Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 201318 AFDBOX Area Forecast Discussion National Weather Service Taunton MA Issued by National Weather Service GRAY ME 918 AM EDT Tue Mar 20 2018 .SYNOPSIS... Dry but unseasonably cold weather will continue today and much of this evening. A coastal storm passes just offshore of Southern New England Wednesday and then moves off to the east Thursday. Upper low pressure lingers over the Maritimes through the weekend with potential for clouds and scattered rain/snow showers. High pressure then brings drier weather early next week. Temperatures remain colder than normal through the period. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 915 AM Update... Low pressure emerging off the mid Atlc coast but confluent flow over New Eng will maintain high pressure and dry air in the column today, so dry weather. High clouds streaming in from the south will lead to increasing clouds through the day. No changes to the first period of the forecast. Previous discussion... Cirrus starting to move into Southern New England. Just thin streamers at this time, but should see increasing cirrus through the morning and midday, then mid-level clouds during the mid- late afternoon. Lots of dry air in place with dew points in the single numbers and teens. Expect dry weather to continue through the day. Temps in the mixed layer should support max temps in the mid 30s to mid 40s. Quebec ridge moves east during the day. As the ridgeline shifts east of us this afternoon, this will allow winds to shift out of the northeast and east. No sign of any low-level marine clouds headed our way at this time, but we will need to monitor for any changes this afternoon. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... Upper lvls become cutoff and meandering in the mid-Appalachian. Upstream and over much of S New England, this enforces ridging during the evening hours. Noting a very dry column particularly below H5 (below the building CI shield), where dwpt depressions remain on the order of 10-15C to start the evening. These two factors will slow the precip shield advancing during the overnight hours, with the likelihood of only a very light snowfall by sunrise Wed AM. POPs reflect this slow timing, keeping the bulk of the overnight hours dry except for the S coast. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Big Picture... Broad trough covers the Eastern USA Wednesday with a closed low along the Mid Atlantic coast. The closed low lifts northeast to the Maritimes Thursday while the broader trough lingers over the Northeast USA into the weekend. One shortwave from the Canadian prairies and a second shortwave from Northern Canada phase as they drop southeast into the broader trough. A shortwave ridge then builds east for early next week. Mass fields show good agreement through Friday, followed by timing differences of shortwaves over the weekend. All then show high pressure centered in varying positions either over New England or to our north in Quebec. There are also signs of another weather system coming out of the plains, but blocked from moving north by the aforementioned high pressure. Overall, confidence is moderate. Details... Wednesday-Thursday... Closed upper low over the Mid Atlantic coast with a strong southern stream jet moving overhead providing dynamic support for lift and deepening of a surface low. With the digging of this upper low, the upper flow feeds north/northeast toward Southern New England. Precipitable water values continue to be forecast between 0.5 and 0.75 inches, above normal but not excessively so. Cold air remains in place at the surface, with dew points expected to remain below freezing in most of Southern New England, and near freezing on the Cape and Islands. So the pieces are in place for a coastal storm, with temperature profiles favoring snow most places. Coastal areas, especially the Cape and Islands, remain warm enough to mix with rain. As the storm passes on Wednesday night, colder air will get drawn back across the Cape and Islands and may change any rain over to snow. Our biggest concern for snow accumulation in this area would be on the Upper Cape which will be closest to the existing cold air. Projected QPF totals range between 0.75 to 1.25 inch. This supports at least 8 inches and possibly 14 inches. This is our snow range in the core of the snow zone. We have opted for upgrading part of the Winter Storm Watch to a Warning. This area represents the most likely area to experience heavy snow from the strong lift. It is also along the core of precipitation, where any further shifting of the storm track would least affect the chances of 6+ inches of snow. In Northern/Northwest Mass the projected sharp northern edge of snowfall has a chance of shifting south, which introduces lingering doubt. In South Coastal Massachusetts the effects of a rain/snow mix linger. In these areas we have maintained the Winter Storm Watch for a few more hours. We have added two areas to the watch... Western Hampshire and Hampden counties, and Cape Cod/Barnstable county. The concern for Cape Cod may be more for the latter half of the storm, as colder air drains in from the northwest. In addition to the