Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 280302 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1002 PM EST Mon Feb 27 2017 .SYNOPSIS... High pressure centered S of New England today moves out to sea Tuesday. This brings back a mild flow of air with above normal temperatures. A couple of weather systems will bring periods of wet weather later Tuesday through Wednesday night. Colder air returns late this week into next weekend. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... Aside from normal adjustments, mainly to time the increasing dwpts through the overnight hours and the hourly/min temperature response (mainly warmer than previous forecast), only minor changes will be made with this evening`s forecast update. Forecast on track otherwise. Previous discussion... A low risk of spotty light showers, otherwise increasing, thickening clouds. Light S flow and slightly milder with lows remaining in and around the low to mid 30s. Leaning towards warmer guidance with the blanket of clouds forecast and S onshore flow with slightly rising surface dewpoints. Not thinking any issues with fog, still dry at the surface. As to the spotty shower chances, rich S moisture encroaching round the low to mid level ridging pattern, undergoing isentropic ascent, albeit weak. The atmospheric column hardly saturates but undergoes top-down moistening gradually. Perhaps support per mid-level forcing via ascent associated with lead impulse energy through the SW to NE gradient flow out ahead of deeper troughing across the W CONUS and lee cyclogenesis of the Rockies. Also additional support up against high terrain. Can`t rule out a spot shower but am somewhat doubtful given lingering dry air, plus supporting ensemble signals of little of any way of outcomes. Going with slight chance PoPs focused over the NW high terrain and S/E over the waters off the MA/RI coast with all other areas remaining dry. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... Tuesday... Increasing wet weather chances, mostly late. Outcomes look mainly light with focus across the entire region however greater emphasis over SE and W portions of S New England. The details below spell out the thinking, otherwise increasing S/SW surface winds and a mild day on tap with usherance of warmer air N. Looking at highs around the low to mid 50s. Holding away from warmest of guidance considering forecast cloud cover. However with any sunshine we could quickly warm into the upper 50s to low 60s, something to watch. As to wet weather chances, continued column moistening with S flow ushering warm high theta-E air within H925-7 layer. Moisture not the issue, rather available lift and forcing. Several synoptic features worth noting: 1.) isentropic upslope and lift along an encroaching warm front, 2.) increasing H925-85 low level jet convergence however stronger focus is across the OH River Valley / E Great Lakes Region with ensemble signals suggesting around +2 standard deviations in precipitable water and low level winds, 3.) supporting lapse rate environments, especially looking at H925-7, and 3.) potential ascent associated with continued lead impulse energy within the mid- levels originating from deeper troughing into the Central CONUS. Altogether looking a low level, synoptic scale lifting environment over SE areas of S New England with some weak instability noted, while the deeper plume and favorable dynamics encroach into W New England late with the accompanying warm front. Will focus chance PoPs in these locales respectively with lower chance PoPs elsewhere. Appears interior central areas of New England will remain dry and end up with a rather quiet day. Bonus if any sunshine. Tuesday night... Light to moderate rain likely. Deeper moisture, low level warming and convergent winds, along with steepening lapse rates, also the concern of some embedded thunderstorms. The increasing gradient flow along with the onset of rain, dewpoints on the rise which will keep conditions mild, and over slightly colder ground temperatures, likely to be contending with a mix of low clouds and fog. Lows around the mid to upper 40s occurring around midnight and then warming thereafter with the good S flow ongoing. As to the rain and thunderstorm threat, deep moist plume of higher theta-E air +2-3 standard deviations, with respect to precipitable waters, pushing into the NE CONUS along with the lifting warm front. Collocated isentropic upslope and convergent forcing of H925-85 low level jet, +1-2 standard deviations, precipitable waters pushing at or above 1 inch along with pockets of H85 dewpoints at or exceeding 10C. Rich, robust, moist sub-tropical environment, atypical for this time of year, within a conditionally unstable profile above boundary layer decoupling / shallow surface inversion. Still potential for lead shortwave energy along with increasing diffluence aloft to aid with ascent. Altogether, weak instability and shear is present, yet however the stronger, deep layer forcing remains well to the W. Synoptically, there is concern with the weak instability and modest shear. With freezing levels around 7 kft, there`s enough forecast guidance to suggest a possible hail / damaging wind threat. However am considering it a low risk for the time- being, yet not ignoring. More focused on the low level jet convergence of anomalous, robust, sub-tropical moisture. Thinking widespread rain with embedded heavy showers and thunderstorms. Quick moving, so think flooding impact is low but something to still think about. So, reiterating, widespread rain, embedded heavier showers and the chance of thunderstorms. Will incorporate areas of dense fog although thinking a mix with low clouds. No specific wording within the weather grids of any severe weather elements. Will continue to evaluate convective threats, especially with later forecast guidance. The whole month of February has been anything but typical, can`t have blinders on with this event, rather have to expect the unexpected. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Highlights... * Above average temperatures for Wednesday * Showers and thunderstorms likely Wednesday into Thursday * More seasonable conditions return on Friday Overview... 27/12Z guidance remain in rather good overall agreement through this week. Confidence in the details diminishes this weekend into early next week. Two distinct camps, with the international models more progressive with a trough moving east from the West Coast late next weekend than the GFS and its ensembles. Prefer a consensus approach for this portion of the forecast for now. Details... Wednesday into Thursday...Moderate confidence. This should be the portion of the forecast with the most significant weather. A low pressure should move from the Great Lakes through the Saint Lawrence River valley. Expecting its cold front to move offshore sometime late Wednesday night into Thursday morning. However, cyclonic curvature and cold air advection persist into Thursday. Above normal temperatures Wednesday. This will, in part, fuel the risk for more thunderstorms Wednesday into Wednesday night. Not a lot of buoyancy to work with, about 500-1000 J/kg of CAPE. However, there should be a large amount of low level shear and helicity. That could maximize the use of whatever energy does develop. Can`t completely rule out a few strong storms at this time, but it is 3 days away, and a lot of things can change. While the entire period don`t be wet, as dry air intrudes in the mid levels at times, it still appears that moderate to heavy rain could fall at times, especially within any thunderstorms. Main threat for these storms will be heavy rain and strong gusty winds. Friday and beyond...Moderate confidence. Northwest winds dominate Friday through Saturday, meaning a return to more seasonable temperatures. Models continue to indicate the potential for a clipper system on Friday/Saturday. The GFS remains slightly more robust then other guidance with this clipper, so leaned towards an ensemble approach. Good mixing and cold air over the ocean may result in ocean effect snow showers over portions of this weekend, before another clipper system comes through towards the end of this weekend. && .AVIATION /03Z TUESDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 21z update... Short Term /through Tuesday Night/... Tonight...High confidence. VFR. SCT-BKN cigs becoming OVC towards morning, then lower to low-end VFR. Light S/SW flow. Mostly dry. Tuesday...Moderate confidence. VFR with low-end VFR cigs but a risk of MVFR late over SE New England terminals, perhaps even some patchy fog and the chance of some -SHRA. Light S/SE flow around 5 to 10 kts. Tuesday night...Moderate confidence. Lowering MVFR to IFR. Widespread RA. Chance 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. KBOS TAF...High confidence in TAF. KBDL TAF...High confidence in TAF. Outlook /Wednesday through Saturday/... Wednesday...Moderate confidence. Periods of -RA through the period will result in MVFR/IFR conditions. Cannot rule out LIFR in fog during the overnight hours. Thursday...Moderate confidence. VFR conditions from west to east during the morning. Blustery westerly winds. Friday and Saturday...Moderate confidence. Mainly VFR, with MVFR possible toward the south coast in -SHSN. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday Night/...High confidence. Winds and seas continue to diminish this evening. Canceled some of the near shore Small Craft Advisories over the past couple of hours, once observations suggested the risk for frequent 25 kt gusts had ended. Current timing of Small Craft Advisories for the outer coastal waters still looks reasonable. Will have to wait for rough seas to diminish, too. Quiet through Tuesday. Increasing S winds with gusts up to 25 kt Tuesday night will also result in building wave action towards 5 to 6 feet by Wednesday morning. Additional Small Craft Advisory headlines will likely be needed. Outlook /Wednesday through Saturday/... Wednesday into Thursday...Moderate confidence. Approaching system from the west will push a cold front through late Wednesday night into Thursday morning, increasing both seas and winds during this period. Heavy rain and a few thunderstorms are possible, lowering vsbys. There is the potential for southerly gales across the waters Wed night, with the better chance for northwesterly gales on Thursday behind a cold front. Friday...Moderate confidence. Gusty west winds continue. Small Craft Advisories likely will be needed for some of the coastal waters. Saturday...Moderate confidence. Possibility of northwest gales for some of the waters. Rough seas continue across the outer coastal waters. && .HYDROLOGY... River levels, especially along the Mid to Lower CT River Valley in MA and CT, are currently cresting or beginning to crest as a result of rains and snowmelt over this past weekend. There is plenty of 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...None. && $$ SYNOPSIS...Belk/Sipprell NEAR TERM...Doody/Sipprell SHORT TERM...Sipprell LONG TERM...Belk AVIATION...Belk/Sipprell MARINE...Belk/Sipprell HYDROLOGY...

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