Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 281158 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 658 AM EST Tue Feb 28 2017 .SYNOPSIS... Unseasonably mild weather on tap for this afternoon and especially Wednesday. Some rain, even thunderstorms will accompany these conditions at times mainly tonight into Wednesday night. Otherwise, mainly dry but blustery weather along with progressively colder air is on tap Thursday through Saturday. Wind chills of -5 to 10 above zero are anticipated by early Saturday morning. Temperatures will begin to moderate some by Sunday and especially next Monday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... 655 am update... An abundance of mid level cloudiness and weak warm advection was resulting in spotty very light rain showers early this morning. Temps were still near or just below freezing across northwest MA, so continued the special weather statement until 9 am for the risk of spotty light freezing rain. Recent mild weather may limit impact to some degree, but icy patches are certainly possible before temps rise above freezing in all areas by mid to late morning. Otherwise, the day will be defined by the increase in low level moisture. Meanwhile, column moisture increases mainly from about H9 to H6, with PWATS reaching just shy of 1.00 inches (nearly 2 std deviations above normal). However, the persistent warm advection/overrunning is very weak, so overall forcing is lacking. Therefore, outside of clouds, expecting much of the region to remain dry into the evening hours other than spotty light sprinkles/light showers. POPs updated to reflect this. Only caveat may be extreme SE MA (mainly the Islands), which will be the first to see the impact of LLJ convergence late this afternoon. H92 temps increase to nearly +4C to +6C by evening. Highs this afternoon are expected to reach well into the 50s to perhaps a few locations near 60 this afternoon. However, sea breezes will develop across northeast MA including Boston and that may result in temps below 50 this afternoon in these locations. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/... Tonight... PWATs remain near 1.00 inches while K-values reach the low-mid 30s suggesting plenty of available moisture. However, while forcing was lacking by day, the convergence of a 40-50 kt LLJ from the S-SW will increase lift through the overnight hours, suggesting widespread wetting rainfall, especially S of the Mass Pike, closest to the exit region of the LLJ. POPs increased to categorical most locations. Will be including low risk for isolated with the widespread RA as TT values increase and mid lvl lapse rates approach moist-adiabatic. Not expecting as widespread a risk as previous days given the overall instability, but enough that at least a mention of TS is worthwhile. Mainly brief heavier precip will accompany any TS that occurs, as strongest jet dynamics occur Wed late. Total rainfall is likely to fall between 0.25 and 0.75 inches, although a few spots may do slightly better. Otherwise, the continued increase in sfc dwpts through the night suggest a risk for pockets of fog, which could be dense at times. Not confident enough in any one place for headlines yet, but something that will need to be monitored. Overnight mins may actually occur early, as continued warming in advance of the LLJ suggests slight increase in sfc temps in spite of typical diurnal curve. Looking at mins mainly in the 40s. Tomorrow... Uncertain forecast regarding precipitation and particularly convective potential later in the day. The widespread rainfall will gradually taper to widespread SHRA during the daylight hours mainly as upper lvl heights form more confluent flow pattern across S New England and a split in the dynamics occurs. General F-gen synoptic forcing remains closer to the parent low across N New England, while LLJ continue to increase (reaching almost 3 std deviations above normal, nearly 50-60 kt between H92 and H85) to the S and SW of S New England. While precip is unlikely to stop entirely there may be a few breaks until as conditions destabilize through the afternoon and evening hours. Convective risk... Remnant EML with mid lvl lapse rates reaching 6.5C/km and 7.0C/km moves over the region by late day tomorrow, allowing the overall MU CAPE profiles to reach 500-1000j/kg, TT to reach 50+. This could combine with the strengthening LLJ reaching nearly 60 kt. High shear values expected, with 0-6km shear nearly 50 kt and 0-3km helicity values exceeding 200. Therefore, expecting another high shear-low CAPE environment late tomorrow, similar if not even with higher shear than this past Sat. While a bkn convective line is the mode of choice here, SREF supercell parameter/sig TOR parameters are high enough to warrant some concern given the values