Area Forecast Discussion
Issued by NWS Boston, MA

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937 FXUS61 KBOX 160758 AFDBOX Area Forecast Discussion National Weather Service Boston/Norton MA 358 AM EDT Tue Jul 16 2019 .SYNOPSIS... Heat and humidity starts to build again today. The remnants of Barry will pass just south of Southern New England Thursday, but will bring showers and scattered thunderstorms, some with local downpours. High pressure builds for Friday through Monday bringing hot and humid weather. A couple of cold fronts will bring scattered showers and thunderstorms Sunday and Monday. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 140 AM Update... Band of high clouds continues to push across the region from the NW, though shifting slowly E as seen on latest GOES-East Nighttime Fog satellite loop. Noting more mid level clouds working into central and W NY state which are trending toward the E-SE. Temps at 05Z remain in the 60s across most areas, except noting upper 60s at KHFD, KPVD up to 75 at KBOS. Dewpts have dropped a bit, back to the upper 50s to around 60, except the lower-mid 60s on the outer Cape and islands. Noting low T/Td spreads there as well, so could see some fog develop there over the next few hours, but expect it to be brief and spotty. May see some breaks in the clouds after about 09Z through around mid morning or so. With light/variable or calm winds across most areas, temps may fall back to the mid to upper 50s if dewpts also fall back after 08Z or so, mainly across the inland valleys, but could hold in the mid-upper 60s along the immediate coast and urban centers, except possibly holding near 70 in downtown Boston. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM WEDNESDAY/... Another mostly dry day on the northern edge of the subtropical ridge. Things start off nice with lower humidity (dewpoints still in the 50s to low 60s) but as a warm front approaches winds turn out of the southwest and moisture advection increases as does humidity. 850 mb temps should support high temperatures in the 80s to near 90. A weak shortwave rotates through the flow today, coincident with increased mid/upper level moisture, expecting some increased cloud cover but minimal shower activity during the day. Can`t rule out at least a few showers in the warm sector late Tuesday into Tuesday night. The moist airmass in place and WAA aloft will keep low temperatures quite warm, in the 70s for many. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Big Picture... Zonal flow across the northern half of the nation while subtropical high pressure controls the southern half. Remnants of Barry lifting north will merge into the zonal flow between the Great Lakes and Ohio Valley, then move east with the zonal flow and crossing Southern New England Thursday. After it moves through, the subtropical high builds north over Southern New England for the weekend. Upper low pressure over the Gulf of Alaska dips south into the Pacific Northwest and then east with the zonal flow, approaching the Northeast USA early next week. Normal 500-mb heights for mid July are 580 to 582 Dm. Forecast heights are 585 to 588 Dm during late week, then build to 591 Dm over the weekend. Falling heights Monday. Expect warmer than normal temperatures through at least the weekend. General agreement among the model mass and thermal fields through Friday and possibly Saturday. The Sunday and Monday fields show small differences but generally favor continued hot temperatures. Forecast confidence is moderate-high. Concerns... Wednesday night to Thursday night... Northern stream shortwave moves through the zonal flow. This drives a cold front southeast through Southern New England Wednesday night. Remants of Barry merge with the front over the Midwest. Model consensus shows the front moving through our area early Thursday morning, then stalling south of New England. The remnants of Barry then move along the front and pass just south of the region. Light flow Wednesday night becomes east-northeast Thursday as the low center passes to our south. This may try to stabilize the surface layer while convection moves over the top. Instability is marginal, shear is weak. Precipitable water values will be 2 to 2.5 inches Wednesday night and much of Thursday...this is above 2 std deviations above normal. Values may diminish north of the Mass Pike but even there expect values over 1.5 inches. The deep tropical environment suggests showers and scattered thunderstorms. Moisture values are high, and the moisture extends deep through the atmosphere. Lift is forecast about 30 mb/hr. This