Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 091828 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 128 PM EST Sat Dec 9 2017 .SYNOPSIS... Low pressure passing east of Nantucket will bring a general 3 to 7 inches of snow northwest of the Cape Cod Canal later this morning continuing into tonight. Mainly dry and chilly weather follows Sunday into Monday other than a brief passing snow shower or two. Low pressure approaching from the west late Monday night into Tuesday will likely bring a period of snow/ice across the interior with mainly rain on the coastal plain. A shot of arctic air follows Wednesday into Thursday and wind chills may drop below zero. && .NEAR TERM /THROUGH TONIGHT/...
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*** Moderate snow event for southern New England will occur today *** *** Snowfall accums nearing 3-7 inches are anticipated *** 127 PM update... Light snow has finally overspread much of southern New England, with any rainfall confined to the Cape and islands. Will need to monitor the progress of the rain/snow line across SE MA, and adjust snowfall as needed. No changes to the headlines, or snowfall forecast, at this time. Minor tweaks to temperatures and dew points to reflect observed trends. Previous Discussion Below... Near term Analysis... Latest water vapor imagery continues to show digging northern shortwave into the Great Lakes/Chicago region. Down south, southern stream wave continues to push moisture plume up the eastern seaboard as the longwave trough begins to sharpen. Continue to watch and see if the northern stream wave will be able to capture the southern to help pull the bulk of the moisture back into the region this morning. Right now the bulk of the precip continues to remain across the Mid-Atlantic and offshore, with Nantucket receiving on and off showers. Overview... 00z guidance members continue to remain in agreement for today`s event. The RGEM however was on the beefier side with its QPF thus went ahead and discounted it. Overall, sharpening upper level trough due to potent northern stream wave will continue to dig through the pattern this morning. Southern stream wave which brought record breaking snow to the Deep South will begin to advect towards the Northeast after developing a surface low off the Carolina coast. These two pieces of the energy will try to phase the pattern later today pushing the low up towards the 40N/70W benchmark. This will help push the deep layer moisture over the region and with approaching upper level jet and QG forcing expect precip to fall for most of the day today and into tonight. Surface low will continue to deepen tonight as it moves towards the Maritimes. Details... QPF: Models remain in agreement with the ongoing forecast. Axis of heavier precip remains across the I-95 corridor and points east near 0.7 inches with 1 inch near Nantucket. QPF lesses as you move westward. Concern about the uptick in the RGEM as moisture associated with this system is from the Gulf leading to 2 STD above normal for PWATS. Something to keep in mind today. Will have to also watch and see how much moisture back towards the region today as upper level wave digs into the Ohio Valley. The HRRR and RAP may be on that trend with less QPF for the first half of the day, and then quickly increasing it later today and into tonight. This may be due to the northern upper level jet streak approaching the region. Still time to adjust if needed. Overall blended the EC with Hi-res guidance to get ongoing QPF forecast. P-TYPE: Mainly a rain and snow event for most of southern New England. Biggest question is where the rain/snow line will set-up. Current temps across the region are still in the mid to low 30s, warmer as you get near the coast. However, dewpoints are well into the 20s and with wet-bulb processes taking place this morning, temps will fall below freezing, resulting in snow. Nantucket may be more mix at the onset, but should remain mainly rain as they will be within the warm layer for much of the event. Hi-res guidance as well as the NCAR and HREF show a coastal front setting up across immediate coastal Plymouth and the Canal. Winds remaining more to the Northeast on the east side of the front will result in temps warming into the mid 30s. Thus more rain than snow. While on the west side of the front, due northerly winds will keep temps below freezing. Guidance has been a bit wobbling on the frontal placement so it will have to be watched closely as amounts could be enhanced just west of the front. Lastly, weak warm nose in the low levels may result in some p-type issues for the immediate south coast. Cannot rule out some sleet late this afternoon into the evening hours. The warm nose will quickly depart tonight as cold air ushers in from the west. One thing