snow headlines, we have also issued a high wind watch for the coastal areas. Low level northeast jet is expected to reach 45-55 knots Wednesday afternoon and night. The strongest winds move over RI and Eastern Mass, and coastal areas will be the most vulnerable. Upper trough moves east of us Thursday, with winds turning from the north to northwest. Winds aloft and favorable mixing 40 knot gusts in Eastern Mass for a time, diminishing in the afternoon. Farther west, 25 to 30 knot gusts are likely. Drying air should bring clearing from west to east through the day. Friday through Sunday... Upper trough remains in place with a couple of northern stream shortwaves moving through. Expect patchy clouds and isolated snow showers. Temps aloft continue to support colder than normal temperatures, topping out each day in the upper 30s and lower 40s. Dry airmass should allow tempertures to drop back into the 20s and low 30s each night. Monday... High pressure builds in for the day, providing dry cold weather. && .AVIATION /13Z TUESDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/...High confidence. Through 04Z tonight... VFR. Increasing high then mid clouds through the day and early evening hours. A ridge moves through midday/early afternoon, at which time NW winds will shift out of the NE. Light NE winds will continue through the evening. After 04Z tonight... Gradual reduction to MVFR first due to low CIGS then with low vsbys in -SN from S-N timing may be off a bit in TAF, but the overall forecast is expected. Northeast winds will increase overnight, with gusts reaching 25 to 30 kt. KBOS Terminal...High confidence in TAF overall. MVFR CIGS will precede SN early Wed AM. Timing of this may be off a bit in TAF. KBDL Terminal...High confidence in TAF overall. MVFR CIGS will precede SN early Wed AM. Timing of this may be off a bit in TAF. Outlook /Wednesday Night through Saturday/...Moderate Confidence. Wednesday: Mainly IFR, with areas MVFR possible. Strong winds with gusts to 45 kt. SN. Wednesday Night: MVFR/IFR conditions possible. Strong winds with gusts to 45 kt. SN likely. Thursday: Mainly VFR, with areas MVFR possible early. Strong winds with areas gusts to 45 kt. Slight chance SN. Thursday Night: VFR. Breezy. Friday: Mainly MVFR, with areas VFR possible. Slight chance SHRA. Friday Night: VFR. Saturday: Mainly VFR, with local MVFR possible. Breezy. Slight chance SHRA. && .MARINE... Short Term /through Wednesday/...High confidence. Quiet winds and seas are expected through this evening. Winds increase and seas begin to build overnight ahead of intensifying surface low pressure east of the Delmarva Peninsular. By daybreak Wednesday, NE wind gusts will be in the lower end of storms with Gales in the sheltered Bays/Sounds. Outlook /Wednesday Night through Saturday/...Moderate Confidence. Wednesday: Low risk for storm force winds with gusts up to 45 kt. Rough seas up to 20 ft. Snow, rain. Visibility 1 to 3 nm. Wednesday Night: Low risk for storm force winds with gusts up to 50 kt. Rough seas up to 22 ft. Rain likely, snow likely. Visibility 1 to 3 nm. Thursday: Moderate risk for gale force winds with local gusts up to 45 kt. Rough seas up to 19 ft. Chance of snow, slight chance of rain. Thursday Night: Winds less than 25 kt. Areas of rough seas. Friday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers. Friday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Saturday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers. && .TIDES/COASTAL FLOODING... The combination of a series of low pressure passages along or just south of the 40/70 benchmark will lead to building seas/surge thanks to NE-E fetch through the day Wed and Wed night. This will culminate around the time of high tide early Thu AM, which at Boston is running about 10.7. Final surge values are likely to approach 2.0-2.5 ft, which would yield low- moderate coastal flooding. This is focused where a Coastal Flood Watch is currently running, from the S Shore of Mass down to the Cape/Nantucket. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Winter Storm Warning from 8 AM Wednesday to 8 AM EDT Thursday for CTZ002>004. MA...High Wind Watch from Wednesday morning through Thursday morning for MAZ007-016-019>024. Coastal Flood Watch late Wednesday night for MAZ019-022>024. Winter Storm Warning from 8 AM Wednesday to 8 AM EDT Thursday for MAZ011>019. Winter Storm Watch from Wednesday morning through Thursday morning for MAZ003>010-020>023-026. RI...High Wind Watch from Wednesday morning through Thursday morning for RIZ006>008. Winter Storm Warning from 8 AM Wednesday to 8 AM EDT Thursday for RIZ001>004. Winter Storm Watch from Wednesday morning through Thursday morning for RIZ005>007. MARINE...Storm Warning from 11 AM Wednesday to 5 AM EDT Thursday for ANZ231>234-254-255. Gale Warning from 2 AM Wednesday to 8 AM EDT Thursday for ANZ230-236-250-251. Storm Warning from 8 AM Wednesday to 5 AM EDT Thursday for ANZ235-237-256. && $$ SYNOPSIS...WTB/Doody NEAR TERM...WTB/KJC/Doody SHORT TERM...Doody LONG TERM...WTB AVIATION...WTB/Doody MARINE...WTB/Doody TIDES/COASTAL FLOODING... is the U.S. government's official web portal to all federal, state and local government web resources and services.