Area Forecast Discussion
Issued by NWS Boston, MA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
000 FXUS61 KBOX 301107 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 707 AM EDT Thu Mar 30 2017 .SYNOPSIS... Mainly dry and seasonable conditions continue today followed by the first impacts of low pressure passing south of the region tomorrow. Low pressure from the Ohio Valley passes south of New England Saturday. This brings a mix of rain and snow Friday and Saturday with several inches possible in northern Massachusetts. High pressure brings dry weather Sunday and Monday, followed by another approaching storm on Tuesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
-- Changed Discussion --
7 am update... Clear skies over the region but with patches of clouds to the east, north, and west. Patchy clouds clipping Outer Cape Cod should move off to the south and out to sea during this morning. Patchy clouds over NH and VT are trending south but also warming/dissipating with time. Mid and high clouds over western and central NY are moving east. These clouds trend to our western areas mid morning and to the eastern areas by late morning. These are mostly thin clouds, but with patches of thicker mid clouds that could move in to our area in the afternoon resulting in partly sunny skies. No significant changes made to the forecast. Previous discussion... Still one of the cooler starts today. However the widespread sunshine should once again yield good mixing today, quite likely better than yesterday. H85 temps are warming slightly through the day but remain at an average of about -4C. This should still allow for a few spots to reach the low 50s with widespread upper 40s, so inched highs a bit warmer than previous forecast. Some increasing clouds expected head of a warm front by late day.
-- End Changed Discussion --
&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... Through 06Z (2AM local)... Mainly dry with increasing clouds expected ahead of an approaching warm front and with moisture loading of the column from the top-down. Initial cooling still expected in spite of the increasing clouds with temps dropping back into the low 30s (maybe a few upper 20s) before precip shield begins to move in from the W. Early tomorrow morning and the remainder of the day... The first of a pseudo-two phased system begins during the early AM hours tomorrow. Weakening warm front from initial parent low pres in the OH Valley region will bring an area of modest warm advective precip into the region from W-E through the morning hours. Given anticyclonic flow aloft combined with a fairly deep layer of dwpt depressions near or exceeding 10C initially, latest trends have slowed the precip shield somewhat as it enters the W reaches of S New England. Latest POPs will reflect this, in fact it`s possible some locations particularly across E MA and RI may not see precip with this feature at all, and may require the added dynamic lift of the developing low pres Fri evening/overnight before any significant precip falls. This trend will have to be watched as it will be one of many factors that will impact initial snow/sleet totals. Even though we start as snow most places as explained below, this timing may contribute to more/less impacts for Fri AM commute. In any case gradual transition to a widespread precip shield through the late morning into the evening hours. The next issue will be p-type. Latest trends not too much help given that GFS remains the weaker outlier with N stream phasing, suggesting a weaker/colder solution overall with ECMWF slightly colder to the N, but allowing S LLJ further inland along the lines of the NAM but not as robust. Taking a blend approach but with slight weight on the ECMWF given it`s only slight shift this update, with some of the original forecast still in play. With precipitation onset it looks to be mostly all snow save for maybe within about 20nm of the S coastline thanks to wet-bulbing through the column below H85 where temps are already mainly below 0C. Initially totals will still be highest N of the MA Pike predominantly due to higher SLRs and a longer period of lift. Looking at a daytime totals of 3-7inches within N extent of the current winter storm watch, (ie along the NH/VT border). Lower amounts toward the S, mainly 1-4 along the Pike within 30 miles or so with a T or less closest to the S coast. The real layer to watch is the 850-700mb layer for the LLJ and how efficiently it will bring it warmer air. Have a gradual S-N transition to a mix of sleet and rain through the daylight hours but this will also be dependent on precipitation rates driven by the overrunning. Therefore, for more on the overnight portions of the storm, see below, but for now will be maintaining current Winter Storm Watch given continued uncertainty in P-type and timing, but this is again based on the overruning remaining as robust as currently forecast. The recent trend to lessen somewhat the initial impact could lead to lower totals of both SN/sleet with the initial warm advective precip. In essence, still uncertainty in P-types and timing. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Big Picture... Longwave flow is nearly west-east with a slight trough over the Northeast USA. Shortwave scale flow shows northern and southern jet streams directing at least three and possibly four shortwaves through New England through the middle of next week. Approximate timing would bring one southern shortwave through on Saturday, a northern shortwave on Sunday, and another southern shortwave on Tuesday. Another shortwave crosses the nation on Wednesday but remains west of New England during the forecast period. Differences in model mass fields with the GFS consistantly faster by 6 to 12 hours. The general pattern is similar but with a wide range of different details either side of that basic pattern. Pattern confidence is moderate through the weekend, diminishing to low by Tuesday-Wednesday. Details... Friday night and Saturday... Midwest weather system redevelops along the Mid Atlantic coast early Friday night. The NAM was farthest north of the group Friday night...the ECMWF farthest north on Saturday. All positions are favorable for a slug of measureable precip as suggested by the precipitable water values which top out between 0.75 inches and 1.10 inches Friday night. The daily mean value for April 1 is about 0.5 inches and the 90 percent exceed value is 0.93, so