Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 201827 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 227 PM EDT Thu Oct 20 2016 .SYNOPSIS... Cold front stalls south of the region today then moves back north as a warm front later tonight. A few showers or drizzle is possible late this afternoon and tonight, then a more widespread heavier rainfall is likely sometime Friday into early Saturday as low pressure tracks across the region. Cooler and mainly dry weather will follow Sunday into early next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 230 pm update... Clouds on the increase given diurnal heating from late this morning along with WAA pattern developing over the region from the south and west. Showers impacting northwest MA will continue moving northeast this afternoon with a little break in the action for this area and then showers redeveloping upstream over the Catskills. Otherwise much of the region remains dry this afternoon with clouds remaining on the increase. Temps have overachieved with highs in the low 70s across portions of RI and CT. Thus have bumped up temps a few degs. Previous discussion... High pressure moves east from northern New England today with sunshine giving way to increasing clouds from west to east as column moisture increases. As a frontal wave lifts NE along the spine of the Appalachians a 40-50 kt low level jet will become focused across central and northern NY this afternoon. The leading edge of the jet approaches from the west late today which will bring a chance of a few showers across mainly western New Eng as weak ascent develops north of the stalled boundary, but best forcing and deep moisture axis remains well to the west where main focus for heavier rain will be. Much cooler today with easterly flow but temps still above normal with highs reaching mid/upper 60s south of the pike, cooler to the north and along the east coast. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... Tonight... As mid level trof amplifies to the west, some mid level drying will lift north into SNE during the night. Low level jet and best moisture transport remains focused to the north and west where best chance of steadier/heavier rain. However, moisture is abundant below 850 mb with shallow lift likely leading to areas of drizzle or light rain, and fog will likely become widespread and locally dense as warm front lifts north into SNE with increasing dewpoints. Expect rising temps into the 60s overnight south of Mass pike with temps closer to 50 near NH border. Friday... High amplitude trof moves east into the Great Lakes with deep southerly flow developing from the surface through the mid levels. Interesting set up developing with one area of heavy rainfall to the west across New York assocd with right entrance region of strong upper jet and strong frontogenesis along the mid level front. Meanwhile, models indicate low pres lifting north toward SNE with area of enhanced low level convergence and some interaction with tropical moisture and elevated instability. This will lead to more widespread rainfall and possible isold thunder but there is low confidence on where the axis of potential heavy rainfall will set up. It may be across SNE but it could also end up to the east. This is something that will need to be evaluated further with later forecasts. SNE will be in the warm sector Fri with temps reaching the upper 60s to lower 70s and quite humid with dewpoints climbing well into the 60s. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Highlights... * Rainy and unsettled Friday night into early Saturday. * Cooler and dry by Saturday night. * Seasonable early next week. Overview and model preferences... With new 00Z update, noting the good agreement at the synoptic scale continues. However, it is the lower level interactions that continue to show a lot of spread, and this includes both ensembles and operational models. At odds is the interaction with weak tropical disturbance and attendant moisture which will be captured early enough for cyclogenesis to occur over or near S New England (ECMWF/CMC, several ensemble members) or offshore (GFS, several other ensemble members). This will ultimately dictate how much rain falls Fri night, and how long the wet wx lingers into Sat. Looking at soundings, noting a fair amount of moisture trapped in the lower lvls even as drier air builds over through the day on Sat. Given this fact, and that ECMWF has been a bit more persistent, will add a bit more weight towards it`s solution with this update. Details... Sat night and Sat... Strong cyclogenesis S or SE of S New England will gradually shift to the NNE as it continues to strengthen and becomes captured by the upper lvl cutoff. With this update, suggest some mod-heavy rainfall during the overnight hours as increased low- lvl convergence and cyclogenesis occurs over portions of S New England. PWAT plume is highest offshore, but noting enough moisture (PWATS near 1.5 inches, 2 std deviations above normal). QPF probabilities are near 70 percent for values of 0.5-1.0 inches. Therefore, would not be surprised to see some areas receive nearly 1.00 inches during the overnight hours and into early Sat in the comma-head precip as low pres shifts into N New England. Still some question on exactly where the peak precip will occur, primarily due to the models continuing to struggle with the tropical interaction. Stay tuned for updates as we approach and better refine the axis of highest precip. Timing wise, will allow pops to linger into mid day on Saturday, slower than previous forecasts. Temps remain near to above normal thanks to H85 not remaining