Area Forecast Discussion
Issued by NWS Buffalo, NY

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000 FXUS61 KBUF 192025 AFDBUF Area Forecast Discussion National Weather Service Buffalo NY 425 PM EDT Fri Apr 19 2019 .SYNOPSIS... A very slow moving front and several waves of low pressure moving along it will bring periods of rain through tonight along with a chance of a few embedded thunderstorms. A cutoff low will then move very slowly from the Ohio Valley Saturday to the Mid Atlantic by Sunday evening. This will produce a few light showers at times over the weekend, but there will be plenty of dry time as well. Temperatures will remain at or a little above normal through the weekend and into early next week. && .NEAR TERM /THROUGH SATURDAY/... A sharp frontal zone bisects the area late this afternoon, stretching from the Lake Erie shore to very near KBUF to the east end of Lake Ontario and just south of KART. This frontal zone will very slowly sag south and east through the rest of the day. Radar imagery showing the first batch of mainly light showers moving north across and east of Lake Ontario early this afternoon. This will be quickly replaced by another expanding area of rain moving north across PA and Ohio, which will move into the area by evening. A cutoff, vertically stacked low will move slowly north across Tennessee tonight. A well defined inverted trough at the surface ahead of this system will stall over NY/PA, with several waves of low pressure moving along it. Meanwhile aloft, a plethora of vorticity maxima will move northward and cross our region in the deep southerly flow regime ahead of the closed low. The strong forcing and diffulent flow downstream of the cutoff low and trough will result in ageostrophic flow adjustments that will produce a number of gravity wave packets, which will then travel northward and cross our region, resulting in numerous hard to time periods of mesoscale ascent. Abundant deep moisture from the Gulf and Atlantic will be transported northward into our region ahead of the cutoff low. PWAT will reach about 1.4 inches, which is 2-3 sigma above normal. The quality of moisture and dynamics coming together will support fairly widespread areas of rain from late afternoon through most of tonight. Much of the rain will be light to moderate, but there will likely be several bands of heavy rain as well tied to subtle mesoscale forcing. Short term mesoscale guidance is still in poor agreement on location and timing of potential areas of heavy rainfall across our area. In a general sense, expect one or two batches of brief heavy rain in Western NY this evening, with the greatest risk then moving to Central NY and the eastern Lake Ontario region from late evening through the overnight. Assuming these corridors of heavy rain materialize and impact most of the region, expect rainfall to range from 0.75-1.25" on average. A few narrow corridors may reach 1.5-1.75" if some training of convective showers occurs. The greater flood risk will be east and southeast of our area where more convection and more training is suggested by the latest guidance. Elevated instability will be advected northward into the region late this afternoon through the first half of tonight, which will support a chance of a few widely scattered thunderstorms as well. Lows tonight will be in the lower 40s close to Lake Erie, the Niagara Frontier, south shore of Lake Ontario, and Saint Lawrence Valley well behind the sharp frontal zone. It will stay much warmer from Allegany County through the Finger Lakes to the southern Tug Hill region on the warm side of the boundary, with lows in the lower 50s by late tonight. Saturday the cutoff low will move from the Tennessee Valley to the Ohio Valley, with a weakening surface low becoming displaced north of the upper level low with time. A substantial mid level dry slot over the Mid Atlantic early Saturday morning will rotate northward into the eastern Great Lakes by late morning and afternoon. This will allow for a substantial decrease in shower coverage through the course of the day, with plenty of rain free time by afternoon. A few spotty convective showers may still develop in the mid level dry slot from boundary layer forcing, but coverage should remain sparse. The frontal zone will remain nearly stalled through Saturday, and continue to bisect the area leading to a big temperature range across the boundary. Highs will only be in the 50s across the Niagara Frontier, and even 40s along the Lake Ontario shore. Meanwhile on the warm side of the boundary, it will be in the upper 60s to lower 70s from Allegany County across the Finger Lakes to points southeast of Lake Ontario. Rochester remains a very difficult call for Saturday, being located right near the boundary. There may be a very large temperature gradient most of the day across Monroe County, with 40s and 50s near Hamlin and 70s near Mendon. && .SHORT TERM /SATURDAY NIGHT THROUGH TUESDAY NIGHT/... Saturday night the closed upper level low will drift from the Ohio Valley into the Virginias...with its attendant surface low devolving into a remnant trough across far western New York. In tandem with a deformation zone aloft...the latter will serve as the focus for scattered to numerous showers across extreme far western New York during the evening...with these decreasing in coverage overnight as available forcing becomes weaker and more diffuse. Elsewhere mainly dry conditions will prevail...with low temperatures ranging from the lower to mid 40s across far western New York to the upper 40s across the Finger Lakes and Eastern Lake Ontario regions. Sunday through Monday the closed upper level low will continue to slowly drift eastward and eventually move off the New England coastline. Periodic spokes of shortwave energy and attendant deeper moisture rotating around the upper low in concert with diurnal heating/destabilization will lead to the development of scattered showers mainly inland from the lakes on Sunday...with these then slowly receding eastward Sunday night and Monday as the upper low pulls further away from our region. Have accordingly maintained chance PoPs for these...with these maximized across inland areas Sunday afternoon and again across our eastern zones Monday afternoon... when daytime heating will lead to the greatest areal coverages. Otherwise...temperatures through this period will remain above average...with