Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 170723 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1132 PM EST Tue Jan 16 2018 .SYNOPSIS... Developing coastal low pressure will emerge off the Delmarva peninsula and track northeast Wednesday. This system is expected to bring a moderate to heavy snowfall to western and central MA and northern CT late tonight into Wednesday. Snow, with a likely transition to rain for the BOS-PVD corridor by the mid morning. However, impacts to the morning commute are likely. Mainly dry weather likely Wednesday night into Sunday, with a warming trend during this weekend. Another low pressure should affect our region early next week with mostly rain. && .NEAR TERM /THROUGH WEDNESDAY/... * Potent Winter Storm expected late tonight into Wednesday * Hazardous morning commute expected 1110 PM Update... Low pressure starting to take shape east of the Delmarva Peninsula at 03Z. As the 00Z models come in, questions continue as to the ultimate track of this low as it either moves near Cape Cod or further S, and how strong either low will be. Latest KBOX 88D radar imagery showing southern edge of light snow running from S of KLWM-near KORH-KBDL. Most areas N and W of this line are reporting -SN, more or less. Northern fringe of precip associated with the developing mid Atlc low is bringing some light precip into Cape Cod and the islands as it moves NE. Pretty dry in between the two precip areas, with some patchy -SN or -RN across S coastal areas. For now, near term forecast in pretty good shape after 03Z update, though temps across S areas have risen to the lower to mid 30s so PTYPE issues in play. Depending upon the track of the coastal low, may see some more colder air work S overnight into Wed. Previous Discussion... Tonight into Wednesday... Overview... More substantial snowfall is expected starting this evening into tonight, as a coastal low pressure system is forecast to emerge off the Delmarva and track northeast along a baroclinic zone close to the NJ/Long Island coast. At the same time, the positive tilt mid/upper level trough will migrate eastward. Appears that a transfer of energy as coastal low jet structure begins to take hold early tomorrow morning will help phase the clipper system. This coastal low track has been key for this forecast as it is projected to track across the Cape and up towards the Maritimes. At the same time, open wave at 700 mb and surface high situated over the Maritimes will help allow the warm air to push into southern New England. The lack of blocking, also allows for this system to be quite progressive. So with the progressive nature, closer surface low track and warm air mixing into the I-95 corridor went ahead and adjusted snowfall amounts and sped the system up. P-type... Ongoing snowfall this evening will overspread across the region as cold front from the west approaches. Coastal low will move up the coast by the morning hours resulting a perhaps a good thump of snow right around the morning commute. Trended the onset of the snowfall with wet-bulbing to help indicate some moderate lift in the snow growth region. However, decent warm air at 925mb and at 950mb pushes into the I-95 corridor right after the rush which will transition any snow into rain. This will help undercut snowfall totals. Several model p-type algorithms also keeps the precip more as snow. However, that does not agree with current synoptics and climatology, especially with a low tracking over the Cape. The other thing we noticed is Hi-res guidance including the HRRR and RAP show a snow hole moving across RI and southeast MA towards the morning commute. This minimum in the guidance is a suggested in jet energy transfer towards the coastal low. Snow will eventually fill into that region, but there could be some lower amounts between 06-12z. Mixing could reach as far as Windham county and up into coastal Essex which is in agreement with EC, GFS and NAM. As the storm moves up towards the Maritimes later in the day, it does strengthen, allowing for any rain to transition back to snow by the late afternoon hours. This is all depending on where precip will be ongoing, with our highest confidence right now for NE MA. Snow Amounts/Hazards... Higher snow amounts remain across western MA for this evening which is climatology supported. Appears to be a good swath of over 0.5 inches across Hartford county and up into northern Middlesex county. Thereafter QPF amounts will be lower towards the Canal, with a secondary max across the Cape and Islands. With higher snowfall ratios, expect about 5-8 inches of snowfall within the current warning, with isolated higher amounts near 9 or 10 inches as you get closer to the Berkshires. This area appears to have good forcing within the snow growth region also some banding near the Berks down into Hartford county as shown by packed thermals in the mid-levels. Thus could see some higher amounts. Thus will keep the current winter storm warning for Western and Central MA as well as Hartford and tolland CT. As you get closer to the I-95 corridor expect around 2-5 as this region has a better shot of mixing during the mid-morning hours. This as well as the snow hole will keep amounts lower and thus have downgraded the I-95 corridor to an advisory. Did not have the confidence to keep the warning up, especially with the warmer trends in all of the models. Closer to the south coast and near the Canal, amounts will be less than 2 inches. Most of the snow will fall tonight into the early morning hours, but will quickly transition to rain by the morning. Thus went ahead an dropped the advisory. Even though amounts and hazards have been lowered, the timing of this system is not ideal as it brings accumulating snow during the morning commute. Motorists should use caution and take their time if heading out in the morning. && .SHORT TERM /WEDNESDAY NIGHT/... Coastal low will continue to move towards the Maritimes Wednesday night. This will pull all the moisture with is so any lingering snow showers will come to an end. Thus will not expect much in the way of issues with the evening commute. Cloud cover will improve by the later half of the night, and if we decouple then temps will bottom out during the overnight hours. Low temps will range from 20s across the Cape and Islands to single digits across Western MA where fresh snow pack resides. