Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 022347 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 747 PM EDT Tue Jun 2 2020 .SYNOPSIS... Variable cloudiness with a few passing showers possible tonight as warm front traverses Southern New England, but many will stay dry. A weak surface low pressure may produce a period of showers and thunderstorms Wednesday afternoon mainly south of the Massachusetts Turnpike and southwest of Interstate 495. Dry and warm on Thursday, then an approaching cold front will bring a chance of showers and thunderstorms Friday, with a better chance on Saturday. Cooler conditions follow Sunday with a risk of a shower, then dry and seasonably warm weather early next week. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/... 730 PM Update... Forecast is largely on track. Main change was to adjust the PoPs for the next few hours as a line of showers move in from New Hampshire into Eastern MA and RI. Even though radar is showing some 30 dBZ reflectivity, a lot of it is falling from virga with dew point depressions between 15 to 20 degrees, especially away from the immediate coast. Otherwise, brought near- term parameters more in line with current obs. PREVIOUS DISCUSSION... Variable cloudiness continues with less further east and more across the interior. Sea breezes have kept nearshore areas in the low 60s, and while it`s been a struggle to break free of mid-level clouds from this morning across the CT Valley and western MA, much of the interior is seeing current temps in the upper 60s to low 70s. Synoptic feature of interest remains a WNW/ESE oriented warm front that trails from north of the Great Lakes through central NY and the mid-Atlantic region. This feature is forecast to progress east with at least a greater increase in clouds as the night progresses. Models are continuing to struggle with indicating where any southeastward-moving showers may develop. Seems there are two real favored general areas: one across central NY that could progress into the CT Valley area overnight tonight, with another weaker area of showers now across the northern Adirondacks that could bring light showers toward the North Shore/Route 2 corridor late tonight. There remains some weak elevated instability in forecast soundings over eastern NY into western CT and that could lead to a rumble of thunder perhaps into Hartford County and part of the Berkshires. However, seems that progged most-unstable CAPE values were a little lower and given fairly weak forcing I`ve left prospects for thunder out of the forecast. Rising moisture levels with weaker southwest winds may allow a window for fog or mist to develop across the South Coast, Cape and Islands overnight into early Wednesday morning, though visbys unlikely to reach into the dense fog level. Lows will be several degrees warmer than last night, with values in the mid to upper 50s. && .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... Wednesday: Early-morning overcast skies with areas of mist or fog along the South Coast, Cape Cod and the Islands should give way to some cloud breaks by mid to late morning. We`ll then have to watch the progression/evolution of a weak mid- level disturbance currently contributing to active convection over western MN, which still remains source of significant uncertainty to the forecast especially for later in the day Wednesday. Today`s 12z synoptic models have shifted the warm front a bit further north/east. What`s less clear is where this disturbance aloft tracks and the northern extent of potential thunder chances. Northward position and location of showers/thunder has been difficult to pinpoint with this weak disturbance aloft; this feature potentially struggling with model-gridscale convective feedback issues. Today`s models have shifted further north and east has resulted in raising a somewhat greater possibility of thunder a bit more, at least from Windsor Locks to Cranston south and west, during the mid- afternoon to early evening (approx. 2 to 7 PM). Forecast soundings show advection of steeper mid-level lapse rates, which range from around 6-6.5 C/km in the GFS to about 1 degree C/km greater on the more robust NAM in the 700-500 mb layer. With rising dewpoints, it leads to most-unstable CAPE values around 1000 J/kg across northern CT into central RI. There is also a healthy amount of effective bulk shear into this instability layer at around 40 kt, less so further north. But as is often the case with these situations, both models show a fairly good cap to convection around 800 mb that will also have to be eroded before any storms were to get going. There are also only a few CAMs which show any particularly robust convection into northern CT into RI, and hardly any at all near/north of the Mass Pike. For the mid to late afternoon hours, at the moment I do think showers with embedded rumbles of thunder are the more likely outcome for areas such as Hartford to Windsor Locks east-southeast, with rain further north and east. If the capping should prove weaker or the shortwave tracks a bit further north and more favorably through Southern New England, a few stronger storms embedded in a larger swath of rain could ensue, possibly capable of hail or localized wind gusts given the shear/instability space. Given the uncertainty described in the track of the initiating disturbance and the degree of capping, however, how strong storms may get and how far north/east any storms make it is something that will need to be refined. While signals in the CAMs seem to be pointing more toward the lower Hudson Valley/Poconos area southward for potential stronger/severe storms (where SPC has delineated higher severe probs in the Day-2 Outlook), did note SPC did bring lower-end severe probabilities - 5%/Marginal Risk - northward into Litchfield and Fairfield counties in CT. Highs in the 70s to lower 80s for many, given southwest winds keeping sea breezes to the immediate coast. Wednesday Night: Any lingering showers or storms clear early Wednesday night. Frontal boundary shifts southward with west winds under building weak high pressure permitting drier weather. Gradually decreasing sky cover towards mostly clear to partly cloudy by daybreak. Lows mid 50s to lower-mid 60s. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Highlights... * Dry and warm Thursday * Few showers/t-storm possible Fri; better chance t-storms Sat * Cooler Sun with continued risk of a few showers * Dry and seasonably warm Mon/Tue Thursday... Deeper moisture and higher PWATs get pushed to the south behind the cold front. Result will be lots of sunshine and warm temps. Soundings show a deep boundary layer to 850 mb and above. 850 mb temps 12-14C will support highs in the low/mid 80s, except 70s along the immediate south coast. Comfortable humidity levels with dewpoints in the 50s, except lower 60s near the south coast. A weak mid level shortwave approaching from the SW may bring a few showers or an isolated t-storm Thu night, mainly south of the Pike. Friday into Saturday... There is some uncertainty with respect to moisture return and higher PWATs on Fri which will have an impact on instability. Given flat flow aloft with better forcing well back across the Gt Lakes, thinking any convection will be widely scattered at best with very marginal instability and weak mid level lapse rates. Most of the day will likely be dry with partly to mostly sunny skies and very warm temps as 850 mb temps increase to 15-16C. Highs Fri will reach mid/upper 80s away from the south coast. More humid as well as dewpoints climb into the 60s. Better chance of showers/t-storms Sat as mid level trough and attendant cold front approach from the Gt Lakes with better forcing and modest instability. CAPES possibly increasing to 1000-2000 J/kg. Can`t rule out a few strong storms with severe wx possible as wind field and deep layer shear increase with the approach of the mid level trough. More cloud cover expected Sat but highs should reach upper 70s and lower 80s. If more sun, temps would likely get into mid 80s. Sunday into Tuesday... High amplitude trough sweeps across New Eng Sun with cooler temps and risk of a few showers. Then it looks dry and seasonable early next week as New Eng will be under NW flow aloft and sandwiched between strong ridge across the midwest and upper trough to the NE. && .AVIATION /00Z WEDNESDAY THROUGH SUNDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. 2330z TAF Update: Through 03z: High confidence with VFR conditions. Chance of light showers affecting the Eastern terminals through 02z. Light SW winds of up to 5 kt except up to 10 kt on the coast. Later tonight: Moderate to high confidence. Mainly VFR, but some MVFR conditions possible after midnight mainly for CEF-BDL-BAF with possible showers. Confidence on how far east these showers extend is fairly low and could stay far enough west. May also see some patchy mist or fog with scattered IFR visibility towards the Cape and the Islands as moisture levels start to rise. But confidence in location and coverage of fog formation is relatively low. Will have to further monitor later tonight. Light SW winds. Wednesday: Moderate confidence. Any IFR-LIFR mist/fog South Coast, Cape Cod and the Islands gives way to BKN VFR ceilings all TAFs. Where there is greater uncertainty in the forecast is Wednesday afternoon into early evening, with the potential for SHRA/-TSRA as a disturbance moves through or south of Southern New England. Biggest challenge is how far north might thunder (were any to develop) might make it. Think best chance for thunder is mainly south and west of the TAFs, but could make it as far north as BDL-PVD and perhaps FMH. Best chance sub-VFR conditions in that corridor. WSW winds of 5 to 15 knots with some gusts into the lower 20 knots possible during the afternoon. Wednesday Night: High confidence. Clearing to VFR early. W winds decreasing to 5-8 kt. KBOS Terminal...High confidence in TAF. KBDL Terminal...High confidence in TAF thru 03z, trends moderate confidence overnight into Wed. Outlook /Thursday through Sunday/... Thursday: VFR. Thursday Night: VFR. Slight chance SHRA. Friday: VFR. Slight chance SHRA, isolated TSRA. Friday Night: Mainly VFR, with local MVFR possible. Breezy. Chance SHRA, slight chance TSRA. Saturday: VFR. Chance SHRA, slight chance TSRA. Saturday Night: Mainly VFR, with local MVFR possible. Slight chance SHRA. Sunday: Mainly VFR, with areas MVFR possible. Slight chance SHRA. && .MARINE... 730PM Update... Fairly quiet weather on the waters as seas remain under 4 ft and winds less than 20 kt. For tonight, high pressure will give way to a weak warm front with winds becoming SW. Rising moisture could allow for areas of fog to develop on the southern waters overnight. Visibilities between 1-3 miles, as low as one-half mile possible. A stronger disturbance moving southeastward Wednesday will allow for southwest winds to increase but still only lead to lower-20 kt gusts. Seas may get close to SCA criteria for southern/southeastern waters and that may need to be considered later but confidence wasn`t high enough to hoist at the time. Rain/embedded thunder possible from Block Island Sound eastward to the waters south of Nantucket/MVY late in the day. Improving conditions with lighter west winds and lower seas for Wednesday night. Outlook /Thursday through Sunday/... Thursday: Winds less than 25 kt. Local rough seas. Thursday Night through Friday: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers. Friday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of rain showers, slight chance of thunderstorms. Saturday: Winds less than 25 kt. Areas of seas approaching 5 ft. Chance of rain showers, slight chance of thunderstorms. Saturday Night: Winds less than 25 kt. Areas of seas approaching 5 ft. Slight chance of rain showers. Sunday: Winds less than 25 kt. Seas up to 5 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...High Rip Current Risk from 6 AM EDT Wednesday through Wednesday evening for RIZ008. MARINE...None. && $$ SYNOPSIS...KJC/Loconto NEAR TERM...Loconto/Chai SHORT TERM...Loconto LONG TERM...KJC AVIATION...KJC/Loconto MARINE...KJC/Loconto

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