Area Forecast Discussion
Issued by NWS Boston, MA

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562 FXUS61 KBOX 252100 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 400 PM EST Sun Feb 25 2018 .SYNOPSIS... Rain will rapidly be ending from west to east through early this evening...but fog and drizzle will linger a bit longer. High pressure will bring dry weather with cool nights and mild afternoons through mid week. The potential continues for a significant coastal storm late this week with several possible weather hazards...but it is not a certainty this far out. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/... Remaining WWAs will expire on time as precip comes to an end this afternoon. Strongest mid-lvl f-gen and moisture continues to dissipate as of the time of this writing, and as such the the precip shield should continue to shift offshore through 22Z. However, as modest subsidence in advance of building ridge/high pres associated with the mid and upper drying is likely to trap moisture in the lowest 100-150hPa for much of the evening and earl overnight hours, especially as long as the flow remains easterly. Therefore, even as precip ends, widespread fog/drizzle and low clouds will linger well into the early portions of the overnight, with the risk for patches of dense fog. Stay tuned for special statements or advisories as this develops as it will not be dense everywhere, especially where N ageostrophic flow becomes strongest this evening particularly in the CT Valley. Otherwise, will need to monitor early AM temps/low lvl moisture. Given the lingering clouds and moisture suggesting little change in overnight dwpts from the upper 20s to low 30s, this may be enough to limit early AM temperature drop, keeping 2m temps above freezing. However, as some drier air is introduced early with winds backing to the N-NW a slight dip is possible, coincident with damp sfcs. Therefore, may have a risk for early AM black ice depending on timing of the temp drop and how long the moisture trapped beneath the sfc remains. Min temps by sunrise could be in the upper 20s and low 30s, coldest across NW MA. Stay tuned as this may impact portions of the AM commute, but it is reliant on timing and microscale processes. && .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... Mon... Low and mid lvl baroclinicity wedged between the low pres moving E of the Maritimes and sfc 1030hPa high pres slackens through the early AM hours. This both raises the H92 temps in the NW interior and cools these same temps across the SE, yielding a nearly uniform +1C to +2C. Early AM lingering clouds should dissipate through mid day, so expect that mixing should reach and more likely exceed H92, allowing temps to reach the mid-upper 40s. Highs may peak out in the low 50s however across portions of E MA/RI thanks to a W-downsloping component to the flow. Overall dry but a bit breezy with winds gusting to 20mph at times. Mon night... High pres in control with continued clearing overnight. Although still not perfectly ideal for radiational cooling as pres gradient remains elevated enough to keep some light winds at the sfc. Only caveat may be the more sheltered valleys of NW MA. Seasonably cool overnight, mins mainly in the upper 20s and low 30s. Slightly cooler where isolated decoupling occurs. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Highlights... * Dry/mild with above normal temperatures Tue and especially Wed * Significant coastal storm possible late this week with several "potential" weather hazards but it is not a certainty this far out Details... Tuesday and Wednesday... Westerly flow aloft with above normal height fields coupled with a surface high pressure to the south will keep our weather dry and mild. High temps should reach into the upper 40s to lower 50s Tue afternoon. Even milder temperatures on Wed as the high moves off the mid Atlantic coast allowing high temps to recover well into the 50s to near 60...away from the cooling marine influence of the south coast. Thursday through Saturday... The potential continues for a significant/long duration coastal storm late this week...but there remains plenty of uncertainty. This event is still 4-5 days out in the model world...where track errors over 150 miles are quite common. really is impossible to lock in any particular solutions or outcomes. Nonetheless...there are several ingredients in place that still have us concerned for the threat of a significant/long duration coastal storm with several weather hazards possible. What exactly unfolds late this week will have to do with timing/strength of many features. A couple main players involve shortwave energy emerging from the southwest and another across the north central plains. Their specific strength/timing will have a large impact on the developing storm that will occur in the Ohio Valley. A strong -NAO/Greenland Block should prevent the storm from cutting to our west and force it least secondary coastal development. As previous forecaster mentioned...a highly anomalous -NAO can sometimes suppress the storm track well south of our region that is not always the case. There have been plenty of big storms with very strong -NAO/s...but a lot of that is a function of where the block sets up along with where exactly the storm develops and intensifies. While it is way too