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Highlights... * Cold and dry Thursday * Continued dry with a warming trend through this weekend * Another storm may bring mostly rain Monday into Monday Night 16/12Z guidance is in reasonable agreement moving a deeper mid level trough offshore Thursday, with the mid level flow becoming more zonal across the eastern USA by Saturday. Still expecting a modest mid level ridge to build over the Eastern Seaboard this weekend. This should set the stage for a modest mid level trough to pass through early next week. Overall, this is looking like a rather quiet period of the forecast. Dry weather should prevail Thursday into Sunday, with a gradual warming trend and above normal temperatures. A low pressure is expected to move through the Great Lakes sometime early next week, and swing a cold front across our region. The timing is still uncertain, since it is still 6-7 days away. It does appear the most likely precipitation type will be rain. However, some light snow, or even freezing rain, will be possible if precipitation were to have a longer duration at night. It will likely be several more days to work out these sorts of details. The most likely period for precipitation would be from Monday afternoon into Tuesday morning. && .AVIATION /04Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/... Overnight and Wednesday...High confidence in trends. Moderate confidence in timing. Mainly MVFR CIGS from interior SE Mass/RI N and W, and VFR along the S coast with local MVFR CIGS. As steadier -SN/-RA moves in, conditions should lower to MVFR-IFR, with lowest conditions across N and W Mass into N central CT. May still see some patchy heavy snow from a KLWM-KORH-KBDL line after 08Z. Snow may mix with or change to rain south of a KGHG-KPVD-KWST line during Wednesday. Conditions improve to VFR across CT and western MA Wednesday afternoon, and across RI and eastern/central MA Wednesday night. Wind gusts 20-25 knots around Nantucket and parts of Cape Cod. Wednesday Night...High confidence. Any lingering IFR/LIFR will improve to VFR from west to east during the overnight. KBOS Terminal...Moderate to high confidence in TAF. Morning push will be impacted. KBDL Terminal...High confidence in TAF. Morning push will be impacted. Outlook /Thursday through Sunday/... Thursday through Friday Night: VFR. Breezy. Saturday: VFR. Windy with local gusts to 30 kt. Saturday Night: VFR. Breezy. Sunday: VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/...High confidence. Seas remain at 5 to 7 feet in easterly swells across the eastern open waters at 04Z, and a bit lower across the southern waters. Seas may lower somewhat through early Wednesday morning. Approaching coastal low during Wednesday will allow winds and seas to build across all waters. The low will track near or SE of the Cape, then head toward the Gulf of Maine. Could see some gusts near 20-25 kts with seas building to near 5-8 feet. Small Crafts have been extended for the outer waters into Wednesday night. Outlook /Thursday through Sunday/... Thursday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Areas of rough seas. Chance of freezing spray. Thursday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Friday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Friday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Saturday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Seas up to 5 ft. Saturday Night: Winds less than 25 kt. Local rough seas. Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft. && .HYDROLOGY... 1110 PM Update... The stage at the Taunton River at Bridgewater has fallen to 7.8 feet, which is below flood stage. The Flood Warning for Bridgewater has been cancelled. A Flood Warning remains in effect for the Connecticut River at Middle Haddam, where ice jams are causing river fluctuations. Colder weather will persist through midweek which will limit additional runoff. Continued ice jams expected on some of the rivers. There will be an increase of snow pack for a portion of southern New England through Wednesday as several inches of snow will fall. For details on specific area rivers, including observed and forecast river stages, please visit the Advanced Hydrologic Prediction Service (AHPS) graphs on our website. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Winter Weather Advisory until 6 PM EST Wednesday for CTZ004. Winter Storm Warning until 3 PM EST Wednesday for CTZ002-003. MA...Winter Weather Advisory until 1 PM EST Wednesday for MAZ017>019. Winter Weather Advisory until 6 PM EST Wednesday for MAZ007- 013>016. Winter Storm Warning until 3 PM EST Wednesday for MAZ002-003- 008>011. Winter Storm Warning until 6 PM EST Wednesday for MAZ004>006- 012-026. RI...Winter Weather Advisory until 1 PM EST Wednesday for RIZ002>004. Winter Weather Advisory until 6 PM EST Wednesday for RIZ001. MARINE...Small Craft Advisory from 7 AM to 7 PM EST Wednesday for ANZ235-237. Small Craft Advisory until 8 AM EST Thursday for ANZ250- 254>256. Small Craft Advisory until 1 AM EST Wednesday for ANZ251. && $$ SYNOPSIS...Belk/Dunten NEAR TERM...Belk/EVT SHORT TERM...Dunten LONG TERM...Belk AVIATION...Belk/Dunten/EVT MARINE...Belk/Dunten/EVT HYDROLOGY...Staff is the U.S. government's official web portal to all federal, state and local government web resources and services.