Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 290227 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 1026 PM EDT SAT MAY 28 2016 .SYNOPSIS... A backdoor cold front begins to drop S over E/NE MA. Ahead of this front on Sunday, scattered showers and thunderstorms redevelop, especially across western MA and CT. Expecting rain late Sunday night through Memorial Day, heavy at times, with the chance of thunderstorms. With tropical moisture streaming north from Tropical Depression Number 2, there is a potential threat for localized flash flooding. A renewed strong to severe weather threat is possible over interior New England late Monday. Dry conditions with warm afternoons follow Tuesday into Wednesday. Unsettled weather may return by the end of next week. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... 1025 pm update... Still very mild late this evening with temps mainly in the 70s to even the lower 80s in Boston. Main story overnight will be a backdoor cold front that will slip south into north central and northeast MA by daybreak. This will allow cooler marine air to work into this region allowing low temps to drop to between 60 and 65 by daybreak. Meanwhile, portions of western Ma and northern CT will see overnight lows mainly in the upper 60s to near 70. Despite some elevated instability, not much forcing for precipitation overnight. Can not rule out a brief spot shower/t- storm but that vast majority of the night will remain dry. Patchy fog may develop in the typically prone locations and may end up a bit more widespread across northeast MA behind the backdoor cold front. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH MONDAY NIGHT/... Sunday... Cooler conditions push into N/E MA associated with a backdoor cold front as shower and thunderstorm activity emerges across the S/W interior. Two airmasses to contend with as there is some trickiness in nailing down specifically where the dividing line will set up especially with respect to temperature forecasts. With confidence, behind the backdoor cold front cooler conditions will prevail with NE flow and low clouds. A definite chill to the air as it will be roughly some 20 degrees cooler than the day prior with highs around 70 degrees. S/W, warmer conditions with highs close to 90, especially in the lower CT River Valley. Likely scattered shower and thunderstorm activity with increasing S winds ahead of a strong synoptic disturbance over the Great Lakes Region. Despite continued mid to upper level ridging and limited upper level support, albeit weakening, slightly stronger shear coupled with robust instability throughout the entire atmospheric layer up to the equilibrium level yields the potential for strong thunderstorms over N/W interior portions of MA and CT. Storms have the potential for producing heavy rain, perhaps even gusty winds and small hail considering the CAPE and shear profiles. But again, missing the upper level support, much of the forcing confined to the low to mid levels, especially more so wherever the increasing S flow converges with high terrain in areas of high instability. This coinciding with SPC`s Day 2 outlook of a general thunder risk. One final note concerning the thunderstorm threat, consensus of models have some drier air working N along with the increasing S winds ahead of the tropical moist plume for Sunday night into Monday. Timing uncertain, it could throw a wrench in the entire cog- wheel process of convection, pushing the focus area of activity further N/W outside of MA and CT. Will watch this closely, but aside the consensus of overall guidance has a potential threat for N/W portions of CT and MA. Sunday Night into Monday... With convection concluding into the evening hours, becoming dry, focus of attention shifts towards a moderate to heavy rainfall event developing late overnight and continuing through Monday as moisture associated with Tropical Depression 2 streams N ahead of a series of trailing cold fronts associated with a synoptic trough disturbance sweeping W to E. Focus is upon areas of deep layer lift, specifically where the right entrance region of the upper level jet aligns with mid level vortex energy out ahead of the deamplifying trough pattern, and with convergent low to mid level forcing along and ahead of an initial sweeping pre-frontal boundary. This with a deep-layer SW to NE training environment of warm-moist tropical-origin airmass with precipitable waters up to around 2-inches, near-record highest for the end of May looking at atmospheric sounding climatologies. H85 dewpoints in excess of 10C, mixing ratios above 12 g/kg, and some elevated instability. In all, and per a consensus of the guidance, expect a sweeping band of moderate to heavy rainfall, W to E, with embedded thunderstorm activity. There is perhaps some model forecast struggle with amounts and location of heavy rainfall / localized flooding, given the uncertainty as to how fast the cold front will sweep S New England and subsequent time in which the tropical-origin moist plume environment will be over any one region. But aside, the overall take- away is that there is a flash flood potential especially for urban centers should tropical-origin rains stream SW to NE over any one area for a prolonged period of time, transitioning through the period from early Monday morning into evening W to E. Keep in mind this is quite an infrequent event for this time of year. Now as this