daytime highs away from lake influences warming from the upper 50s to mid 60s on Sunday to the 65-70 range on Monday as the upper low and its attendant cool pool aloft slide off to our east. Monday night any leftover showers across our eastern zones should finally come to an end early as the upper low drifts out to sea... with weak high pressure otherwise providing our region with a mainly dry and quiet night with lows mainly in the mid to upper 40s. Finally...Tuesday and Tuesday night should feature at least the approach (if not outright passage) of the next frontal system... though the exact timing of this feature remains questionable as the 12z runs of the GFS/GEM have slowed considerably compared to both their previous continuity and the 00z ECMWF. Given the latter and our previous forecast which also favored a much faster solution... for now have continued to lean more heavily toward the latter with another cold frontal passage and highest potential for some showers/embedded thunder Tuesday afternoon and evening...though this eventually could be delayed to as late as Tuesday night and/or Wednesday should the slower guidance turn out to be right. && .LONG TERM /WEDNESDAY THROUGH FRIDAY/... Near to above normal temperatures are anticipated during this period...as a progressive split flow will make its way across the Lower 48. The northern tier of the country...including the Great Lakes region...will be under the influence of the low amplitude northern branch...so the threat for any significant weather will be greatly minimized. That being said...the confidence in the details is notably lower than normal. All of the ensemble packages have shown a wide array of solutions with little if any run to run consistency. Will thus use a blend of HPC guidance with ECMWF as the basis for this three day period. Improving conditions are expected across the region on Wednesday... as a strong shortwave and its attendant surface low will cross New England and exit off the coast. In its wake...high pressure will gradually build across our forecast area. After some leftover showers east of Lake Ontario...this should support generally decreasing clouds for the midday and afternoon. A progressive mid level ridge within the northerly branch will cross our region with its associated sfc high Wednesday night and Thursday morning. This will promote fair dry weather for the bulk of the 24 hour period...although there are suggestions that some moisture could sneak north within the return flow late Thursday to support showers in the Southern Tier. Will not try to get too `cute` with the forecast from this range...especially given the inconsistencies in the guidance...so will broad brush with 15 pops for now. Conditions should deteriorate Thursday night and Friday though as a shortwave in the northern branch will sweep across the Great Lakes and attempt to phase with a closed low over the Deep South. While there is very little confidence in whether these systems can phase...there is some belief that some of the GOMEX moisture in the closed southern system can get entrained into the passing feature over our region. Have thus introduced low chc pops for Thursday night and Friday. && .AVIATION /21Z FRIDAY THROUGH WEDNESDAY/... A frontal zone bisects the area early this afternoon, stretching from Lake Erie to near KBUF to the east end of Lake Ontario. IFR will be widespread to the north of this boundary as low level moisture becomes trapped beneath a steep/shallow frontal inversion. To the south of the boundary VFR will prevail much of the afternoon. The frontal zone will sag slowly southward during the late afternoon and evening, forcing IFR to become more widespread across the area. IFR will continue through early Saturday morning before drier air moves back into the region, with conditions improving to MVFR and VFR through the course of the day. Several waves of low pressure will move along the stalled frontal boundary this afternoon through tonight, bringing with them periods of rain. The rain may be briefly heavy at times with local further reductions in VSBY. Much of the steadier rain will taper off Saturday morning as a mid level dry slot moves into the eastern Great Lakes. A few spotty showers will remain possible Saturday afternoon, but much of the time will be rain free. Outlook... Saturday night and Sunday...VFR/MVFR with a chance of showers. Monday through Wednesday...Mainly VFR with a chance of showers. && .MARINE... Northeasterly winds will increase this afternoon on Lakes Erie and Ontario as low pressure moves from the Tennessee Valley towards the Ohio Valley, and high pressure remains anchored from the western Great Lakes to central Quebec. Moderate to strong northeasterlies will continue tonight and early Saturday before diminishing late in the day Saturday as the low over the Ohio Valley rapidly weakens. Small Craft advisory conditions will continue in most locations through Saturday before improving Saturday night. Winds and waves will be much lower on Sunday. && .EQUIPMENT... The KBUF WSR-88D radar will be down most of next week for a scheduled upgrade. The radar will be down beginning Monday morning April 22, with a planned outage lasting 5 days until Friday April 26. During the upgrade, the transmitter will be refurbished including the installation of new fuses and cables. Some of the components being replaced are original to the radar, which is now over 20 years old. The transmitter refurbishment is the second major project of the NEXRAD Service Life Extension Program, a series of upgrades and replacements that will keep our nation`s radars viable into the 2030`s. This is the second project in the NEXRAD Service Life Extension Program. The first was the installation of a new signal processor. The last two phases of the project are the refurbishment of the pedestal, and refurbishment of equipment shelters. All phases are expected to be complete in 2022. && .BUF WATCHES/WARNINGS/ADVISORIES... NY...None. MARINE...Small Craft Advisory until 10 AM EDT Saturday for LEZ040-041. Small Craft Advisory until 1 PM EDT Saturday for LOZ030. Small Craft Advisory until 5 PM EDT Saturday for LOZ043>045. Small Craft Advisory until 8 PM EDT Saturday for LOZ042. && $$ SYNOPSIS...Hitchcock NEAR TERM...Hitchcock SHORT TERM...JJR LONG TERM...RSH AVIATION...Hitchcock MARINE...Hitchcock EQUIPMENT...Hitchcock

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