that was noticed this morning was that once winds switch to the northwest late this evening/tonight, some of the hi-res guidance is increasing 2m temps into the upper 30s to low 40s. Perhaps they are indicating that mixing will take place resulting in a brief warm-up. If this does occur, believe models temps may be a bit to warm as soundings keep a good portion of the profile at or below freezing. Snowfall & Intensity: Forecast snowfall amounts are similar to the previous shift with just a few slight adjustments. Generally anticipating a 4-7 inch event with higher amounts along the I-495 belt down towards the Rhode Island hills. This region has enough QPF, cooler temps, and the high terrain to help result in a few 8 inch amounts. While there weren`t a lot of changes to the snowfall amounts, one region that will have to be watch is the MA coastal plain. If the coastal front sets-up east then currently forecast then this area may see higher amounts and thus an upgrade to the warning may be needed. Snowfall intensity will be generally light with some modest bursts today. There is some descent lift in the snow growth region, but overall it is fairly marginal through the entire event. F-gen banding may develop as indicated by some guidance members which could help produce some banding. Perhaps one over southeast MA and the other across the interior/western MA. Still have to wait for these potential bands to set-up as they could still change. Overall a light snowfall intensity with some on-off bands which could lower vsbys and help increase snowfall rate to near 1 inch. Generally speaking this is a moderate snow event for the region with ranges between 3-7 inches. Higher amounts if bands end-up setting up across the region. But overall anticipate snow to be light with some brief moderate bursts through the day. Just a reminder, this is the first snowfall of the season so motorist should use caution if traveling. Snowfall will begin to taper off from west to east during the overnight hours with perhaps lingering snow showers across the east coast towards daybreak.
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&& .SHORT TERM /SUNDAY/... Sunday... Strong shortwave energy will swing across the region Sunday. This will probably result in a few lingering snow showers Sunday morning, especially across eastern MA where moisture is deeper. Otherwise, a mainly dry but blustery day is on tap. High temps should recover well into the 30s to near 40 with partial sunshine. Any lingering road issues should improve by afternoon with temps above freezing. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Highlights... * Chilly weather Sunday night and Monday * Some snow/ice likely late Mon night/Tue AM across the interior, but mainly rain along and southeast of the BOS-PVD corridor * Shot of arctic air Wed/Thu and it will be windy for a time. Details... Sunday night and Monday... Upper trough over the northeast will result in continued chilly weather. Low temps Sun night will mainly be in the upper teens to the middle 20s. High temps on Mon will recover well into the 30s to near 40. While the majority of this time will feature dry weather, a shortwave may bring a brief passing snow shower or two late Sun night/early Mon with the best chance across southern sections. Monday night and Tuesday... Low pressure will be tracking eastward from the Great Lakes late Mon night and Tue, likely passing to our north. This should generate a decent low level jet over an antecedent cold airmass. This will probably generate a period of snow/ice northwest of I-95 and especially the distant interior late Mon night/Tue am. While this does not look to be a big storm there is the potential for an impact to the Tuesday morning rush hour. Along and southeast of the Boston to Providence corridor, enough southeast boundary layer wind will probably result in precipitation mainly in the form of rain. Tuesday night into Thursday... Impressive northern stream shortwave/arctic cold front may generate a few snow showers Tuesday night and Wednesday. Given the potential strength of the shortwave, can not rule out at least a low risk for a period of some inverted trough snow with developing ocean storm. Latest guidance indicates that it may develop too late for a classic Noreaster in our region, but still bears some watching in the Tue night early Wed time frame. Otherwise, main story will be shot of arctic air Wed into Thu with potential for wind chills to drop below zero. It also may become quite windy for a time. Thursday night and Friday... Temperatures will moderate by Friday ahead of another shortwave. Will have to keep an eye on the potential for a period of wintry precipitation, but confidence is quite low at this time. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/...
