Area Forecast Discussion
Issued by NWS Boston, MA

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584 FXUS61 KBOX 230228 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1028 PM EDT Sun Oct 22 2017 .SYNOPSIS... High pressure will push slowly offshore, but will keep unseasonably mild and dry weather across the region into Monday. A slow moving cold front will bring rounds of heavy rain Tuesday into Wednesday with a period of strong to damaging wind gusts possible. A few lingering showers are possible Thursday, but the trend towards drier and more seasonable temperatures are expected Thursday into Friday. Above normal temperatures may return by next weekend. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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1020 PM Update... Areas of low clouds and fog developing across eastern MA late this evening. While the fog may come and go to some extent, thinking the low clouds are here for the rest of tonight. Already some patchy dense fog, so will need to give some more thought toward a Dense Fog Advisory. Visibility is quite variable, so will start with a Special Weather Statement. Adjusted the forecast to bring it back in line with observed trends. 7 PM Update... The weak backdoor cold front has made little headway this evening, however within the last hour or so, thanks to a bit of diurnal cooling it is now draped from Bedford SE to Marshfield and slowly drifting SSE. The stratus deck associated with it is lagging the front by about 30 miles or so, so timed temps/dwpts and skies with this thinking in mind during the evening. Otherwise, used mainly RAW 2m temps as the MOS guidance is not interpreting the front at all. Also, given the expansion of the clouds, RAW temps were warmer and therefore favored with this update. Previous Discussion... While skies remained mainly clear except for a veil of high clouds across the region, kept an eye on some low clouds that pushed into coastal Cape Ann that pushed westward from the Gulf of Maine as seen on GOES-16 (prelim, non-operational data) starting around midday and through the afternoon. The onshore wind also helped in bringing the low level moisture to those areas. Otherwise, as very pleasant day across the region as temps rose t the mid 70s to around 80 except remaining in the 60s along the immediate E coast with the sea breezes. For tonight, expect the sea breezes to end around or after sunset. Winds will become light E-SE during the night along the shore, but will diminish to light/variable or calm elsewhere. With a continued E-NE flow just off the deck and low level inversion set up at around 1000 ft or so as seen on 12Z BUFKIT soundings, will likely see low clouds and patchy fog develop especially across central and E Mass, RI into NE CT probably by around midnight. Could see local visibility below 1/2 mile, so will need to monitor for potential of locally dense fog at times from around 2 AM to 6 AM. The high will drift slowly E overnight, so winds will slowly veer to S during the night as well. Expect temps to fall back to the lower-mid 40s across NW Mass where the lower clouds should not push in, but could see some brief patchy light fog for a few hours late tonight. Elsewhere, readings will bottom out in the upper 40s and 50s, mildest along the immediate coast.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... Monday... Expect the patchy fog to dissipate by around mid morning. Slow but steadily increasing S winds around the western periphery of the exiting high, along with the slow eastward approach of low pressure out of the Great Lakes, will see clouds increase during the day especially across western areas. With the S wind flow in place, will see low level moisture steadily increase as well. Some of the global and high res models try to develop light precip ahead of the approaching front, mainly across western areas. Should be a dry day for most areas, but could see some spotty showers push into central and western areas after 20Z or so. Will see another mild day by late October standards, though about 10 degrees cooler in some areas than today. Highs will range from the mid 60s across the higher inland terrain to the mid 70s across the coastal plain away from the coast. Monday night... With the southern inflow as well as increasing PWATs (up to 1.3 to 1.5 inches after midnight Mon night), will see precip become more organized well ahead of the approaching front. Noting strong winds just off the ground, with 35-40 kt jet from H925 to H85 pushing across the region. However, low level mixing is not the greatest with low level inversion in place, but could see some S wind gusts up to around 20-25 kt after midnight along S coastal areas. Will also see area of showers fill in especially across the CT valley where up to 0.2 inches of precip is possible by daybreak Tuesday. Heavier precip looks to hold off until Tuesday. With the mild S winds in place, temps will not fall back much. Lows will range from 55 to 60, though a bit milder along the immediate coast. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Highlights... * Period of strong to damaging wind gusts possible Tue into Wed am * Heavy rain will result in an urban/street flooding threat Tue-Wed * More seasonable late October temps finally return Thu/Fri Overview and model preferences... Strong and acute blocking high pres now looks to cutoff completely near Labrador based on 22.12Z guidance. This promotes deepening, and redevelopment of upstream longwave trof/cutoff across the CONUS, yielding a highly meridional flow pattern through much of the week. The first longwave trof begins negative tilt process through Wednesday, and is progged to be a bit closer to the downstream ridge. The trof is linked to gradually occluding low pres which will slide into Canada but drag a slow moving front into New England through mid week, coupled with Gulf of Mexico moisture. Lifting N late week thanks to a slight resurgence of the subtropical high, a second trof, deepens in response to arctic influenced phasing near the continental divide. With blocking remaining firmly entrenched across the W Atlantic, this second trof deepens in place through the weekend. Overall, a complex pattern with some disagreements in the mass fields particularly beyond Wed. Will continue to lean most heavily on the ensembles at this time, although with a little weight on the ECMWF given its overall slower progression which is favorable in such an amplified pattern. Details... Tue and Wed... Met. Overview... Slow cold front becomes parallel to upper flow as trof takes on negative tilt near the OH valley and parent low pres moves into Quebec. Guidance continues to favor the front slowly drifting across the region Tue into Wed. This front will be associated with very deep layer moisture, with PWATs near the 2.00 inch mark, or nearly 3 std deviations above normal. A series of weak LLJ impulses will also traverse the front. Mean H92 S wind speeds rest mainly around 40 kt, however, with each impulse 50-60 kt are also forecast, especially early Wed as the front shifts E. The increase in low lvl moisture and anomalously warm airmass associated with the high downstream (temps mainly in the upper 60s to low 70s by day) combined with dwpts in the mid- upper 60s as well and a conditionally unstable profile, do indicate some modest CAPE values near 500j/kg especially late Tue into early Wed. All ingredients point to a wet and highly unsettled environment. It is interesting to note that both ensemble probabilities, and latest runs of the CIPS analogs support heavy rain and strong winds as has been advertised. Wind risk... With 40-45 kt steady and impulses reaching near 60 kt at H92 as modest BL destabilization occurs late Tue into early Wed, at least some of this momentum is expected to reach the sfc. The peak will be along E MA/coastal plain, as the jet increases early Wed morning across this area. 35-45 mph gusts are possible, potentially as far inland as the Worcester hills and central CT, with gusts 50+ mph possible overnight Tue night into Wed mainly in the coastal plain mentioned above. Will need to especially monitor the potential for a fine convective line along the frontal boundary as this could more efficiently yield the strong-damaging gusts. With leaves still on most trees, this will exacerbate the risk for downed limbs or even some trees with localized power outages. Wind headlines will likely be needed. This risk coincides with the latest SPC SWODY3. Heavy rain... With very high PWATs, a strong frontal boundary and LLJ, combined with modest instability, pockets of convectively driven heavy rain is possible. Widespread shield of light-mod rain begins early Tue, but pockets of rainfall with 1+ inch/hr rainfall rates are likely late Tue into early Wed as the front gradually shifts E. Any thunderstorms will enhance these rainfall rates. Given the winds, some stripped leaves could clog area storm drains as well, exacerbating issues with the rainfall. At this time, the primary risk will be pockets of urban/poor drainage flooding. Overall totals above 2.00 inches are looking more likely given good agreement on GEFS/EPS and CIPS. However, with the potential for high rainfall rates locally, some areas of 3-4 inches cannot be ruled out. Thu... Transitional day as secondary frontal wave is able to shift N of the region, allowing for mid-upper lvl dry slot to begin to shift in from the S. This is likely to trap some lower lvl moisture, given PWATs remain elevated. Therefore, risk for drizzle and continued SHRA activity through the day on Thu even as heavier precip shield shifts into N New England. POPs will gradually decline because of this. The introduction of colder air within the trof lifting NNE will allow temps to drop closer to seasonal normals, mainly the upper 50s and low 60s as drier air slowly mixes to the sfc. Dwpts drop through the day as well. Fri through the weekend... Weak mid lvl ridging replaces the tilted trof as its axis shifts well NNE of S New England. High pres 1020+ hPa shifts over Fri into the weekend with drier air expected. The cool airmass is short