suggests potential for downpours and potential for street and poor- drainage flooding. Heaviest rainfall looks to be across parts of CT, RI and Southeast Mass during the day Thursday. The last of the remnants moves east of our area Thursday night, and drier air moves in above 700 mb. This should wind down the showers. However, a lingering surface flow out of the east may hold low clouds and widely scattered showers into Friday morning, especially across Eastern Mass. Friday through Monday... Note that the ECMWF and GGEM hold onto showers Friday morning in Eastern Mass before ending precipitation. Could be widely scattered showers lingering into the start of Friday. All models play with showers to our west and east Friday, but little or nothing over us. They also show a weak surface trough moving through. At the same time, we see the upper ridge building over the Northeast USA Friday and lingering over us Saturday. Temps at 700 mb are expected to reach above 10C Friday afternoon, peak around 13C Saturday, and linger above 10C until late on Sunday. Tend to favor a rain- free forecast for Friday and Saturday. Cold front then works through on Sunday and may support a few showers/tstms, mainly in Western MA. Mixing reaches to 850 mb each day. Temps at that level are at least 18-20C, and data for Saturday suggests 20-22C. So sfc max temps will be in the 90s each day, and could approach 100F on Saturday. Dew points will be 70 to 75 in this period, leading to heat index values of 95 to 103F Friday and Sunday...and 100 to 105F on Saturday. Heat Advisory headlines may be needed Friday to Sunday. A second cold front approaches on Monday with another round of shower/tstms especially in the west. && .AVIATION /08Z TUESDAY THROUGH SATURDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/...High confidence. 06Z Update... Through 12Z...VFR. Light/variable or calm winds for most areas, except NW 5-10 kt across the ORH hills and INVOF KBOS, which should diminish by around 09Z or so. May see spotty MVFR VSBYS in patchy fog across portions of the CT valley, interior E Mass into N RI and along the S coast from 08Z-11Z. Today...VFR. Light/variable winds early shifting to S-SW 5-10 kt. Gusts to around 15 kt across S coastal terminals during the afternoon. Tonight...Mainly VFR. May see patchy fog develop after midnight across N central/W Mass with local MVFR-IFR VSBYS. Widely SCT SHRA/Isold thunder possible across W Mass into N Central CT after midnight, slowly shifts E toward daybreak. Wednesday...Mainly VFR through midday. Conditions lower to MVFR as areas of SHRA/SCT TSRA move in across N and central areas during Wed afternoon, which may include KBOS-KORH terminals. KBOS Terminal...High confidence in TAF through tonight, then moderate confidence Wednesday. KBDL Terminal...High confidence in TAF through tonight, then moderate confidence Wednesday. Outlook /Wednesday Night through Saturday/...High confidence. Wednesday Night: MVFR in clouds and fog. SHRA likely, chance TSRA. Thursday through Thursday Night: MVFR in clouds and fog. Chance SHRA, slight chance TSRA. Friday: VFR. Slight chance SHRA, isolated TSRA. Friday Night and Saturday: VFR. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Wednesday/...High confidence. 2 AM update...Current forecast remains on track. Relatively tranquil boating weather across the coastal waters through tonight. Seas generally 4 feet or less with winds 20 kt or less. Outlook /Wednesday Night through Saturday/...High confidence. Wednesday Night: Winds less than 25 kt. Rain showers likely, chance of thunderstorms. Thursday through Thursday Night: Winds less than 25 kt. Chance of rain showers, slight chance of thunderstorms. Friday: Winds less than 25 kt. Slight chance of rain showers. Friday Night: Winds less than 25 kt. Seas locally approaching 5 ft. Slight chance of rain showers, isolated thunderstorms. Saturday: Winds less than 25 kt. Seas locally approaching 5 ft. && .CLIMATE... Record highs for Wednesday July 17 are as follows: BOS: 98F in 1977 & 1999 BDL: 97F in 1999 PVD: 97F in 1977 & 1999 ORH: 93F in 1900 Record highs for Saturday July 20: BOS: 99 in 1991 BDL: 100 in 1991 PVD: 101 in 1991 ORH: 92 in 1901 Record highest min temps for Saturday July 20: BOS: 75 in 2016 BDL: 73 in 1975 PVD: 77 in 1983 ORH: 72 in 2013 Record highest min temps for Sunday July 21: BOS: 81 in 1991 BDL: 74 in 1977 PVD: 77 in 1980 ORH: 73 in 1991 && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...WTB/EVT NEAR TERM...EVT SHORT TERM...EVT LONG TERM...WTB AVIATION...WTB/EVT MARINE...WTB/EVT CLIMATE...

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