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Forecaster Confidence Levels...Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/... Through tonight...high confidence on trends, but lower confidence on the precise details, especially timing. Mainly MVFR early this afternoon with -SN/SN overspreading the area. Lower vsbys in stronger snow bands. SN to RA across the Plymouth county coast and Cape Cod this afternoon, and perhaps briefly onto the immediate south coast. Thinking runways are going to remain mostly wet along the coast, with temperatures lingering around freezing, and rain mixing in. Although, for those along the north coast into this evening, expecting snow accumulation up through around midnight, when snow is expected to taper off. Preliminary Forecast Terminal Snow Totals... KBOS...4-7" KPVD...3-6" KBDL...3-6" KBOS Terminal...High confidence in TAF trends. Snow should taper after 10/03Z. SNRA approaches terminal from S/E towards 10/00Z, but likely remains mostly SN. Runways may remain wet through early afternoon, accumulating later into evening with forecast SN that tapers off overnight. KBDL Terminal...High confidence in TAF trends. Light to occasionally moderate snow continues into early evening. Snow tapering during the late evening into the overnight hours. Sunday...High confidence in TAF trends. Any lingering snow showers end Sunday morning with conditions improving to VFR. NW wind gusts of 25 knots. Outlook /Sunday Night through Wednesday/... Sunday Night: VFR. Breezy. Slight chance SHSN. Monday: VFR. Breezy. Monday Night: Mainly VFR, with areas IFR possible. Breezy. SN. Tuesday: Mainly MVFR, with areas IFR possible. Windy with local gusts to 30 kt. Chance RA, chance SN, slight chance SHRA. Tuesday Night: MVFR/IFR conditions possible. Windy with local gusts to 30 kt. Slight chance SHSN. Wednesday: Mainly IFR, with local MVFR possible. Windy with gusts to 35 kt. Slight chance SHSN.
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&& .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Sunday/...High confidence. Today and tonight... Low pres off the NC coast this morning tracks over or near 40N/70W this evening. NE winds increase to 20-25 kt by evening resulting in the building of seas. SCA have been issued. Rain and snow will reduce vsby today and tonight and slowly taper by daybreak. Sunday...Northwest wind gusts of 25 to 30 knots anticipated in the cold air advection pattern. Marine headlines continue. Outlook /Sunday Night through Wednesday/... Sunday Night: Moderate risk for Small Craft Advisory winds with gusts up to 25 kt. Rough seas up to 10 ft. Slight chance of rain showers, slight chance of snow showers. Monday: Low risk for Small Craft Advisory winds with gusts up to 25 kt. Local rough seas. Monday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Rain likely. Tuesday: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Local rough seas. Rain likely, slight chance of rain showers. Tuesday Night: Moderate risk for Small Craft Advisory winds with gusts up to 30 kt. Areas of rough seas. Slight chance of rain showers, slight chance of snow showers. Areas of visibility 1 to 3 nm. Wednesday: Moderate risk for gale force winds with gusts up to 40 kt. Rough seas up to 13 ft. Slight chance of snow showers. Visibility 1 to 3 nm. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Winter Weather Advisory until 7 AM EST Sunday for CTZ002-003. Winter Storm Warning until 7 AM EST Sunday for CTZ004. MA...Winter Weather Advisory until 7 AM EST Sunday for MAZ002-003- 008>011-016>021. Winter Storm Warning until 7 AM EST Sunday for MAZ004>007- 012>015-026. RI...Winter Weather Advisory until 7 AM EST Sunday for RIZ002- 004>007. Winter Storm Warning until 7 AM EST Sunday for RIZ001-003. MARINE...Small Craft Advisory until 7 AM EST Monday for ANZ231>235-237- 250-251-254>256. && $$ SYNOPSIS...Frank/Dunten NEAR TERM...Belk/Frank/Dunten SHORT TERM...Frank LONG TERM...Frank AVIATION...Belk/Dunten MARINE...Frank/Dunten is the U.S. government's official web portal to all federal, state and local government web resources and services.