Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 210516 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 116 AM EDT Mon Aug 21 2017 .SYNOPSIS... High pressure builds over the region tonight and then offshore Monday, providing dry and comfortable weather. Hot and humid weather is forecast for Tuesday as the high moves farther offshore. A cold front will approach the area late Tuesday, then sweep across the region by mid-afternoon Wednesday, bringing the risk of showers and thunderstorms. Any storms could produce locally heavy rain and gusty winds. Skies clear out later Wednesday. Then strong high pressure brings fall-like weather with mainly sunny skies and clear nights Thursday through Sunday...with highs in the 70s and lows in the 50s. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... 115 AM update... Clear skies and light winds will allow temperatures to fall into 50s overnight, except lower 60s along immediate coast and in urban areas. Patches of valley fog possible as well. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TUESDAY/... Updated 4 PM... Monday... After a cool start to the day return flow (SW winds) becomes established by late morning as high pres slips offshore. Low level WAA results in slightly warmer and more humid conditions tomorrow with highs in the mid to upper 80s away from the south coast. Dew pts still tolerable (U50s to L60s) as warm sector airmass remains west-southwest of New England. Dry weather prevails along with mostly sunny conditions. Thus good viewing of the solar eclipse (just remember to wear safe glasses)! Could be some mid and high clouds arriving late in the day across western MA/CT, but by then the eclipse will have ended. Monday night... Low amplitude lead short wave and attending warm sector approach from the west. Convection over PA/NY/NJ late Monday likely weakens/dissipates as it approaches southern New England given loss of daytime heating and weak mid level lapse rates. Nonetheless low pops will be included in the forecast to account for any leftover showers especially western MA/CT. Much warmer and humid than previous nights given warm sector overspreads the area. This may be accompanied by stratus/patchy fog and possibly spotty light drizzle. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Big Picture... At upper levels, closed low pressure will be moving slowly across Canada, from west of James Bay Tuesday to eastern Quebec by Saturday. The trough axis extending southward from the low will remain to our west until Saturday. The cold pool of air will arrive over our region around Thursday and remain in place into next weekend. A weak ridge begins to build over the area on Sunday, as the trough finally exits. At the surface, high pressure will pass southeast of the region Tuesday. A cold front moves through Wednesday. Then strong high pressure builds over the Great Lakes states, with its influence extending eastward into New England Thursday into Saturday. The strong high pressure becomes centered around New Brunswick on Sunday. As the previous forecaster noted, there has been a tendency for the Canadian model to generate a tropical system off the southeast coast of the U.S. then head northeast towards us for the past several runs. Now, the UKMET, GFS, and to some extent the ECMWF all show the potential for tropical development late in the week, off the Florida coast. However, the significant change is that they all now focus the northeastward movement more along a persistent frontal boundary that currently exists several hundred miles south of New England. That would appear to make more meteorological sense, as tropical systems sometimes do form on the tail end of old frontal boundaries. The strong high pressure over the northeast states is expected to dominate our weather. However, it is possible that late next weekend there could be increasing potential for rip currents once again on southern beaches. Anyway, that`s a week away. Details... Tuesday... Summer is back. It will be hot and humid as high pressure shifts offshore and southwest breezes increase, well in advance of an approaching cold front. With 925 mb temperatures reaching 25-26C and a good deal of sunshine, expecting highs to soar to 90-95. Dewpoints will be rising to near 70 by late afternoon and heat indices will be reach the mid 90s. Our new criteria for Heat Advisories is 95 degrees, but it needs to be for 2 consecutive days, unless it hits 100, which would only require 1 day. Since it will probably not be reached on Monday, we may issue a Special Weather Statement to draw attention to it, as opposed to a Heat Advisory. Southwest winds increase to 30-40 kts at 925 mb Tue afternoon, so expect gusts at the surface to 20-25 mph at times, especially in southeast MA and RI. CAPE is maximized over NY state, well to our west, close to the cold front and that is where the best potential exists for strong/severe thunderstorm activity. However, a few showers and thunderstorms could form over the higher terrain of western MA and northwest CT in the mid-late afternoon hours. Tuesday night... Precipitable water values increase to 2.0 to 2.2 inches across the region Tue night, becoming maximized in eastern sections toward daybreak. We will have lost some heating as the front moves into western