Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 180700 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 300 AM EDT Fri Aug 18 2017 .SYNOPSIS...
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Out ahead of a deep low pressure center lifting into Southern Canada, rain is to be expected, possibly heavy at times along with the threat of localized flooding, for Friday into Friday night. A low risk of strong to severe thunderstorms mainly over S/W portions of MA and CT. A cold front moves through New England Saturday with a few leftover showers and thunderstorms, then moves off to the east Saturday night. High pressure returns with fair seasonable weather Sunday through Tuesday. Another cold front sweeps through New England Wednesday, followed by more dry weather Thursday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/... 1000 PM Update... Overall trend in the forecast remains on track. High level clouds continue to move into the region from the west. Dewpoints are beginning to rise in advance from the approaching warm front. Dry weather will prevail over the next several hours as light showers are still back in upstate NY. Still a lot of dry air per sounding data for the moisture to overcome, so do not expect wet weather until closer to sunrise. Previous discussion follows... Quiet weather into this evening, clouds lowering and thickening overnight into early morning Friday. High pressure shifting S/E, winds beginning to increase out of the S with increasing surface dewpoints. Dry and clear conditions likely to prevail longest over the N/E resulting in the opportunity of radiational cooling and lows getting down into the low 60s, while all other locations drop down around the mid to upper 60s as showers encroach. Chance to likely PoPs into N/W portions of MA and CT by morning as tropical moisture begins to over-run out ahead of the lifting warm frontal boundary. More on that below. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SATURDAY/... Friday into Friday night... Main concern of a heavy rain / flooding threat, with a lower risk of strong to severe thunderstorms. An atypical event for mid-August, on the order of +1-2 standard deviations above average, with anomalous S flow of tropical moisture with precipitable waters in excess of 2- inches into a deep area of low pressure over SE Canada with surface dewpoints rising up into the low 70s. Will focus on 1.) a potential heavy rain threat associated with a low-level jet impinging upon a lifting warm front during the morning into afternoon period, and possibly again overnight lingering into Saturday morning, and 2.) afternoon-evening thunderstorms from the S/W, some of which may be strong to severe, at a minimum containing heavy rain threats. 1.) Potential heavy rain threat. Synoptically low-mid level forcing is apparent given convergence of a S H925-85 jet rearward of a lifting warm frontal boundary beneath ascent associated with lead shortwave energy, though the morphology and strength differs among the forecast guidance. Absent is upper- level support given lack of height falls, neighboring anticyclonic motions with the immediate mid-upper level ridge E, as well as the H3 jet axis N/W. But can`t ignore the anomalously moist environment and weak elevated instability being acted upon. K-indices exceeding 40 indicative of a potential heavy rain threat. Potential renewed heavy rain across E/SE New England during the overnight period with the warm front lingering SW-NE just offshore out ahead of a H5-7 westerly convergent dry-punch. Continued low- level confluence of tropical air brought N by H925-85 SW flow. Along with elevated instability, main threat of heavy rain with embedded thunder that lingers into Saturday morning ahead of a slow moving cold front and the aforementioned mid-level dry punch. Prefer high-res guidance which is in good agreement with the NCAR ensemble signaling significant probabilities of an inch per hour rain rates with storm total rainfall amounts in excess of 3 inches over areas of S New England, especially over the S-coast W to E on Friday. Heaviest rain S/E to which CIPS Analogs agrees. Overall, despite a slight risk of excessive rainfall with tropical downpours exceeding flash flood guidance per WPC, given uncertainty of outcomes plus differences in forecast solutions, will forego head- lines with this forecast package. Leaning towards the potential of an inch or more of rainfall in a short period of time that would lead to urban, poor-drainage flooding. Likely PoPs. Will go with a heavy rain mention with a lesser threat of thunder given the low-level convergent forcing and warm-moist column up to around 14 kft coinciding with the tropical airmass. Warm-rain processes snailed. Likely low clouds and visibility issues associated through the day and overnight. 