Provincetown MA to Chatham MA to Nantucket MA out 20 NM Marine Forecast
|This Afternoon...E Winds 5 To 10 Kt. Seas 2 To 3 Ft. A Slight Chance Of Showers With Isolated Tstms.|
|Tonight...Se Winds Around 5 Kt, Becoming Sw After Midnight. Seas 2 To 3 Ft. Patchy Fog. Vsby 1 Nm Or Less.|
|Sun...Sw Winds Around 5 Kt, Becoming S In The Afternoon. Seas 2 To 3 Ft. Patchy Fog. Vsby 1 Nm Or Less.|
|Sun Night...Sw Winds 5 To 10 Kt. Seas 2 To 3 Ft.|
|Mon And Mon Night...Sw Winds 5 To 10 Kt With Gusts Up To 20 Kt. Seas 2 To 4 Ft.|
|Tue Through Wed Night...Sw Winds 10 To 15 Kt. Seas 2 To 4 Ft. Winds And Seas Higher In And Near Tstms. Seas Are Reported As Significant Wave Height, Which Is The Average Of The Highest Third Of The Waves. Individual Wave Heights May Be More Than Twice The Significant Wave Height.|
Area Forecast Discussion|
National Weather Service Boston/Norton MA
1013am EDT Sat August 8 2020
A weak upper level disturbance will move across the region today bringing scattered showers and isolated thunderstorms. Very warm to hot weather with uncomfortable humidity levels for much of the upcoming workweek. Limited storm chances Monday and Tuesday, with better chances on Wednesday through Friday.
Near Term - Until 6pm This Evening
No major changes to the forecast in the latest update. The shortwave trough is currently located over northern New England. This feature will lift northeastward through the afternoon. Still think there will be scattered showers. However, have knocked down the thunder mention as we remain under the poleward entrance region of at 250 hPa jet and the shortwave is exiting. These dynamic factors should limit activity today. Best chance is across far western MA/northern CT where there will be more instability. Will see a few hundred to around 1000 J/kg of surface based CAPE.
Though most are generally partly to mostly cloudy, a couple trouble spots early this morning. Water- vapor imagery shows shortwave disturbance aloft over eastern NY fostering some early-day scattered showers across the Berkshires, with some thunder being noted across southern VT. Expect scattered showers and thundershowers to eventually develop across the interior as the shortwave trough moves across New England. However not anticipating any stronger storms today with only a limited window to destabilize.
Meanwhile, a passing surface low SE of Nantucket is leading to onshore NE/ENE flow across eastern MA and the South Coast. Areas of low clouds and fog were still prevalent across the South Coast and the Cape and Islands and should continue to be thru mid-morning. Did increase sky cover a bit here with this update. However, already seeing modest improvement and expecting most areas here to scatter out.
Forecast otherwise looks to be on track.
Previous discussion... Zonal flow across the Northern USA with two shortwaves embedded in that flow moving across New England this weekend.
The first shortwave was moving into New England this morning and will cross the region during the morning and early afternoon before moving off through the Maritimes. Cold pool aloft moves through with the shortwave axis. The cold advection as the pool approaches will mean destabilization and increasing instability. CAPE values vary among the models, but agree on 400-600 J/Kg during the late morning and early afternoon. Indices such as the total-totals are forecast in the upper 40s and around 50 at that time. CAMS models show scattered convection firing over NW Mass this morning and early afternoon. With the cold pool(a region of relatively cold air) moving off later in the afternoon, the resulting warming aloft would work against convection firing late in the day.
Favor chance probability of precipitation for scattered showers/thunder in the interior.
Light flow with high pressure centered over Northern New England supports a light east flow across the region, a sea breeze flow in Eastern MA. This will keep high temperatures lower along the coast. Temps at 850-mb will be 13-14C, supporting max surface temps in the low to mid 80s, especially in the CT River Valley. It is possible mixing goes a little higher, which would allow max temps a couple of degrees warmer. Meanwhile the sea breeze should keep coastal areas in the 70s.
Short Term - 6pm This Evening Through 6pm Sunday
Shortwave ridge overhead and surface ridge both suggest subsidence and dry weather tonight. Dew points in the 60s suggest similar values for min temps across the region, although urban areas may remain around 70 or low 70s. Dew points in the 60s will be noticeable but not oppressive.