mentioned above. Exacerbated by LCLs below 1000ft. Therefore, can`t rule out a risk of strong winds or even an isolated TOR in any T-storms that develop late in the day, however there are mitigating factors. At odds here, even with very modest instability and high shear, there is some concern that best forcing will remain both N and S of S New England. Think of the many very unstable airmasses that have been wasted in the past due to lack of forcing. Even with the high numbers in place across New England, instability/moisture and LLJ is strongest to the S. Therefore, any convection which fires during the afternoon upstream could actually shift S of the region by evening, usurping the moisture/instability and limiting the strength of any convection across S New England. For what it`s worth tail end of latest meso-scale guidance is actually suggesting this may occur. The best chance of any strong convection would be mainly across CT and MA W of the Worcester Hills, as areas further E will be under stronger Marine influence from SW flow. This coincides with latest SPC SWODY2 with an upgrade to Marginal Risk in these areas. Therefore, stay tuned to updates as we approach, even day-of- event. While there is a risk that the strongest convection completely misses us, there is a high enough risk for severe weather that we must continue to watch. Otherwise, a warm day as H92 temps above +10C should allow many locations to reach the 60s. Low moisture/clouds will limit the ability for further warming. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Highlights... * A few strong to severe thunderstorms are possible Wed evening but the higher risk will be southwest of our region * Dry but windy and much cooler Thu * Colder Friday and watching band of light snow that probably remains to our south * Brief shot of very cold air Fri night/Sat am: Wind chills -5 to +10 * Milder Sun and especially by next Mon Details... Wednesday night... Highly anomalous setup Wednesday night as strong low pressure lifts into northern New England and a cold front approaches the region. Extreme jet dynamics will be in place resulting in 0 to 6 km shear values increasing to over 80 knots. Steep mid level lapse rates coupled with very mild/moist southwest flow will result in dewpoints well up into the 50s with surface temps remaining in the upper 50s to the middle 60s for the first half of the night /away from the south coast/. This should result in a few hundred J/KG of cape with perhaps 500 to 1000 j/kg across far western MA/northern CT. The biggest uncertainty is if we are able to generate a few strong to severe thunderstorms given the marginal instability. The bulk of the synoptic scale forcing will be lifting well to our north into Quebec and northern New England with the intensifying surface low. Meanwhile the focus for surface based convection may end up to our southwest given higher instability/deeper low level moisture. If this occurs...much of our region may be spared the worst weather. So while highest risk for severe weather will probably be south and west of our region...we are certainly not out of the woods. Very anomalous airmass in place/steep mid level lapse rates and extreme jet dynamics are concerning. Therefore...a few strong to severe thunderstorms can not be ruled out with the highest risk in northern CT. Main severe weather threat would be locally strong to damaging straight line wind gusts...but hail would also be concern given decent mid level lapse rates/cold temps aloft. Lastly...0 to 1 km helicity values are on the order of 300 to 400 units so the low risk for a tornado exists. Again though...all of this hinges on the idea on whether or not we generate enough instability to generate a few strong to severe thunderstorms, which remains highly uncertain. Thursday... Dry and much cooler air works in behind the cold front on Thursday. Temps during the day Thu will mainly be in the 40s with some mid to upper 30s across the high terrain. Bufkit soundings indicate excellent mixing so expect northwest wind gusts of 25 to 40 mph. Friday... Digging upper trough over the northeast will allow progressively colder air to work into the region. High temps on Friday will mainly be in the 30s. Most of the guidance has a fast moving weak low pressure system passing for enough south of our region to keep us mainly dry. The exception is the GFS, which shows the potential for 1 to 3 inches of snow south of the MA turnpike. While the GFS solution can not be ruled out at this point...feel its a low probability and will just carry slight chance pops. Friday night and Saturday... A brief shot of very cold air on tap for Friday night and Saturday as shortwave energy passing east of the region. While we should remain dry, 850T should drop to near or even below -20C for a