Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 150916 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 416 AM EST Thu Feb 15 2018 .SYNOPSIS...
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Dry but much colder weather returns Friday night into Saturday. A fast moving low pressure system will likely bring accumulating snow to much of the region Saturday night into early Sunday morning...but amounts remain uncertain. Above normal temperatures will quickly return early next week with the potential for near record high temperatures Tuesday and possibly Wednesday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/... A nearly stationary boundary continues to provide a focus for a modest frontal wave this morning. Given the strongest baroclinicity rests closer to the S coast, and low-mid lvl moisture flux is W-E parallel to the mid lvl flow, this has kept the bulk of the precip associated with the wave mainly S of the Mass Pike, and this will be the case as it rapidly ends by about 12Z (7am local). However, there is one other issue at hand, given that the early half of the overnight was mostly clear, some decoupling allowed temps to drop below freezing before precip onset around 06Z, and therefore localized light FZRA is occurring but should be ending over the next few hours. The location this seems to overlap most, based on meso-west data is along the Mass Pike corridor of SW MA, so will issue SPS for the localized ice risk. Even though precipitation ends early, WAA following the wave passage is enhanced, allowing H92 temps to warm to near +10C while H85 remains +4C. However, at the same time as WAA continues, so does moisture flux, with PWATs by 00Z about 3 std deviations above normal. This is likely to manifest as low clouds early, trapped beneath the inversion developing. Therefore, in spite of the low lvl warming, mixing is likely to remain limited through the day unless any breaks are observed. In fact, have highs slightly below previous progs given temps this morning are cooler than initially forecast. In spite of this, highs will still be 15-20F warmer than seasonable normals, mainly in the 60s, but shy of the record highs. The low clouds and warm moist airmass will lead to another impact, particularly through the morning. Following the wave passage, sfc dwpts should increase to the upper 30s and low 40s, which is near/above SSTs along the S coast. Given S-SW flow continuing, this could lead to localized dense fog, especially across CT/RI and SE MA which may require advisories. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... Overnight into Fri AM... A stronger slightly more acute wave will slide E from the Great Lakes, attendant to sfc cold frontal passage late. Given high PWATS, around 1.10 inches or nearly 4 std deviations above normal, will likely see more widespread RA during the overnight with this wave passage/fropa. Given the fact that this will be associated with a sfc convergent boundary and high PWATs (K-values exceed 30 as well) rain could be heavy enhanced by SLIs near 0 and high TTs. Will also include a very low risk for TS, mainly across the S coast where these convective indices are highest. Widespread QPF is mainly between 0.25-0.50, mainly because the overall system/wave is progressive. However, given the factors suggesting brief heavier downpours, would not be surprised if localized spots exceed reach 0.50-1.00. Overall, a mild wet night for Feb standards. Rain should gradually end from NW-SE after sunrise, as drier air W winds push in from the W. Remainder of Fri... Dry air and CAA follow the cold frontal passage. This will allow drier air and clearing through the day. The coldest airmass lags, so highs, especially with afternoon sunshine will remain mild, in the 40s-50s. Given these anomalously warm sfc temps, afternoon mixing should also easily tap strong westerlies 30-40 kt at H92, yielding windy conditions through the afternoon. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
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Highlights... * Dry but turning much colder Fri night/Sat * Accumulating snow likely Sat night/early Sun AM but uncertainty on amounts/location and thermal profiles * Period of rain showers likely Mon afternoon/evening * Near record high temperatures likely Tue and possible Wed Details... Friday night and Saturday... Dry but much colder moves in behind the front Friday evening on blustery northwest winds. High pressure building in from the west will allow winds to diminish later Friday night and especially toward daybreak Saturday. This will allow low temps to drop into the teens across many locations...while lower to middle 20s will be common in downtown Boston/Providence as well as the Cape/Islands. High temps on Saturday will reach well into the 30s...but light winds/sunshine will take the bite off the chilly temperatures. Saturday night... Accumulating snow appears likely Sat night...but considerable uncertainty remains regarding amounts/locations and if there are ptype issues on the coastal plain. Shortwave energy approaching from the west will combined with surface low lifting northeast out of the southeast states. The issue is that the upper level flow is very progressive and given the tight packing in the height fields...the track of this fast moving system is quite uncertain. However...given the system/s origins it will be loaded with moisture so a fast moving snowstorm is a possibility if it tracks close enough to the coast. The strength and amplification of the shortwave energy will determine how close to the coast this fast moving low pressure system tracks. The individual ensemble runs remain more