889 FXUS61 KBOX 072204 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 504 PM EST Tue Jan 7 2020 .SYNOPSIS... An offshore low will bring light accumulating snow to southeast Massachusetts especially Cape Cod and the Islands Tuesday night into early Wednesday. Behind the departing ocean storm, blustery northwest winds will bring colder than normal temperatures to the region Wednesday night into Thursday. However, this will be short lived as a warm front lifts across the area Thu night with a possible light wintry mix changing to rain in the interior. A warming trend follows Friday and possibly near record highs Saturday. The warm weather will likely be accompanied by scattered showers from time to time Fri and Sat but a washout not expected. A cold front sweeps across the area early Sunday followed by dry and seasonable weather later Sunday into Monday. Shower chances return on Tuesday. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... An intensifying but progressive low pressure system will move out of Chesapeake Bay this evening and track close to the 70W/40N benchmark later tonight. This system has had a history of producing 1"/hour snowfall rate in parts of Northern VA, Central MD and Central PA. Low to mid-level warm advection combined with a negatively tilted trough over New England will produce an area of banded precipitation north and west of the surface low. However, this will not a big snow maker along and west of the I-95 corridor because there is an elongated piece of 500mb shortwave energy tracking over the Upper Midwest that will serve as the 'kicker' for the system. In other words, instead of tracking closer to the coast, the system will eject rapidly into the Canadian Maritimes and produce blizzard conditions for Nova Scotia later on Wednesday. So in all, not a big snow event but there is a window of saturation in the favorable snow growth region of -12 to -18C overnight Tue into early Wed in Southeast MA that would help boost snowfall. Rain may mix in with snow on the Outer Cape and Nantucket, especially during precipitation onset but should change over to snow as precipitation rate intensifies due to evaporative cooling Snow rates could exceed one inch per hour with reduced visibility for a brief time overnight. Overall, given that the window for heavy snowfall rate is only for a few hours, have gone with a general 2-4" heavy wet snow for Southeast MA and the Cape. It looks like the snowfall moves out of the area before Wed AM rush but early commuters, especially across southern Bristol, Plymouth and Barnstable County as well as the Southern half of RI could encounter snow-covered roads, since snow sticks to surfaces better at night. So give yourself extra time on the roads if you are planning to travel late Tue night into early Wed morning. .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... Quiet weather for most of the day on Wednesday but with the strong cold air advection and steep low-level lapse rates, will see strengthening winds as the day progresses, reaching wind advisory and gale force levels in the interior high elevations and coastal waters respectively. There will also be scattered snow showers throughout the day, especially across interior MA and CT. Wind chills drop down into the single digits and as cold as -5F for the school bus stop on Thursday morning. So be sure to bundle up and hold on to your hats when out and about Wednesday night into Thursday morning! Also, with waters in the low to mid 40s, low temps in the 20s and rough seas, there is a low risk for some freezing spray late Wed night into early Thu morning, though the coverage is not widespread enough to issue an advisory. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Main Concerns: A light wintry mix is possible during the day on Friday across the higher elevations of northwestern Massachusetts. This same system will bring a large warm sector making for well above average temperatures and rain for the weekend. Detail... Thursday... A potent trough begins to move out of the Northeast bringing strong northwesterly 850 hPa cold air advection in excess of -10 to -15 degrees Celsius air advecting in. As this trough moves out of our area, the pressure gradient created between a large high pressure system out over the great lakes will weaken resulting in a calming of the winds at the surface. A jet streak in the downstream side of the upper ridge will cause the ridge to weaken over time. High clouds can be expected to increase in coverage as strong moisture advection at 300 hPa begins to move in later in the day. The 850 hPa flow will shift to southwesterly as the ridge weakens yielding warm air advection. Highs on Thursday can be expected to be in the 20s inland to 30s along the coast. Friday... The high pressure system begins to move out of our region throughout the day. This allows a shortwave trough to lift from northern Minnesota into eastern Ontario/western Quebec. With this trough, a warm front will gradually lift into the Northeast. All of the models have decreased precipitation chances compared to their previous runs. Precipitation will transition from a light wintry mix to rain across western MA. Temperatures will start to increase as stronger southwesterly flow accompanies warm air