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Area Forecast Discussion
Issued by NWS Buffalo, NY
Issued by NWS Buffalo, NY
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509 FXUS61 KBUF 171037 AFDBUF Area Forecast Discussion National Weather Service Buffalo NY 637 AM EDT Sat Aug 17 2024 .SYNOPSIS... A large storm system over the Upper Great Lakes will slowly drift eastward across our forecast area through Monday. This will result in unsettled conditions with occasional showers and a chance of thunderstorms...although there will also be periods of dry weather. Some of the showers could be locally heavy...with a stronger storm or two not also out of the question. Following the passage of this system...Canadian high pressure will build across our region and provide us a prolonged period of fair weather and comfortable conditions that will last through the rest of next week. && .NEAR TERM /THROUGH TONIGHT/... Broad and vertically stacked low pressure will slowly drift east across Lower Michigan today. During this morning...the weak warm front extending eastward from the low should continue to help generate fairly numerous showers/possibly some embedded thunder across the North Country and Saint Lawrence Valley. Meanwhile further to the west...the combined effects of dry slotting aloft and weak subsidence behind the warm front should help to yield partly sunny and mainly dry conditions across much of western New York... with only a 20-30 percent or so chance of a few pop-up showers or storms. This being said...the partial sunshine that will be experienced across WNY this morning will also lead to increasing amounts of diurnally-driven instability within our already warm and humid airmass. As we push through this afternoon and the vertically stacked low to our west draws a bit closer...shortwave energy rotating around the low will impinge upon this more unstable airmass and should lead to the development of more numerous showers and thunderstorms from about the Genesee Valley and Finger Lakes westward...for which some likely PoPs remain in place from continuity. Meanwhile...convective coverage should tend to be more scattered across the eastern Finger Lakes and North Country this afternoon as this area will lie in between the departing warm frontal pcpn and the shortwave energy crossing far western New York. With PWATs remaining up in the 1.5 to 2 inch range...the potential will exist for some of this convection to produce locally heavy rainfall...especially if any slow-moving heavier cores can manage to train over the same areas. The amount of available instability will also support the potential for an isolated stronger storm or two featuring strong gusty winds...with environmental shear values continuing to appear too low to support an organized severe threat/ greater coverage of stronger storms. Otherwise...it will be a warm and muggy day with highs in the upper 70s to mid 80s...and surface dewpoints running between the mid 60s to lower 70s. As we move into tonight...the large stacked area of low pressure will meander its way into southern Ontario. With this feature slowly drawing ever closer and additional shortwave impulses circulating across far western New York...expect the western half of the area to continue to see a general likelihood of showers and a chance of thunder...though this activity may tend to diminish somewhat in coverage through the night as diurnally-driven instability wanes with the loss of heating and convective overturning. Further east... pcpn coverage should remain more scattered across the eastern Finger Lakes and North Country. Otherwise we can expect another warm and humid night...with lows again ranging from the mid 60s across the interior of the Southern Tier and North Country to around 70 near the lakeshores. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... Very unsettled weather will continue Sunday through Monday as an amplified pattern matures across North America. A strong mid level ridge will move from the Rockies into the Plains Sunday through Monday, with a deep longwave trough carving out over the eastern United States. A mid level low within the trough over Lake Huron Sunday morning will gradually evolve into a open wave as it slowly crosses the eastern Great Lakes Sunday night through Monday. At the surface, a weak low vertically stacked beneath the mid level circulation will approach the eastern Great Lakes Sunday and Sunday night before weakening into a trough by Monday. Sunday through Sunday night, DPVA and height falls will continue to spread east across the eastern Great Lakes, providing a background environment of large scale ascent. Numerous convectively augmented vorticity maxima will propagate through the trough, bringing periodic enhanced ascent to the region. Modest daytime heating will allow for weak diurnal instability to develop, with subtle differential heating and terrain boundaries providing low level focus for shower and thunderstorm development. In general, expect the most widespread rain to be across Western NY in the morning, and then expand eastward across the remainder of the area through the day. There is some suggestion in guidance that a dry slot and/or modest lake shadow may lower rain chances later in the afternoon over and northeast of Lake Erie. Occasional showers will continue Sunday night as the trough axis slowly crosses the eastern Great Lakes. Thunder chances should peak Sunday afternoon and evening with the diurnal cycle, although it`s possible some thunder may linger through Sunday night as weak instability persists, especially over and near the warm lakes. PWAT values will continue to run in the 1.5"-1.75" range Sunday through Sunday night with surface dewpoints approaching 70F. The ample low level moisture and persistent forcing will continue to support a risk of locally