Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 232101 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 501 PM EDT Mon Oct 23 2017 .SYNOPSIS... A few showers may develop well ahead of a slowly approaching cold front tonight into early Tuesday morning. As the front gets closer, expect rounds of heavy rain late Tuesday into Wednesday along with a period of strong to damaging wind gusts possible. A few lingering showers are possible Thursday, but the trend towards drier, less humid and more seasonable temperatures are expected Thursday into Friday. Above normal temperatures may return by this weekend. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... A stubborn area of very low clouds and dense fog has lingered from the mid coast of Maine SW to Cape Ann down to S of Boston as seen on GOES-16 prelim, non-operational satellite loop. Noting light E-SE winds from about KOWD-KBED-KLWM east to the coast as well as into the seacoast of Maine, while the remainder of the region has generally S winds at 20Z. This is likely due to the leftover remains of a backdoor cold front that pushed into E Mass Sunday. As the S pressure gradient increases with the slow approach of a cold front, expect winds to finally shift and allow the low clouds and fog to push offshore. Still some question when this will happen, but feel it should for a time this evening. With some onshore wind component in place tonight, will still see more low clouds and patchy fog, some locally dense, to push back along E coastal Mass. Also, with dewpoints in the upper 50s to lower 60s, will see patchy fog develop away from the coast as well as temps approach to dewpoints, mainly after midnight. Scattered showers will develop early tonight across western Mass this evening, which should become more numerous after midnight. As an H5 short wave wraps around cutoff low pressure over the western Great Lakes, will see the vorticity line push slowly E overnight, pushing the front eastward toward western New England toward daybreak. Not a lot of instability with the leading edge, so expect only showers at this point. QPF amounts generally 0.1 inches or less across central and eastern areas, possibly up to 0.2 inches across the E slopes of the Berkshires as plume of higher PWAT air approaches. The mild S winds will keep temps well above seasonal normals overnight. Temps will bottom out in the mid 50s to lower 60s. Could see record tying or setting high minimum temps for 10/24 at our four long term climate sites. Details will be in the climate section. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH 6 PM TUESDAY/... Expect showers to move slowly across the region during the day Tuesday. Very sultry airmass in place with increasing S winds. PWAT values up to 1.6 to 1.7 inches will slide up the coast ahead of the front. Will also see some heavier downpours develop with this airmass mainly across western areas during the afternoon. Heaviest rainfall looks to hold off until Tue night. Good instability also develops across central and western areas during Tue afternoon. TQ values increase to the upper teens, with K indices reaching the lower-mid 30s across central and western areas. Have mentioned some thunderstorms in the forecast as well. Also, with strong low and mid level jet associated with the approach surface front and mid level short wave, expect S winds gusting up to 30-40 mph, highest along the S coast. High temps will reach to upper 60s to mid 70s. && .LONG TERM /TUESDAY NIGHT THROUGH MONDAY/... Highlights... * Period of strong to damaging wind gusts possible Tue into Wed * Heavy rain will result in an urban/street flooding threat Tue- Wed along with a risk of isolated flash flooding * Less humid & more seasonable weather finally return Thu/Fri Overview and model preferences... Blocking across the W Atlantic continually sharpens up toward Labrador through the long term. This allows for a near constant trof reloading pattern over the CONUS, the first of which will be associated with the heavy rain/strong winds Tue into Wed, but after taking on negative tilt and lifting NNE late week, a secondary longwave trof develops in the lee of the continental divide thanks to reinforcement of arctic influence. Will continue with a baseline of ensembles given the highly amplified pattern, which will take into account most solutions in the envelope. Details... Tue Night and Wed... Met overview... A cold front becomes gradually parallel to the low-mid lvl flow as strong upper lvl trof digs and takes on a negative tilt late Tue night into Wed. The progression of the front across S New England is very slow as a