Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 240815 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 415 AM EDT Sat Jun 24 2017 .SYNOPSIS... Potential flooding and severe weather threats through the early half of the day, especially across Southeast New England, as the remnants of Tropical Storm Cindy clash with a sweeping cold front that will push offshore during the later half of the day, improving quickly and continuing overnight. Seasonably warm and less humid conditions Sunday will be followed by cooler weather and few diurnally driven showers and isolated thunderstorms Monday into Wednesday. A warming trend begins for the latter half of the week with a return to summer heat and humidity by Friday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... */ Highlights... - Potential flooding threats across Southeast New England - 1 to 2 inch rainfall rates per hour possible - Additional threats of gusty winds and possible brief tornado */ Discussion... Scattered showers and thunderstorms becoming more focused along and ahead of a sweeping well-defined cool frontal boundary. Collision with the remnants of Cindy and associated tropical plume, there is the threat of heavy rain and flash flooding, especially across SE New England, including the possibility of gusty winds and perhaps a brief tornado. Yet, there remains about a N-S spread of the axis of heaviest rain that leads to a level of uncertainty. This has been a tricky forecast. Let`s start with a broad overview. Ahead of the front, strong, convergent H925-7 moisture transport pushes precipitable waters to 2.5 inches, all in association with Cindy remnants becoming focused along a well-defined cool frontal boundary. Elevated instability up to 1k j/kg, but all below the freezing level of around 15 kft within an atmospheric column that is saturated up to around H3. Deep layer forcing ahead of the mid-level long-wave trough axis and cool front, expecting efficient warm-rain processes with rainfall rates of 1-2"/hr that usher a threat for localized flash flooding. Secondary threats of gusty winds, but not thinking wet microburst owing to the lack of mid-level dry air. Also, importantly, have to watch out for the potential of a brief tornado given strong 0-1 km shear with helicity values modeled as high as 250 m2/s2. Low LCLs with surface dewpoints well into the 70s, again along the well-defined cool frontal boundary, aside from the weak instability, a brief tornado is possible and this was seen yesterday across the Pittsburgh PA region. Very similar setup with a few brief tornadoes observed. SPC, in agreement, has the S-coast in a MARGINAL risk of severe weather with a 2-percent tornado risk within 25 miles of a point. So which guidance is preferred? Interrogating initialization, the WRF and Canadian model solutions are out. The EC is the southern- most solution. And a consensus of HRRR/NAM/GFS/UKMET/RAP/SREF focus mainly on SE New England. But even within consensus there is some wavering N to S, however it is focused on SE New England overall. Looking at upstream trends and considering above discussion, will go with definite PoPs over SE New England. Expecting a roughly SW to NE band of storm total rainfall within a 6-hour timeframe, possibly less, of 1 to 2 inches. Collaborating with WPC will highlight the region for a MARGINAL risk of EXCESSIVE RAINFALL. What is keeping rainfall amounts from being higher is the progressive flow over the region. So in thinking of isolated to scattered impacts, not wide- spread, and with 1 and 3 hour flash flood guidance at 2 to 4 inches, will hold off on a FLASH FLOOD WATCH. Thinking nuisance urban, poor- drainage flooding. Will have to keep an eye on coastal communities given coincident timing of high tide, however a tide lower than astronomically high tides that have occurred as of late. With the high tide, potential for drainage issues. Will issue a SPECIAL WEATHER STATEMENT to highlight the potential flooding concerns, as well as severe weather threats, more importantly the possibility of a brief tornado, which was discussed above. Behind the front, incredible drying discerned from model forecast soundings. The airmass aloft relatively unchanged with respect to temperature, notched downward slightly from +18C to +15C at H85, the sun immediately coming up, we will warm up quickly into the mid to upper 80s with some locations in the low 90s. However much less humidity as behind the cool front dewpoints drop gradually with dry air advection, down around the mid to upper 50s given deep layer mixing up to H8 of a much drier airmass building in aloft. So in summary, the later half of the day should turn out fairly nice and pleasant. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/... Tonight... Overall becoming quiet. Some initial light shower activity across N/W MA and CT, up against the high terrain, as mid-level energy advects through the continued cyclonic flow promoting localized theta-E / moisture convergence. But soon after weak height rises overnight out ahead of a more stout mid-level feature through the longwave trough over the Great Lakes Region. Surface high pressure emerging, drying out and clearing out, W winds becoming light, there is the opportunity for radiational cooling. Indications of low to mid 50s in the coolest of guidance at typically prone locations. Dry air advection with dewpoints fall, not anticipating fog threats. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Highlights... * Seasonably warm/less humid Sun with a spot shower possible interior * Cooler with a few showers/isold t-storm possible Mon into Wed * Warming trend toward end of the week with return to summer heat/humidity by Fri Details... Sunday... Fairly robust mid level shortwave moves NE from Gt Lakes. Brunt of energy remains well to the north and west, but cooling aloft will result in diurnal cu developing and can`t rule out a few afternoon showers, mainly in the interior. Instability parameters are not favorable so no mention of thunder. 850 mb temps near 12C and deep mixing supports highs in the low to mid 80s but cooler south coast with SW flow. Dewpoints in the 50s will make it feel much more comfortable. Monday into Wednesday... Anomalous mid level trof moves eastward from the Gt Lakes Mon pushing into New Eng late Tue and may linger into early Wed. Temps continue to cool aloft Mon which may trigger a few showers or an isold t-storm in the interior, but best chance of showers/t-storms will likely be on Tue as the trof approaches and core of coldest air aloft moves overhead. Timing of when the trof exits is somewhat uncertain but GFS and ECMWF still have -21C at 500 mb 12z Wed before temps aloft warm in the afternoon. So expect isold showers and t-storms again on Wed with focus more across eastern half New Eng. Temps through Wed will average near or slightly below normal with highs mainly 75-80 and lows in the 50s. Thursday and Friday... Pattern change signaled as upper trof moves out with rising heights and zonal flow across New Eng. This will result in a warming trend with temps into the 80s Thu and possibly 90+ on Fri. Humidity levels should still be comfortable Thu as dry air remains in the lower levels, but higher dewpoints will be moving in by Fri. Mainly dry weather, but developing warm advection may bring a few showers Thu afternoon to northern areas, then chance of afternoon showers/t- storms Fri as atmosphere destabilizes. && .AVIATION /08Z SATURDAY THROUGH WEDNESDAY/... 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... MVFR to LIFR mainly across SE New England with accompanying lower VSBYs as RA/+RA spreads across the region this morning with the threat of TSRA and SW gusty winds up around 30 kts. In addition there is the threat of LLWS as winds 2 kft agl will be out of the SW around 50 kts. All of this clears out during the later half of the day, becoming SKC, with winds shifting W. Quickly becoming VFR with Cape and Islands holding out longest, 21z. Tonight... W winds becoming light. Mainly SKC. VFR. KBOS Terminal...LLWS threats should remain S/E of the terminal. Looking at RA/+RA threats around 12z, yet the bulk of those threats look to remain S/E. Will hint for a brief 3-hour period in the TAF. Breezy SW winds with gusts up around 25 kts possible towards noon, then winds become light and W. KBDL Terminal...RA/+RA potential moving in around 9z. TSRA is possible. TEMPO IFR VSBYs with +RA. Could see some gusty winds. Outlook /Sunday through Wednesday/... High confidence. Mainly VFR. A few diurnally driven showers and perhaps an isold t-storm each day. && .MARINE... Low clouds and fog across the SE waters especially as the remnants of Cindy advect from the SW out ahead of a sweeping cool front, most of the activity mainly through the afternoon hours. Moderate to heavy rain, along with the threat of thunderstorms, some of which could be strong to severe with gale force wind gusts. Mariners may also need to be on the lookout for possible water spots as the environment is conducive for brief tornadic spin-ups. Improvement towards the evening hours as the cool front sweeps the waters. Quickly clearing out as winds shift to the W and become light. The 6 to 8 foot wave action on all outer waters and S inner waters should begin to diminish towards Sunday morning. Outlook /Sunday through Wednesday/... Sunday...High confidence. Some lingering 5 ft seas possible over the southern waters, otherwise winds will remain below SCA. Monday through Wednesday...High confidence. Quiet boating weather with winds and seas below SCA. A few hours of nearshore lower 20 knot wind gusts are possible each afternoon. && .TIDES/COASTAL FLOODING... Astronomical tides are rather high through this weekend during the night time cycles. Boston has a 12.4 ft high tide around midnight tonight and 12.2 ft just after midnight Sunday night. While, offshore winds are forecast current conditions suggest a 0.4 surge which will result in minor splashover. Thus will go ahead an issue a coastal flood statement for tonight`s high tide. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...High Surf Advisory until 8 PM EDT this evening for MAZ020-023- 024. RI...High Surf Advisory until 8 PM EDT this evening for RIZ006>008. MARINE...Small Craft Advisory until 2 PM EDT this afternoon for ANZ231>234. Small Craft Advisory until 11 AM EDT Sunday for ANZ235-237. Small Craft Advisory until 2 AM EDT Sunday for ANZ250. Small Craft Advisory until 8 PM EDT Sunday for ANZ254>256. && $$ SYNOPSIS...KJC/Sipprell NEAR TERM...Sipprell SHORT TERM...Sipprell LONG TERM...KJC AVIATION...KJC/Sipprell MARINE...KJC/Sipprell TIDES/COASTAL FLOODING...WFO BOX Staff

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