Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 241409 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1009 AM EDT Mon Jul 24 2017 .SYNOPSIS... Widespread rain and unseasonably cool temperatures impact the region today. Rain tapers off to light showers and areas of drizzle later today, but it remains rather cloudy and unseasonably cool on Tuesday. High pressure will bring a return to dry and pleasant weather for Wednesday. Another cold front should bring a period of showers and perhaps isolated thunderstorms sometime late Thursday and/or Friday. Early signs are for dry weather returning in time for most of the weekend, but it is possible clouds/showers linger into Saturday if the cold front slows up. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Watching northward progress of a dry slot towards the south coast of New England. Strong convection well south and southeast of Nantucket this morning has limited inflow farther north, which in turn has reduced the threat of really heavy rainfall. Also noted another band of higher reflectivity farther west, which is associated with a low level frontogenetical band. This is much more typical of a winter Nor`easter than late July. Temperatures continue to hold nearly steady or slowly fall. Made some adjustments to reflect observed trends. Thinking we have already reached our maximum temperature earlier this morning. Previous Discussion... A morning light to moderate rain event with the greatest risk of heavy rain for Southeast New England, mainly S-coast and Islands. Lead H5-7 shortwave energy within the diffluent flow pattern aloft and beneath the right rear quadrant of the upper level jet is invoking a surface area of low pressure off the Delmarva Peninsula which will sweep E along the quasi-stationary frontal boundary (QSFB) immediately S of New England through afternoon. Accompanying anomalous low-level convergent focus, per isallobaric response, of sub-tropical moisture with precipitable waters in excess of 2-inches advects from the S along the QSFB as a wedge of stable, cooler air is drawn in from the N/E across the interior of New England by a 4-5 standard deviation low-level E jet. A pattern more typical for the cool season, a N-S cross-sectional analysis shows a pronounced warm frontal frontogenetical slope associated with the QSFB with modest moisture convergence all the way up through H4 associated with ageostropic flow signaling a good deep-layer frontal circulation. It`s along this slope a consensus of forecast guidance is signaling two distinct areas of mesoscale banding and upward forcing: 1.) along the QSFB, and 2.) beneath the lead H5-7 shortwave, mid-level deformation zone. But it remains a challenge as to exactly where these two areas emerge and how they evolve over time, especially with regards to magnitude and with respect to associated spatial mesoscale features (i.e., strongest low-level jet). As alluded to earlier, an anomalous event per analysis of the GEFS and SREF. This as the top 5 CIPS analogs signal a heavy rain event for the immediate S-coast. So there is some concern, especially in regards to potential flooding. However not all that certain after interrogating the NCAR ensembles and evaluating the latest HRRR. Only thing we can comfortably hang our hat on is that much of the rainfall will occur this morning within the 12z-18z (8a-2p) timeframe and that much of the heavy rain focus will lie S upon the QSFB collocated with the greatest focus of sub-tropical moisture and instability. In all, evaluating latest radar trends and HRRR guidance as of 330a, storms are hinging to the QSFB over the waters. The islands look to be under the greatest threat of seeing amounts around 2 inches in a short period of time, Nantucket especially. A flood potential, yet during a time of high tide transitioning to low tide. NCAR ensembles signal the potential for 1 inch in 1 hour over Nantucket, with the potential for 2 inches out of the entire event. The remainder of the area, probabilities are low for seeing over an inch, thinking the result of the wedge of stable air out of the N/E plus the storm environment along the QSFB robbing the environment N of sub-tropical moisture and instability. Will not be issuing any headlines for this event. Low risk of thunder, believe it will stay mainly S/W of our area as we find ourselves beneath the stable wedge. Wet weather tapering for the remainder of the day as a mix of light showers and/or drizzle under a deck of low clouds. Brisk N/E wind with gusts up to 30 mph for the E MA coast.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH TUESDAY NIGHT/... Cool, damp weather at first, clearing out late, mainly overnight Tuesday night, as surface high pressure builds into the region. Bulk of sub-tropical moisture pushed out to sea, a more continental air- mass resides out ahead of the deamplifying mid-level positively- tilted shortwave trough with precipitable waters around 1.25 inches. Weakening ascent upon lingering moisture as drier air works into the region and heights rise. Indications of a building dry inversion from forecast model soundings. Expecting low clouds to linger with initial showers tapering to drizzle tonight into Tuesday with continued N/E winds keeping a cooler, stable airmass in place at the surface. Chance PoPs accordingly with highest confidence up against high terrain with orographic support (N/W MA). A clearing trend begins to emerge late Tuesday with clouds becoming broken to scattered, continuing overnight as a 1020+ high pressure builds across the area. Winds becoming light, an opportunity may emerge for radiational cooling allowing temperatures to drop well down into the 50s along with the possibility of patchy ground fog given recent damp conditions. Unseasonable temperatures with highs mainly in the 60s with lows around the 50s. && .LONG TERM /WEDNESDAY THROUGH SUNDAY/... Highlights... * Dry and pleasant weather Wednesday * Some showers/isolated t-storms likely late Thu and/or Fri * Early signs are for pleasant weather for most of the weekend Details... Wednesday... High pressure remains in control but slowly slides east of the region. This should result in dry/pleasant weather Wed with partly sunny skies. May see some more clouds develop as the day wears on across the interior with weak warm advection beginning. Regardless, high temps should recover into the upper 70s to around 80 along with comfortable humidity levels. Thursday and Friday... An impressive shortwave for July standards will drop southeast from the Great Lakes, but still quite a bit of timing differences amongst the models. It is possible that some showers/isolated t-storms arrive during Thu afternoon, but the greater risk for more widespread activity may end up sometime Thu night/Fri. High temps should be in the upper 70s to lower 80s Thu and perhaps some middle 80s if more sunshine is realized. Friday will probably be a bit cooler with a greater risk for showers at this time. Saturday and Sunday... Still a lot of uncertainty this far out in the forecast, but most current model guidance indicates trough axis moves to our east. This would allow for dry and pleasant weather with high temps well up into the 70s to lower 80s Sat and Sun. However, if the shortwave trough ends up moving slower clouds and showers could linger into Saturday, especially along the coast. && .AVIATION /13Z MONDAY THROUGH FRIDAY/...
