Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 011528 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1028 AM EST Wed Mar 1 2017 .SYNOPSIS...
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Unseasonably mild, wet weather with the chance of thunderstorms through tonight. Dry weather on tap for Thursday, but it will be very windy with much cooler temperatures. An arctic cold front crosses the region late on Friday, bringing dry but unseasonably cold weather Friday night and Saturday with temperatures 15 to 20 degrees below normal. Moderating temperatures begin on Sunday and particularly by Monday and Tuesday along with a chance for a period or two of precipitation.
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&& .NEAR TERM /INTO EVENING/...
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1030 am update... Severe Weather: The risk of strong to severe weather remains low. Potent H5-7 mid level impulse will quickly sweep across the region over the next several hours into the early afternoon with accompanying widespread rain and embedded heavier showers and thunderstorms given how saturated the environment has become and the parallel SW-NE mean cloud bearing layer and shear. Through mechanical mixing / precipitation drag down to the surface with heavier showers / thunderstorms, have continued the isolated thunderstorm wording to contain the threats of heavy rain and gusty winds. This given the high shear, weak instability setup. Recognize the astonishing values of helicity and shear but won`t reiterate here. It`s behind this wave that already clearing is being observed over W Upstate NY. A gut feeling that subsidence will evolve over the region at the prime time when destabilization will take place (aforementioned meso-ridging as noted by the previous forecaster). Indications of column drying within model forecast data around H85-7. A lot of model signal that suggests a lack of convergent forcing over S New England. That rather the line convection presently W of the Appalachains over the E Ohio River Valley follows the corfidi vector flow E/SE towards regions of higher instability and thetaE. Perhaps the present line of convection stretches N towards region of MUCAPE into Central Upstate NY as the SPC Mesoanalysis suggests. But thinking such an evolution N will be across PA and progress E towards the NJ coast perhaps clipping the S coast of New England. Feel the previous forecast discussion states things succinctly: "Late afternoon/evening...Following the initial band moving through early this afternoon, brief meso-ridging will yield a lull as secondary convection fires upstream in an area of very high CAPE/shear across PA/MD/NJ which will begin to slide E. With this convection, there is an upward pulse in shear with 0-3km nearing 50 kt and helicity values exceeding 200 and EHI almost 2.0 across CT and W MA especially. MU CAPE values also increase with 500-1000 j/kg with some mid lvl lapse rates exceeding 7.0C/km. These values are in fact stronger than those during the Conway, MA tornado of 2/25. Therefore, if any of these TSRA are able to slide into CT/RI, will need to be vigilant and monitor for not only strong, damaging winds (which will be the primary SVR mode), but also another isolated tornado. However, again there are mitigating factors. First is the lack of a low lvl vorticity/convergence source (the cold front provided that this past Saturday). The other, is the brief mid lvl subsidence inversion that develops in the meso-ridge. Bufkit profiles show a layer of dry air which may limit CAPE profiles somewhat and a recent trend in guidance is to reduce QPF in from 18Z 00Z, a potential red flag. In fact, the strongest convection may follow the Bunkers storm motion to the S, toward the peak LLJ core and region of higher CAPE values." Another concern is when the cold front sweeps across the region late during the evening and overnight hours. Still a decent environment of high thetaE air along with strong 0-1 km and 0-6 km bulk shear with SW to NE mean cloud bearing layer with the aforementioned bulk shear vectors, parallel. There are subtle hints that prior to cold frontal passage that a decent slug of instability creeps N just prior. However dry air could deter convection. Something that will need to be watched closely. It is of some question whether the convection ongoing to the S/W will rob the better environment from advecting N. This is quite the chaotic setup to which higher confidence may not come till 1 to 2 hours prior to an event. Right now, we still feel there is a low risk of strong to severe storms. Fog: Will continue to be an issue along the immediate coast given the surge of higher dewpoint air over the colder waters. Focus mainly along the S/SE coast given breezy SW winds, thus the E coast of MA should clear out as the latest webcams are already showing. Only low clouds remain. Feel this issue continues all the way through into the evening hours. It`ll take the cold front to push this anomalous, moist airmass out of here. However with the slug of widespread rain with embedded heavier showers and thunderstorms, could see some possible improvement. Still a very soupy, warm, moist airmass across the region. Not entirely optimistic in viewing signals from the HRRR.
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&& .SHORT TERM /OVERNIGHT/...
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Cold frontal passage after midnight... One more last push with the actual cold front tonight, the primary issue would be brief heavy shower with gusty winds however, mainly due to the fact that drier air will already be training into the column aloft which may limit the ability for any further convective threat. However, following the cold fropa, during the early morning hours cold advection could yield some strong wind gusts thanks to increased mixing. A wind advisory will be issued, with more on this below. Admittedly, all of this is to say there are a few rounds of isolated severe weather but with several mitigating factors which cannot be overlooked, and may, ultimately result in little to no actual impacts. However given that there remains a low-mod threat for an isolated mod-high impact event it would be premature to completely remove the risk altogether. The best bet will be to stay tuned to last minutes updates and warnings as the day progresses.