time. Low temps Friday night should mainly be in the teens with highs on Saturday generally in the lower 30s. Gusty northwest winds will result in wind chills between -5 and 10 above late Friday night into early Saturday morning. Certainly a big change from our recent mild weather. Sunday and Monday... The upper trough lifts northeast of our region allowing somewhat milder air to work back into the region. We can not rule out a brief period of light over running precipitation...but dynamics/forcing look limited so the majority of the period appears to favor dry weather. && .AVIATION /12Z TUESDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/... Today...Moderate to high confidence. Mainly VFR, although some MVFR CIGS may develop by early evening. Tonight...Moderate confidence. Mainly IFR with spotty MVFR. Widespread RA. Low risk for TSRA. Areas of fog developing. Increasing winds out of the S with gusts up to 20 kts. There is a LLWS risk for SE New England terminals with 50 kt SW winds 2 kft agl. Wednesday...Moderate confidence. Rainfall tapers to SHRA with breaks during the day Wed. More widespread SHRA/TSRA possible mainly during the late afternoon/evening hours, especially across W MA/CT. Widespread MVFR/IFR continues, but a few breaks to VFR during the day cannot be ruled out. KBOS TAF...High confidence in TAF. Sea breezes kick in by afternoon. KBDL TAF...High confidence in TAF. Outlook /Wednesday Night through Saturday/... Wednesday night...Moderate confidence. Mainly MVFR to VFR conditions in a broken deck of cloudiness, but brief IFR/LIFR conditions are possible in localized heavy showers along with the risk for a few thunderstorms. LLWS also expected with strong southwest low level jet. Lastly, dense fog is possible at times across the south coast, Cape and Islands. Thursday through Saturday...Moderate to high confidence. Mainly VFR conditions with northwest wind gusts of 20 to 30 knots at times. Low risk for a period of MVFR conditions in light snow on Friday...but odds favor that remaining to our south. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/... Through tonight...Moderate confidence. Quiet boating weather today, with a low risk for fog/showers tonight. S winds increase late tonight into tomorrow, with gusts 20-25 kt possible by the early morning hours. However, the persistence of the S flow will allow seas to reach 5-6 ft even with winds below SCA criteria. Therefore, will be hoisting advisories beginning early tomorrow morning. Wed...Moderate confidence. Small craft advisories continue as winds increase 25-35 kt at times out of the S and seas rise to 8-10 ft. However, there is a risk for some Gale force winds by late Wed, Wed night. Gale Watches may be needed in future updates prior to tomorrow/tomorrow night when winds will peak. Outlook /Wednesday Night through Saturday/... Wednesday night...Moderate confidence. Strong low level jet will result in southwest wind gusts of 25 to 35 knots. Inversion may keep the strongest winds just off the deck...but later shifts will still need to consider potential Gale Watches and/or warnings. Seas should build over 10 feet across our southern waters. Areas of dense fog are also possible. Thursday through Saturday...Moderate to high confidence. An extended period of northwest wind gusts of 25 to 35 knots expected as a result of cold air advection behind several fronts. Strongest winds look to occur Thursday and again late Friday night/Saturday, when gale headlines may be needed. Lastly, areas of freezing spray expected Friday night into Saturday morning as very cold air moves over the waters. && .HYDROLOGY... River levels, especially along the Mid to Lower CT River Valley in MA and CT, are currently cresting. There is still snowpack across N New England and subsequent melt water that can move downriver. In addition, there is a decent slug of rain forecast across the Northeast beginning Tuesday, and continuing through Wednesday into Wednesday night. These additional rains combined with snow melt will likely result in river rises in the near future. Those with interests along the mainstem rivers should stay aware of the latest forecasts as conditions will likely change through the midweek period. The river gauge at Middle Haddam CT along the CT River has been fixed by USGS. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 10 AM to 7 PM EST Wednesday for ANZ230>234-236-251. Small Craft Advisory from 7 AM to 7 PM EST Wednesday for ANZ235-237-250-254. Small Craft Advisory from 4 AM to 7 PM EST Wednesday for ANZ255-256. && $$ SYNOPSIS...Doody/Frank NEAR TERM...Frank/Doody SHORT TERM...Doody LONG TERM...Frank AVIATION...Doody/Frank MARINE...Doody/Frank HYDROLOGY... is the U.S. government's official web portal to all federal, state and local government web resources and services.