activity moves out, there is still the mid to upper level synoptic trough disturbance and accompanying surface cold front ahead of which partial clearing is forecast in wake of a pre- frontal boundary and tropical moisture. A rebound in instability beneath stronger W/SW flow and subsequent shear, SPC Day 3 forecast has a marginal threat mainly N/W of S New England for strong to severe thunderstorms with threats of heavy rain, strong to damaging winds, and small hail. Timing is an issue as to how quickly tropical- origin activity will exit to the E and the atmosphere over W New England can rebound and destabilize. During the late afternoon and towards evening, it is quite possible that thunderstorm activity could enter W New England. Something to watch out Monday Night... Convective activity withers with the lack of diurnal heating. The associated cold front weakening sweeps SE across S New England behind which W/NW follows pushing into the region much cooler and drier air, while shunting tropical moisture out to sea. Will see lows drop back into the 50s. && .LONG TERM /TUESDAY THROUGH SATURDAY/... Highlights... * Mainly dry with above normal temperatures Tue/Wed * Unsettled weather may return toward next weekend Details... Tuesday and Wednesday... Dry weather with above normal afternoon highs are anticipated away from the immediate coast. Dewpoints mainly from 55 to 60, so it will still feel humid. Thursday into Saturday... Forecast confidence remains rather low for this time frame. Increasing humidity with south winds ahead of an approaching cold front, especially Friday into Saturday. Timing this front remains one of the biggest challenges. Showers, and perhaps a few thunderstorms, possible. Way too early to try to narrow down the timing, although afternoon and evening hours would be climatologically favored. Kept just a chance for measurable rainfall in the forecast, until the details become more clear. && .AVIATION /02Z SUNDAY THROUGH THURSDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/... Overnight...Moderate to high confidence. Mainly VFR but may see some patchy fog resulting in some MVFR conditions after midnight. Also, backdoor cold front may allow a deck of 1000 to 1500 foot stratus and perhaps some fog to slide into Northeast MA including Boston near 12z. Sunday...Moderate Confidence. Winds back NE across E/NE MA with MVFR/IFR cig and vsby impacts. Towards the SW of this wind shift, SCT SHRA/TSRA expected with TEMPO MVFR/IFR impacts and possible 20-40 kt gusts. Focus around midday into afternoon. S winds overall for those areas S/SW of the NE wind shift. Sunday Night into Monday...Moderate Confidence. MVFR/IFR as RA/+RA with possible TSRA sweep W to E through the day. Cig and vsby impacts with strong S winds gusting 20 to 25 kts. For those areas W that clear out earlier, there is the potential for additional SHRA/TSRA activity late in the day. Low confidence with respect to impacts/outcomes. KBOS TAF...Moderate to high confidence in TAF. Mainly VFR overnight but probably will see low end MVFR CIGS arrive near 12z behind the backdoor cold front. KBDL TAF...Moderate to high confidence in TAF. Outlook /Tuesday through Thursday/... High confidence VFR. Low probability for MVFR in isolated showers or thunderstorms Thursday. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Monday Night/...High confidence. S/SW winds prevailing with gusts up around 20 kts and seas holding below 5 feet through Sunday evening over the S waters. Some visibility restrictions with mist or patchy fog not out of the question with warm-moist air streaming over the cooler waters. Whereas the E waters winds will shift out of the NE briefly tonight and through Sunday before reverting back out of the S/SW. Late Sunday night into Monday will see tropical moisture associated with TD 2 yield rain, locally heavy at times, along with the chance of thunderstorms across all waters. Likely visibility restrictions in accordance with rain, along with mist or fog forecast. Pronounced S/SW winds ahead of the front with gusts up to 25 kts look to yield seas of 5 to 6 feet Monday into Monday evening. Small Craft Advisories may be needed. All this activity clears out late into Monday night as a cold front sweeps the waters behind which winds back out of the W. Outlook /Tuesday through Thursday/... Tuesday...High confidence. Lingering 5 foot swell possible across our southern waters , but should diminish by late Tuesday night. Wednesday and Thursday...Moderate confidence. Relatively tranquil boating weather. Increasing east winds Thursday could lead to some rough seas across the outermost coastal waters Thursday afternoon and evening. && .CLIMATE... Record highs for today, Saturday May 28... Boston... 92 Tied (also set in 1931) Providence... 91 set in 1931 Hartford... 93 Tied (also set in 1977) Worcester... 89 New Record (previously 88 in 1929, 1911) Milton/Blue Hill...91 New Record(previously 90 in 1929) && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Air Quality Alert until 11 PM EDT this evening for CTZ002>004. MA...Air Quality Alert until 11 PM EDT this evening for MAZ004>007- 009>018-026. RI...Air Quality Alert until 11 PM EDT this evening for RIZ001>005. MARINE...None. && $$ SYNOPSIS...Belk/Sipprell NEAR TERM...Frank SHORT TERM...Sipprell LONG TERM...Belk AVIATION...Frank/Belk/Sipprell MARINE...Belk/Sipprell CLIMATE...Staff

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