Area Forecast Discussion
Issued by NWS Boston, MA

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000 FXUS61 KBOX 222322 AFDBOX Area Forecast Discussion National Weather Service Taunton MA 722 PM EDT FRI JUL 22 2016 .SYNOPSIS... Scattered thunderstorms with a few possibly strong to severe this evening before diminishing in the late evening. More thunderstorms will likely develop Saturday afternoon and early evening, mainly across northern and eastern Massachusetts. A few of those storms may be severe. Hot weather is expected to continue this weekend. An approaching cold front will bring another round of scattered showers and thunderstorms Monday night and Tuesday. More warm weather follows midweek. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... 715 PM Update... Scattered thunderstorms, a few severe, finally developed and will probably see continued scattered thunderstorms throughout the evening. Thinking that the potential for severe will begin to diminish around 10 or 11 PM. Thus far, main hazard from these storms have been strong wind gusts. Have made just minor adjustments in temperatures and wind fields for tonight. Prior discussion... There remains below average confidence on convection through this evening. Sufficient instability for deep convection. Vertical wind shear is marginal and believe convection is most likely to take the form of individual pulse cells or small clusters. A few of these storms may produce strong wind gusts or large hail. Model instability parameters and high resolution model output points toward SW MA and CT as a somewhat more favored region for scattered thunderstorms. The area of focus may shift to RI for a time this evening. Since there is not a clear transitory large scale forcing boundary, outflow from one cell could initiate a subsequent cell. There is a signal of drier air attempting to work its way into northern MA per soundings and model K index fields. Have POPs this evening at barely chance category across northern MA. Low level jet across SE MA resulting in significant surface wind gusts, especially along portions of the Cape. This will likely continue into early evening and then gradually diminish. The strongest wind gusts are over land rather than water given the better low level momentum transfer above the hotter ground. Have generally gone with a mode blend for temperatures tonight. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... Updated at 2 PM... * Risk of strong to severe thunderstorms Saturday afternoon into early evening across northern and eastern Massachusetts... Have increasing confidence of a severe thunderstorm threat across northern and eastern Massachusetts Saturday mid afternoon to early evening. Main potential hazards look to be locally damaging wind gusts and hail. The 12Z GFS, ECMWF and NAM all continue to paint an area of instability across northern MA and eastern MA Saturday afternoon/early evening. This is being helped by an impressive 500 mb cold pool of temperatures lowering to around -13C in the 21Z to 00Z time frame per both the GFS and ECMWF in connection with a short wave trof embedded in a broad WNW flow. In addition, the vertical wind shear looks more impressive Saturday afternoon/evening than early this evening with 500 mb winds 35 to 40 knots. Other considerations include a low level moisture convergence signal from what is left of the surface frontal boundary and the presence of the left front quad of a 90+ KT upper jet over NE MA. Saturday will be another hot day as H85 temps ramin near 18C until evening. Anticpate max temperatures lower to mid 90s. Dewpoints will probably be 5 to 7 degrees lower than today, however, as somewhat drier air seeps into the region in the low levels. Heat indices should remain below advisory thresholds. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Big Picture... High pressure covers the Southern and Central USA much of the week. A zonal flow extends across Canada and the USA Northern Tier. Shortwaves in that zonal flow will move through New England Monday night and again on Friday. In between, parts of the large high will push up over our area. Forecast contour height fields remain higher/warmer than normal levels through the period, so expect temperatures to run warmer than normal most of the time. Details... Sunday... High pressure in place through the day. Some signs on the GFS and ECMWF that a twist may develop in the flow in Eastern MA that could generate an eastern coast sea breeze during the afternoon, but low confidence at this time. Solar mixing to near 850 mb with temps at 850 mb forecast at 16-17C. This would support max sfc temps inland at 87-91F, with ocean buffering keeping levels near 80 Cape and Islands. Northwest flow will bring lower/drier dew points of 55 to 60 inland and lower 60s along the South Coast. Light wind Sunday night. Some increase in high clouds and slowly increasing dew points should keep min sfc temps in the 60s, although upper 50s not out of the question in some normal cool spots. Monday... Shortwave drives low