Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 240211 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1011 PM EDT Sun Jul 23 2017 .SYNOPSIS... Cooler, damp conditions are forecast for Monday and Tuesday. High pressure brings pleasant conditions back for Wednesday and most of Thursday with a return of more unsettled conditions for Thursday night into the early weekend. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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10 pm update... Heightened concern Monday morning (roughly 8a-11a) for heavy rain, especially across the S-coast, and the possibility of flooding. To put it briefly, a quasi-stationary frontal boundary immediately S of Long Island and the S-coast of the New England acts as a convergent focus of sub-tropical moisture as an area of low pressure traverses W to E overnight into Monday morning, parent with a H5-7 mid-level vortmax and right rear quadrant of an upper level jet. Strong SW inflow as the E jet strengthens along the backside per isallobaric response tightening up the baroclinic zone along which mid-level dynamics act. Indications of a N-S frontal circulation along the warm frontal slope with strong frontogenesis and moisture convergence brought about by pockets of modest omega (upward forcing). Subsequent banding, think we`ll see 2 areas of heavy rain, one in the vicinity of the immediate S-coast associated with the low-level forcing of more rich, sub- tropical moisture and accompanying instability, the second across the interior with the more dominant mid-level forcing, though absent rich moisture and instability given the wedge of stable air brought about by the E/NE flow over the interior per isallobaric response to the traversing surface low. Again, moist of this occurring Monday morning. With the S-band, can not rule out 1 to 2 inch rainfall rates per hour as precipitable waters will exceed 2 inches. But echoing the previous forecaster, given lowering heights within the low to mid levels, the surface boundary may lift slightly N with the localized low-level convergence. It`s a tricky forecast to say the least. A lot of convective elements involved evolving in a more typical synoptic pattern observed during the cool-season. Will hold off on any headlines with the evening forecast package, making the decision overnight into the early morning hours as things evolve upstream. No changes made to the going forecast. Still advertising around 1.5 inches of storm-total precipitation, mainly falling 12-18z (8a-2p) Monday over SE New England.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... Impressive 5 to 6 standard deviation easterly flow at 925 mb Monday. Very rare event for late July. Precipitable water values also about 2 standard deviations above normal. Thinking a widespread rain will result, despite the strongest lift staying over the south coast of New England during the morning. Locally heavy rain will be possible at times, especially toward the south coast of New England. Generally thinking one half to one inch of rainfall, with locally higher amounts. The steadier rain should taper off to showers Monday afternoon into Monday night. This should be forced mainly by a cold pool aloft and a trailing mid level shortwave. Rainfall amounts expected to be quite a bit less that what happens Monday morning and early afternoon. With our region remaining on the north side of this low pressure, expecting temperatures to remain generally below normal. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Highlights... * Unsettled/wet conditions persist Tue and Tue night. * Dry and pleasant Wed and probably into much of Thu afternoon * Unsettled/wet weather likely returns by Thu night and/or Fri * Temps will remain below normal through the end of the week Overview and model preferences... With the 23.12Z update, models continue to show the same general synoptic pattern they have for the past 48-72 hours, only minor differences mainly with the development of the late week-weekend long-wave trof. Given this, no particular model will be favored and will continue to blend in some persistence with this forecast update. In essence, weakening trof Tue begets weak ridging for mid week followed by a return of shortwave digging late week into the weekend. The latter is fairly robust by June standards as noted by previous forecaster, and acts to promote long-wave trof digging toward the end of this long-term forecast cycle. Details... Tue... Noting one last vorticity-max rotating through the remnants of the weakening trof across the region on Tue. Lingering PWATs between 1.00-1.25 inches suggest that with this feature rotating through, remnant SHRA could continue through at least the daylight hours on Tue. Continued E-flow will enhance this risk and also help maintain the widespread BKN-OVC cloudiness as well. Not expecting a washout as PWATS will be diminishing and mid lvl heights are rising through the period. Otherwise, the E flow, little change in airmass and overall cloud cover will maintain the cool temps across the region, with daytime maxes struggling to break out of the 60s or low 70s. Wed and Thu... Brief ridging follows the exiting shortwave and actually shows signs of a modest and slow building process enhanced by the deepening, more robust wave skirting the N CONUS. As such, am favoring a slightly slower progression in the advancing front for Thu, suggesting it, along with Wed will remain mostly dry. Although the additional cloud cover and slight moderation of the airmass is expected, mid lvl temps still hover mainly around +10C, which is slightly cooler than seasonal normal values. Hence, highs are likely to remain mainly in the upper 70s