early to get into specifics about this potential strong coastal storm...there are several possible weather hazards. These include significant amounts of rain and/or snow depending on thermal profiles...which are impossible to say at this point. If ptype favored rain there would be a concern for fresh water flooding. A period of strong to damaging winds especially along the coast. The GEFS ensembles are already indicating an easterly LLJ 4+ standard deviations above normal at 12Z Friday...which is quite unusual to see in an ensemble 120 hours out. Lastly...a persistent and potentially strong onshore flow may result in significant coastal flooding/beach erosion during several high tide cycles along the eastern MA coast given high astro tides. In a nutshell...the potential hazards listed above are if this storm comes to fruition but we are still 4 to 5 days out. If shortwave timing/strength changes a weaker storm suppressed further to the south could spare the region many issues. Nonetheless...this system does have significant potential and bears watching. Sunday... Low confidence this far out...but looks like bulk of the precipitation will have departed with seasonable temperatures. && .AVIATION /21Z SUNDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/... Through 21Z...High confidence. Mainly a mix between MVFR and upper end IFR with RA/fog and lowered CIGS. The RA/fog will dissipate initially late afternoon. 21Z this evening through 10Z Mon morning...Moderate confidence. Evening and early overnight, expect IFR/LIFR conditions in fog, especially as winds remain 100-040. These winds will be backing toward the NW during the early morning hours, which should allow improvement to MVFR/VFR although timing may be off a bit in TAF. Some sites, especially those typically prone to fog, may stay IFR/LIFR longer. After 10Z and into Mon night...High confidence. Winds settle NW around 10 kt. Lingering IFR/fog conditions dissipate through the early AM, with VFR then remaining through Mon night. KBOS Terminal...MVFR in RA/BR gives way to some IFR CIGS/Fog this evening. Once winds shift mainly to the N-NW between 04Z- 06Z, conditions should improve to VFR then remain. High confidence in these trends, but lower confidence in timing. KBDL Terminal...Moderate confidence. Lingering RA/BR keeps conditions mainly MVFR through 21Z. Then with winds shifting N, then NW late, lingering BR should dissipate, with a trend toward VFR after. Timing somewhat uncertain, as some lower ground fog cloud form as CIGS rise. Outlook /Tuesday through Friday/... Tuesday through Wednesday...High confidence in mainly VFR conditions. Thursday: Increasing threat for MVFR to perhaps even IFR conditions as the day wears on Thu...but that will depend on the timing of a developing coastal storm. Thursday Night and Friday: MVFR-IFR conditions likely. Windy with gusts to 40 to 50 knots possible along the coast depending on track/timing/strength of coastal storm. RA and/or SN likely. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/...High confidence. Through tonight... Lingering Gales mainly E waters give way to all small craft advisory conditions this evening. While the winds dissipate and back to the N-NW, seas remain elevated through the early AM hours. Therefore small craft advisories may have to linger through early Mon. Otherwise, fog through the evening expected with reduced vsby. Mon and Mon night... Brief lull in conditions Mon morning. By Mon afternoon, wind gusts should once again reach around 25 kt offshore out of the NW and linger through the evening, seas only build to about 5 ft, but some Small Craft Advisories will likely be needed even for the marginal conditions. Outlook /Tuesday through Friday/... ***Potentially dangerous storm for Mariners late Thursday into Friday with strong winds and high seas which are dependent on the track/strength of a potential strong coastal storm*** Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft. Tuesday Night: Winds less than 25 kt. Wednesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Wednesday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Thursday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain. Thursday Night and Friday: Gale to storm force wind gusts between 40 and 50 knots may develop depending on track/strength timing of coastal storm. Seas may approach or exceed 20 feet! && .TIDES/COASTAL FLOODING... Thursday through Saturday... The potential exists for significant coastal flooding/beach erosion over several high tide cycles late this week...particularly along the eastern MA coast. While this is far from a certainty given the potential event is 4 to 6 days in the future...there are several ingredients in place coupled with high astronomical tides that this will need to be watched closely. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory until 7 PM EST this evening for ANZ230>234. Small Craft Advisory until 10 PM EST this evening for ANZ235- 237. Gale Warning until 7 PM EST this evening for ANZ250-254. Small Craft Advisory until 7 AM EST Monday for ANZ251-255-256. && $$ SYNOPSIS...Frank/Doody NEAR TERM...Doody SHORT TERM...Doody LONG TERM...Frank AVIATION...Frank/Doody MARINE...Frank/Doody TIDES/COASTAL FLOODING... is the U.S. government's official web portal to all federal, state and local government web resources and services.