pressure across Eastern Canada with a cold front sweeping across NY during the day and New England Monday night. Most of the day should be rain-free. Some signs of marginal instability crossing during the morning which may be a warm front with a couple of light showers. Better instability moves in from the west during the afternoon and evening. Totals forecast in the upper 40s and mid-level lapse rates 6.2 to 6.5 C/Km. Precip Water values climb to near 2 inches. This would support thunderstorms, some with heavy downpours. Winds of 20-25 kt at 850 mb and 35 kt at 500 mb suggest potential for strong gusts in convection. Expect scattered afternoon/early night convection with local downpours. Temps of 18C at 850 mb supports another day of max temps in the low 90s. Tuesday-Wednesday... High pressure takes control and brings fair weather. Temps aloft will again be 16-18C which will support upper 80s and lower 90s. Thursday-Friday... Next upper shortwave moves through during this time. Both GFS and ECMWF seem inconsistant with their surface features but indicate at least one cold front moving through during this time. We will show chance pops in western sections Thursday afternoon and all areas Friday. Low confidence in details at this time. && .AVIATION /23Z FRIDAY THROUGH WEDNESDAY/... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/... This evening...Scattered thunderstorms have been increasing in coverage and intensity over the past hour. Have adjusted POPs accordingly. A few of these thunderstorms may produce strong wind gusts. VFR conditions expected to prevail except briefly lower in vicinity of scattered evening thunderstorms. The surface wind gusts will gradually diminish this evening across the Cape. Overnight...Moderate confidence. Areas of MVFR cigs/vsbys and even pockets of IFR tonight in areas of stratus and fog, mainly along the south coast and a few interior valley locations that receive rain. Winds shift to the NW overnight. Saturday...Moderate confidence. After any early fog dissipates. VFR dominates again most of the day. Scattered afternoon/early evening thunderstorms are possible, mainly across northern and eastern MA. A few of these thunderstorms may contain strong wind gusts and hail. Winds mainly W. KBOS TAF...Moderate confidence. Looking like an increased chance of a thunderstorm given the cluster now in Worcester County and moving eastward at 715 PM. There looks to be another chance of thunderstorms in the vicinity of the terminal Saturday late afternoon to early evening. A few of those thunderstorms could also contain strong wind gusts. KBDL TAF...High confidence of VFR prevailing. There is a risk of thunderstorms in the vicinity of the terminal through mid evening. Any thunderstorms could produce brief strong wind gusts. If it rains over or near the airfield, there would a possibility of MVFR visibilities in overnight fog patches. Outlook /Sunday through Wednesday/... Moderate-high confidence. Sunday...VFR. Possible areas of IFR in early morning fog. Monday... VFR. Possible IFR in early morning fog. Brief MVFR/IFR cigs/vsbys in afternoon and early night showers/tstms. Tuesday-Wednesday... VFR. && .MARINE... Forecaster Confidence Levels. Low - less than 30 percent. Moderate - 30 to 60 percent. High - greater than 60 percent. Short Term /through Saturday Night/...High confidence. Gusty winds will persist across most east facing nearshore waters through early evening, mainly within 3 miles of shore. This is why Boston Harbor and Ipswich/Massachusetts Bays were added. As the evening progresses, wind will drop below Small Craft Advisory thresholds but the SW flow will have produced seas to at least 5 feet across most south coastal waters and the outer potions of the east coastal waters. Thunderstorms that form over eastern Massachusetts during Saturday afternoon may impact the adjacent coastal waters during the late afternoon and early evening time period. A few of these thunderstorms may produce strong enough wind gusts to warrant a Special Marine Warning or two. Outlook /Sunday through Wednesday/... Moderate confidence. Winds less than 20 knots and seas less than 5 feet through the period. A cold front will bring scattered showers/thunderstorms Monday afternoon and night, with potential for strong wind gusts and brief poor vsbys in heavy downpours. && .BOX WATCHES/WARNINGS/ADVISORIES... CT...Air Quality Alert until 11 PM EDT this evening for CTZ002>004. MA...Air Quality Alert until 11 PM EDT this evening for MAZ005>007- 012>021. RI...Air Quality Alert until 11 PM EDT this evening for RIZ001>008. MARINE...Small Craft Advisory until 8 AM EDT Saturday for ANZ231>234- 236-250-254. Small Craft Advisory until 11 PM EDT this evening for ANZ230- 251. Small Craft Advisory until 2 PM EDT Saturday for ANZ235-237- 255-256. && $$ SYNOPSIS...WTB/Thompson NEAR TERM...Thompson SHORT TERM...Thompson LONG TERM...WTB/Thompson AVIATION...WTB/Thompson MARINE...WTB/Thompson is the U.S. government's official web portal to all federal, state and local government web resources and services.