heavy rainfall and isolated flash flooding, mainly in locations where embedded convection repeats over the same areas. Monday, the trough will continue its slow march eastward. The surface front will become increasingly anabatic with time, allowing deep moisture and rain to linger on the cooler northwest side of the boundary. A broad area of stretching deformation and frontogenesis arcing from Quebec south-southwestward across the eastern Great Lakes will maintain wet weather through much of the day. There is still some chance of a little embedded thunder, but in general the thunder chances will diminish from northwest to southeast as instability wanes with the arrival of a cool airmass. && .LONG TERM /TUESDAY THROUGH FRIDAY/... The long stretch of unsettled weather will come to an end by Tuesday, with dry weather from midweek through next weekend. Tuesday through Wednesday a mid level trough will remain over the eastern US, however the surface front and deep moisture will be located along the eastern seaboard by Tuesday, then off the coast by Wednesday. Meanwhile, high pressure will build into the upper Great Lakes Tuesday and then expand into the eastern Great Lakes by Wednesday, with associated subsidence and drying supporting the return to dry weather. Thursday through Friday expansive high pressure surface and aloft will dominate the entirety of the Great Lakes, Ohio Valley, and New England with dry weather prevailing. Temperatures will start off quite cool midweek. Highs Tuesday will range from the upper 60s for lower elevations to the lower 60s for higher terrain. High temperatures will then add on a few degrees each day for the rest of the week, getting back into the upper 70s by Friday for lower elevations. Nights will be quite cool, with lows in the 50s close to the lakes and 40s in cooler inland locations. && .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/... Broad low pressure will slowly drift east across Lower Michigan today. During the morning hours...the weak warm front extending eastward from the low should continue to help generate fairly numerous showers/possibly some embedded thunder across the North Country...with ceilings there lowering to the lower end of the VFR range...and perhaps to MVFR across the higher terrain. Meanwhile further to the west...the combined effects of dry slotting aloft and weak subsidence behind the warm front should help to yield mainly dry and VFR conditions across much of western New York...with just a low chance of a few pop-up showers and storms. Any IFR/MVFR ceilings across the Southern Tier will likely mix out by late morning/early afternoon. As we push through this afternoon...the combination of building diurnal instability and another impulse rotating around the larger- scale low should help to trigger the development of another round of more numerous showers and thunderstorms from roughly a KROC-KDSV axis westward...with more scattered convection expected further east across the remainder of the area. Flight conditions should be mainly VFR...except for brief/localized reductions within any heavier showers and storms...and some lingering lower ceilings across the higher terrain of the North Country. As we move into tonight...the area of low pressure will meander its way into southern Ontario. With this feature slowly drawing ever closer and additional impulses circulating across far western New York...expect the western half of the area to continue to see a general likelihood of showers and a chance of thunder...though this activity may tend to diminish somewhat in coverage through the night as instability wanes. Further east...convective coverage should remain more scattered across the eastern Finger Lakes and North Country. With respect to flight conditions...expect these to generally be VFR...again save for brief/localized reductions within any heavier showers and storms...some lingering lower ceilings across the higher terrain of the North Country...and redeveloping lower ceilings across the western Southern Tier overnight. Outlook... Sunday...Occasional showers and scattered thunderstorms with associated reductions to IFR/MVFR...and MVFR/VFR otherwise. Monday...MVFR/IFR ceilings with showers likely and a chance of thunderstorms. Tuesday...Improvement to VFR. Wednesday...VFR. && .MARINE... Over the weekend expect a general southerly to southwesterly flow on Lake Erie and a southerly to southeasterly flow on Lake Ontario... with winds generally remaining at or below 15 knots. The above said...there will be a couple periods of slightly stronger (15-20 knot) SSE flow on eastern Lake Ontario...with the first of these coming early this morning...and the second then following later tonight and early Sunday. Fortunately...the offshore component of the flow will direct the bulk of the greater wave action across Canadian waters and should preclude the need for any Small Craft Advisories during these time periods. Of greater concern to marine interests this weekend will be the potential for scattered thunderstorms...with locally higher winds and waves possible in and near any such storms. Following the passage of a cold front...northerly flow will develop later Sunday night...then will strengthen across the Lower Great Lakes Monday. This will at least bring some choppy conditions to Lake Erie...with somewhat stronger winds across Lake Ontario potentially leading to some advisory-level conditions along the south shore of that lake Monday and Monday night. Conditions should then gradually improve on Tuesday as Canadian high pressure begins building into the region...with a slackening pressure gradient allowing for winds and waves to subside. && .BUF WATCHES/WARNINGS/ADVISORIES... NY...None. MARINE...None. && $$ SYNOPSIS...JJR NEAR TERM...JJR SHORT TERM...Hitchcock LONG TERM...Hitchcock AVIATION...JJR MARINE...JJR