result, and is likely to take until Wed evening to fully shift offshore. The parent trof has a draw of subtropical air with PWATS reaching very close to 2.00 inches (3 std deviations above normal). As dwpts respond to this moisture in the low lvls, soundings become conditionally unstable, resulting in 300-500j/kg of CAPE available. Finally, a strong LLJ, which peaks at about 60 kt at H92 and near 70 kt at H85 shifts N-S through the area late night into early Wed. This will lead to a very unsettled, prolonged period of wet wx, potentially exceeding 24 hrs. Wind/severe threat... LLJ peaks late Tue night into early Wed, with H92 speed values around 60 kt even on the typically more conservative ECMWF/EPS members. Combination of precip drag and some BL mixing thanks to conditionally unstable lapse rates will likely lead to some of this momentum reaching the ground. Widespread 35-45 mph wind gusts are likely, even into interior New England. However, 50-60 mph are also possible, especially across SE MA/RI where the core of the LLJ peaks. With some instability, thunderstorms, and/or even a fine line of convection will more efficiently mix these winds down, potentially leading to localized higher impact damage. While this is possible anywhere in New England, highest winds will still be featured across the SE. Wind advisories will be hoisted, and given fully leaved trees, will highlight all of S New England for this risk, but still monitor for localized higher impacts in thunderstorms/fine convective line. This coincides with the SWODY2 upgrade to a slight risk, given localized gusts could produce more damage in convection. Peak will be overnight into Wed morning. One final note, cannot rule out a brief spinup given the high shear (helicity 300+) and low LCLs within the moist airmass. This is a very low/localized risk. Heavy rain... High PWATs and the influence of convective potential could yield pockets of heavy rain with rainfall rates exceeding 1in/hr at times. The combination of the slow movement of the front, and strong S LLJ suggests a risk for training heavier cells, which could lead to prolonged periods of high-rate rainfall. A slight E component to the low lvl flow could lead to enhanced upslope in E slopes of the Berkshires and Litchfield hills. Even though dry, with FF guidance nearly 3-4 inches/hr, the long timeframe could exceed 3 or 6 hr FF guidance. Flashier streams will see a risk for flooding. Therefore, will be hoisting a FF Watch with this update mainly for W portions of the forecast area, where the enhancement is most apparent. Further E, training/heavy rain is still an issue, but the risk would be more in the form of urban/poor drainage flash flooding, exacerbated if stripped leaves clog drains. We may need to expand the FF watch for this risk in later updates. Overall widespread 1.5-2.5 inches are expected with localized 3-4 inches certainly possible. Thu... Dry air moves in through the column aloft first, trapping some lower lvl moisture into the day on Thu. The combination of this, some cold advection and strong cyclonic curvature aloft suggest a risk for lingering clouds and SHRA through much of the day, but not nearly as widespread as the previous 24-48 hours. H92 temps drop to about +6C on average, so expecting seasonable temperatures with highs mainly in the upper 50s and low 60s while weak flow overnight and gradual clearing should allow mins to drop into the 40s. Fri and Sat... Dry period as weak mid lvl ridging forms between the exiting cutoff/trof to the NE and deepening longwave trof shifting E into the prairies. H92 temps hover around +9C to +10C each day, suggesting highs will be seasonably warm, mainly in the mid 60s, although overnight lows will be seasonable thanks to some radiational cooling especially Fri night. Dwpts in the low-mid 40s during the afternoon suggest these will be near the overnight lows. Sun into Mon... With a very amplified pattern timing/mesoscale features are not resolved well this far out, but given a similar setup with a gradually tilting trof with some subtropical moisture associated with it, will have to watch for another round of low pres with widespread rainfall. Will favor the slower timing of ensembles given the meridional nature of the pattern. At the very least, another unsettled wet period is possible early next week. && .AVIATION /21Z MONDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/.... Through 00Z...Moderate confidence. Lingering IFR-LIFR conditions across portions of E coastal Mass through 00Z. Elsewhere, areas of MVFR CIGS/VFR VSBYS across the remainder of the region. May see a few isolated