important at this stage of the event than the specific operational runs. Unfortunately...they continue to show a great deal of spread given the overall pattern. Solutions range from a miss/glancing blow to a fast moving snowstorm. In fact...given that the high is moving away from the region and antecedent airmass is not spectacularly cold if the system comes too close to the coast some ptype/boundary layer issues would be a concern especially on the coastal plain. In a nutshell...while a complete miss can/t be ruled out that looks to be a lower probability at this point given upper air pattern and ensembles. We will say that accumulating snow appears likely...but amounts remain uncertain along with potential ptype issues on the coastal plain and especially Cape/Islands. Sunday... What ever happens with this system...the bulk of the precipitation will have come to an end by daybreak Sunday. So if we do end up with a fast moving snowstorm...it will be lower impact than usual given the timing of the event. The one thing will have to watch is if the storm follows the more amplified ECMWF and results in a swath of 6+ inches of wet snow along the coastal plain...there would be the low risk for isolated power outages. Otherwise...temperatures on Sunday will rise into the 40s so travel will be fine by late Sunday morning/afternoon. Monday... A warm front will approach the region and probably result in a period of showers sometime Monday afternoon and/or night. High temps Monday should be mainly in the 40s. Tuesday/Wednesday... Very anomalous upper level ridge over the southeast coast builds northward bringing above to well above normal temperatures to southern New England. Just how mild temps get will depend on the amount of sunshine and whether we eventually get shallow cool air to slip south...which can often happen this time of year. The guidance is in pretty good agreement in near record high temps in the 60s Tue and perhaps again on Wed. Lower confidence on Wed given the potential for a backdoor cold front...but if it does not come through there would even be a low risk for temps to exceed 70 in a few locales. However...on the flip side of the coin a backdoor cold frontal passage could result in Wed highs in the 40s to lower 50s.
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&& .AVIATION /09Z THURSDAY THROUGH MONDAY/...
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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 Friday/... Through 17Z...Moderate confidence. VFR (except ACK, which will remain IFR/LIFR) through 07Z, with MVFR/IFR conditions in rain and fog mainly S of the Pike through 11Z. After this the rain will likely be replaced by fog, with IFR vsbys/CIGS possible mainly S of a line from BDL-PYM. Further N, mainly VFR through this period, although occasional MVFR CIGS possible. 17Z - 03Z... Mainly VFR N of the BDL-PYM line with a mix of MVFR CIGS S of this line. Occasional BR mainly S of the line as well. 03Z tonight through 10Z Fri... Widespread light-mod RA. Mainly IFR CIGS, MVFR VSBYs with the RA/FOG. Winds shift to the W early AM. Fri daytime... Lingering early AM RA/BR dissipates after sunrise. This allows IFR/MVFR conditions to gradually shift to VFR all terminals through the day. W winds gust 25-30 kt during the afternoon. KBOS Terminal...Moderate confidence in TAF. Main issue is the possibility of some brief MVFR conditions this morning. KBDL Terminal...Moderate confidence in TAF. Brief period of MVFR/IFR conditions possible this morning in both SHRA/BR. Outlook /Friday Night through Monday/...Moderate Confidence. Friday Night: VFR. Windy with gusts to 30 kt. Saturday: VFR. Saturday Night: MVFR/IFR conditions possible. SN likely perhaps mixed with RN across portions of the coastal plain. Sunday: VFR. Breezy. Sunday Night: VFR. Washingtons Birthday: Mainly VFR, with areas MVFR possible. Breezy. SHRA likely.
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&& .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 Friday/...High confidence. Through early Fri... Although S-SW winds remain below 25 kt through the period, it is persistent, leading to building swells 5-8 ft late tonight into early Fri on the S waters. Small craft advisories will be issued for this risk and will continue until the westerlies develop early Fri. Otherwise, fog on the waters today and rain overnight will lead to visibility restrictions. Fri... W winds increase, and although seas may dip initially because of this, wind gusts are likely to reach 35-40 kt by the late afternoon. Gale Watches have been issued. Outlook /Friday Night through Monday/...Moderate to High Confidence. Friday Night: Strong small craft to gale force winds with gusts up to 35 kt. Gale Watches hoisted for our eastern waters...where confidence is highest. Rough seas up to 11 ft. Saturday: Winds less than 25 kt. Local rough seas. Saturday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Rain, snow. Local visibility 1 nm or less. Sunday: Winds less than 25 kt. Slight chance of rain showers. Sunday Night: Winds less than 25 kt. Washingtons Birthday: Winds less than 25 kt. Chance of rain showers.
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&& .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Gale Watch from Friday afternoon through late Friday night for ANZ231-250-251-254. Small Craft Advisory from 7 PM this evening to 4 PM EST Friday for ANZ235-237-254. Small Craft Advisory from 4 PM this afternoon to 4 PM EST Friday for ANZ255-256. && $$ SYNOPSIS...Doody/Frank NEAR TERM...Doody SHORT TERM...Doody LONG TERM...Frank AVIATION...Doody/Frank MARINE...Doody/Frank

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