Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 300813 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 413 AM EDT Thu Mar 30 2017 .SYNOPSIS...
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Mainly dry and seasonable conditions continue today followed by the first impacts of low pressure passing south of the region tomorrow.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Mainly SKC conditions currently ongoing per latest IR although a few low lvl clouds continue to move across the interior high terrain. This clearing has allowed for some decoupling across S New England, but with modest pres gradient in place still noting a few spots of 5-10kt winds which have kept early AM temps mainly in the upper 20s to low 30s, a bit warmer than previous guidance might have suggested. 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.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/...
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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.
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&& .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Highlights * Snow sleet and freezing rain north of the Pike Fri into Sat * Rain snow and sleet changing to rain south of the Pike * Several inches of snow/sleet accumulation and light ice accumulation possible north of the Pike with greatest risk over higher elevations Friday through Saturday... Models agree on overall pattern with potent mid level low over Ohio valley tracking south of New Eng with intensifying secondary low tracking neat 40N Fri night into Sat. There are differences with the interaction of this mid level low with northern stream trof in Canada which impacts QPF and thermal profile. GFS is weaker with northern stream trof and less interaction which translates to a weaker low level jet and a bit less QPF but much colder thermal profile. GFS would suggest mostly snow north of the Pike with potential for up to a foot, especially higher elevations. However, NAM and ECMWF to a lesser extent are stronger with the northern stream trof with stronger low level jet, heavier QPF but much more aggressive with warm nose above 850 mb. This would imply less snow but a prolonged period of sleet and freezing rain. A very uncertain ptype and snowfall forecast as minor changes in thermal profile will result in much different outcomes. We are still 48-72h out in the model world so really can`t lock into any one solution as changes are likely. Due to uncertainty we used a blend of model temp profiles to derive snow and ice accum. Timing and precip types/accumulation... Initial shot of warm advection precip will overspread SNE Fri morning assocd with the mid level warm front. The column is starting out rather cold but warmer air in the 850-700 mb layer will be approaching from the SW. Expect snow or mixed snow/rain/sleet south of the Pike and especially near the coast as warmer air aloft moves up from the SW. North of the Pike expect mainly snow on Friday. Precipitation will be heavy at times late Fri and especially Fri night into early Sat as the secondary low tracks to the south and comma head moves across the region. The biggest uncertainty is how far north the warm nose aloft gets. We will mention a mix of snow sleet and freezing rain north of the pike Fri night. Precip should begin to taper off Sat afternoon as the low moves east of New Eng and may transition back to snow before ending as deeper cold air move back southward. Potential for 6+ inches of snow north of the Pike with greatest risk over higher elevations, so we issued a winter storm watch for this area. Some ice accumulation is possible as well if the warm layer advances northward. South of the Pike across CT/RI and SE MA expect amounts mainly under 2 inches and confined to the grass and trees due to temps remaining above freezing. However, this is not a high confidence forecast due to uncertainty in precip type. Gusty NE winds developing along the coast during Sat and may need wind advisories for gusts over 40 mph. Sunday onward... A low confidence forecast concerning a very active weather pattern. Evaluating upstream, persistence of a low over the Gulf of Alaska promotes downstream ridging promoting equatorward flow along its E periphery. Pacific-origin energy sheared S, cyclogenesis occurs over the 4-corners region of the SW CONUS. Pushing E it undergoes further amplification and negative tilt to the lee of higher terrain before shooting off downstream through the progressive flow. It then comes down to the environment ahead, on the strength and location of any preceding high pressure noting isallobaric / ageostrophic flow and any subsequent cold air damming above which initial overrunning in advance of C CONUS storm systems is likely to occur. Despite uncertainty with low level thermal fields coupled with the magnitude and track of synoptic features, considering the late-March into early-April timeframe, increasing length of daytime heating, as well as lack of Arctic air and near-neutral teleconnections, am left to believe that any wintry weather outcomes, will be marginal with a low to moderate impact potential. Much of the focus in N/W MA, along the high terrain as we`ve seen recently with recent weather. Initial focus on the Monday night into Tuesday timeframe. Consensus blend of forecast data preferred. Ensemble members still exhibiting a large amount of spread. && .AVIATION /08Z THURSDAY THROUGH MONDAY/... 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 through Saturday...Moderate confidence. Widespread IFR/LIFR expected with some improvement from west to east Sat afternoon. Snow/sleet/freezing rain north of the pike and rain/snow/sleet to the south transitioning to all rain by Fri night. Gusty easterly winds near the coast Fri night becoming NE Sat. Sunday into Monday... VFR. NW winds becoming E with time. Increasing clouds late in the period. && .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 Friday/... Winds and seas diminish through the day today. 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-Saturday...Moderate confidence. Easterly winds below SCA Fri but increasing Fri night with risk for gale force gusts to 35 kt. Winds becoming NE Sat with gales possible. Seas building up to 10 ft over the outer waters. Sunday into Monday... Blustery NW flow initially becoming light into Monday while turning E beneath high pressure. Should see wave action diminish throughout the period.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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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. Small Craft Advisory until 8 AM EDT this morning for ANZ230- 235>237. 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.
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&& $$ SYNOPSIS...WTB/Doody NEAR TERM...Doody SHORT TERM...Doody LONG TERM...WTB AVIATION...WTB/Doody MARINE...WTB/Doody

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