and low 80s, still slightly on the cooler side of normal. Mins, thanks to afternoon dwpts dropping into the 50s should be quite pleasant, mainly in the 50s themselves. Thu night into Fri... Cold front will be crossing the region. As mentioned above, due to uncertainty in the deepening/digging process of the attendant shortwave, there remains some uncertainty in the timing of this feature. Favored the slightly slower timing with this update given the ridge building process initially. However, by the overnight hours, noting a rapid drop in mid-lvl heights and moisture loading of the column. Modest destabilization of the column also noted as H5 values drop to about -10C. Therefore, will be continuing trend of increasing risk for SHRA and isolated TSRA given the conditionally unstable profile. This risk will linger into Fri thanks to the fronts slowed movement and diurnal heating leading to further destabilization. POPs mainly at chance. Temps still running mainly at or below seasonal normals given minimal airmass change in the low-lvls. Sat... Conditions for Sat contingent on timing of front. The slower solution suggests it may hang near enough to the S coast to provide weak forcing for lift during peak heating. Therefore, will continue to highlight slight to low chance POPs at this time given the uncertainty in timing. Sun... Signals suggesting improvement as the cyclonic curvature associated with the digging longwave trof shift a bit further S allowing for influence of sfc high pres. Still relatively cool given mid lvl temps cool with the digging trof. && .AVIATION /02Z MONDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/... Tonight...High confidence. CIGs lowering SW to NE towards MVFR-IFR as -RA begins to spread across the region, but these lower categories likely hold off until after 10Z. Continued E flow with gusts up to 25 kts towards the south coast also mainly after 10Z. Monday...Moderate confidence. MVFR-IFR CIGs. Impacts to the AM push possible with RA/+RA. VSBY restrictions expected. Continued breezy NE flow with gusts up to 25 kts. Monday Night...Moderate confidence. MVFR-IFR CIGs linger with -SHRA/DZ. LIFR conditions possible towards the Cape and islands. NE winds continue with gusts up to 25 kts possible during the evening, especially across E MA. KBOS Terminal...Moderate confidence in TAF. Timing of MVFR/IFR conditions may be off by a couple hours. KBDL Terminal...Moderate confidence in TAF. Timing of MVFR/IFR conditions may be off by a couple hours. Outlook /Tuesday through Friday/... Tue and Tue night...Moderate confidence. Lingering IFR/MVFR conditions in low CIGS and occasional vsby restrictions in SHRA/fog. These begin to dissipate mainly during the overnight hours Tue night. E flow begins to shift toward the S by early Wed morning. Wed and Thu...High confidence. Mainly VFR. S winds gust to near 20 kt especially E coastal MA on Thu. Also, overnight SHRA/isolated TSRA mainly Thu night. Fri...Moderate confidence. Mainly VFR, but lingering risk for SHRA/isolated TSRA with localized lower categories. && .MARINE... FORECASTER CONFIDENCE LEVELS... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/... A more winter-like low pressure is expected to travel south of New England Monday morning. This will result in an increasing easterly flow with gusts up to 30 kt at times. Its possible for a brief period of gale force gusts around the Cape and islands Monday morning, but have low confidence it will persist long enough to warrant a Gale Watch or Warning at this time. Issued a Small Craft Advisory for all waters. Outlook /Tuesday through Friday/... Tue and Tue night...High confidence. Low pres moves E of the waters. Winds gradually shift from NE to the S by early Wed morning with gusts through the period mainly below 20 kt. However, lingering higher seas on the ocean waters 5-6 ft suggest small craft advisories will need to continue into the overnight hours. Otherwise, showers/fog linger. Wed through Fri...Moderate confidence. Overall quiet boating weather as high pres moves over the waters Wed and Thu. A slow moving cold front will cross late Thu night into Fri, but outside of showers and isolated thunderstorms winds/seas should remain below small craft advisory thresholds. && .TIDES/COASTAL FLOODING... Strong easterly flow, at least by July standards, will continue tonight. Latest tide gage data shows a residual between 0.5-0.8 feet along the east coast of MA. Issued a Coastal Flood Advisory for the time of high tide this evening. continued easterly flow into Monday may also cause some issues with the daytime high tide. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...Small Craft Advisory from 8 AM to 10 AM EDT Monday for MAZ023. Coastal Flood Advisory until 2 AM EDT Monday for MAZ015-016- 019-022-024. Small Craft Advisory from 8 AM to 6 PM EDT Monday for MAZ020- 021. RI...Small Craft Advisory from 8 AM to 10 AM EDT Monday for RIZ008. MARINE...Small Craft Advisory from 8 AM to 6 PM EDT Monday for ANZ232- 234-236. Small Craft Advisory from 6 AM to 6 PM EDT Monday for ANZ233. Small Craft Advisory from 10 AM Monday to 6 AM EDT Tuesday for ANZ230-250-251. Small Craft Advisory from 8 AM to 11 PM EDT Monday for ANZ231. Small Craft Advisory from 6 AM to 11 PM EDT Monday for ANZ235- 237. Small Craft Advisory from 8 AM Monday to 6 AM EDT Tuesday for ANZ254. Small Craft Advisory until 6 AM EDT Tuesday for ANZ255-256.
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&& $$ SYNOPSIS...Belk/Doody NEAR TERM...Belk/Doody/Sipprell SHORT TERM...Belk LONG TERM...Doody AVIATION...Belk/Doody/Sipprell MARINE...Belk/Doody/Sipprell TIDES/COASTAL FLOODING...WFO BOX Staff

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