Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 222034 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 434 PM EDT Sat Apr 22 2017 .SYNOPSIS... Low pressure moves out to sea tonight with high pressure building into New England late tonight through 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 AM SUNDAY MORNING/... Area of rain will gradually push south and move off the coast late this afternoon and evening as forcing for ascent moves offshore and some mid level drying comes in from the north. Lingering light rain possible this evening along the south coast and especially the islands. Otherwise a dry night with clearing developing from west to east late tonight as mid level shortwave moves to the east and high pres builds in from the west. Low temps will settle into the mid 30s to around 40 degrees. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/... Sunday... High pres in control will bring sunshine, light winds and seasonably mild temps. Seabreezes are likely along the coast during the afternoon. Highs will range from the upper 50s along the immediate coast to the upper 60s in the CT Valley. Sunday night... High pres remains in control. Clear skies and light winds although some increase in high clouds from the south is expected overnight. Good radiational cooling with lows ranging from mid 30s to lower 40s. && .LONG TERM /MONDAY THROUGH SATURDAY/... Overview... Development of H5 cutoff low pres off the Carolinas early next week will slowly progress northward up the eastern seaboard while high pres ridge noses W-SW across central New England through around Wed night. Will likely see periods of precip and gusty onshore winds. Further west, noting digging H5 long wave trough moving slowly SE out of the eastern Rockies into the Plains states. This will develop a responding downstream SW flow as ridging builds over western Quebec into eastern Ontario by late next week. This should help to lift the east coast cutoff low, but timing is in question. The upper ridging over eastern Canada looks to be short lived as the midwest upper trough shifts slowly eastward late next week. Southern stream ridge builds off the Carolinas/Florida coast, so SW flow aloft should remain in place. Model solution spread increases late next week into next weekend as to where the approaching front may push, or stall in the upper flow if the E coast ridging up the eastern seaboard. */ Discussion... Monday... A cold front will slowly push S as low pressure becomes organized off the SE U.S. coast. 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: Will see a prolonged period of rain beginning Monday night persisting through Tuesday night. Decent subtropical 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. Will also see persistent onshore E flow beginning Monday night thru Wednesday. This, along with building onshore seas and swells, as well as astronomical high tides, will have to monitor potential for possible coastal issues during this timeframe. Details are in the Tides section below. 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 /20Z SATURDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday Night/... Through 00z...IFR/MVFR cigs improving to VFR CT valley late afternoon. Rain focused near the south coast. Tonight...Moderate confidence. MVFR/IFR along and E of FIT-ORH-PVD with VFR cigs to the west, improving to VFR late tonight with clearing from west to east. However, areas of IFR persisting over Cape/islands. Sunday...High confidence. VFR, except MVFR/IFR stratus lingering over outer Cape/Islands in the morning. Seabreezes developing. Sunday night...High confidence. VFR. KBOS TAF...High confidence in trends, moderate confidence in timing. Seabreeze developing 15-16z Sun. KBDL TAF...High confidence in trends, moderate confidence in timing. Outlook /Monday through Thursday/... 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 Night/...High confidence. Tonight and Sunday... Some northerly gusts to 20 kt over the eastern waters this evening, otherwise diminishing winds later tonight and especially Sunday as high pressure gradually builds over the waters. Winds becoming onshore around 10 kt Sun afternoon as seabreezes develop. Lingering swell on the eastern waters will lead to SCA seas which will gradually subside during Sunday. Sunday night... Mainly southerly winds 15 kt or less as high pres gradually moves offshore. Some gusts to 20 kt possible over NE MA waters where seas may build to 5 ft. Outlook /Monday through Thursday/... 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. && .TIDES/COASTAL FLOODING... Continue to see potential of an easterly wind flow beginning late Monday through around midday Wednesday. Could see sustained winds up to 15 to 20 mph with gusts up to around 30 mph, but the question is whether the higher winds above the surface will mix down due to low level inversion. Looks like there could be a chance for the stronger winds to mix down around the late Tuesday into Wednesday timeframe, as the H95 jet of 30-35 kt moves across. Here are the times of heights of the high tide for Boston Harbor... 10.9 feet / Monday 10:33 pm 10.9 feet / Tuesday 10:59 am 11.5 feet / Tuesday 11:20 pm 11.2 feet / Wednesday 11:50 am With a surge of up to 1 foot brought about by onshore flow, particularly at the time of the Tuesday night high tide, there could be some splashover issues along vulnerable E MA roadways. Have continued to mention this potential in the Hazardous Weather Outlook. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory for hazardous seas until 6 PM EDT Sunday for ANZ250-254. Small Craft Advisory for hazardous seas until 2 PM EDT Sunday for ANZ251-255. && $$ SYNOPSIS...KJC/EVT NEAR TERM...KJC SHORT TERM...KJC LONG TERM...EVT AVIATION...KJC/EVT MARINE...KJC/EVT TIDES/COASTAL FLOODING...EVT is the U.S. government's official web portal to all federal, state and local government web resources and services.