Area Forecast Discussion
Issued by NWS Buffalo, NY

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000 FXUS61 KBUF 280743 AFDBUF Area Forecast Discussion National Weather Service Buffalo NY 343 AM EDT Sun May 28 2017 .SYNOPSIS... Quiet and dry weather will continue to prevail through this morning... before an approaching low pressure system brings increasing chances of showers and thunderstorms to western New York this afternoon... followed by more widespread showers and embedded thunderstorms tonight. After this system exits our region Monday...a large upper level trough will then meander its way across the region through much of the upcoming week...bringing a continuation of somewhat unsettled weather along with near to slightly below average temperatures. && .NEAR TERM /THROUGH TONIGHT/... During the course of today...surface-based ridging over eastern New York and New England will slide east and out to sea...with the axis of its parent upper level ridge also sliding from western New York into New England. Meanwhile...yet another upper level low will begin digging across the Upper Great Lakes...with an attendant broad surface low over Illinois gradually deepening and lifting northeastward to near Detroit. In the process...this latter feature will push its associated warm front into southwestern New York state this afternoon. With our area remaining under the influence of the passing upper level ridge through this morning...we can expect the first half of the day to be quiet and dry...with a good deal of sunshine early giving way to increasing diurnal cumulus by midday/early afternoon with strong diurnal heating of the still-moist lower levels. This said...it still appears that any real chances for showers and thunderstorms will hold off until the warm front approaches/arrives during the mid to late afternoon hours... resulting in slowly increasing convective potential from about the western Finger Lakes westward...with the greatest chances for this found over the western Southern Tier. As for temperatures...the warm advection regime out ahead of the approaching low will result in 850 mb temps climbing to the +10C to +14C range this afternoon...which will be enough to support fairly widespread highs in the mid to upper 70s...with a few of our normally warmer spots potentially reaching the 80 degree mark. This said...areas along the Lake Ontario shore from Niagara County over to Rochester will be kept noticeably cooler by a modest east-northeast flow off the much cooler lake waters. As we move into tonight...the surface low will lift northeastward into nearby southern Ontario and push its warm front across our region...before swinging its trailing cold front into far western New York late. The combination of continued warm air advection across our region...increasing height falls aloft...and the aforementioned surface features will provide ample lift in the presence of an increasingly moist airmass (PWATs climbing to around 1.5 inches)...resulting in widespread showers developing across our region from west to east. There will also likely be some embedded thunderstorms as well given the presence of some weak to modest instability... though the relatively limited nature of this and the unfavorable nocturnal timing should result in any storms remaining on the tame side. Otherwise we can expect a rather mild night...with the general warm air advection regime and a southeasterly to southerly surface flow both helping to keep temps from falling below the upper 50s to lower 60s...resulting in overnight lows that will be some 10 to 15 degrees above late May normals. && .SHORT TERM /MONDAY THROUGH WEDNESDAY NIGHT/... An expansive...nearly stationary stacked upper level storm system centered between Lake Superior and the heart of Ontario province will be our dominant weather feature through this period...and beyond to the end of the week. The resulting broad cyclonic flow of relatively moist unstable air over the Great Lakes region will support occasional shower activity...with the showers being enhanced from time to time by shortwaves that will rotate through the base of the broad cyclonic flow. Depending on the timing of these often subtle impulses...diurnal instability will add fuel to this activity as well. Interestingly...while the bulk of this period will be rainfree...it will be fair to say that the period will be characterized by many as being unsettled. While temperatures will average close to normal...there will be an almost imperceptible day to day cooling trend. As for the day to details... A cold front will be exiting our forecast area as we open this period Monday morning. The bulk of the leftover convection associated with the front will be focused on areas east of the Genesee Valley and particularly over the eastern Lake Ontario region. Meanwhile conditions will already be improving over the far western counties. As the front pushes east across the Adirondacks to western New England during the midday and afternoon...similar improvement can then be expected for the remainder of the forecast area. Temperatures for Memorial Day cook outs and other activities will be reasonable for this of year...with H85 temps around 10c supporting max temps in the low to mid 70s for most areas. Readings will be a bit lower over the North Country as that region will get a later start on any sunshine. While fair weather should be in place Monday evening to close out the holiday weekend...conditions will deteriorate during the overnight. The first of several shortwaves during this period will cross our