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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 Tuesday Night/... Today... MVFR to IFR conditions develop as widespread rain lowers cigs/vsbys this morning. Heaviest rain tapers to lighter showers/drizzle later today, but low end MVFR-IFR conditions continue. E winds becoming N over a majority of terminals, VRB along the S coast. Strongest winds across E MA sustained around 20 kts with gusts up to 30 kts. Tonight... Mostly IFR CIGs, lowest across high terrain terminals, with SHRA/DZ. Winds turning N, strongest along E MA coast sustained around 15 kts with gusts up to 25 kts. Tuesday into Tuesday night... Improving trends. MVFR-IFR mix gradually lifting, eroding into the overnight hours Tuesday night. N winds diminishing becoming lighter towards morning, still a bit breezy along the E coast with sustained winds up around 10 kts, gusts potentially to 20 kts. KBOS Terminal...E winds becoming strong through the day with the potential for gusts up around 30 kts. Winds back late in the day out of the NE remaining breezy during which time IFR CIGs are possible. KBDL Terminal...CIGs lowering with areal coverage of -RA. Low conditions persisting through roughly midday Tuesday as E winds becoming N. Outlook /Wednesday through Friday/... Wednesday...Moderate confidence. VFR conditions. Wednesday night and Thursday...Moderate confidence. VFR conditions probably dominate, but will have to watch for localized MVFR-IFR conditions in some low clouds/fog patches especially during the late night/early morning hours. Thursday night and Friday...Moderate confidence. A period of MVFR- IFR conditions likely in showers and perhaps isolated thunderstorms, but timing is uncertain.
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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 Tuesday Night/... A more winter-like low pressure is expected to travel south of New England this 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 today, but have low confidence it will persist long enough to warrant a Gale Watch or Warning at this time. Small Craft Advisory for all waters continues, concluding late Tuesday into Tuesday night as winds subside and seas diminish. Outlook /Wednesday through Friday/... Wednesday...High confidence. High pressure slowly moves east of the waters, but weak pressure gradient will keep winds/seas below small craft advisory thresholds. Thursday and Friday...Moderate confidence. Winds and seas will generally remain below small craft advisory thresholds. However, a period of southwest wind gusts of 20 to 25 knots and choppy seas are expected Thursday afternoon. && .TIDES/COASTAL FLOODING...
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Continued strong E/NE flow continue today into tonight with sustained winds around 10 to 15 mph with gusts upwards of 30 mph. This yields a surge upwards of a foot for locations along the E MA coast N of a low sweeping W to E along the waters south of New England. Another Coastal Flood Advisory has been issued for the evening high tide accordingly, given surge on top of high astronomical high tides.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...Coastal Flood Advisory from 11 PM this evening to 3 AM EDT Tuesday for MAZ007-015-016-019-022-024. RI...None. MARINE...Small Craft Advisory until 5 PM EDT this afternoon for ANZ232>234. Small Craft Advisory until 2 AM EDT Tuesday for ANZ230. Small Craft Advisory until 2 PM EDT Tuesday for ANZ231-251. Small Craft Advisory until 2 PM EDT this afternoon for ANZ236. Small Craft Advisory until 5 PM EDT Tuesday for ANZ235-237. Small Craft Advisory until 2 AM EDT Wednesday for ANZ250- 254>256.
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&& $$ SYNOPSIS...Frank/Sipprell NEAR TERM...Belk/Frank/Sipprell SHORT TERM...Sipprell LONG TERM...Frank AVIATION...Frank/Sipprell MARINE...Frank/Sipprell TIDES/COASTAL FLOODING...Staff

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