Area Forecast Discussion
Issued by NWS Boston, MA

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129 FXUS61 KBOX 010301 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1001 PM EST Tue Feb 28 2017 .SYNOPSIS... Unseasonably mild weather continues into Wednesday evening. Some rain, even thunderstorms will accompany these conditions at times mainly tonight into Wednesday night. Then dry but blustery and progressively colder air arrives Thursday and continues into Saturday. In fact by Friday night into Saturday temperatures will average 15 to 20 degrees colder than normal along with wind chills of -5 to 10 by Saturday morning. Temperatures will begin to moderate some by Sunday and especially next Monday. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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10pm update... Area of more widespread rain co-located with deepest overrunning associated with a warm front which still remains to our S-SW this evening. As this warm front continues its slow push to the N, along with a gradual increase in column moisture will allow the band of rain to shift into portions of S New England through the early AM hours. Have POPs gradually increasing through the remainder of the night. However, have tapered them back from previous forecasts as the strongest LLJ energy will actually remain to the S suggesting that best chances for rain will remain especially S of the Pike. Otherwise, forecast on track, expect temps to remain where they are or even increase as dwpts continue a slow climb. Previous discussion... Tonight... Widespread showers with an area of heavier rain and embedded thunder- storms. Focus along a convergent h925-85 nose of high thetaE air and anomalous precipitable waters roughly +2-3 standard deviations with elevated instability, accompanying H5-7 vorticity maximum. Modest accompanying shear in a conditionally unstable environment above a shallow stout surface inversion. Expect widespread shower activity given the lift and forcing mechanisms over the region with heavier rain and embedded thunderstorms over SE areas of S New England with the better environment of convergent focus and ascent. Progressive, low risk of flooding. Dewpoints on the rise with increasing S/SW flow, gusts potentially up around 20 to 25 mph along the coastline. Anticipate a non-diurnal temperature trend with lows observed around midnight in the low to upper 40s. Likely going to be contending with low clouds, fog given the abundance of moisture and aforementioned surface inversion with the pronounced warming immediately aloft.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... Wednesday into Wednesday night... Potential for strong storms, however an underlying uncertainty with respect to outcomes. Some keys to this forecast: 1.) instability and specifically whether destabilization occurs yielding surface based mixing, and 2.) available forcing / lift, in particular convergent, support via ascent. Lots of variables with this low instability, high shear setup. Am confident concerning shear: parallel jet pattern throughout much of the column yielding similar vector orientation of the cloud bearing layer and bulk shear. Roughly parallel to the pre-frontal trough and later cold front, with any instability, storm mode in the form of line convection with potential bowing segments, preference towards N book-end vorticies. Of considerable note is values of 0-1 km bulk shear upwards of 50 kts and helicity on the order of 300 m2/s2. This along with 0-6 km bulk shear upwards of 80 kts. The parallel flow regime, these storms are going to be booking it E/NE at a rapid pace with an embedded threat of damaging winds. However, this convective setup requires instability and forcing. The thermal and instability axes focus towards the low/mid Hudson Valley also SW New England ahead of a pre-frontal trough and later cold front. Forecast models signal MUCAPE of several hundred to 1000 j/kg potentially pushing into S New England collocated with a significant H5-7 elevated mixed layer with lapse rates of around 7 C/km, though more pronounced S of the region. Yet convergence seems absent over S New England much of the day until the cold front sweeps through the region towards late afternoon into evening. Convergent forcing does not appear as sharp as last weekend. Uncertainty and forecast challenges. Could see some morning - midday convection associated with a fairly potent H5-7 shortwave suggested by a majority of the models. Clouds/precipitation subsequent issues, perhaps remnant boundaries. In addition, possible that convection developing upstream robs the unstable environment from the N, storms potentially turning E/SE with the Corfidi vector flow. Lots of uncertainty making this only a moderate confidence forecast at most. Not willing to hang my hat on any one solution. Even as the prior forecaster noted, forcing may be split across the area with synoptics to the N and convection to the S. Though not ignoring some of the recent SREF parameters highlighting the severe weather threat as the sig-tor probability of values of 1 and 3 are high, up to 90% with the probability of a sig-tor of 1. With focus on instability values > 500 j/kg and shear > 30 kts, there is concern for W/SW portions of MA and CT. Given the potential environment, should forcing/instability/shear come together, its possible that with any line convection an embedded tornado within a N book-end vortex is possible, exacerbated by the low LCLs forecast. Greater concern with any S/SE surface winds, watching closely the CT and Pioneer River Valleys. In all we need to continue to watch this closely. There remains the potential for line convection to sweep the region. Damaging winds possible along with the threat of a tornado if it materializes. Storm Prediction Center has extended the marginal risk further E across most of S New England except the Cape, with a slight risk into CT, mainly for the late day storm threat. Focus is upon the risk of rapidly moving, low-topped line convection with bowing segments that when traversing the area enhance the surface wind threat. There is some underlying uncertainty similar to concerns noted above, but felt it prudent to account for the possibility of damaging winds and thus the marginal / slight risk was extended E. A mild day with the surge of warmer temperatures and increasing moisture. Highs topping in the low to mid 60s, holding fairly stable into the evening hours before dropping out behind the sweeping cold front and increasing W flow, cold air advection. Lows down into the low to mid 40s with mid to upper 30s W especially over the Berkshires. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Updated 420 pm Highlights... * Dry but turning windy and much cooler Thu * Brief shot of arctic air Fri night into Saturday * Milder Sun afternoon and especially early next week Details... Thursday... 985 mb low over Maine continues to intensify as it tracks into the the maritimes as a sub 980 mb low Thu afternoon. Its attending surface cold front whips across the region early Thu morning with strong CAA during the day as 850 temps fall from zero to about -10C by 7 pm. Thus highs for the day will likely occur early in the morning and then fall into the 40s thru the afternoon. Temps could be near 50 at sunrise across southeast MA. Bufkit soundings support gusty west winds up to 40 mph which will make it feel even colder. Model moisture profiles suggest lots of post frontal strato-cu, so likely could be more clouds than sunshine with decreasing clouds toward sunset as diurnal heating is lost. Friday... at 12z Arctic front and weak wave located across the eastern Lakes into northern-central MA...then sweeps across the area as the day during the evening and overnight. Models not offering much moisture with this feature so any meaningful snowfall is looking to be a low probability. However these small scale frontal waves offer very low predictability, probably not much more than 36 hrs in advance. Thus will go with slight chance pops but probably more clouds than precip. Then arctic air overspreads the region Fri night along with blustery NW winds with wind chills -5F to +10F by sunrise. Sided with the coldest guidance here. Saturday... Anomalous cold airmass over the region Sat morning with both 925 mb and 850 mb 2 standard deviations colder than climo! GFS and its ensembles GEFS are the coldest guid with 850 temps down to -27C to -28C over the area 12z Sat. However the 12Z EC and its ensembles EPS not as cold at -20C to -22C, still anomalous for early March. Even using the EC and EPS temps aloft, highs Sat will likely remain in the mid to upper 20s, perhaps 30/31 along the south coast. These temps are 15-20 degs colder than normal! Nonetheless a gusty NNW winds will provide bitterly cold wind chills especially during Sat morning. Could be a risk of a few snow showers or flurries late Sat/Sat night as potent s/wv moves thru. In fact could be some ocean effect snow showers over southeast MA if winds turn to the northeast. As for highs Sat, leaned toward the coldest guid given temp anomalies at 925 and 850 are 2 standard deviation below climo. Sunday... Very cold start to the day with mins in the teens throughout the region with single digits possible too. Given it`s March the higher sun angle combined with lack of snow cover will help temps rebound to 35-40 during the afternoon. It will feel even milder given lots of March sunshine and light winds with high pressure overhead. Early next week... Deep trough over the maritimes continues to move eastward allowing height rises over New England. This will support milder temps but also the risk of precip as another potent system approaches. && .AVIATION /03Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/...Moderate confidence. 00z update... Not much change from previous TAFs. Uncertainty on how widespread IFR-LIFR in areas of low clouds and dense fog overnight into Wed morning. Latest guidance suggest highest probability of IFR-LIFR in dense fog will be across CT/RI and southeast MA. Earlier discussion below. ================================================================== Tonight... MVFR lowering to IFR with BR / FG issues, low cigs. Widespread RA developing late towards morning. Focus especially across S/SE New England. Low risk TSRA. Increasing winds out of the S with gusts up to 20 kts. LLWS threat for SE New England terminals with 50 kt SW winds 2 kft agl. Wednesday into Wednesday night... Potential shot of RA/TSRA morning into midday. Another episode late afternoon into evening possible. MVFR-IFR a majority of the time. SW winds around 15 kts with gusts 25 to 35 kts, strongest over S/SE coastal terminals. LLWS threats with 60 kt SW winds 2 kft agl, mainly over SE New England. Winds back W late, strengthen with widespread gusts 30 to 35 kts after midnight into Thursday morning. KBOS TAF...Moderate confidence in TAF. KBDL TAF...Moderate confidence in TAF. Outlook /Thursday through Sunday/... Updated 420 pm... Thursday through Saturday...Moderate confidence. Mainly VFR conditions with northwest wind gusts of 20 to 30 knots at times. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday Night/... Increasing SW winds throughout the period with a potential for gales for the SE waters going into Wednesday / Wednesday evening ahead of a cold front. Increasing wave action towards 8-12 feet on the S/SE waters with wind stress as rain / fog issues impact the waters with reductions in visibility. Small craft advisories extended for all areas not encompassed within the gale watch. Outlook /Thursday through Sunday/... Updated 420 pm... Thursday through Sunday...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. Could be a second surge of strong NNW winds late Sat-Sat night. Lastly, areas of freezing spray expected Friday night into Saturday morning as very cold air moves over the waters. May need a freezing spray advisory during this time to account for the risk of moderate freezing spray. && .HYDROLOGY... River levels, especially along the Mid to Lower CT River Valley in MA and CT, are dropping from Monday`s crest, aside from Middle Haddam, CT which is approaching crest later this afternoon. 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 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. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Gale Watch from Wednesday afternoon through Wednesday evening for ANZ232-254>256. Small Craft Advisory from 10 AM to 1 PM EST Wednesday for ANZ232. Small Craft Advisory from 10 AM Wednesday to 7 AM EST Thursday for ANZ230-231-233-234-236-251. Small Craft Advisory from 7 AM Wednesday to 7 AM EST Thursday for ANZ235-237-250. Small Craft Advisory from 4 AM to 1 PM EST Wednesday for ANZ254>256. && $$ SYNOPSIS...Nocera/Sipprell NEAR TERM...Doody/Sipprell SHORT TERM...Sipprell LONG TERM...Nocera AVIATION...Nocera/Sipprell MARINE...Nocera/Sipprell HYDROLOGY...

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