Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 170811 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 311 AM EST Sat Feb 17 2018 .SYNOPSIS...
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Fast moving low pressure will bring a plowable snow to southern New England tonight, with the heaviest amounts near and south of the Massachusetts turnpike. The snow will be over by daybreak Sunday with rapidly improving conditions thereafter as temperatures rise well above freezing. A warm front will then approach the region Monday afternoon and night with a period of rain showers likely. Near record high temperatures likely follow Tuesday and especially Wednesday before a cold front brings cooler temperatures by Thursday and Friday.
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&& .NEAR TERM /UNTIL 6 AM THIS MORNING/... 10PM update... Working toward the SKC conditions, only remnant CI across outer Islands at the time of this writing and some CAA/Lake enhanced SC across the E slopes of the Berkshires/Litchfield Hills. However, monitoring pres rise/fall couplet, which is shifting orientation, suggests that winds should shift more to the W over the next few hours, cutting off the Lake enhancement. Therefore, trended sky grids toward SKC. In spite of the clearing the pres rise/fall couplet remains a player at least through the early AM, which will keep lower lvl winds in play, and not allow for radiational cooling to really get going until closer to sunrise. However, in a scenario like this, it would really only take an hour or two for temps to rapidly drop, especially as sfc dwpts continue to drop overnight. Therefore, did adjust temps/mins a bit, suggesting a quick dip around sunrise. Mins are now mainly in the low-mid teens in the normal decoupling spots of NW MA, with ranges from the mid teens to low 20s elsewhere. Typically chilly Feb night. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 AM SUNDAY/... Saturday... Expect mostly sunny skies through midday as the high moves off the east coast, then clouds will quickly increase as the broad mid level ridge also pushes E. Winds will veer to S-SE by late in the day, but will be light. Will start to see good isentropic lift start to approach from the SW as a potent fast moving short wave and associated surface low moves out of the Tennessee Valley to the mid Atlantic coast. With the cold air in place, will start to see light snow develop across the mid and lower CT valley toward sunset. The main show is expected Saturday night...see below. Saturday Night... * Winter Storm Watch expanded onto Cape Cod (for upper Cape) * Quick hitting storm will bring accumulating snows Still watching potential for a quick shot of snow across SNE Saturday night as fast moving southern stream short wave deepens as it tracks just outside 40/70 benchmark. 12z Ensembles favor locations from Route 44 corridor in RI/MA down to South Coast for higher snowfall totals, which is generally in line with previous forecast. Although this is a fast moving system, it is expected to intensify rapidly as it passes SNE as PV fields show tropopause lowering to around 550 mb. It will also inject plenty of southern stream moisture, with origins from Baja California, as currently seen on water vapor imagery. Despite decent agreement in larger scale features, 12z models are showing differences in thermal fields with GFS bringing milder air (925 mb) into much of RI and eastern MA around 09z Sunday, while NAM is much colder and ECMWF is in between, but leans toward colder NAM. Our forecast will trend toward cold side, especially since we think soundings will end up being isothermal near South Coast, so the snow will be able to pile up despite what looks like marginal boundary layer temperatures. Peak of snowfall is expected between 10 pm Sat and 5 am Sun, when we should be under best mid level frontogenesis and maximum lift in snow growth zone. Although it is still a bit early to accurately determine where this will be, it looks like RI and SE MA are favored. We`ll need to watch later model trends to see if things shift a bit north or south, or if it remains where it is now. Since we are still 36+ hours away from this event, and our confidence in where banding will set up is not overly high, we will keep Winter Storm Watches posted and allow next shifts to get a look at high-res guidance/ensembles as we get within the 36 hour window for these sources. We did expand the Watch onto Cape Cod, since we do see an increased potential for 6 inches of snow on the upper Cape. Our forecast has storm totals of 4-6" in the Watch area with as much as 6-8" closer to the South Coast. Farther to the north, accumulating snow is still expected despite lower QPF, since snow to liquid ratios will be higher in the colder air. We think 2-4" is reasonable right now. Conditions should improve quickly early Sunday morning, as snow rapidly exits between 4 and 6 am. && .LONG TERM /SUNDAY THROUGH FRIDAY/...
