Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 230811 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 411 AM EDT Sun Apr 23 2017 .SYNOPSIS... Weak high pressure builds across the area today and Monday providing dry and warm weather. A coastal storm slowly tracks up the eastern seaboard Tuesday and Wednesday bringing periods of moderate to heavy rain along with gusty winds and the risk of coastal flooding to the region. A drier trend along with warmer temperatures is forecast Thursday into early next week with chances of showers and thunderstorms at times. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 4 AM Update... Clearing trend continues from northwest to southeast with back edge of the cloud shield across CT/RI and eastern MA. By sunrise back edge of clouds should be over Cape Cod and the Islands. As mid level trough moves offshore early this morning, anticyclonic mid level flow overspreads the area resulting in strong subsidence and mid level drying. This will promote abundant sunshine today and blyr mixing above 850 mb. Given this leaned toward the warmer MOS guid and even added a few degs. This brings highs this afternoon to 65-70 away from the coast. Will feel even warmer given light winds. Spectacular spring weather! Enjoy. Weak pres gradient results in seabreezes keeping the coastline in the MU50s. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/... Tonight... High pres remains in control with dry/quiet weather continuing. Good radiational cooling with high pres overhead. Thus leaned toward the cool MOS temps. Could have some patchy fog late. Monday... Dry weather continues as mid level confluent flow provides subsidence across the region. Morning sunshine will likely fade behind increasing mid and high cloudiness on the southern horizon. Nonetheless still very pleasant with highs in the MU60s inland and cooler along the shore with seabreezes. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... */ Highlights... - Rain Monday night thru Wednesday night - Accompanying fresh water and coastal flooding concerns - Back and forth pattern late into early next week */ Discussion... Monday night through Wednesday night... Period of moderate, possibly heavy, rain forecast especially Tuesday night into Wednesday morning. In brevity, energy presently sweeping over the SE CONUS lifts back N into the mid-latitude flow ahead of which there is a decent sub-tropical connection. Per GEFS/ECens/SREF there remains anomalous signals on the order of +1-2 SD with respect to precipitable waters and H925-85 winds converging into New England along with increasing probabilities of a decent slug of rain within a 6-12 hour window, again mainly around Tuesday night into Wednesday morning. Subsequent low-mid level ascent ahead of main H5-7 vortmax of rich theta-E sub-tropical air and H85 dewpoints exceeding +10C, with freezing levels pushed above 11 kft making this practically a warm-rain process, very atypical as noted per sounding climatology for Chatham which has a record max of 1.42" 12z Wednesday April 26th when the GFS is forecasting a 1.63" precipitable water. There are a number of concerns and a couple of unknowns discussed below. 1.) Fresh water flooding. Potential 1.0 to 1.5 inches of rain in a 6- 12 hour period centered around Tuesday night into Wednesday morning. Per USGS most streamflows in S New England are within their normal capacity based on climatology. Latest ensemble river forecasts per GEFS/SREF show slight rises but no indications of exceeding minor flood stage. So present thinking is that rivers will remain within their banks, yet there remains a risk of typical nuisance flooding such as ponding of water on roadways, urban / poor drainage issues. We`ve seen a fair amount of rain in the last 72-hours and prior to the heavier rain, beginning Monday night there will be some initial warm advective rains that`ll likely saturate soils leading to more runoff. Focus of risk especially within the I-95 corridor and areas S/E with the upslope onshore flow and signals discerned from CIPS analogs. There is also some additional concern up against the E slopes of higher terrain. 2.) Coastal flooding. Persistent E fetch beginning Monday night and through Wednesday morning with the strongest winds mainly due to the gradient flow between the N/E wedge of high pressure up against the approaching low. Fairly stout inversion above the surface which will limit mixing. Still can`t rule out 0.5-1.0 foot surge brought about by the persistent E flow around 15 to 20 mph sustained with gusts up to 30 mph during astronomically high tides Tuesday into Wednesday, the Boston tides listed below. Keeping in mind of minor flood stage in Boston is 12.5 feet and we`re dealing with multiple tide cycles, with a 1.0 foot surge, considering minor coastal flood issues with inundation of vulnerable shore roadways in addition to splashover, perhaps even some beach erosion. Particular focus on Tuesday night into Wednesday morning high tides. Boston High Tides... 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 Please see the COASTAL FLOODING section below for additional details which also highlights the threat of poor drainage flooding for coast- line communities during periods of heavy rain coincident with high tides. 