region late Monday night...and it will be accompanied by a cool front that will help to generate a passing line of showers and possible thunderstorms. Will increase pops by 10 to 20 `points` regionwide. Subsidence behind the previous nights cool frontal passage should offer us mainly dry weather for the first half of Tuesday. Diurnal instability and the chance for another shortwave passage will then hike up the risk for more showers and possible thunderstorms for the afternoon. The exception would be in a corridor from the Niagara Frontier to the North Country where lake shadowing should prevail. Temperatures Tuesday should be a few degrees lower than those from Monday...with max temps in most areas ranging from the upper 60s to lower 70s. Given the difficulty in timing the passages of the subtle shortwaves...the short and medium range deterministic models increasingly diverge as we move from Tuesday into Wednesday. Will thus trend towards a more generic...ensemble approach at an earlier stage in the forecast process. This will result in less specificity...ie more broad brushing of pops. That being said... The daily shower activity may become a little more widespread on Wednesday...as there is some suggestion that at least one shortwave will cross the region...and possibly the base of the longwave trough itself. Will thus maintain high chc pops outside of the influences of the Lake Erie `shadow`. As will be the case with temperatures... will aim a few degrees lower than the previous day...with highs forecast to generally range from 65 to 70. && .LONG TERM /THURSDAY THROUGH SATURDAY/... The persistent closed low centered over Ontario in the days leading up to this period will slowly fill and push east as we end the week. While our region could experience more scattered showers in the process...shortwave ridging will build across the Lower Great lakes in the wake of the exiting stacked low...and this should support generally fair weather for the start of the weekend. It should be noted though that model consistency remains poor...so forecast confidence remains relatively low. That being said...day to day details will be limited. On Thursday...low level ridging and somewhat drier air is expected to promote mainly dry weather for our region. Will maintain slight chc pops to account for the larger scale cyclonic flow and model inconsistencies. Max temps will generally be in the 60s. A greater number of ensemble members favor a frontal passage sometime late Thursday night or Friday...so have raised slgt chc pops to chc to cover any possible showers. Again...afternoon highs will largely be in the 60s. A progressive shortwave ridge should build across the region in the wake of the front on Saturday...and this would offer a brief opportunity for some welcomed fair dry weather. && .AVIATION /08Z SUNDAY THROUGH THURSDAY/... Through sunrise...general VFR conditions will prevail under lingering high pressure...though the combination of mainly clear skies...light winds...and narrowing dewpoint depressions could allow for some patchy IFR/MVFR fog to develop just about anywhere. Whatever fog that does form will quickly mix out after sunrise this morning...leaving behind widespread VFR conditions for the balance of the morning and early afternoon hours under a developing cumulus field. An approaching warm front will then bring slowly increasing chances for showers and thunderstorms to areas from about KROC-KDSV westward during the mid to late afternoon hours...with some of these capable of producing brief/localized reductions to MVFR. Tonight surface low pressure will move into our region...along with more widespread showers and embedded thunderstorms. This will result in areas of MVFR and embedded pockets of IFR...which should tend to become more widespread late as low level moisture increases. Outlook... Monday...Showers and thunderstorms tapering off from west to east... with conditions improving back to VFR. Tuesday through Thursday...Mainly VFR...with localized MVFR possible in scattered (mainly afternoon) showers and thunderstorms. && .MARINE... As surface high pressure slides off to our east and a warm front approaches from the Ohio Valley today...easterlies will freshen a bit on Lake Ontario...though winds and waves will remain well below advisory levels. The only real ill effects from this will be choppy conditions on the western end of Lake Ontario and especially near the mouth of the Niagara River...where the winds will oppose the outflow from the river. Of greater concern will be an increasing potential for scattered thunderstorms during the mid to late afternoon hours as the warm front pushes into the area...with the threat of these greatest over Lake Erie. Showers and thunderstorms will then become widespread over our region tonight as surface low pressure slides eastward from the Central Great Lakes. Meanwhile...winds will continue to freshen while veering to the south following the passage of the aforementioned warm front. On Memorial Day...the surface low will swing its trailing cold front across the region during the morning...with the showers and storms quickly winding down from west to east following its passage. In the wake of the front...moderate southwesterlies will also develop across the Lower Lakes...however winds and waves should remain below small craft advisory levels. && .BUF WATCHES/WARNINGS/ADVISORIES... NY...None. MARINE...None. && $$ SYNOPSIS...JJR NEAR TERM...JJR SHORT TERM...RSH LONG TERM...RSH AVIATION...JJR MARINE...JJR/RSH

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