2.) The risk of strong to severe thunderstorms. Two areas of focus for thunderstorm development where both shear and instability warrant the MARGINAL RISK of severe weather advertised in the Day 2 outlook from the Storm Prediction Center: Upstate NY and PA, and around the Delmarva Peninsula. The expectation is that within these two regions of focus shower and thunderstorm activity will develop during the afternoon hours and advect into S/W portions of New England late. As prior forecaster mentioned, there is marginal instability, however strong shear which can potentially compensate. There is, subsequently, a risk for strong to severe thunderstorms, along with heavy rain given the tropical environment. Though a lot of this will rely on how quickly the morning warm front lifts N/E and whether the S/W zones can destabilize with partial clearing. That remains unclear at this time. From the NCAR ensembles, lightning activity level encroaches from the W, diminishing E towards evening seemingly with lack of daytime heating and, more likely, a renewed focus of the low-level jet along the warm front S/E. Indications of a better convective environment further S/W of Worcester, with helicity noted in the 0-1 / 0-3 km layer given the turning hodograph profile. K-indices get up to 40 as lapse rates are conditionally unstable throughout the column, 5-6 C/km. Overall a high shear, weak instability event. Not overly confident given activity from the W will be entering a region of more anticyclonic flow within the mid-upper levels S/E, and it is quite possible that activity around the Delmarva Peninsula robs the environment N. Support given via hazard guidance from CIPS analogs. Will go with likely PoPs for both showers and thunderstorms over W/SW MA and CT towards the later half of the day, diminishing into evening. Keep with heavy rain but won`t mention gusty winds. && .LONG TERM /SATURDAY THROUGH THURSDAY/...
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Big Picture... Subtropical high remains in control across the Southern USA while a general zonal flow continues across Canada and the Northern USA. The flow becomes less zonal next midweek as Alaska closed upper low digs south over the Gulf, subtropical upper ridge builds north over the Western USA, and upper low over Hudsons Bay digs south into the Northeast USA. One shortwave moves across New England Saturday night, with brief cooling aloft that destabilizes the airmass Sunday. A second shortwave crosses New England Wednesday as the Hudsons Bay low approaches and moves a supporting jet max across the region. Contour heights are normal to a little higher than normal through early next week. As the Quebec closed low deepens, heights over New England dip below normal midweek. Model mass and thermal fields from the 00Z suite are in general agreement through Wednesday, building forecast confidence through that time. Less agreement in the fields Thursday. Details... Saturday... Moderate confidence. Models hold the the cold front over Southern New England while moisture fields race east and drier air moves in during the late morning and afternoon. PW fields show 1.5 inch values across much of the region in the morning, but moving east of Cape Cod by evening. CAPE fields show 1000-1500 J/Kg over Easter CT/RI/Eastern Mass midday. Winds aloft are marginally significant, especially at 500 mb with speeds of 40-45 knots. Stability parameters are mixed, with Totals in the mid to upper 40s in the early night, and LI values subzero over RI and Eastern Mass. Expect the main area of showers/heavy downpours to move east of Massachusetts during the morning. Drier air moves in at low levels, with lingering moisture around 850 mb. This suggests at least some clouds during the day, but with potential for sunny breaks. This heating should push temps into the 80s, with enough instability to generate scatterd showers/thunder along and ahead of the cold front. Saturday night... Cold front moves offshore, and surface winds become northwest. Skies will either clear or partially clear. Dew points will linger in the 60s, with min temps close to that level. Sunday... Upper shortwave and the coldest of the cold mid-level temps will be over New England. The more significant cold temps as well as cloud- level moisture will be over NH-VT. Cross-sections show Southern New England will have a moist layer based at 850 mb, but with drier air above and below that layer. Expect diurnal clouds, probably a bit more than guidance is suggesting, but otherwise a fair day with less noticeable humidity. Temps aloft 12-14C support max sfc temps in the upper 70s to mid 80s. Surface high pressure builds in with light variable wind Sunday night. With dew points in the upper 50s and around 60, expect min temps around this same range. Guidance temps were nudged a couple of degrees colder. Monday-Tuesday... High pressure builds over the region Monday and shifts offshore south and east by Tuesday. Cross sections show dry air in place at most levels through Tuesday evening, so expect dry weather both days. Increasing mid and high clouds Tuesday ahead of the next weather system. Temps aloft 14-15C Monday and 15-17C Tuesday, supporting low-mid 80s Monday and 85 to 90 on Tuesday. Tuesday night-Wednesday-Thursday... Closed low digs over Quebec and the Northeast USA with shortwave driving a cold front into/through New England. Expect showers/scatterd tstms with the front Tuesday night/Wednesday. The airmass dries out Thursday, but lingering patches of moisture in the mixed layer should generate some diurnal sky cover.