above +6C until Sat afternoon. Looking at highs in the upper 60s, with lows only falling into the low 60s. Also, strong NW flow especially if mixing increases with some sunshine late Sat. 30-40 mph gusts possible, with a low risk for some wind advisories. Sat night through Sun... Strong NW flow continues with drier air moving in aloft first. Therefore, it may take some time for all low clouds to clear out Sun night. Otherwise, cold advection allows H85 temps to drop below 0C while H92 temps fall to near +4C. Highs Sun should remain in the 50s, while lows drop into the 30s. Save for a low-lvl instability induced sprinkle, conditions remain mostly dry. Mon... Reinforcing shortwave quickly rushes through the region with some moisture and weak insatiability below H7. This should be enough for some quick shra across the area. These should end with the evening hours and loss of any diabatic support. Otherwise, winds shift from W, to more N-NE by the overnight hours as the isallobaric couplet shifts. Increased risk for ocean effect clouds/rainfall with this trajectory for the Cape, lower Plymouth county as SSTs remain near +15C and H85 temps drop to -7C. Highs in the 50s, but lows may actually dip into the low-mid 30s, cooler than the weekend. Tue into Thu... The strong trof settling in Atlantic Canada will lead to building ridge upstream and allow for high pres to build across the region. H85 temps drop to about -5C at its lowest while H92 temps drop near 0C. Therefore, looking at a seasonably cool period with this high pres. Overnight mins especially could be chilly with radiational cooling. && .AVIATION /18Z THURSDAY THROUGH TUESDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/... 230 pm update... Thru 00z...high confidence. A combination of VFR and MVFR with any rain showers likely confined to northwest MA and then a chance of showers along the south coast of MA and RI toward 00z. After 00Z...High confidence on trends but lower confidence on exact timing and details. Widespread MVFR to start and then lowering to IFR/LIFR after 02z/03z in drizzle/fog and scattered showers. Localized LLWS possible with light easterly winds at the surface and southerly winds 20-30 kt developing at 2K ft. Friday...High confidence on trends but lower confidence on exact timing and details. Widespread IFR/LIFR conditions persisting in the morning...possibly lifting to MVFR at times in the afternoon with rain becoming more widespread. Isold t-storm possible. Friday night...High confidence on trends but lower confidence on exact timing and details. IFR/LIFR likely in periods of rain/drizzle and fog. Isold t-storm possible. KBOS TAF...High confidence in trends. KBDL TAF...High confidence in trends. Outlook /Saturday through Monday/... By Sat, although rains gradually diminish low clouds could linger into early afternoon, suggesting MVFR conditions at least continue through much of the day. Otherwise, winds shift to the W, with LLWS overnight giving way to gusty (20-30 kt) gusts during the day on Sat. Sat night into Sun night...High confidence. Some gradual improvement with more widespread VFR through this period. Winds remain an issue, with gusts (especially during the day) reaching 20-30 kt out of the W. Mon and Tue...High confidence. Isolates shra on Monday, otherwise VFR throughout. Winds shift to the NW and continue to gust 20-30 kt. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/...Moderate confidence. 10 am update... ENE winds of 15-20 kt continue today. Given this long fetch seas will build to greater than 5 ft across the ocean waters of MA and RI. Hence SCA remain in effect for this afternoon for southern RI and MA waters and then this evening for eastern MA waters. ================================================================= Today...Increasing easterly winds with gusts to 20-25 kt developing and building seas. SCA issued for the outer waters. Tonight into Friday...Winds becoming SE later tonight and persisting into Fri with gusts to 20-25 kt at times, strongest eastern waters. Seas building to 6-8 ft outer waters during Fri. SCA will be needed for outer waters. Outlook /Friday Night through Monday/... Fri night into Sat...High confidence. Winds will gradually shift to the W, and as they do so, expect an increase in gusts over the waters 25-30 kt. This will actually allow seas to diminish slightly initially but remain above 5 ft over much of the waters. Rain/fog reduce vsbys. Small crafts likely. Late day Sat into Sun...Moderate confidence. W-NW winds continue but rainfall ends. Wind gusts could reach 35 kt at times this entire period, so a low risk for Gales. Otherwise building seas to 8+ ft on the ocean waters and high end Small Craft Advisories can be expected. Mon and Tue...Moderate confidence. Winds shift to the NW fully, with another low end risk for Gale force gusts late Mon and Mon night. Otherwise 5-7 ft seas remain and winds 20-30 kt suggest small crafts continue. && .TIDES/COASTAL FLOODING... Astronomical high tides are subsiding with each tidal cycle. There may still be one or two high tides Thursday and Friday that threaten minor splashover at the more vulnerable spots on the coastline, but the general trend is now moving away from that. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 8 PM EDT Friday for ANZ250-254. Small Craft Advisory until 2 AM EDT Friday for ANZ255-256. && $$ SYNOPSIS...KJC/Doody NEAR TERM...KJC/Nocera/Doody SHORT TERM...KJC LONG TERM...Doody AVIATION...KJC/Nocera/Doody MARINE...KJC/Nocera/Doody TIDES/COASTAL FLOODING... is the U.S. government's official web portal to all federal, state and local government web resources and services.