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Highlights... * Snow ends by daybreak Sun: Rapidly improvement thereafter * Period of rain showers likely Mon afternoon/night with a warm front * Near record warmth likely Tue and especially Wed * Turning cooler Thu/Fri but temps still above normal Details... Sunday... Rapidly departing southern stream shortwave will have allowed the snow to have come to an end by daybreak. Improvement occurs quickly Sunday morning and with plenty of sunshine temps will quickly rise above freezing. In fact..highs should reach well into the 40s by Sun afternoon. Therefore...a good portion of the snow that falls Saturday night will melt by the end of Sunday. Sunday night... A ridge of high pressure builds in from the west resulting in mainly clear skies/light winds. This will allow a good night of radiational cooling with low temps bottoming out in the upper teens and 20s across many locations. This might result in a bit of patchy black ice developing from the snow melt that occurred during the afternoon. Monday and Monday night... Dry weather anticipated Monday morning...but clouds will be on the increase ahead of a warm front. This warm front will likely bring a period of showers sometime Monday afternoon and/or night. High temps Monday should be in the 40s to around 50 as there should be an inversion in place. Temps Monday night will either remain steady or slowly rise given low clouds and increasing southwest flow. Tuesday and Wednesday... Impressive setup for near record warmth Tue/Wed as anomalous upper level ridge of 594 decameters sets up off the southeast coast. The southwest flow/high height fields result in 850T between +12C and +14C in SNE. Good mixing on southwest flow should result in high temperatures well into the 60s both days...away from the cooling marine influence of the south coast/Cape/Islands. In fact...70+ degree readings are possible in some locations on Wednesday based on support from most model guidance as long as enough sunshine is realized. The one fly in the ointment would be if the cold front moves through earlier than expected...which would result in cooler temps than our going forecast on Wed. If current guidance holds steady...many record highs will be broken on Wednesday. As for precipitation chances Tue/Wed...upper level ridging and lack of synoptic scale forcing will result in mainly dry weather. We can not rule out a few spot showers...but again appears much of this time will feature dry weather. Thursday and Friday... While the southeast upper level ridge will maintain itself...strong 1040 mb high pressure system will allow significantly cooler air to work back into the region. While temps will still average above normal...it will be noticeably cooler.
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&& .AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...
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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 Tonight/... Through 00Z this evening...High confidence. VFR. NW winds diminish this morning, then gradually shift to the E then SE through the day today, but remain around 10 kt. Overnight...Moderate confidence. A quick moving winter storm passes SE of New England with -SN developing between 00Z and 02Z across all of New England, then periods of +SN with 1-2in/hr snowfall rates continue from about 03Z-09Z especially across portions of CT/RI and SE MA. Accums on runway expected. The SN then tapers off between 09Z and 12Z from W-E. Peak uncertainty across Cape/Islands, where some rain may mix in. Winds out of the S with a few gusts to about 20 kt across coastal areas. KBOS Terminal...High confidence in TAF overall, timing of lower categories this evening may be off a bit with beginning of light SN this evening. Accumulations on runways can be expected. KBDL Terminal...High confidence in TAF overall, timing of lower categories this evening may be off a bit with beginning of light SN this evening. Accumulations on runways can be expected. Outlook /Sunday through Wednesday/...Moderate to High Confidence. Sunday: VFR. NW wind gusts up to 25 knots. Sunday Night: VFR. Washingtons Birthday: Mainly VFR, with local MVFR possible. Breezy. Chance SHRA. Monday Night: Mainly MVFR, with local IFR possible. Breezy. SHRA likely. Tuesday: Mainly VFR, with areas MVFR possible. Breezy. Tuesday Night: Mainly VFR, with local MVFR possible. Breezy. Wednesday: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA.
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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 Tonight/...High confidence. Tonight... W-NW winds continue with gusts of 30-35 kt, highest across the eastern waters where Gale Warnings continue. Small Craft Advisories in effect on the remaining waters. Good visibility. Saturday... NW winds and seas diminish as high pressure moves across the waters. Winds become light E-SE during the afternoon. Leftover Small Craft Advisories will end by mid morning. Saturday night... Winds become S-SE and increase during the night, gusting up to 25 kt at times on the eastern waters overnight. Small Crafts may be needed. Seas should remain below 5 ft. Visibility will be reduced in rain and/or snow, lowering to 1 SM or less across the eastern waters after midnight in moderate to heavy snow. Outlook /Sunday through Wednesday/...Moderate to High Sunday: High risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of seas approaching 5 ft. Sunday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Washingtons Birthday: Risk for Small Craft Advisory winds with gusts up to 25 kt. Seas locally approaching 5 ft. Chance of rain showers. Monday Night: Risk for Small Craft Advisory winds with gusts up to 25 kt. Seas up to 5 ft. Rain showers likely. Tuesday: Risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Slight chance of rain showers. Tuesday Night: Risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Wednesday: Risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Slight chance of rain showers.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Winter Storm Watch from this evening through Sunday morning for CTZ002>004. MA...Winter Storm Watch from this evening through Sunday morning for MAZ011>013-015>022. RI...Winter Storm Watch from this evening through Sunday morning for RIZ001>007. MARINE...Small Craft Advisory until 6 AM EST early this morning for ANZ232. Small Craft Advisory until 5 AM EST early this morning for ANZ230-233-234. Small Craft Advisory until 7 AM EST this morning for ANZ231- 235-237-251. Small Craft Advisory until 10 AM EST this morning for ANZ250- 254>256.
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&& $$ SYNOPSIS...EVT/Frank NEAR TERM...Doody SHORT TERM...EVT LONG TERM...Frank AVIATION...MD/EVT/Frank MARINE...EVT/Frank

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