze starts 14-15Z KBDL TAF...High confidence in TAF. Outlook /Saturday Night through Tuesday/... Saturday night into Sunday...Moderate confidence. Areas of cigs and patchy fog developing later Sat night and possibly lingering into Sun morning, then improving to VFR. Low risk of showers and T-storms late Sun (toward sunset) across western areas. Sunday night into Monday...Moderate confidence. Lowering to MVFR/IFR from west to east as showers and sct t-storms move through later Sun night into early Mon, then improving to VFR during Mon. Gusty south winds becoming northwest late mon night. Tuesday...High confidence. VFR.
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&& .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/... High confidence. Winds and seas remain below small craft advisory thresholds as high pres builds over, then moves slightly east of the waters over the next 48 hours. Outlook /Saturday Night through Tuesday/...Moderate confidence. Saturday night into Sunday...Fairly tranquil boating weather S/SE winds generally under 20 kt. Mainly dry weather and good vsby prevail. Sunday night into Monday...South winds may briefly gust to 25 kt ahead of frontal boundary Sun night. Periods of heavy rain and fog late Sun night into early Mon, then improving conditions. Wind shifts to NW Mon morning with gusts to 25 kt possible Mon afternoon and evening. Tuesday...NW winds 10-20 kt. Excellent vsbys. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Nocera/Doody NEAR TERM...Nocera/Doody SHORT TERM...Doody LONG TERM...Nocera AVIATION...Nocera/Doody MARINE...Nocera/Doody

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