Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 130238 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 938 PM EST Fri Jan 12 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 SATURDAY MORNING/...
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935 pm update... ***Periods of heavy rain overnight along with significant urban street flooding*** ***Rain changing to Freezing rain/Sleet across western MA towards 12z*** 1) Heavy Rain/Flooding... Anomalous airmass in place with Pwats 4-5 S.D. above normal along with strong forcing ahead of a cold front will continue to result in periods of heavy rainfall overnight. Rainfall totals of 2-3 inches with localized 4 inch amounts are possible. The main focus for the heaviest rain should be across eastern MA/RI where a potent low level jet taps this anomalous airmass. Given the strong forcing in place and MUcapes of a few hundred J/KG...a rumble or two of thunder can not be ruled out. The result of this heavy rainfall and the rapid snowmelt will result in a continued significant urban street flooding threat. Also...some minor river/stream flooding is also expected. 2) Strong Winds on the Coastal Plain: Impressive 925 mb low level jet will increase further towards midnight to over 80 knots along the southeast New England coast. Despite the inversion in place...some of those winds may be brought down in heavier rain and any potential convection. While LLJ is not quite as strong in the BOS-PVD corridor...milder temps around 60 will allow for better mixing. Therefore...Wind advisory continues through daybreak along and southeast of the Boston to Providence corridor. This is for southwest wind gusts of 40 to 50 mph at times. 3) Dense Fog along the south coast/Cape/Islands: Opted to extend the dense fog advisory through daybreak across the south coast, Cape and Islands. Very high dewpoint air continues to stream over the relatively warm ocean. This will result in continued fog...which will be dense at times. 4) Rain changing to a bit of ice towards 12z across western MA: Impressive surge of very cold air will work into western MA by 12z behind a strong cold front. Many locations with current temps in the upper 50s to near 60 will see readings drop below freezing in a matter of just a couple hours in western MA. Still some timing differences...but the cold air comes in initially shallow enough for ptype to change to some light freezing rain/sleet in this region with enough residual low level moisture. 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 SATURDAY MORNING THROUGH 6 PM SATURDAY/... ***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 /SATURDAY 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 /02Z SATURDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/... Tonight...Moderate confidence. Conditions lowering with widespread IFR/LIFR expected as additional rain overspreads the region from the south. Rain changing to a period of FZRA western MA 09-12z. South wind gusts to 40-45 kt through tonight across RI and eastern MA with winds likely peaking between midnight and daybreak Sat. Widespread LLWS. Saturday...Moderate confidence. Rain changes to a brief period of FZRA interior before ending in the morning. Low risk of brief FZRA making it into the coastal plain before the precip ends. Greater concern is flash freeze as temps rapidly drop from 50s to below freezing in 2-3 hours. Widespread IFR/LIFR improving to VFR in the afternoon from west to east. KBOS Terminal...Moderate confidence in TAF. KBDL Terminal...Moderate confidence in TAF. Outlook /Saturday Night through Wednesday/... Saturday Night: Mainly VFR, with local MVFR possible. Breezy. Slight chance FZRA. 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. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday/... 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 /Saturday 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. && .CLIMATE... Record high temps for today January 12 BOS 61 in 2017,1975 PVD 60 in 2017 BDL 57 in 1913 ORH 57 in 2017 && .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Flood Watch through Saturday afternoon for CTZ002>004. Winter Weather Advisory from 9 AM to 4 PM EST Saturday for CTZ002>004. MA...Flood Watch through Saturday afternoon for MAZ002>024-026. Dense Fog Advisory until 5 AM EST Saturday for MAZ020-021. Winter Weather Advisory from 11 AM to 4 PM EST Saturday for MAZ005>007-013>019. Wind Advisory until 9 AM EST Saturday for MAZ018>024. Wind Advisory until 6 AM EST Saturday for MAZ013-015>017. Winter Weather Advisory from 5 AM to 4 PM EST Saturday for MAZ002-003-008>011. Winter Weather Advisory from 9 AM to 4 PM EST Saturday for MAZ004-012-026. Dense Fog Advisory until 9 AM EST Saturday for MAZ022>024. RI...Flood Watch through Saturday afternoon for RIZ001>008. Dense Fog Advisory until 5 AM EST Saturday for RIZ006>008. Winter Weather Advisory from 11 AM to 4 PM EST Saturday for RIZ002>005. Wind Advisory until 6 AM EST Saturday for RIZ002-004-006-007. Winter Weather Advisory from 9 AM to 4 PM EST Saturday for RIZ001. MARINE...Gale Warning until 9 AM EST Saturday for ANZ231>234-250-251- 254>256. Gale Warning until 7 AM EST Saturday for ANZ230-235-236. Small Craft Advisory until 7 PM EST Saturday for ANZ237.
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&& $$ SYNOPSIS...WTB/KJC NEAR TERM...Frank SHORT TERM...KJC LONG TERM...WTB AVIATION...WTB/KJC MARINE...Frank/KJC/WTB CLIMATE...Staff

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