3.) Uncertainty. Still some question on the strength and timing of the low center as it lifts up the coast, perhaps complicated by the wedge of 1035 high pressure N/E off of Newfoundland. There is some spread beginning Tuesday night. With this forecast will go with a non-NAM consensus. Still advertise categorically high PoPs as well as coincident moderate rainfall. More confident of embedded heavier showers, but still no indication of instability as we remain capped and thus no mention of thunder. Heaviest rainfall amounts forecast over S/E New England and along the E-slopes of high terrain. Will continue to highlight potential threats within the hazardous weather outlook. As the low passes across S New England Wednesday - Wednesday night there will be a lot of moisture associated trapped within the low levels with a measure of ascent associated with the main mid-level vortmax as winds become light. Practically textbook, expecting a period of low clouds, drizzle, and fog, quite soupy. Gradually improve going into Thursday, clearing. Thursday and beyond... Back and forth pattern. Sweeping cold frontal boundaries ahead of which warm-moist, more unstable air surges N yielding the potential for showers and thunderstorms. Immediately followed by cooler air associated with high pressure out of Canada sliding E, winds turn onshore ahead of a warm front approaching from the S/W lifting back N before another surge of warm-moist, unstable air ahead of another cold frontal boundary. Looking at an active weather pattern with periods of wet weather and unseasonably warm temperatures followed by cool, dry, more seasonable conditions. Going with a blend of the global models, showers and thunderstorms possible Friday, turning cooler and dry into Sunday, return of wet weather into early next week. && .AVIATION /08Z SUNDAY THROUGH THURSDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday/...High confidence. Thru 12z... IFR over eastern MA lifts to MVFR-VFR as NNW usher in drier air across the region. Elsewhere, MVFR will lift to VFR by 12z. High forecast confidence. After 12z... Any lingering IFR-MVFR over Cape Cod and Nantucket at 12z quickly improves to VFR. Elsewhere, VFR with light NW winds becoming west. However near the coastline light NNW winds become onshore around midday. Dry weather prevails. High forecast confidence. Tonight... VFR. Monday...H VFR and light winds. KBOS TAF...High confidence in trends, moderate confidence in timing. Seabreeze developing 15-16z. KBDL TAF...High confidence in trends, moderate confidence in timing. Outlook /Monday night into Thursday night/...Moderate confidence. Monday night through Tuesday 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 15 to 20 kts along the coast with gusts up to 30 kts possible. Height of the winds Tuesday - Tuesday night. Wednesday into Wednesday night... IFR / LIFR cigs with -DZ/DZ. Light winds initially N/E becoming S/W. Thursday into Thursday night... Improving VFR during the day, lowering MVFR / low-end VFR evening into overnight. Increasing SW winds. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday/...High confidence. Sunday... NNW winds 15 to 20 kt over the eastern MA waters relax later this morning and become locally onshore by midday. Dry weather and good vsby prevail. Only issue will be leftover east swells of 4 to 7 ft. Tonight... High pres over the region provides light winds and good vsby with easterly swells continuing to erode. Monday... More of the same, weak high pres over the waters provides light winds, dry weather and good vsby. Outlook /Monday night into Thursday night/...Moderate confidence. * SMALL CRAFT ADVISORIES likely for a majority of the period. Monday night through Tuesday 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, 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 8 to 10 feet into Wednesday. Rain is also likely and there may be some visibility impacts on the waters. Wednesday into Wednesday Night... While the low passes overhead Wednesday into Wednesday night and winds become light, seas will be at their height Wednesday diminishing into Wednesday night. It is also during the low passage that there will likely be visibility impacts over the waters with drizzle. && .TIDES/COASTAL FLOODING... Persistent E fetch along with astronomically high tide cycles from Monday night through Wednesday, with the height of the winds Tuesday into Tuesday night. Strongest sustained winds, mainly along the E MA coast, of 15 to 20 mph, gusts up to 30 mph. This presents the possibility of a 0.5 to 1.0 foot surge on top of the high astronomical tides. Along with wave action, an increasing confidence of minor flooding impacts, if not at least splashover. Inundation of vulnerable shoreline roads is possible. Wave action is forecast to build through Tuesday and continue into Wednesday with waves offshore of 8-10 feet. Can not rule out some beach erosion along E-facing Atlantic beaches. Boston High Tides... 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 A final word, can`t rule out additional impacts as a result of heavy rain. Combined with high tides there is the possibility of poor drainage issues along all coastlines, including the S coast of MA. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory for hazardous seas until 6 AM EDT Monday for ANZ250-254. Small Craft Advisory for hazardous seas until 2 PM EDT this afternoon for ANZ251-255. && $$ SYNOPSIS...Nocera/Sipprell NEAR TERM...Nocera SHORT TERM...Nocera LONG TERM...Sipprell AVIATION...Nocera/Sipprell MARINE...Nocera/Sipprell TIDES/COASTAL FLOODING...Sipprell

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