lived however as H92 temps warm to +9C on Fri, then +12C by Sat and Sun. This suggests temps remain generally above normal for highs with near normal lows thanks to radiational cooling. Regarding the influence of the secondary longwave trof developing late week, the highly meridional pattern favors a slower progression than the current GFS/CMC and some ensemble members suggest. Will lean on a slower/drier period through the weekend based on other guidance sources than those listed previously. && .AVIATION /02Z 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 Monday Night/.... Overnight...Moderate confidence. IFR cloud deck already across the eastern 1/3 of MA this evening. IFR vsbys in fog spills over the region from SE to NW, likely getting as far inland as KORE-KORH-KPVD line. This drop will generally occur between 02Z-06Z this evening. Further inland, expecting VFR with some light night ground fog possible. Otherwise, very light and variable winds. Monday...High confidence. Early morning fog and low clouds with MVFR-IFR conditions should improve to VFR by around 15Z. However, MVFR CIGS may push into western MA/N central CT during the late afternoon. May see isolated showers move into western areas after 18Z. S winds increase to around 10 kt. Monday night...Moderate confidence. Conditions lower to MVFR-IFR across N central and western areas Mon 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 mainly 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. KBOS Terminal...Moderate to high confidence in TAF. Expect VFR conditions through around 02Z-03Z. MVFR-IFR CIGS/VSBYS move into terminal by around 04Z-05Z, continuing through the morning push, then should improve by around mid morning Monday. KBDL Terminal...High confidence in TAF. Mainly VFR. May see brief MVFR-IFR VSBYS in patchy fog after 06Z or so but should improve by morning push. CIGS may lower to MVFR around midday Monday. Outlook /Tuesday through Friday/... Tue and Wed...High confidence. Mainly IFR with occasional MVFR in low CIGS rain/fog. Some occasional TS also possible with heavy RA at times. Winds gust 20-30 kt most of the region out of the S-SE with a few gusts 30-40 kt mainly SE MA/Cape/Islands. Cannot rule out stronger gusts everywhere in heavier rainfall/thunderstorms. LLWS likely, with winds as high as 50 kt at 2kft. Thu...Moderate confidence. Gradual improvement as winds shift to the west and diminish. Some lingering SHRA and IFR/MVFR conditions but with improvement to VFR into the overnight hours. Fri...High confidence. Mainly VFR.
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&& .MARINE...
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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 Monday Night/... Relatively light winds and seas continue across the waters into Monday evening. Increasing south winds ahead of a cold front will also mean building seas Monday night. Poor visibility in areas of fog across the southern coastal waters, likely becoming more widespread late tonight. Visibility will slowly improve after daybreak Monday. Outlook /Tuesday through Friday/... Tue and Wed...High confidence. S-SE Gales developing through the day Tue and continuing into the early half of the day on Wed. Wind gusts up to 40 kt, with a few gusts higher possible at times. Seas build to 12-14 ft on the SE outer waters. Rain, fog and occasional thunderstorms possible. Overall, dangerous boating conditions. Thu and Fri...High confidence. Winds shift to the W, and gradually diminish although gusts to 20 kt possible both Thu and Fri afternoon. Seas will take a long time to diminish with a lingering swell, and may require small crafts on the outer waters at least well into Fri. Otherwise improving weather, with dry conditions overall by Fri.
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&& .FIRE WEATHER... While minimum RH values were at 25 to 35 percent across central and western areas today, winds were either light E-NE or calm with the lower winds where the lower relative humidities occurred. This should be the last day of potential fire weather issues. Winds become south on Monday with some gusts up to 15-20 mph as a frontal system slowly approaches. This will bring higher RH values across the region. Forecasted minimum RH values will be from 60 to 75 percent on Monday. A few showers may also move into the Connecticut River Valley region into NE CT during Monday afternoon as well. This means that there will be a low red flag warning threat. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 6 AM to 6 PM EDT Tuesday for ANZ231>234-236. Small Craft Advisory from 8 AM to 6 PM EDT Tuesday for ANZ230- 251. Small Craft Advisory from 2 AM to 6 PM EDT Tuesday for ANZ235- 237. Small Craft Advisory from 4 AM to 6 PM EDT Tuesday for ANZ250- 254. Small Craft Advisory from 11 PM Monday to 6 PM EDT Tuesday for ANZ255-256. && $$ SYNOPSIS...Doody/EVT NEAR TERM...Belk/Doody/EVT SHORT TERM...EVT LONG TERM...Doody AVIATION...Belk/Doody/EVT MARINE...Doody/EVT FIRE WEATHER...

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