these values are well above average. Precip type will be quite varied but trending through sleet and freezing rain to rain through the period. Even southern areas could see some sleet at times Friday night. But those southern areas should change to rain, while northern areas start as snow and change to sleet/freezing rain Friday night...then to rain Saturday. Winter Storm Watch continues north of the Mass Pike. Potential remains for 6 inches or more along the northern tier of MA with lesser amounts farther south. Additional concern is the water content of the snow, which is expected to be high. Looking at a sharp gradient in expected snow amounts between Northern MA and the Mass Pike corridor, in part due to the expected change to sleet/rain. This has limited our forecast confidence both in and adjacent to the watch area. So no changes in headlines planned at this time. Low level southerly jet starts feeding the best moisture into our area starting Friday evening in CT and expanding east by midnight. We have 100 pct pops during this time frame. The moisture inflow diminishes on Saturday. Models show a band of strong low level winds around the coastal low as it passes Southern New England Saturday with speeds of 45 to 55 knots at 2000 feet above the South Coast and Islands. The ocean induced temperature inversion may keep the highest winds aloft, but potential for 30-35 knot gusts along the South Coast and Islands Saturday. The low moves east of CT by afternoon and east of Nantucket by Saturday evening. Expect pcpn to diminish west to east during this time. It is possible the rain may change back to snow before ending. Sunday and Monday... High pressure builds in from the west bringing fair weather Sunday. Sufficient mixing during the afternoon from cold air aloft and daytime sunshine...gusty afternoon NNW winds. Temps aloft suggest max sfc temps in the mid 40s to lower 50s. Continued dry weather and lighter winds Sunday night as the high builds overhead. Dew points in the 20s suggest min temps in the 20s and lower 30s. Some increase in clouds Monday as the next weather system approaches, but otherwise a dry day under the high. Tuesday... Low pressure ejecting from the Southwest USA is projected to approach our area on Tuesday. This system bears some similarity to the Friday-Saturday system, including a weak cold air damming signal and a coastal redevelopment. This could be another coastal rain/inland mix situation, but currently expected temperatures support mostly rain. Wednesday... High pressure builds in with dry weather and near seasonable temperatures. && .AVIATION /12Z THURSDAY THROUGH MONDAY/...
-- Changed Discussion --
Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/... Today...High confidence. VFR with light winds. Mid to high level cigs increasing late. Tonight...Moderate confidence. Cigs lower and thickening towards morning, but mainly after 06Z and especially N/W. -SN develops mainly after 06Z as well and slowly spreads W-E across portions of MA/CT. This will lead to a reduction rapidly to IFR/LIFR vsbys before CIGS drop below MVFR. Light NE winds. Fri...Low confidence. High confidence in a trend toward IFR/LIFR everywhere as both CIGS and vsby lower. Confidence is low mainly a transition from SN to a mix of -SN/PL/FZRA and pure RA through the day. The heaviest precip waits until Fri evening, but this mix could still occur during the daylight hours Fri. KBOS TAF...High confidence in TAF through 06Z tonight. KBDL TAF...High confidence in TAF through 06Z tonight. Outlook /Friday Night through Monday/... Friday night... Moderate confidence. Widespread IFR/LIFR expected into Saturday morning, with some improvement west to east Saturday afternoon. Precip types will be mostly rain along the coast. Inland areas will start as snow or a rain/sleet mix and then change to rain during Saturday. Gusty easterly winds near the coast Friday night and Saturday morning with gusts 30 to 35 knots possible. Winds become NNE Saturday afternoon. Sunday-Monday...Moderate confidence. VFR. Northwest winds Sunday become variable Monday and east Monday night.
-- End Changed Discussion --
&& .MARINE...
-- Changed Discussion --
Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Friday/... 7 am update... Winds and seas have diminished enough to drop Small Craft headlines on RI Sound and Block Island Sound as well as Boston Harbor and Narragansett Bay. Otherwise no changes planned. Previous discussion... Winds and seas diminish through the day today. First ground Dropping below small craft thresholds this morning for Bays/Sounds and then through the afternoon elsewhere across the waters. After small craft advisories are dropped a period of mainly quiet boating weather is expected through tomorrow evening as E winds remain light and seas take time to increase. Only caveat will be for rainfall across the waters beginning Fri morning. Outlook /Friday Night through Monday/... Friday night-Saturday...Moderate confidence. Increasing east winds with potential gusts to 35 knots on the waters south of Provincetown and the South Coast. Sesa will build to 5 to 10 feet on the exposed waters. The coastal weather system causing this will move east of the waters Saturday night. A Gale Watch has been posted for the southern and southeast waters and includes Cape Cod Bay. Winds turn from the north or northeast as the weather system moves off Saturday night. Sunday-Monday...Moderate confidence. High pressure builds over the waters. This will bring diminishing winds and seas. Small Craft Advisory may linger on the outer waters Sunday.
-- End Changed Discussion --
&& .BOX WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
CT...None. MA...Winter Storm Watch from Friday morning through Saturday afternoon for MAZ002>006-008-010-026. RI...None. MARINE...Gale Watch from Friday evening through Saturday evening for ANZ232>235-237. Small Craft Advisory until 2 PM EDT this afternoon for ANZ231>234-251-255-256. Gale Watch from late Friday night through Saturday evening for ANZ231. Gale Watch from late Friday night through Saturday evening for ANZ250-254. Small Craft Advisory until 11 PM EDT this evening for ANZ250- 254. Gale Watch from Friday evening through Saturday evening for ANZ255-256.
-- End Changed Discussion --
&& $$ SYNOPSIS...WTB/Doody NEAR TERM...WTB/Doody SHORT TERM...Doody LONG TERM...WTB AVIATION...WTB/Doody MARINE...WTB/Doody

USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.