Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 160835 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 335 AM EST Mon Jan 16 2017 .SYNOPSIS...
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High pressure maintains dry and seasonably cold conditions into Tuesday morning. Low pressure pushing E out of the Ohio River Valley will transition S of New England into Wednesday ahead of which there is the likelihood of snow and ice impacting much of the interior, specifically late Tuesday into Wednesday morning. 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.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/... A rather quiet January day, weather-wise. A large high pressure will be overhead, as it continues to move to the Mid Atlantic coast from the Great Lakes. Expecting a light west wind early, to become southwest later this morning, then increase slightly this afternoon. With plenty of sunshine, should see temperature slightly above normal this afternoon. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/... High pressure remains in control tonight, even with its core to our southwest. Not much moisture to work with, so a weak cold front late tonight should move through without much fanfare. Forecast gets much more complicated during Tuesday. Only have moderate confidence in the details at this point. 16/00Z guidance continues to suggest precipitation arrives during Tuesday afternoon, particularly across the western half of southern New England. The main issue there, will be low level humidity. It`s not expected to be very high to start, and am thinking it will take a while for the column to saturate, and permit the precipitation to reach the ground. However, that will have the consequence of evaporational cooling, which could lead to a wintry mix. Thinking a rain/snow mix would be most likely at the onset, before the arrival of warmer air aloft introduces the possibility of sleet and freezing rain as well. The Tuesday afternoon commute could get messy, particularly across the higher terrain of central and western MA. These details are not set in stone. Will need to monitor over the next 24-30 hours. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
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*/ 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... Tuesday Evening into Wednesday... An over-running setup ahead of low pressure emerging out of the C CONUS against a 1030+ high over SE Canada. Focus upon a front-end thump with low-mid level isentropic lift of a warm-moist conveyor belt of more continental-origin ascending above a shallow surface cold dome courtesy of cold air damming. Anticipating an interior wintry mix consisting of snow to sleet to freezing rain, possibly changing over to all rain, with precipitation onset roughly around late Tuesday into Tuesday evening W to E across S New England, concluding into Wednesday as low pressure transitions S of New England in regions of better baroclinicity and out to sea. Areas of greatest concern with regards to ice accretion are across the high terrain, namely elevations in excess of 500 feet, definitely at or above 1000 feet. Some keys to the forecast: 1) Low level thermal profile especially at the surface, 2) the presence of ice within the column, and 3) the magnitude of lift / forcing upon available moisture. Focus closely on a non-GFS consensus and probabilistic guidance for late Tuesday into Wednesday morning timeframe. To put it simply, the warm intrusion aloft around H8 as the layer below down to the surface saturates below-freezing, down to the wet- bulb, yields a transition of precipitation across the interior as most likely we`re all aware. Greatest concern is for those areas of high elevation above 500 ft especially 1000 ft where surface temperatures are most likely going to be at or below freezing, maintained by a N ageostrophic / isallobaric flow from regions with notable snow depth. Precipitation falling, and likely transitioning from, sleet and/or freezing rain potentially not going over to plain rain throughout the entire event across the interior. Some snow is possible at the very beginning but feel it will be brief for two reasons: 1) the warm intrusion around H8 is already pushing N/E across the region into NH at onset of precipitation, and 2) drier air within the mid levels is not far behind, eroding moisture within the dendritic growth zone around roughly midnight. Mainly rain for the coastal plain. Continued strong signals among the probabilistic guidance. From CIPS analogs there is an increasing signal of a significant freezing rain event. EPS/GEFS/SREF signal precipitation onset closer to Tuesday evening, targeting higher elevations with the greater likelihood of seeing a wintry mix. While the high-res guidance targets the high elevations such as the Berkshires, Worcester, Litchfield and Tolland Hills, SREF plumes exhibit some chaos with respect to precipitation outcomes at point locations. Thus there is a level of uncertainty. But despite the level of uncertainty, Winter Weather Advisories have been issued for Western and Central MA as well as N CT. There has been a persistent signal of sleet / freezing rain over the advised area by a consensus of forecast guidance and probabilistics. Only need a trace of freezing rain to reach advisory criteria. Significant ice accretion expected with one to two tenths forecast in the Berkshires in agreement with the forecast from WPC. Adverse travel impacts accordingly. Also forecasting up to an inch of snow and/or sleet accumulation prior to the transition to freezing rain. Most of these snow / sleet accumulations across the Berkshires and Worcester Hills. Continued concern of ice accretion for those areas across the interior that see a change over to rain. Not expected to warm quickly and given the likelihood that the ground will be near- freezing there is the possibility of continued ice accretion even with air temperatures above freezing. Something to keep in mind. 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.
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&& .AVIATION /09Z MONDAY THROUGH FRIDAY/...
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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 Tuesday/...High Confidence. VFR. Dry weather and mainly light winds through Tuesday morning. MVFR developing from W to E Tuesday afternoon. Increasing risk fora wintry mix of RA, SN, FZRA and IP across the higher terrain of central and western MA Tuesday afternoon. Timing is not yet certain. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Tuesday night through Saturday/...Moderate confidence. Conditions lowering late Tuesday into Tuesday night to IFR-LIFR. Threat of SN/IP/FZRA especially across the interior. Ice accretion forecast along with some potential low visibilities. 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.
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&& .MARINE...
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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 Tuesday/...High Confidence. Today and tonight...High pressure moves south of the waters today. Southwest winds should develop as it does. Brief increase in the pressure gradient expected this afternoon and evening, leading to some gusts up to 25 kt. It`s marginal, but think these gusts will be widespread enough to warrant a Small Craft Advisory for some of the eastern coastal waters, especially the outermost waters. Dry weather and good visibility prevail. Tuesday...A weak cold front should move through during the morning, followed by a quick wind shift to the east; ahead of an approaching warm front. Good visibility should prevail, with only a chance of rain across the RI coastal waters late in the day. Outlook /Tuesday night through Saturday/...Moderate confidence. Warm front lifting across the waters ahead of which easterly flow increases, strongest Tuesday night into Wednesday as an 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.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...Winter Weather Advisory from 7 PM Tuesday to 7 AM EST Wednesday for CTZ002>004. MA...Winter Weather Advisory from 4 PM Tuesday to 7 AM EST Wednesday for MAZ002-008-009. Winter Weather Advisory from 7 PM Tuesday to 7 AM EST Wednesday for MAZ003-004-010>012-026. RI...None. MARINE...Small Craft Advisory from 7 PM to 11 PM EST this evening for ANZ231-251. Small Craft Advisory from 4 PM this afternoon to midnight EST tonight for ANZ250. Small Craft Advisory from 6 PM this evening to 1 AM EST Tuesday for ANZ254.
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&& $$ SYNOPSIS...Belk/Sipprell NEAR TERM...Belk SHORT TERM...Belk LONG TERM...Sipprell AVIATION...Belk/Sipprell MARINE...Belk/Sipprell

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