Area Forecast Discussion
Issued by NWS Boston, MA

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236 FXUS61 KBOX 242037 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 337 PM EST Sat Feb 24 2018 .SYNOPSIS...
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A fast moving low pressure system will bring a mixture rain...sleet and freezing rain early Sunday morning into Sunday afternoon . The bulk of the sleet and freezing rain will be found across the high terrain and areas north of the Massachusetts turnpike. High pressure will bring mainly dry and mild temperatures Monday through Wednesday. There is the potential for a strong coastal storm late next week that if impacts our region may bring significant rain and/or snow...strong winds on the coast along with potential coastal flooding. This storm is far from a certainty at this point...but given the ingredients in place does bear watching.
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&& .NEAR TERM /UNTIL 2 AM SUNDAY MORNING/...
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Leading weak shortwave associated with a slight increase in column moisture (PWATs shift above 0.5 inches) has led to a modest precip shield which continues to slide across S New England this afternoon. Focus should mainly be through CT/RI and SE MA, as areas N have too much dry air to overcome, and the wave itself will be subject to ridge building into the overnight hours. Therefore, expect mainly light RA across these areas through the early evening hours, at which point the combination of modest subsidence and drier ageostrophic flow should cause this band to dissipate and mainly shift offshore. After this dissipation, flow will shift around to the NE, and potentially N thanks to strong isallobaric flow during the late night/early Am hours. This should allow for enough initial cooling before precip start to yield the wintry mix as advertised. See below for this risk.
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&& .SHORT TERM /2 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/...
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*** Mixed Wintry Precipitation Early - Late Day Sunday *** Overview... Moisture flux shifts from W-E to mainly SW-NE late this evening into early Sun AM, which allows PWATs to reach near 1.00 inches by 15Z Sun, which is nearly 3 std deviations above normal. This should provide plenty of moisture for overrunning ahead of a stalling warm front. Meanwhile confluent longwave pattern undergoes a gradual shift thanks to deepening wave upstream, the result is actually a modest building ridge through the mid lvls, which provides the needed mid lvl warming while temps below H92 remain below -2C through about 15Z. At the same time, it enhances the inverted ridging, as mass fields continue to highlight strong cold air damming signal, which should yield a shift from snow to sleet before a change to all rain as the warm layer fills and lowers closer to the sfc. Isallobaric flow remains strong northerly through at least 21Z. As is often the case, guidance looks to be a bit too warm at the sfc for this signal. Leaned most heavily on the 12Z WRF/HRRR with this update particularly for 2M temps. Icing... Warm layer looks to move in between H85 and H6 while the layer below to the sfc starts between -2C and -4C. This, especially in the lower terrain, suggests more of a sleet profile, so have leaned more heavily on sleet for wx type with this update. In fact could see after a quick change from SN at onset, accumulating sleet. Given the high PWATs and lift, would not be shocked if up to an inch of pure sleet is observed especially N of the Mass Pike. So will expand Winter WX Advisories into NE MA, where this is also possible. Elsewhere, mainly closer to the SE coast and RI, 2m temps should warm sufficiently above freezing for mainly straight RA. Meanwhile higher terrain, as the warm air fills and descends will change to RA with marginal 2m temps near or just below freezing. Two mitigating factors for ice accretion are these marginal ambient temps and the fact that warm conditions leading up to the event should keep actual sfc temps above freezing. In either case, whether icing due to sleet or freezing rain, travel would be hazardous with the risk for scattered power outages possible. One area of concern, especially since as mentioned above, guidance is not handling the damming signal well, are the deeper valleys such as the CT valley, which could remain colder longer thanks to N ageostropic flow. Timing... Precip onset (as RA mainly for S CT/RI and SE MA, mix of snow/sleet elsewhere) occurs between 2AM and 5AM from SW-NE, such that precipitation should be occurring everywhere by 6AM. The gradual transition from S-N from mixed wintry precip will then occur between 6AM and noon, with areas of the Worcester Hills and Berkshires remaining freezing rain until the precipitation ends between 2PM and 6PM tomorrow. Uncertainty... Given this is a mixed precipitation event and temps may hover very close to the freezing mark through the daytime hours on Sun, areas where freezing precip, and accumulation of ice/sleet or snow are likely to change somewhat, even as the event is ongoing. In any case, expanding the winter weather advisories due to the risk for hazardous travel conditions under ice or sleet, and keeping the rest where they currently reside.
