Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 130640 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 140 AM EST Sat Jan 13 2018 .SYNOPSIS... Multiple low pressure areas along an approaching cold front will bring heavy rainfall, potential flooding and gusty winds through tonight. A sharp turn to much colder weather occurs rapidly Saturday which will allow rain to end as a bit of light freezing rain and sleet, but more importantly a Flash Freeze is expected across all of southern New England. Mainly dry but cold weather is expected Sunday and Monday. Low pressure diving south from the Canadian Arctic will bring a chance of snow Tuesday and Wednesday. Dry weather may return Thursday and Friday. We will be watching a potential coastal storm that could change that Thursday forecast. && .NEAR TERM /UNTIL 6 AM THIS MORNING/...
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135 am update... 1) Heavy Rain/Flooding And Coastal Wind... Rain continues to impact most of the region very early this morning. The heaviest rain was across southeast New England where an 80+ knot low level jet was tapping into Pwats 4-5 SD above normal. This heavy rain will continue to impact RI/SE MA over the next several hours...bringing the potential for significant urban street flooding. Enough elevated instability is available for a rumble or two of thunder. In addition to the heavy rain...strong low level jet will allow for 40-50 mph southwest wind gusts along and southeast of the BOS-PVD corridor so the wind advisory continues. Another band of decent rain was across western MA/northern CT which was being driven by strong low level frontogenetic forcing. Other than significant urban street flooding...isolated ice jam and river/stream flooding need to be watched. 2) Dense Fog Along The South Coast... High dewpoint air being advected over the cold ocean continue to result in visibility between 1/4 and 1/2 mile along the south coast, Cape and Islands. The Dense Fog Advisory continues in this region through daybreak. 3) Strong Cold Front/Ice Threat In Western MA Toward Daybreak... The strong cold front is now entering extreme western MA and North Adams was down to 41 as of 1 AM...While the rest of the region was still between 55 and 60. Temperatures across western MA will likely drop below freezing by 12z. Since the cold surge initially comes in shallow...there should be a period of freezing rain and sleet across western MA with southwest flow aloft. The bigger issue will be a Flash Freeze that will sweep across the rest of the region Saturday with rapidly falling temps.
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&& .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SUNDAY/... ***Flash Freeze all SNE along with a brief period of freezing rain/sleet, especially interior*** Cold front moves through rest of SNE Sat morning followed by a rapid temp drop. Best chance for a period of freezing rain and possibly some sleet before the precip ends will be in the interior. Lower chances for freezing rain toward the coast as guidance shows precip moving out as colder air rushes in. It will be a race between the colder moving in and precip moving out. However, the greater concern is the flash freeze potential as temps rapidly drop from the 50s to below freezing in about a 3 hour period. Exact timing of the colder air is a bit uncertain due to timing differences in the models and flash freeze timing may be off by a few hours. Our forecast has the flash freeze occurring during the morning in the interior and late morning into early afternoon in the coastal plain. Even with NW winds, the rapid temp drop below freezing and leftover wet surfaces will result in icy spots developing. Temperatures by late afternoon will be falling into the teens and 20s. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Big Picture... Longwave pattern through next Friday shows trough over the Eastern USA and a ridge over the Western USA. Shortwave pattern shows a shortwave over the Canadian Arctic sweeping south this weekend and then digging over the Great Lakes. This trough maintains a progressive axis through Tuesday. The GFS and ECMWF show the trough then forming a closed low along the NE USA coast Wednesday-Thursday. Model mass fields are similar through Wednesday morning, then show differences through Friday. Thermal fields also are similar through Wednesday morning, then diverge. Confidence is high through Tuesday night, then low-moderate the latter half of the week. Details... Saturday night through Monday... High pressure over the Midwest/Plains will be in transition as it builds into New England. The axis starts the weekend NNW-SSE, a cold weather signature, and morphs to a NE-SW axis Sunday before lifting out through the Maritimes Monday. The departing ridge leaves a cold- air damming signature across New England Monday night. Expect dry weather during this time with colder-than-normal temperatures. Temperatures are borderline for ocean-effect clouds/snow. But wind direction turns from the north Sunday and northeast Monday. Moisture/RH cross sections show increasing values below 850 mb. North wind supports clouds and scattered showers on the Cape and Islands Sunday. Northeast wind supports this across East coastal Mass Monday. Temps in the mixed layer support max sfc temps in the 20s to around 30. Dry air will support min temps in the single numbers and lower teens, except a little higher on the Cape/Islands. Tuesday-Wednesday... Low pressure from the Canadian Arctic will sweep south to the Great Lakes Monday, then turn east and cross New England on Tuesday/Wednesday. Timing of fast-moving systems is a challenge, especially 4-5 