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&& .LONG TERM /THURSDAY THROUGH TUESDAY/... Highlights... * Very windy and turning much cooler Thu * Band of snow probably remains to our south Fri * Brief shot of arctic air Fri night and Sat * Moderating temps begin Sunday afternoon * Unsettled weather possible at times Mon/Tue with milder temps Details... Thursday... Intensifying low pressure lifting into the Canadian Maritimes will result in strong northwest winds and much cooler temps. Early morning temps in the 40s to the lower 50s will fall into the 30s to lower 40s by mid afternoon. Excellent mixing with very steep surface to 850 mb lapse rates support northwest wind gusts of 40 to 50 mph with the low risk for a few gusts up to 55 mph. Have gone ahead and issued a wind advisory for the entire region. These winds coupled with the falling temps will make it feel rather chilly given our recent stretch of unusually mild temperatures. Should see a fair amount of strato-cumulus clouds across the region but it will be dry other than the low risk for a passing sprinkle/flurry. Thursday night... Winds diminish some Thursday evening, but cold advection will continue. Low temps should drop into the upper teens and 20s, but dry weather will continue. Friday... The upper trough continues to amplify over the northeast allowing progressively colder weather. High temps on Friday should only be in the 35 to 40 degree range, but with less wind than Thursday. Will need to keep an eye on a fast moving wave of low pressure and its associated snow shield. Based on most of our model guidance feel that this band of snow should pass to our south, resulting in dry weather. We should mention that the 00z NAM was alone in showing a snowstorm south of the MA Pike. This appears to be very unlikely and there may be some convective feedback issues in the model. Friday night and Saturday... Vigorous shortwave and closed low will be dropping southeast into Northern New England. This will allow a shot of arctic air to spill into the region with 850T dropping below -20C. Quite the drastic change from our recent stretch of weather. Low temps Friday night should be well down into the teens...with perhaps some single digits in northwest MA. High temps on Saturday will only recover into the 25 to 30 degree range with a gusty northwest wind. Wind chills late Friday night and Saturday morning will be in the -5 to 10 above zero range. Dry weather should prevail despite the very cold weather Friday night and Saturday. The one exception may be across the outer-Cape. If winds take on a NNW component there could be a period of ocean effect snow showers. Sunday... Upper trough begins to move northeast and away from the region allowing for somewhat milder temps. High pressure should keep the weather dry with afternoon highs recovering well into the 30s to near 40. Monday and Tuesday... Low confidence this far out but should see moderating temps continue with digging trough across the north central part of the country. May see a period of light snow/ice/light rain late Sun night/Mon in the warm air advection pattern...but upper level ridging may keep precip amounts light and spotty. Better chance for a period of precip sometime on Tue with a cold frontal passage, but with the main low likely tracking to our northwest ptype will probably be mostly rain showers. && .AVIATION /15Z WEDNESDAY THROUGH SUNDAY/...
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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 Tonight/...Moderate confidence. Today into tonight... SCT-BKN SHRA and isolated TSRA throughout the period. MVFR across the interior and E coast, whereas IFR-LIFR over the S coast with BR/FG. SW winds 15 with gusts 25-35 kts. Yet LLWS, mainly uni-directional shear, threat aloft with SW 60-65 kt flow 2 kft agl. Improving conditions late towards midnight as winds back out of the W and gust with greater intensity, around 30 to 35 kts, into Thursday morning. KBOS TAF...Moderate confidence in TAF. Not expecting much in the way of visibility issues given a SW persistent wind. KBDL TAF...Moderate confidence in TAF. Outlook /Thursday through Sunday/... Thursday...High confidence. VFR but with northwest wind gusts of 35 to 40 knots with brief/localized gusts up to 45 knots possible. Friday through Sunday...High confidence in mainly VFR conditions. Northwest wind gusts of 25 to 35 knots expected on Saturday.
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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. 1030 am update...No major changes to the forecast. Short Term /through Tonight/... S-SW winds increase today reaching 25+ kt across all waters by this afternoon. On the S waters gusts to 35 kt possible. Otherwise showers/fog will lead to continued low vsbys. By tonight, once winds shift to the W, gusts 35-45 kt are expected across all waters. Therefore, Gale Warnings will be issued, first for the S waters starting late this afternoon, then for all waters late tonight. Outlook /Thursday through Sunday/... Thursday...High confidence. Northwest wind gusts of 35 to 45 knots in the cold air advection pattern. Gale Warnings posted for all waters. Thursday night and Friday...Moderate to high confidence. Northwest wind gusts drop to small craft levels Thursday night and probably below those levels for a time on Friday. This a result of a weakening pressure gradient. Friday night and Saturday...Moderate to high confidence. An arctic cold front crosses the region and the result should be northwest wind gusts of 30 to 40 knots late Friday night and especially on Saturday. Gale warnings will likely be needed again. In addition...arctic airmass will generate widespread light freezing spray and probably some pockets of moderate freezing spray late Fri night/Sat am. A freezing spray advisory may be needed for some waters. Sunday...Moderate to high confidence. Northwest wind gusts should diminish to small craft levels as a ridge of high pressure slowly builds in from the west.
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&& .HYDROLOGY... There is still snowpack across N New England and subsequent melt water that can move downriver. In addition, there is more rain forecast across the Northeast through tonight. 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 late week period. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Wind Advisory from 1 AM to 7 PM EST Thursday for CTZ002>004. MA...Dense Fog Advisory until 10 AM EST this morning for MAZ020>024. Wind Advisory from 1 AM to 7 PM EST Thursday for MAZ002>024- 026. RI...Dense Fog Advisory until 10 AM EST this morning for RIZ003>008. Wind Advisory from 1 AM to 7 PM EST Thursday for RIZ001>008. MARINE...Gale Warning from 6 AM to 7 PM EST Thursday for ANZ231>235-237. Gale Warning from 7 AM to 7 PM EST Thursday for ANZ230-236. Gale Warning from 4 AM to 7 PM EST Thursday for ANZ250-251. Gale Warning from 1 PM this afternoon to 7 PM EST Thursday for ANZ254>256. && $$ SYNOPSIS...Frank/Doody NEAR TERM...Frank/Sipprell SHORT TERM...Doody LONG TERM...Frank AVIATION...Frank/Doody/Sipprell MARINE...Frank/Doody/Sipprell HYDROLOGY...WFO BOX Staff

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