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&& .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... Highlights... * Patchy black ice poss late Sun night/early Mon esp across interior * Mainly dry/mild with above normal temps Mon/Tue and especially Wed * Strong/long duration coastal storm "possible" sometime Thu into Sat but not a lock and timing/ptypes/potential impacts uncertain Details... Sunday night... Any lingering light rain/drizzle along the coast should be coming to an end early Sunday evening as wave of low pressure exits the region. Low clouds will be slow to dissipate...but should see at least partial clearing after midnight. Many locations should see low temps drop into the 20s...to the lower 30s. This coupled with light winds may result in the risk for some patchy black ice forming...especially across the interior late Sun night into the early Mon am commute. Monday through Wednesday... Westerly flow aloft with above normal height fields are anticipated Monday through Wednesday. This will combined with a surface high pressure system to our south...resulting in mainly dry/mild weather with above normal temperatures. High temperatures should mainly be in the upper 40s to the middle 50s Mon/Tue...but as the high shifts off the coast may see high temps reach well into the 50s to near 60 by Wed. Thursday through Saturday... A strong/long duration coastal storm is possible late next week...but considering the time frame there is significant uncertainty if this transpires. We will be watching a couple pieces of shortwave energy. The first will be from the southwest U.S. with a second piece of northern stream energy. Timing/strength of this shortwave energy means everything and obviously uncertain in this time frame. The guidance does show the potential for a rather strong storm to develop. As mentioned above...while plenty of uncertainty exists with this "potential" strong coastal storm there are some ingredients in place that we have not seen in quite sometime. The first is a very strong -NAO depicted to be 4-5 standard deviations above normal over this time range by the GEFS/EPS. In addition...the GEFS anomalies are showing an upper level ridge 3+ standard deviations above normal across Greenland. A very strong -NAO/Greenland Block often prevents a storm from cutting to our west but can force them eastward and lead to the development of strong coastal storms. However...timing still has to be ideal and there certainly is a risk that the system gets suppressed to our south. Nonetheless...it is also possible we see a long duration/strong coastal storm late this week. Ptypes would be quite uncertain at this point...but a significant precipitation event would be possible in the form of rain and/or snow. Strong winds would also be a concern on the coastal plain...bringing the potential for coastal flooding along the eastern MA coast given rather high astronomical tides. In a nutshell...it is too early to say much more other than the "potential" for a significant coastal storm late next week but it is far from a lock given the time range. Nonetheless...given the ingredients in place it certainly bears watching. && .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/... Through 06Z tonight...High confidence. Mainly VFR, although brief MVFR vsbys are possible in -SHRA at terminals S of the Mass Pike late this afternoon/evening. Winds NW will shift around to the E-NE late this evening. After 06Z through Sunday...Moderate confidence. Precipitation shield begins to build SW-NE mainly between 08Z and 12Z. Initially, mix of SN/PL possible, with gradual transition to PL/FZRA after 10Z though 16Z from S-N, with all RA expected eventually outside of the highest terrain. Timing may be off a a bit in TAF, and this will be refined through the overnight updates. Mainly IFR/LIFR as the precipitation gets going, both due to low CIGS and vsbys. KBOS Terminal...Moderate confidence, mainly due timing uncertainty. VFR into early AM, then gradual reduction to IFR as precipitation moves over the terminal. Will likely see some PL mixing in with the RA for the first 2-4 hrs before a change to just rain until the precipitation ends by late Sun afternoon. Wind gusts during the afternoon tomorrow 20-25 kt out of the E. KBDL Terminal...Moderate confidence, mainly due timing uncertainty. Precip may start early Sun AM as mix of SN/PL then transition to period of PL/FZRA before becoming all RA. Timing of this in TAF may change. Mainly IFR once precipitation begins. If Winds remain predominantly N, PL/FZRA lingers longer. Outlook /Sunday Night through Thursday/...Moderate to high confidence. Sunday Night: Improvement from west to east overnight...but some ground fog may develop resulting in localized MVFR/IFR continuing through daybreak Monday. Monday through Wednesday: Mainly VFR Thursday: Highly dependent on track/timing of potential coastal storm but conditions may deteriorate to MVFR/IFR thresholds in rain and/or snow sometime Thu or Thu night. && .MARINE...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/... Overnight...High confidence. Light rain on the southern waters this evening. Winds will shift from W-NW to NE by the early morning. Rain overspread during the early AM hours. Otherwise, winds/seas mainly below headline criteria until early Sun morning. Early Sun morning into Sun evening...High confidence. Marginal E-NE Gales on the waters between 12Z and 21Z from W-E. Small craft advisory conditions on the near shore waters. These small craft conditions linger into the overnight as seas will build to 8-10 ft on the SE outer waters by late afternoon, and will take some time to diminish thereafter. Otherwise, rain/fog leads to reduced vsbys. Outlook /Sunday Night through Thursday/...Moderate to high confidence. Sunday Night: Winds diminish below small craft headlines Sunday evening...but lingering swell will keep seas above thresholds across the outer- waters into the overnight hours. Monday: Winds less than 25 kt. Seas up to 5 ft. Monday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. 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. Thursday: Highly dependent on timing/track of potential coastal storm. However...increasing ENE winds and building seas will become a concern sometime Thu and especially Thu night.
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&& .TIDES/COASTAL FLOODING... Thursday through Saturday... The potential for coastal flooding and beach erosion exists on several high tide cycles late next week. This will depend upon the development of a potential strong/long duration coastal storm. While it is too early to give more specific information...high astronomical tides and the potential for a strong coastal storm will need to be watched especially along the eastern MA coast. && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Winter Weather Advisory from 1 AM to 1 PM EST Sunday for CTZ002>004. MA...Winter Weather Advisory from 1 AM to 1 PM EST Sunday for MAZ002>012-014-026. RI...None. MARINE...Gale Warning from 10 AM to 4 PM EST Sunday for ANZ231>234-250. Small Craft Advisory from 6 AM to 5 PM EST Sunday for ANZ230. Small Craft Advisory from 6 AM to 1 PM EST Sunday for ANZ236. Gale Warning from 7 AM to 1 PM EST Sunday for ANZ235-237. Small Craft Advisory from 6 AM to 7 PM EST Sunday for ANZ251. Gale Warning from 7 AM to 4 PM EST Sunday for ANZ254>256.
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&& $$ SYNOPSIS...Frank/Doody NEAR TERM...Doody SHORT TERM...Doody LONG TERM...Frank AVIATION...Frank/Doody MARINE...Frank/Doody TIDES/COASTAL FLOODING...

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