days in the future. The GFS favors Tuesday, the 00Z ECMWF favors Wednesday. Expect a period of light accumulating snow during this time. Thursday-Friday... Low-moderate confidence, but the GFS and ECMWF show potential for an upper low to close off near the Mid Atlantic coast with upper SSW jet along the Eastern USA coast. The jet axis placement favors development of a coastal low off the Carolinas. The GFS and GGEM maintain enough of a progressive pattern to move the low out to sea, while the ECMWF turns the upper flow from the south and directs the coastal low up the coast to Outer Cape Cod. This latter solution shows enough warm air for rain in Eastern Mass/RI, and possibly all the way to the Berkshires. As it is the minority solution, we opted for the drier GFS forecast. && .AVIATION /06Z 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 10z This Morning...Moderate to high confidence. Low end MVFR to IFR conditions in heavy rain through 10z with locally dense fog along the south coast. LLWS along with southerly wind gusts of 35 to 40 knots expected along and southeast of the BOS- PVD corridor. Strong cold front should be through western MA 10z allowing Ptype to change to -FZRA/IP. After 10z Today...Moderate to high confidence. IFR to MVFR conditions should improve to mainly VFR from west to east this afternoon. Strong cold front reaches the BOS-PVD corridor by 15z and probably off the southeast New England coast by 18z. Most of the light -FZRA/IP should be confined to western and north central MA...but a brief period of spotty -fzdz may occur for an hour or two behind the front across the rest of the region. Main issue will be a Flash Freeze as temperatures tumble below freezing and into the 20s in most locations by mid afternoon. KBOS Terminal...Moderate to high confidence in TAF. KBDL Terminal...Moderate to high confidence in TAF. Outlook /Sunday through Wednesday/... Sunday: VFR. Breezy. Sunday Night: VFR. Martin Luther King Jr Day: Mainly VFR, with local MVFR possible along the east coast of Massachusetts. Breezy. Monday Night: Mainly VFR, with local MVFR possible. Tuesday: MVFR/IFR conditions possible. Chance SN. Tuesday Night: Mainly IFR, with local MVFR possible. Chance SN. Wednesday: Mainly VFR, with areas IFR possible. Breezy. Chance SN.
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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 Sunday/... Tonight...Moderate confidence. Gale warnings continue and expanded to Boston harbor and Narragansett Bay. 70-80 kt low level jet develops through tonight. Strongest gusts to 40 kt will occur over nearshore waters where mixing is better. Less wind expected over the outer waters due to inversion but risk of gales continue, especially in any convective elements. Vsbys below 1 mil e in areas of heavy rain and fog. Saturday...High confidence. Southerly gales in the morning, then winds diminishing as low level jet moves offshore. Winds shifting to NW late morning through afternoon with gusts 20-25 kt. Outlook /Sunday Night through Wednesday/... Saturday Night: Winds less than 25 kt. Rough seas up to 12 ft. Slight chance of snow showers. Local visibility 1 to 3 nm. Sunday: Winds less than 25 kt. Areas of rough seas. Local visibility 1 to 3 nm. Sunday Night: Winds less than 25 kt. Seas up to 5 ft. Slight chance of snow showers. Local visibility 1 to 3 nm. Martin Luther King Jr Day: Winds less than 25 kt. Seas up to 5 ft. Slight chance of snow showers. Areas of visibility 1 to 3 nm. Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Tuesday: Winds less than 25 kt. Local rough seas. Chance of snow, chance of rain. Visibility 1 to 3 nm. Tuesday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of snow. Visibility 1 to 3 nm. Wednesday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of snow. Visibility 1 to 3 nm.
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&& .CLIMATE...
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Record high temps set yesterday: BOS 61 in 2017,1975 PVD 62 in 2018 BDL 60 in 2018 ORH 59 in 2018
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Flood Watch until 1 PM EST this afternoon for CTZ002>004. Winter Weather Advisory from 9 AM this morning to 4 PM EST this afternoon for CTZ002>004. MA...Flood Watch until 1 PM EST this afternoon for MAZ002>024-026. Dense Fog Advisory until 5 AM EST early this morning for MAZ020-021. Winter Weather Advisory from 11 AM this morning to 4 PM EST this afternoon for MAZ005>007-013>019. Wind Advisory until 9 AM EST this morning for MAZ018>024. Wind Advisory until 6 AM EST early this morning for MAZ013- 015>017. Winter Weather Advisory from 5 AM early this morning to 4 PM EST this afternoon for MAZ002-003-008>011. Winter Weather Advisory from 9 AM this morning to 4 PM EST this afternoon for MAZ004-012-026. Dense Fog Advisory until 9 AM EST this morning for MAZ022>024. RI...Flood Watch until 1 PM EST this afternoon for RIZ001>008. Dense Fog Advisory until 5 AM EST early this morning for RIZ006>008. Winter Weather Advisory from 11 AM this morning to 4 PM EST this afternoon for RIZ002>005. Wind Advisory until 6 AM EST early this morning for RIZ002-004- 006-007. Winter Weather Advisory from 9 AM this morning to 4 PM EST this afternoon for RIZ001. MARINE...Gale Warning until 9 AM EST this morning for ANZ231>234-250- 251-254>256. Gale Warning until 7 AM EST this morning for ANZ230-235-236. Small Craft Advisory until 7 PM EST this evening for ANZ237.
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&& $$ SYNOPSIS...WTB/KJC NEAR TERM...Frank SHORT TERM...KJC LONG TERM...WTB AVIATION...Frank/WTB MARINE...WTB/KJC CLIMATE...

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