Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 162200 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 500 PM EST Mon Jan 16 2017 .SYNOPSIS... Low pressure will move from the Great Lakes to New York state with secondary low pressure forming near Long Island Tuesday and passing east of Nantucket by Wednesday. A mixed bag of wintry weather will impact much of southern New England from Tuesday afternoon into Wednesday. Could see leftover spotty showers along the shore late this week. High pressure will build along the eastern seaboard with dry, milder conditions into next weekend. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... 450 PM update... A high pressure ridge will remain over New England tonight with mainly clear skies to start, then increasing mid and high level cloudiness from southwest to northeast overnight. Low temperatures should bottom out in the upper teens in northwest MA, ranging to the mid 20s elsewhere, except lower 30s on the Islands. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... Low pressure moving eastward from the Great Lakes to New York state will spawn a secondary low pressure area near Long Island Tuesday morning, which will head east toward Nantucket late Tuesday night. It will cause a wide variety of messy weather across our region. All models are coming into good agreement on the timing. Precipitation is expected to break out in western MA and northern CT in the afternoon and spread eastward toward evening. The most difficult part of this forecast is the precipitation types. For this forecast package, we have expanded the Winter Weather Advisory to include central Middlesex and western Essex Counties in northwest MA. We have upped snowfall totals significantly over interior and especially northern MA. Although the typical 1540m value for 850-700 mb partial thickness will be to our northeast through the bulk of the event implying warm air aloft, cold air will remain in the low layers of the atmosphere. The 1000-850 mb partial thickness values remain below 1310m over a large part of MA through Tue. night. In fact, the ECMWF and GFS show a pronounced lowering of the 1540m line down to 1535m in northeast MA as heavier precipitation moves in Tuesday night and evaporational/dynamic cooling takes place. All model snowfall algorithms and the NCEP WPC forecast have at least 3 inches of snow, with perhaps as much as 6 inches occurring in the Route 2 corridor of northern MA. Sleet and freezing rain will be the predominant type of precipitation in northern CT and in western and central MA south of the MA pike, before likely changing to all rain. Up to one-tenth inch of ice accretion is expected, although we cannot rule out local two- tenths amounts, especially in the slopes of the Berkshires. In northern RI and interior eastern MA, an inch or two of snow is possible before turning to rain. All rain is expected in southern RI and southeast MA. A lot could change with this situation. A small change in temperatures, both at the surface and aloft, could result in big changes of precipitation type and corresponding snowfall amounts. A thin coating of ice on untreated roadways can be dangerous. Please slow down and use caution if driving late Tuesday and Tuesday night. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... */ Highlights... - Winter Weather Advisories posted for a wintry mix - Western and Central MA, as well as N CT - Tuesday evening into Wednesday - Ice accretion forecast up to two tenths, impacts to travel */ Discussion... Remainder of the week into the weekend... Individual shortwaves either emerging from the N-branch of Pacific flow or from the preferred troughing regime over the SW CONUS. Feel there is poor handling within deterministic model forecasts and thus preference to ensemble means to smooth out the noise. In the end, the squeeze of high pressure between a downstream blocking pattern as inferred from a -NAO trend and preferred low pressure development to the lee of the Rockies may yield a longer period of dry weather till we get into next week. Low confidence. Will keep any outcomes simply chance for now and not get into any particulars. Much of the focus is on the next 24 to 48 hours where confidence is higher. && .AVIATION /22Z MONDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday Night/...High Confidence. 2150Z update... Tonight...VFR. Tuesday...VFR to start. MVFR developing from W to E in the afternoon. Wintry mix of SN, IP, FZRA, and RA spreading east during the afternoon. Conditions then becoming IFR throughout the region. Snow may be locally heavy reducing visibility in the higher terrain areas of northern and western MA. Tuesday night...IFR in SN, IP, FZRA interior, changing to rain and in RA in RI and southeast MA. Easterly winds gusting to 20-30 kt near the coast late at night. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Wednesday through Saturday/...Moderate confidence. Northerly winds throughout, gusts up to 30 kts at time especially across the E coast of MA. Improvement through Thursday with only ocean effect MVFR ongoing across E MA. Winds diminishing out of the NW. Low confidence forecast Friday and beyond. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday Night/...High Confidence. 450 PM update... Tonight...Have dropped Small Craft Advisories for this evening, as wind gusts have diminished to below 25 kts. Dry weather and good visibility. Tuesday...Winds becoming easterly and increasing to 10 to 15 kt. A chance of rain late in the afternoon, especially along the RI coast. Tuesday night...Low pressure will be passing near or just south of Nantucket. Small Craft Advisories have been issued for the overnight hours into Wednesday for the eastern outer waters. Seas will be increasing to 3 to 6 ft as easterly winds persist...and gusts increase to 20-30 kt over the eastern outer waters. Outlook /Wednesday through Saturday/...Moderate confidence. Area of low pressure develops S offshore deepening N/E towards SE coastal Canada. Gusts up to 30 kts during this development with waves potentially building to 10 feet on the E outer waters. Will see wind and wave activity diminish into Thursday as the storm continues to slip E and winds turn out of the NW becoming light. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Winter Weather Advisory from 1 PM Tuesday to 7 AM EST Wednesday for CTZ002>004. MA...Winter Weather Advisory from 1 PM Tuesday to 7 AM EST Wednesday for MAZ003>005-010>012-026. Winter Weather Advisory from noon Tuesday to 7 AM EST Wednesday for MAZ002-008-009. Winter Weather Advisory from 4 PM Tuesday to 7 AM EST Wednesday for MAZ006. RI...None. MARINE...Small Craft Advisory from 1 AM to 5 PM EST Wednesday for ANZ250-251-254-255. && $$ SYNOPSIS...EVT/GAF NEAR TERM...GAF SHORT TERM...GAF LONG TERM...EVT AVIATION...EVT/GAF MARINE...EVT/GAF is the U.S. government's official web portal to all federal, state and local government web resources and services.