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&& .AVIATION /07Z FRIDAY THROUGH TUESDAY/...
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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/... Rest of today...High confidence. VFR. Light winds S/W winds with an immediate sea-breeze along the E coast. Tonight...High confidence. VFR dominates most of the of the overnight hours, with MVFR/IFR CIGS not arriving until mainly after 10Z across the west. Winds generally light, shifting mainly S. Friday into Friday night...Moderate confidence. MVFR-IFR CIGs prevailing especially 12Z-16Z from W-E. -RA/RA with embedded +RA with lower risk TSRA accompanies the dropping categories., holding across E terminals late and overnight as SCT RA/+RA/TSRA develop into S/W portions of MA and CT during the afternoon, dissipating into evening. Strong S winds around 10-15 kts with gusts up to 25 kts, strongest over S/SE New England. Potential LLWS overnight with 40-45 kt SW jet across S/SE New England. All conditions improving as winds turn W into early morning Saturday. KBOS Terminal...Moderate confidence. VFR most of the overnight hours with a gradual transition through the morning to MVFR/IFR along with a band of SHRA and occasional +SHRA moving in mid morning. Low risk for occasional TSRA through the day. KBDL Terminal...Morning SHRA with IFR possible especially after 10Z. Some improvement is possible into the later-half of Friday prior to RA/+RA/TSRA encroaching from the S/W. May dissipate before reaching the terminal. Low confidence concerning timing and outcomes with 18-22z TAF issuance. Outlook /Saturday through Tuesday/... Saturday... Moderate confidence. MVFR/IFR cigs and vsbys in showers and fog Saturday morning, then lingering showers/thunderstorms in Eastern Mass and RI in the PM. Trend all places will be to VFR. A cold front over the region will move offshore to the east by Saturday evening with winds shifting from the west. Areas of IFR cigs/vsbys in fog and low clouds later Saturday night. Sunday through Tuesday...Moderate-High confidence. VFR. Areas of IFR in early morning fog/low clouds, then improving to VFR by each mid morning and continuing the remainder of each day.
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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. Short Term /through Tonight/...Moderate confidence. High pressure and light winds will lead to good boating weather through this evening. But S winds will be on the increase over- night, gusting up to 25 kts across the S/SE waters Friday into Friday night. SMALL CRAFT ADVISORY issued accordingly as winds will result in heightened seas up around 5 feet. Heavy rain with embedded thunderstorms sweeping W to E across the waters Friday into Friday night. Will likely be some visibility restrictions. Outlook /Saturday through Tuesday/... Saturday...Moderate confidence. Lingering potential for scattered showers/tstms through the day, diminishing west to east late Saturday. Diminishing wind through the day. Some gusts 20-25 knots in the morning. Seas near 5 feet on the outer waters and on RI Sound, but trending lower through the day. Sunday through Tuesday...Moderate-High confidence. High pressure builds over the waters. Winds remain 20 knots or less, and seas 4 feet or less.
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&& .BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 5 PM this afternoon to 9 AM EDT Saturday for ANZ231>234. Small Craft Advisory from 5 PM this afternoon to 2 AM EDT Saturday for ANZ236. Small Craft Advisory from 2 PM this afternoon to 11 AM EDT Saturday for ANZ235-237. Small Craft Advisory from 8 PM this evening to 2 PM EDT Saturday for ANZ250-254. Small Craft Advisory from 2 PM this afternoon to 8 PM EDT Saturday for ANZ255-256.
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&& $$ SYNOPSIS...WTB/Sipprell NEAR TERM...Dunten/Sipprell SHORT TERM...Sipprell LONG TERM...WTB AVIATION...WTB/Sipprell MARINE...WTB/Sipprell

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