Another shortwave moves through the flow on Sunday. Much of its curvature will be focused on Northern New England, which suggests any potential for showers would be well to our north. Expect mostly sunny skies. Temps at 850-mb will be a little higher, around 15-16C, so expect max surface temps in the mid to upper 80s inland, cooler near the shore. If mixing goes a little higher, the normally warmer spots could touch 90.
Long Term - Sunday Night Through Friday
Highlights... * Very warm/hot weather with uncomfortable humidity levels for much of the workweek. Monitoring for heat advisories. Potential heat wave in some locations.
* Limited t-storm chances Mon-Tues, better t-storm coverage mid to late week with slow-moving cold front.
Details... (1) Temperatures/Humidity
Main story for a good part of the upcoming workweek is the return to hot and humid weather. Potential heat wave in some locations (at least 3 straight days of 90+ high temps), but a multi-day heat wave also is in the cards if high temps overachieve on downslope flow Sunday and/or if we can eke out any 90-degree days toward mid to late week. Heat Advisories may be needed if it looks more obvious that we'll reach the consecutive days of >95-99 F criteria.
Only significant break from the heat/humidity arrives toward the end of the workweek with a slow-moving cool front. Models have trended slower with its passage with each successive run, so that break in the heat continues to be kicked further down in time and is now looking more like Friday. 850 mb temps around +16 to +18C should be common through Thursday, though more in the way of cloudiness and rain chances by mid to late week that could keep some or all areas from reaching 90 degrees (widespread mid/upper 80s, possible low- 90s). With more sun than clouds Monday and Tues, those project as the warmest days (mid 80s to lower-mid 90s). Coolest highs will be near the South Coast and the Cape/Islands given SW flow (and resulting in potential for a few days of early-day fog/mist with higher moisture levels coming in). Little relief at night as well with muggy lows in the upper 60s to mid 70s.
In terms of humidity levels, hottest of the days (Mon/Tues) are still fairly humid but should have slightly lower dewpoints mix in during the afternoon. By midweek, somewhat lower high temps amid more clouds and rain may be offset by oppressive low to mid 70s dewpoints areawide. The need for Heat Advisories in parts of the upcoming week still is in consideration but not clear if or on which days we'll reach the consecutive days criteria. Nonetheless, it's going to be uncomfortable to oppressive even if heat indices fall just short of headline criteria.
(2) Precip/Thunder Chances
To begin the week (Mon/Tues), shower/storm chances are looking more limited due to background subsidence. Any storms likely mesoscale- forced (near sea-breezes and terrain). Really similar look to model soundings across the region both days: strongly-heated PBL with significant mid-level dry air and fairly limited CAPE values. Thus t- storm coverage and strength thru early-week looks limited, but any storm that were to develop would likely be slow-moving and be capable of localized downpours given weak flow (< 20 kt speeds thru entire depth of atmosphere). Most areas are dry much of the time, but any town(s) that do get a storm stand to get at least a temporary reprieve from the heat.
Mid to late-week offers the best chance for greater t-storm coverage across a larger portion of Southern New England, due to deeper vertical moisture profiles combined with the very slow approach to the cold front. Most global models indicating the best upper-level forcing remains locked in northern New England/adjacent Canada and mid-level flow trending more parallel to the boundary. There's some guidance that keeps the front from really clearing our area until late Fri. Will continue to focus higher Probability of Precipitation toward the Wednesday thru Fri timeframe. Too unclear/uncertain at this time range if we'll be looking at any strong to severe storms in this timeframe. One concern which I'll also mention in terms of generalities into the mid to late week is localized heavy-rain footprints, given the high PWAT (Precipitable Water) air and that we could be potentially dealing with successive day(s) of somewhat better shower/storm chances.
Weak high pressure to the north will keep overall winds on the light side, favoring east winds and a sea breeze along the Mass East coast. High pressure shifts south tonight and Sunday, bringing a shift to winds out of the southwest. Winds Sunday may be light enough to allow a sea breeze to again form, but confidence is low on this possibility.
Not expecting wind/sea conditions to exceed small craft advisory thresholds.
Outlook /Sunday Night through Wednesday/... Sunday Night through Tuesday: Winds less than 25 kt.
Tuesday Night: Winds less than 25 kt. Seas locally approaching 5 ft.
Wednesday: Winds less than 25 kt. Chance of rain showers.
NOAA Boston MA Office: Watches - Warnings - Advisories
CT...None. MA...None. RI...None. MARINE...None.