Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 221327 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 927 AM EDT Sat Apr 22 2017 .SYNOPSIS... Low pressure remains over S Quebec today, with a secondary low passing S of New England this evening. Onshore winds will keep low clouds, fog, and patchy rain and drizzle in the area this morning. Drying late today once the offshore low pressure moves out to sea. Continued dry weather Sunday night into Monday. A prolonged period of wet weather beginning Monday night through Wednesday night, as a low pressure off the southeastern US lifts N across the region. A brief period of dry weather for Thursday prior to a cold front and some scattered showers. Mild on Friday under high pressure and light winds while cooler on Saturday with onshore flow. Warm and dry on Sunday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 925 AM update... Area of rain moving east into SNE this morning in response to amplifying shortwave moving into the Gt Lakes. Backing flow across New Eng allowing deep moisture plume to lift NE into the region along with low level fgen just offshore. As shortwave moves east it will induce an inverted trof between offshore low and low pres to the north. Deep moisture axis and mid level omega will keep rain focused primarily south of the Pike and especially south coast and islands into the afternoon where a wet and raw day is expected. Updated PoPs to reflect current radar trends and HRRR forecast into the afternoon. A chilly day near the coast and eastern New Eng with highs reaching upper 40s to lower 50s warming to mid/upper 50s CT valley where a bit of sun is possible later today. Previous discussion... No surprises in the 22/00Z guidance. Primary low pressure remains in S Quebec today, with a triple point low pressure developing and passing by to our S by this evening. Thinking the last few runs of the HRRR have the best idea for the forecast today. Low levels remain quite moist, and it will not take much to wring out some drizzle or light rain. As such, have gone with likely rainfall chances for very light rainfall amounts as this low pressure moves by. The greatest chance for accumulating rainfall will be toward the south coast of New England, which will be closest to the offshore low pressure. GFS and NAM have similar ideas, but keep the northern edge of their respective precipitation shields slightly farther south. Until the drier air arrives late today, will have to contend with areas of drizzle and fog. Not expecting much sunshine this morning, but with winds expected to be more light northerly, should see some breaks this afternoon across the western half of southern New England. That should be enough to bump temperatures up toward 60 degrees in the CT River valley. Elsewhere, temperatures will remain well into the 50s, lowest near the coasts. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... High pressure starts to arrive tonight, but really does not move overhead until Sunday. Strong drying in the mid levels will eventually be in place, too. Anticipating decreasing clouds through the night, with plenty of sunshine for Sunday. Near normal temperatures tonight and Sunday. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... */ Highlights... - Rain Monday through Wednesday, nuisance/river flooding concerns - Accompanying persistent E onshore flow, possible coastal issues - See-saw of airmasses for the late week into weekend timeframe - With any attendant frontal boundaries, chances for wet weather */ Overview... Still favoring a sub-tropical connection ahead of deeper troughing over the N-Central Pacific weighted by MJO signals towards phase 8. This acts to heighten the H5 ridge pattern over the NW Pacific and subsequently amplifies the downstream H5 trough-ridge pattern over the CONUS with the ridge setup by late next week over the E CONUS into the NE Atlantic. Thus two things to consider: 1.) a measure of subsidence across our region given +2 standard deviations (SD) of H5 height anomalies advertised per ensemble means, and 2.) warmer than average temperatures aloft signaled by ensemble mean H85 temperature anomalies on the order of +15C. However not everything is as clear- cut as it may seem. With ridge amplification comes perseverance of surface high pressure N/E downstream of H5 troughing and surface low pressure over the Central CONUS. Subsequent onshore flow, likely will end up cooler until synoptics push any ridge axes E allowing for S flow ahead of low pressure centers and trailing cold fronts. Good global model agreement overall and will weigh a consensus blend with this forecast. Will hit upon any targets of opportunity in the discussion below. */ Discussion... Sunday night through Monday... Squeeze play. Cold front pushing S as low pressure becomes organized off the SE CONUS. An W-E corridor of drier air emerges beneath mid- level ridging at H85. The cold front from the N subsequently stalls across the NE CONUS, mainly N of our region, becoming diffuse with time as it slowly lifts back N as a warm front. Should end up being mild with light S winds as clouds lower and thicken from the S towards Monday evening ahead of the SE CONUS low. Chilly Sunday night with an opportunity of radiational cooling. Seasonable Monday, highs up around the low to mid 60s. Monday night through Wednesday night... Will see a fair amount of rain associated with a cutoff low lifting N out of the SE CONUS with a decent sub-tropical connection. All guidance succinct on low-mid level features lifting N/E out of the SE CONUS yet weakening in the process ahead of deepening H5 trough upstream. Associated anomalous features per ensemble guidance with both +1 SD precipitable waters and E onshore flow N of the low. Up against a N/E wedge of a 1035 high, there are two main concerns: 1.) A prolonged period of rain beginning Monday night persisting through Tuesday night. Decent sub-tropical anticyclonic branch of the warm-moist conveyor belt and high theta-E air isentropically ascending and frontogenetically converging up against a N/E wedge of high pressure within the low to mid levels. Heightened more so with the crux of the H925-85 wind max around