Area Forecast Discussion
Issued by NWS Boston, MA

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236 FXUS61 KBOX 211057 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 657 AM EDT Mon Aug 21 2017 .SYNOPSIS... High pressure moves offshore today. Southwest winds bring warm humid air Tuesday night, which will feed developing showers and thunderstorms. The wet weather will continue into Wednesday, until a cold front sweeps across Southern New England. High pressure brings dry weather Thursday through Sunday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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7 am update... Patchy clouds and fog at sunrise. Much of this will dissipate as the sun warms the air. Observed temp were a couple of degrees cooler than forecast, but are now moving into the warming part of the diurnal cycle. No significant changes planned for the forecast. Previous discussion... Temperatures will rebound quickly this morning thanks to plenty of sunshine. Model cross sections show enough low level moisture to result in diurnal cumulus once again today, but coverage should not be enough to affect viewing of the partial solar eclipse this afternoon (unless one of those clouds ends up in the wrong place at the wrong time!) Any higher clouds will remain thin enough to allow good viewing. Remember to wear safe glasses in order to protect your eyes. The only other issue today is whether or not a weak sea breeze develops for a few hours from late morning into early afternoon, before gradient tightens and SW flow gets underway. Forecast soundings show deep enough mixing in Boston which should prevent sea breeze from making it onto land, although it`s certainly possible any sea breeze could stay parked over Boston Harbor. Highs will top out in mid to upper 80s across much of area.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/... Mid level short wave is expected to dampen out as it reaches SNE tonight. This will cause any convection that develops to our west today to weaken as it reaches out area, especially given loss of daytime heating and little overall support for convection this far east. It`s still possible that a few showers survive the trip, especially given increase in low level moisture, mainly in western New England and near South Coast. Cold front approaches from Great Lakes Tuesday, but will be far enough to our west to keep better chances of convection outside of SNE. Models are fairly aggressive in trying to bring at least scattered showers/storms into region, especially given surface- based CAPE in excess of 1000 J/kg and plenty of low level moisture as precipitable water values climb to nearly 2 inches. However better large scale lift should be located closer to cold front, so we`re not convinced we`ll see much in way of showers/storms until very late in day across western New England. It`s possible a few showers develop due to heating and moist airmass in place however. Other issue is with high temperatures. NAM keeps much more low level moisture locked in which keeps highs in low to mid 80s, while GFS does not and highs end up closer to 90. Persistent SW flow certainly favors NAM and it also has support from high-res models (NMM in particular). It may end up that south coastal areas hang onto clouds/fog for much of day, while areas farther inland are able to break out into sunshine. Based upon model consensus, nudged highs down slightly (mid to upper 80s). && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Big Picture... Low pressure from Central Canada digs over Quebec Tuesday night and swings the trough axis across the Northeast USA during the late week. Cyclonic flow lingers over New England through next weekend. With upper heights lowering after midweek, expect temperatures to trend seasonably cool during the period. GFS and ECMWF mass and thermal fields are similar through the period, although with small differences developing Saturday and Sunday. Thus, forecast confidence is moderate to high. Details... Tuesday night and Wednesday... Upper shortwave moving up the St Lawrence Valley will swing a cold front across the Northeast USA. Consensus of timing would bring the front across New York State Tuesday night and across Southern New England on Wednesday morning. Deep moisture along with PW values of 2+ inches will be acted on by favorable dynamics in the right entrance region of the upper jet. This suggests potential for thunderstorms with locally heavy downpours. Winds at 850 mb are forecast at 35-40 knots and 500 mb winds around 50 knots. So there is also a potential for a few strong wind gusts. Thursday through Sunday... 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 Friday. Temperatures at 500-mb will minimize at -16C to -18C 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 partly cloudy and dry pattern, but the instability shows some potential for a few showers. For now we will maintain pops at less than 15 percent. The mixed layer reaches to between 850 and 800 mb, with temps at the top of the layer supporting max sfc temps in the 70s. Dew points will be in the 50s Thursday-Friday and 45-55 over the weekend...with light winds allowing min temps in the 50s each night. The light wind flow will allow sea breezes to develop each late morning and afternoon Friday through Sunday. && .AVIATION /12Z MONDAY THROUGH FRIDAY/...
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Forecaster Confidence Levels... Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Tuesday/... VFR today, with W winds becoming 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 a sea breeze in the 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/... Areas of MVFR cigs/vsbys in showers/thunderstorms Tuesday night and Wednesday morning. Otherwise VFR all areas, with patchy IFR cigs/vsbys in fog Wednesday night/early Thursday and Thursday night/early Friday.
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&& .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. SW flow gets underway today as high pressure moves offshore. That said, gradient may weaken enough to allow for a sea breeze to develop on eastern MA waters from late morning into early afternoon before winds shift to S/SW. Small Craft Advisories are being posted from Tuesday afternoon into Tuesday night for south coastal and outer coastal waters. Gusts should reach 25 to perhaps 30kt, along with building seas up to 6-7 ft offshore. Outlook /Tuesday Night through Friday/... Tuesday night... Southwest winds near 25 knots Tuesday night will maintain building seas with heights 5 to 7 feet on the outer waters and on RI Sound and Block Island Sound. Wednesday through Friday... Cold front moves across the waters, turning winds from the west- northwest. High pressure then builds in with winds diminishing to less than 20 knots. Winds further turn from the north on Friday. Diminishing wind will bring diminishing seas, with some leftover 5 foot seas Wednesday followed by seas 3 feet or less Thursday and Friday. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...None. MA...None. RI...None. MARINE...Small Craft Advisory from 2 PM Tuesday to 6 AM EDT Wednesday for ANZ231>235-237-250-254>256. && $$ SYNOPSIS...WTB/JWD NEAR TERM...WTB/JWD SHORT TERM...JWD LONG TERM...WTB AVIATION...WTB/JWD MARINE...WTB/JWD

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