Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 170301 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1001 PM EST Fri Feb 16 2018 .SYNOPSIS... High pressure will build into the region, bringing a return to seasonably wintry temperatures tonight into Saturday. Fast moving low pressure will pass south of New England. This will bring plowable snows Saturday night into very early Sunday morning, with the heaviest amounts near and south of the Massachusetts turnpike. High pressure brings dry and cool weather Monday. A surge of much warmer air returns Monday night and may linger through Wednesday. A cold front drops through either late Wednesday or Thursday. High pressure brings cooler weather Thursday and Friday. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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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.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... 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/... Big Picture... Upper pattern starts with flat flow Sunday, but this quickly amplifies Monday-Tuesday. Shortwave from the Alaska coast dives south over California. This digs a deep trough over the Western USA while ridges build along the East Coast and the Eastern Pacific Ocean. The eastern ridge flattens late in the week. Per the NA Reanalysis the average values of 500 mb heights over Southern New England for February are 5420 meters to 5460 meters. The average values of 850 mb temperatures are -4C to -7C. Model values start near normal Sunday, then quickly climb well above normal Monday through Wednesday before dropping back closer to seasonable late week. Per the SPC Sounding Climatology, record 850 mb temps are roughly 10-11C. GFS, ECMWF, and GGEM forecast 850 temps at or above this value on Tuesday and Wednesday. This projects record warmth by midweek, and even when cooler air moves in it is forecast at near seasonable temperatures. Model mass and thermal fields are similar through Tuesday. The GFS is faster with the subsequent cold front with passage on Wednesday while the ECMWF and GGEM show a Thursday fropa. All models agree on high pressure and dry seasonable weather during the day Friday. Details... Sunday-Monday... Upper shortwave races through on Sunday, pushing the offshore surface low out to sea and making room for cooler drier air. This will mean clearing skies. Pressure rises behind the storm will average 1 mb per hour, tightening the pressure gradient and bringing gusty northwest winds. Mixing should reach to 900 mb, which would support max sfc temps in the low to mid 40s. High pressure moves overhead Sunday night. Should feature mostly clear skies, light winds, radiational cooling. With dew points in the 20s and low 30s, expect min temps in a similar range. High pressure moves off to the east Monday, with an increasing southwest flow aloft. This will bring warm advection aloft, lowering the mixing depth to around 950 mb...temps at that level support highs mostly in the 40s. The flow will also bring increasing moisture aloft, leading to increasing mid and high clouds mainly in the afternoon. Monday night through Wednesday... Warm front moves through during Monday night. A 45 to 55 knot low level jet works over the region Monday night and should set up a period of lift that generates showers. The clouds and incoming warm air will keep temperatures from falling much. More likely we will have steady or slowly rising temperatures during the night. The warm front lifts to the Canadian border Tuesday and Wednesday, leaving Southern New England deep in the warm sector. Temps at 950 mb will be 12-14C each day, supportive of highs in the 60s except cooler along the South Coast in the southwest flow. One uncertainty is whether the southwest flow producing clouds where the flow comes off the ocean. The moist flow may also bring areas of drizzle, especially along the south coast. Cloudier skies would suggest lower max temps. For now we will stay near guidance which has 60s with 50s along the south-facing coasts. Wednesday night through Friday... Multiple low pressure waves move along the front midweek. The final wave will swing the front south as a cold front. Our other uncertainty with this system comes with the timing of that trailing cold front. The GFS brings the cold front through Southern New England during the day Wednesday while the ECMWF does so Wednesday night and the GGEM toward Thursday morning. Whatever the timing, we expect low-end pops with the passage. We took the middle solution for timing, which lines up with the ECMWF. As the cold front moves off to the south, high pressure builds over nearby Canada. This brings a north flow of cooler air to our region. The flow turns from the northeast with time, and eventually turns from the east off the ocean. Expect temperatures trending near normal. Warm air ahead of the next weather system approaches Friday night. This may bring another chance of showers at the end of the forecast period. && .AVIATION /03Z 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 Night/... Tonight...High confidence. Conditions have lifted to VFR in all areas, with satellite images showing clearing skies entering from Upstate NY. Wind gusts have reached 20-30 kt. The rate of pressure rises should keep gusty winds with us early tonight, but winds will diminish overnight. Saturday...High confidence. Expect VFR conditions through most of the day. Conditions then lower to MVFR by 22-24Z with the onset of -SN across portions of N CT/RI. Light NW winds, but then shift to S-SE at 10 kt or less after 18Z. Saturday night...Low to moderate confidence. Conditions lower to IFR in -SN/SN, heaviest from 03Z-10Z with local LIFR CIGS/VSBYS in +SN at times. PTYPE may be -RA/RA at times through around midnight along immediate S coast/Cape Cod and the islands. Conditions may quickly improve across western and central areas after 10Z. KBOS Terminal...Moderate to high confidence in TAF. KBDL Terminal...Moderate to high confidence in TAF. Outlook /Sunday through Wednesday/...Moderate Confidence. Sunday: VFR. Breezy. Sunday Night: VFR. Washingtons Birthday: Mainly VFR, with areas MVFR possible late. Breezy. SHRA likely. Monday Night: Mainly MVFR, with local IFR possible. Breezy. SHRA likely. Tuesday through Tuesday Night: VFR with local IFR along the South Coast Breezy. Slight chance SHRA. Wednesday: VFR, with local IFR possible along the South Coast. Breezy. Chance SHRA. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/...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: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas locally approaching 5 ft. Sunday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Washingtons Birthday: Winds less than 25 kt. Seas locally approaching 5 ft. Chance of rain showers. Monday Night: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Rain showers likely. Tuesday: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Slight chance of rain showers. Tuesday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Slight chance of rain showers. Wednesday: Winds less than 25 kt. Local rough seas. Chance of rain showers. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Winter Storm Watch from Saturday evening through Sunday morning for CTZ002>004. MA...Winter Storm Watch from Saturday evening through Sunday morning for MAZ011>013-015>022. RI...Winter Storm Watch from Saturday evening through Sunday morning for RIZ001>007. MARINE...Small Craft Advisory until 6 AM EST Saturday for ANZ232. Small Craft Advisory until 5 AM EST Saturday for ANZ230-233- 234. Gale Warning until 2 AM EST Saturday for ANZ231-251. Small Craft Advisory until 1 AM EST Saturday for ANZ236. Small Craft Advisory until 7 AM EST Saturday for ANZ235-237. Gale Warning until 3 AM EST Saturday for ANZ250-254. Small Craft Advisory until 10 AM EST Saturday for ANZ255-256. && $$ SYNOPSIS...WTB/Doody/EVT NEAR TERM...Doody SHORT TERM...EVT LONG TERM...WTB AVIATION...WTB/EVT MARINE...WTB/EVT

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