sections Tue night. Nevertheless the 12Z ECMWF keeps 1000+ J/kg CAPE overnight. There will be increasing wind fields aloft, mainly unidirectional. The Storm Prediction Center has a "marginal risk" of severe weather the western part of our area. Expect that we will mainly have remnants of the strong convection that had been to our west/southwest during the day, but there should be some new development as well. Any storm could produce locally heavy rainfall and gusty winds. Any training of storms or clusters of storms could lead to a flooding/flash flooding threat. Wednesday... Cold front swings across southern New England. Timing is still in question, but consensus would have the front exiting the coast in the afternoon. Southern New England will be in the right rear quadrant of a 300 mb jet max of 100-110 kts, which would be favorable for strong thunderstorm development, especially given high PW values still in place in eastern MA and RI through early afternoon. Skies will become mostly sunny from west to east during the mid and late afternoon. Highs in the 80s. Thursday through Saturday... High pressure builds in at the surface while an upper trough digs south from Quebec. The trough will carry a cold pool with cold advection aloft on Thursday and Friday. Temperatures at 500 mb will minimize at around -16C early Saturday morning. Moisture fields show a moist layer between 850 and 700 mb during this period, but dry air above and below. This looks like a mostly sunny and dry pattern, but the instability shows some potential for a few showers on Saturday. The mixed layer reaches to between 850 and 800 mb, with temps at the top of the layer supporting max sfc temps in the a fall-like feel. Overnight low temperatures will be in the 50s each night. Sunday... High pressure becomes centered over New Brunswick. Sunny, cool weather will continue across southern New England with highs in the 70s and northeast to east wind flow. It is possible that southernmost coastal waters could see increasing swell and potential rip currents if there is any tropical development several hundred miles south of the region. && .AVIATION /05Z MONDAY THROUGH FRIDAY/... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /Today Through Tuesday/... Patchy valley fog will bring local MVFR visibilities through sunrise. Otherwise VFR today as W winds become SW this afternoon. It`s possible a weak sea breeze develops near eastern MA coast 15z-18z, but confidence is not high enough to include in forecast. VFR conditions persist through midnight, but MVFR/IFR ceilings should develop overnight, especially from western New England to South Coast. Ceilings should slowly lift Tuesday to VFR, except along South Coast, Cape, and Islands where IFR may last much of the day due to persistent SW flow. Scattered thunderstorms are possible Tuesday afternoon across interior, mainly near Berkshires. KBOS Terminal...High confidence in TAF. Low chance of a weak sea breeze 15z-18z but currently expecting it to remain offshore. KBDL Terminal...High confidence in TAF. Outlook /Tuesday Night through Friday/... Tuesday Night into early Wednesday afternoon... Moderate confidence. VFR except areas of MVFR/IFR in showers and scattered strong thunderstorms mainly Tuesday night and Wednesday morning to early afternoon. Patchy fog. Later Wednesday afternoon through Friday... High confidence. VFR. The only exception will be patchy IFR in fog Wednesday night and early Thursday morning. && .MARINE... Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/...High confidence. 715 PM update... Tonight...Modest west winds become light and variable overnight. Good vsby and dry weather prevails. Monday...High pres moves offshore yielding modest SW winds. Dry weather and good vsby continue. Monday night...Modest SW winds continue and will likely be accompanied by low clouds/patchy fog along with a low risk for spotty drizzle. Thus vsby may be reduced at times especially southern waters. Outlook /Tuesday Night through Friday/... Tuesday...High confidence. High pressure shifts east of the waters. Winds 20 knots or less, and seas 4 feet or less. Tuesday night-Wednesday...Moderate confidence. A cold front approaches from the Great Lakes, crossing the waters late Wednesday afternoon. Showers and scattered thunderstorms will move across the waters Tuesday night and Wednesday, with locally heavy rain and gusty winds. Southwest winds increase with gusts 25 knots. Winds shift from southwest to northwest Wednesday night and diminish to 15 knots or less. Seas build Tuesday night and Wednesday reaching 5 to 6 foot heights on the outer waters. The southwest flow may also nudge those higher seas into RI Sound as well. Once the cold front moves through and winds become northwest, seas will subside to 3 feet or less Wednesday night. A Small Craft Advisory may be needed on some of the waters Tuesday night and Wednesday. Thursday-Friday... High confidence. Generally light north to northeast wind flow with seas below 3 ft. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...None. && $$ SYNOPSIS...Nocera/GAF NEAR TERM...JWD SHORT TERM...Nocera LONG TERM...GAF AVIATION...JWD MARINE...Nocera/GAF is the U.S. government's official web portal to all federal, state and local government web resources and services.