showers across the E slopes of the Berkshires by 22Z-23Z. S-SE winds increase to around 10 kt. Tonight...Moderate confidence. Conditions lower to MVFR-IFR across N central and western areas this evening in scattered showers and patchy fog. VFR conditions to start across S central and eastern areas, but CIGS lower to MVFR to local IFR after midnight as showers slowly move E. Patchy fog will lower VSBYS to MVFR to local IFR as well. S winds gusting to 20-25 kt along S coastal areas after midnight. Tuesday...High confidence in trends. Moderate confidence in timing. Expect mainly MVFR-IFR conditions in low CIGS and areas of fog. Expect areas of -SHRA/SHRA to push slowly into the region during the day. Rain heavy at times late in the day across W areas. Occasional TSRA at times Tue afternoon. S-SE winds gusting up to 25-30 kt, possibly to 40 kt in any thunderstorms. Highest gusts along S coast, Cape Cod and the islands. Areas of LLWS likely, mainly after 15Z. KBOS Terminal...Moderate confidence in TAF. Mainly IFR-LIFR CIGS and IFR VSBYS lingering through at least 04Z, possibly later. E winds up to 10 kt should shift to SE-S overnight. Remaining IFR during at least early portion of the morning push. KBDL Terminal...Moderate confidence in TAF. VFR through 00Z, then CIGS lowering to MVFR. May see MVFR-IFR VSBYS in scattered showers by around 10Z or so. S winds gusting up to 20-25 kt by around midday Tuesday. Outlook /Tuesday Night through Saturday/... Tuesday Night: Mainly IFR, with local MVFR possible. Strong winds with gusts to 40 kt. RA, chance TSRA, patchy BR. Wednesday: Mainly VFR late, with local IFR possible. Windy with gusts to 35 kt. RA, patchy BR. Wednesday Night: Mainly VFR, with local MVFR possible. Breezy. Chance SHRA, chance RA, patchy BR. Thursday: VFR. Breezy. Chance SHRA. Thursday Night through Friday: VFR. Friday Night: VFR. Patchy BR. Saturday: VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/... Have converted Gale Watches to Warnings with this package for all waters. Expect increasing S-SE winds tonight, gusting up to 20-25 kt after midnight. Seas build up to 5-6 ft. Winds and seas highest on the southern waters. Reduced visibility in areas of fog. Scattered showers across the southern waters late tonight. On Tuesday, S winds continue to increase, with gusts up to 30-35 kt. Seas continue to build, up to 6-9 ft. Visibility restrictions develop in areas of locally heavy rain and patchy fog. Scattered thunderstorms possible. Gale force wind gusts likely Tue afternoon across most of the waters. Outlook /Tuesday Night through Saturday/... Tuesday Night: Gale force winds with gusts up to 45 kt. Rough seas up to 11 ft. Rain, chance of thunderstorms, patchy fog. Local visibility 1 to 3 nm. Wednesday: Gale force winds with gusts up to 40 kt early. Rough seas up to 12 ft. Rain, chance of thunderstorms, patchy fog. Local visibility 1 nm or less. Wednesday Night: Winds less than 25 kt. Rough seas up to 11 ft. Rain showers likely, rain, patchy fog. Thursday: Winds less than 25 kt. Areas of rough seas. Chance of rain showers. Thursday Night: Winds less than 25 kt. Local rough seas. Friday: Winds less than 25 kt. Seas up to 5 ft. Friday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Saturday: Winds less than 25 kt. Seas locally approaching 5 ft. && CLIMATE... Overnight lows tonight will likely remain mild with S-SE winds in place. May tie or set record high minimum temperatures for October 24. Here are the records... October 24 Record High Min Boston 60/1900 and 2001 Windsor Locks 61/1959 Providence 61/1959 and 2001 Worcester 59/1959 && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Wind Advisory from noon Tuesday to 11 AM EDT Wednesday for CTZ002>004. Flash Flood Watch from Tuesday evening through Wednesday morning for CTZ002. MA...Wind Advisory from noon Tuesday to 11 AM EDT Wednesday for MAZ002>024-026. Flash Flood Watch from Tuesday evening through Wednesday morning for MAZ002-003-008>011. RI...Wind Advisory from noon Tuesday to 11 AM EDT Wednesday for RIZ001>008. MARINE...Gale Warning from noon Tuesday to 11 AM EDT Wednesday for ANZ231>234. Gale Warning from 8 PM Tuesday to 9 AM EDT Wednesday for ANZ230-251. Gale Warning from 2 PM Tuesday to 8 AM EDT Wednesday for ANZ236. Gale Warning from 11 AM Tuesday to 8 AM EDT Wednesday for ANZ235-237. Gale Warning from noon Tuesday to 2 PM EDT Wednesday for ANZ250-254. Gale Warning from 11 AM Tuesday to 11 AM EDT Wednesday for ANZ255-256. && $$ SYNOPSIS...Doody/EVT NEAR TERM...EVT SHORT TERM...EVT LONG TERM...Doody AVIATION...Doody/EVT MARINE...Doody/EVT

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