Area Forecast Discussion
Issued by NWS Buffalo, NY

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000 FXUS61 KBUF 192040 AFDBUF Area Forecast Discussion National Weather Service Buffalo NY 440 PM EDT Sat Aug 19 2017 .SYNOPSIS... A weak upper level disturbance will cross the region with a few widely scattered showers and thunderstorms through this evening. High pressure will then bring a return to dry weather Sunday and Monday with gradually warming temperatures. A strong cold front will cross the region Tuesday, with widespread showers and thunderstorms. This will be followed by much cooler and less humid conditions for the end of next week. && .NEAR TERM /THROUGH SUNDAY/... A rather sharp mid/upper level trough axis will move across the region through this evening. A moderate southwesterly flow ahead of this has resulted in lake shadows northeast of the lakes this afternoon, but the lake shadows should break down as a surface trough pushes across the region this evening. Expect mainly scattered showers and thunderstorms, which will be most widespread across the Western Southern Tier which is closer to the strongest DPVA and instability located to the south of the region. Showers and storms will also develop across Niagara/Orleans counties and these should weaken as they push southward into the lake shadow. Afternoon temperatures will be in the 70s. A surface trough trailing the upper level trough will drop from north to south across the region this evening. This will shift winds to the northwest and may result in another round of lake/upslope enhanced showers east of Lake Ontario. Farther west, increasing subsidence and drying will bring clearing skies. Expect fog to develop in river valleys across the Southern Tier. Lows will range from the lower 60s on the lake plains to the mid 50s across the interior Southern Tier and Lewis County. On Sunday, high pressure will ridge into the region with dry weather and partly to mostly sunny skies. In the morning there may be some lingering lake induced cloud cover, then in the afternoon diurnal cumulus should develop inland of the lake breeze. In each case, cloud coverage should be partial at most. Otherwise, temperatures will be a bit warmer with highs ranging from the mid 70s to lower 80s. && .SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY NIGHT/... Sunday night and Monday will be characterized by generally fair weather as upper level ridging transits the region. Speaking of transits, the ridging should provide for good viewing conditions for the upcoming solar eclipse - while we may see some diurnal fair- weather cu developing inland of the lakes during the afternoon, skies will remain mostly clear during the early afternoon hours on Monday. Skies will be clearest northeast of the lakes, where the prevailing southwesterly flow should generate a lake shadow. The aforementioned southwesterly flow will be the product of broad return flow around a prominent western extension of the Bermuda High that will be solidly in place across the southeastern U.S.. While temperatures Sunday night will be seasonable, with clear skies and light winds allowing for radiational cooling that will in turn cause temperatures to drop into the upper 50s in the coolest Southern Tier/North Country spots, with low to mid 60s elsewhere, readings will climb into the mid 80s on Monday as the return flow advects increasingly warm and moist air into the region, with 850mb temps climbing to +18C by Monday afternoon. The southwesterly flow and associated downsloping will tack on an additional couple of degrees across the Genesee valley and the Lake Ontario plain, where highs will reach the upper 80s. The warm temperatures will be accompanied by increasing mugginess as well, as GOMEX moisture is drawn across the Mississippi and Ohio valleys and into the Great Lakes. Dewpoints will surge into the mid to upper 60s Monday night, resulting in lows that will fail to dip below 70 across much of the Lake Erie and Lake Ontario plains, while the rest of the forecast area will only dip into the mid 60s, making for a sticky night. The hot and muggy weather will persist into Tuesday. The warm and moist airmass will become increasingly unstable on Tuesday, with the approach of a deep upper level trough tracking across the Great Lakes. This will likely drive showers and thunderstorms that will develop across the forecast area Tuesday afternoon ahead of a strong cold front that will be crossing the region Tuesday night. An anomalously strong 40+ LLJ ahead of the trough will advect additional moisture and significantly increase low-level shear as well, and latest 12Z runs are a little faster with the timing of the trough driving the convection, with lift arriving during peak diurnal heating Tuesday afternoon. Given the very warm and moist airmass preceding the system, and the strength of the synoptic system and associated vertical shear, severe weather is looking increasingly likely Tuesday afternoon into Tuesday evening, as noted in the SPC Day 4 Outlook. After another day in the mid to upper 80s, readings will fall off into the low to mid 60s in the wake of the cold frontal passage. && .LONG TERM /WEDNESDAY THROUGH SATURDAY/...
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A much cooler airmass will flow over the region this period, behind a strong cold front from Tuesday. Initially Wednesday a few lake effect rain showers will be possible east of the lakes as cool 850 hPa temperatures of +7 to +8C flow over the lakes. As the upper level trough passes Wednesday additional scattered showers will be possible across the region. In addition winds will be brisk Wednesday especially over the lakes and along the shoreline where WNW winds upwards to 25 knots and increasing wave heights will be likely. Thursday we will still have anticyclonic flow aloft, and with the core of the cold air passing over the lakes, additional lake effect showers and clouds will be possible...though with a thinner moisture layer...expect the lake response to be more clouds than showers. Both Wednesday and Thursday will be cool with highs in the upper 60s to lower 70s. Sprawling high pressure will then reach our region for Friday and Saturday with abundant sunshine and temperatures around 70 Friday increasing to the lower 70s for Saturday.
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&& .AVIATION /21Z SATURDAY THROUGH THURSDAY/... Scattered showers and thunderstorms will develop inland of lake breezes this afternoon, with lake shadowing early this afternoon gradually eroding late afternoon and early evening. Outside of this, expect partly cloudy skies with VFR conditions. Showers and a few thunderstorms may produce brief/local MVFR to IFR conditions The convection will end this evening, although a few upslope showers may continue across the higher terrain east of Lake Ontario where some MVFR CIGS may also develop. Valley fog will develop late tonight across the western Southern Tier with local IFR. Otherwise VFR will prevail for the rest of the region. Any valley fog will quickly dissipate Sunday morning, with mostly clear skies and VFR conditions during the day Sunday. Outlook... Sunday night and Monday...VFR except for late night and early morning valley fog with local IFR. Tuesday...Mainly VFR, with a chance of showers and thunderstorms in the afternoon. Tuesday Night...MVFR/IFR, with showers and thunderstorms likely. Wednesday and Thursday...VFR/MVFR with a chance of showers. && .MARINE... A moderate to strong WSW flow will continue this afternoon, then will diminish and shift to the NW tonight. This will bring Small Craft Advisory conditions to much of Lakes Erie and Ontario through this evening. Winds will then diminish from west to east tonight. There is a chance of waterspouts through this evening. Although the 850mb to lake temperatures difference is marginal, equilibrium levels will increase significantly with the arrival of the upper level trough. High pressure will bring a return to lighter winds Sunday and Monday. Then a strong cold front will result in increasing winds Tuesday which may require another round of small craft headlines before and after the cold frontal passage. There also may be strong thunderstorms along and ahead of the front. && .BUF WATCHES/WARNINGS/ADVISORIES... NY...Beach Hazards Statement until 11 PM EDT this evening for NYZ006-007-010-019-085. MARINE...Small Craft Advisory until 11 PM EDT this evening for LEZ020- 040-041. Small Craft Advisory until 8 PM EDT this evening for LOZ042. Small Craft Advisory until 5 AM EDT Sunday for LOZ044-045. && $$ SYNOPSIS...APFFEL NEAR TERM...APFFEL SHORT TERM...CHURCH LONG TERM...THOMAS AVIATION...APFFEL MARINE...APFFEL

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