advection across southern New England. High temperatures can be expected to be in the 40s across the CWA. Saturday... Split flow develops over the plains and allows an increasingly negative tilted shortwave trough to lift from the Southern Plains to the Ohio River Valley during the evening. The models are still not in complete agreement about where the surface low develops along the stationary front draped over the Ozark mountains. With the extended amount of time that the Northeast spends in the warm sector of this developing system, 850 hPa warm air advection will bring temps at 850hPa to +5 to +12 degrees C. This warm air can mix down to the surface resulting in possibly record breaking temperatures. Surface temperatures will rival the record highs for January 11th for our four climate cites. For reference, the daily highs on these days are as follows: BOS 62F in 1975, ORH 58F in 1924, BDL 60F in 1983 and PVD 61F in 1975. Sunday... Model consensus diverges further as they all have the low in different positions and moving at different speeds. The GFS as always continues to be the fastest and weakest with the system while the Euro is the slowest and strongest. To give the best forecast, we continued to go with a blend of guidance. High temperatures will range from the upper 40s to mid 50s. Best chance for rain comes on Sunday as the upper trough moves into New England. Monday/Tuesday... A weak upper ridge builds into our region making for dryer and more pleasant weather on Monday. From the passing of a cold front, temperatures will slightly cooler before another system makes its presence known with the return of showers late Monday/Tuesday. Model guidance is in decent disagreement on the details of this system at this time, so confidence remains low. && .AVIATION /22Z TUESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 21Z Discussion... Tonight...VFR conditions initially with ceilings gradually lowering as moisture spreads in from south to north. MVFR and then IFR conditions mainly for Eastern MA terminals with snow spreading in after 03z. Longest period of IFR conditions and possibly LIFR conditions for Cape and the Islands. Up to 1 inch of snow for PVD and BOS. FMH, HYA and ACK could see 2 to 4 inches of snow, with rain mixed in at times. Snow moves out after 15z. Wednesday...VFR. Partial clearing. Winds gusting to 20-30 kt. Slight chance of snow showers, mainly in interior higher elevations of MA. KBOS Terminal...High confidence in TAF. Moderate confidence after 03z. KBDL Terminal...High confidence in TAF. Outlook /Thursday through Sunday/... Thursday: VFR. Windy with local gusts up to 30 kt. Thursday Night: VFR. Breezy. Friday: Mainly VFR, with areas MVFR possible. Breezy. Slight chance FZRA. Friday Night: Mainly VFR, with local IFR possible. Breezy. Slight chance SHRA. Saturday: Mainly VFR, with areas MVFR possible. Windy with local gusts up to 30 kt. Chance SHRA. Saturday Night: Mainly VFR, with areas MVFR possible. Windy with gusts up to 30 kt. Chance SHRA. Sunday: Mainly VFR, with local MVFR possible. Windy with local gusts up to 30 kt. Chance SHRA, slight chance FZRA. && .MARINE... *Gale Warnings going into effect for all waters on Wed at noon. Tonight... Low pressure passes offshore during the night. This will bring increasing east and northeast winds through midnight, turning out of the northwest by morning. Periods of moderate snowfall along with reduced visibility, especially for waters south and east of the Cape. Winds will increasing overnight, reaching 20 kt and possibly 25 kts toward morning. Seas mostly less than 5 feet, but may be near 5 feet southeast of Nantucket. Wednesday... Cold advection over the waters and deepening of the departing low pressure will mean increasing wind, with sustained values reaching 20-25 kt and gusts near 35 kt. Seas will build through the day, reaching 5 to 10 feet on the exposed waters. Outlook /Thursday through Sunday/... Thursday: Low risk for Small Craft Advisory winds with local gusts up to 30 kt. Areas of rough seas. Freezing spray. Thursday Night: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Seas locally approaching 5 ft. Friday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Seas up to 5 ft. Friday Night: Winds less than 25 kt. Areas of rough seas. Slight chance of rain showers. Saturday: Strong winds with gusts up to 30 kt. Rough seas up to 11 ft. Chance of rain showers. Saturday Night: Strong winds with gusts up to 35 kt. Rough seas up to 14 ft. Chance of rain showers. Sunday: Moderate risk for Small Craft Advisory winds with areas of gusts up to 30 kt. Rough seas up to 14 ft. Chance of rain showers. && .CLIMATE... Record highs for Saturday Jan 11th ... BOS 62/1975 ORH 58/1924 BDL 60/1983 PVD 61/1975 && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Wind Advisory from 3 PM Wednesday to 1 AM EST Thursday for CTZ002>004. MA...Wind Advisory from 3 PM Wednesday to 1 AM EST Thursday for MAZ002>004-008>012-022>024-026. Winter Weather Advisory from 10 PM this evening to 5 AM EST Wednesday for MAZ020>024. RI...None. MARINE...Gale Warning from noon Wednesday to 8 AM EST Thursday for ANZ230>237-250-251-254>256. && $$ SYNOPSIS...BW/Chai NEAR TERM...Chai SHORT TERM...Frank/Chai LONG TERM...MM AVIATION...BW/Chai MARINE...BW/Chai CLIMATE...RC/BW