early Tuesday thru Wednesday morn. No instability indications, however given such forcing of low to mid level 1.0 to 1.5 inch precipitable waters along W-E banding, may need to monitor with respect to minor river flooding and perhaps some nuisance issues. While a majority of forecast guidance has the mid-Atlantic progged for heavy rain, there are subtle hints of rain- fall amounts of around an inch within a 6-12 hour window. Can see some slow movement of the system indicated by initial 0-6 km winds ahead of the low as +10C H85 dewpoints converge across the region with the H925-85 flow. This before everything races out late in the period with the low lifting back N. Categorically high PoPs. Will prevail with moderate rainfall. Can`t rule out some embedded heavier showers. Do not see any indication of thunder. Will advertise heavier storm total rainfall amounts than the previous forecast with areas S/E having the greater chance of seeing rainfall amounts exceeding an inch, considering upslope flow off the ocean and orographic lift along the E slopes of high terrain. 2.) Persistent onshore E flow beginning Monday night thru Wednesday. Again the crux of faster low-level winds beginning early Tuesday through Wednesday morning. Decent inversion around H95 will limit mixing of faster momentum down to the surface. However the driving component will be the gradient winds. Focusing on tides late Monday through Wednesday with Boston... 10.88 feet / Monday 10:33 pm 10.87 feet / Tuesday 10:59 am 11.47 feet / Tuesday 11:20 pm 11.15 feet / Wednesday 11:50 am With a surge of 0.5 to 1.0 foot brought about by persistent E flow, particularly at the time of the Tuesday night high tide, there could be some splashover issues along vulnerable E MA roadways. An initial guess for now to which we`ll likely have a better handle and greater confidence as we get closer to the event. Will continue to highlight the threat of possible coastal issues in the hazardous weather outlook. Thursday and beyond... See-saw of airmasses. A brief warm-up and drying out on Thursday prior to a sweeping cold front late with some light showers. Fairly decent dry punch within the mid-level advertised as mid to upper level heights remain stable or rise ahead of deeper troughing out to the W. Weak high pressure for Friday, will likely be mild with light winds and available sunshine, however cooler along the shores with expected sea-breezes. The cold front lifts back N as a warm front Saturday into Saturday night. E onshore flow out ahead will likely result in temperatures near-seasonable for the day. Rising dewpoints and some weak lift associated, could squeeze out some light showery weather along the warm front prior to entering the warm sector. Sunday looks to be very warm out ahead of a stout cold front pushing through Sunday night into Monday as the H5 trough-ridge pattern shifts E. && .AVIATION /13Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/... 12z update... Today...Moderate confidence, mainly due to timing issues. Widespread IFR CIGS early this morning. CIGS improving to VFR along and west of the CT River by late afternoon. IFR likely to persist along the east coast of MA, with patchy MVFR CIGS in between. Light rain likely south of a line from KGHG-KSFZ-KIJD. Elsewhere, mainly dry except for areas of drizzle this morning. N winds, breezy along the E coast with gusts up to 20 kts. Tonight...Moderate confidence. IFR initially E of MHT-PVD-BID terminals lingering towards roughly midnight and then lifting and eroding S/E. Clearing back over the W. N winds diminishing. Sunday...High confidence. VFR. Winds becoming S while remaining light. KBOS TAF...Continued high confidence in trends, but moderate confidence on exact details, including timing. KBDL TAF...High confidence in trends, moderate confidence in timing. Outlook /Sunday Night through Wednesday/... Sunday night into Monday... VFR. Lowering and thickening clouds from the S. Expect low-end VFR along the S-coast by Monday evening. Light S winds turning E. Monday night through Wednesday night... Cigs lowering to a mix of MVFR / IFR with -RA/RA. Likely visibility impacts with RA. Higher confidence of lower conditions over the high terrain and across the S/E coastal plain terminals. Increasing E flow with sustained winds around 10 to 15 kts along the coast with gusts up to 30 kts possible. Height of the winds Tuesday into Tuesday night. Winds becoming light Wednesday into Wednesday night turning gradually S. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/...High confidence. Today... Rough seas expected to continue across the outer coastal waters as a low pressure passes by to out south. Small Craft Advisories will continue. Tonight and Sunday...Moderate confidence. High pressure gradually moves over the waters, particularly on Sunday. Light winds overall. Lingering swell will occur on the eastern outer coastal waters. Tweaked end time of Small Craft Advisories based on latest expectations. Outlook /Sunday Night through Wednesday/... Sunday night into Monday... Weak area of high pressure stretched W to E across interior S New England and out across the E waters. Winds will mainly southerly around 5 to 10 kts. Little wave action. Monday night through Wednesday night... Coastal low slowly approaching from the S expected to sweep N/E over the S/E waters Wednesday night into Thursday morning. It is ahead of this low that E flow will become persistent through Monday night, the height of the winds Tuesday into Tuesday night with sustained flow around 10 to 15 kts with gusts possibly as high as 30 kts. This will result in wave action over the outer waters to build up around 7 to 9 feet. SMALL CRAFT ADVISORIES will likely for Tuesday into Tuesday night. Both wind and wave action diminish Wednesday into Wednesday night with the core of the low passing over the waters. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 11 AM EDT Sunday for ANZ250-254-255. Small Craft Advisory until 9 AM EDT Sunday for ANZ251. && $$ SYNOPSIS...Belk/Sipprell NEAR TERM...Belk/KJC SHORT TERM...Belk LONG TERM...Sipprell AVIATION...Belk/Sipprell MARINE...Belk/Sipprell

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