Area Forecast Discussion
Issued by NWS Buffalo, NY

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000 FXUS61 KBUF 230532 AFDBUF Area Forecast Discussion National Weather Service Buffalo NY 132 AM EDT Mon Oct 23 2017 .SYNOPSIS... Warm and dry weather will continue through Monday afternoon. A cold front will then slowly cross the area Monday night and produce periods of rain. A few scattered showers and lake effect rain showers will then continue through much of the week as temperatures turn progressively cooler. && .NEAR TERM /THROUGH TODAY/... Strong high pressure will remain anchored off the New England coast overnight. Subsidence and dry air will continue to support fair weather with additional cirrus level cloudiness crossing the area. Model point soundings continue to show low level moisture increasing across central PA overnight, and some of this may move northward into interior sections of the Southern Tier late tonight and Monday morning. Otherwise increasing southerly flow and mixing will keep temperatures very mild overnight, with lows in the upper 50s to lower 60s on the lake plains and low to mid 50s across the interior Southern Tier and Lewis County. To put this into proper perspective... such readings will actually range near or above normal HIGH temperatures for this time of year. On Monday high pressure will drift farther east off the eastern seaboard. Subsidence and dry air will continue to hold moisture ahead of the next system at bay for the majority of the day, with mid level clouds increasing from west to east. A strong mid level trough will dig into the western Great Lakes through the course of the day. DPVA from the first shortwave will begin to reach Western NY by the end of the day along with deeper moisture. This may allow a few showers to develop at the far western end of the state by late afternoon. Temperatures will be well above normal again, with highs in the low to mid 70s on the lake plains aided by SSW downslope flow, while higher terrain remains in the mid to upper 60s. && .SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/... Time to `pay the piper`. After a prolonged stretch of absolutely gorgeous weather...we can expect a significant shift in the overlying weather pattern across the country. This change will not include a polarity shift in temperature anomalies over the nation but will also include a much more dynamic and progressive pattern...one that will involve three DISTINCT cold shots. We will only deal with the first of these colder intrusions in this short term period. A phasing of a split flow will take place as we open this period late Monday and Monday night...as a robust shortwave within the northern branch will dive south across the plains. This will carve out a full latitude longwave trough that will capture a southern stream storm system already found over the Upper Ohio Valley. As the mature southern stream system ejects to the north Monday night...it will push a surface occlusion across our forecast area. Strong frontogentic forcing associated with this boundary will be accompanied by some lift under the right rear entrance region of a 110kt H25 jet over southern Ontario...providing plenty of forcing within an airmass that will include PWAT values in the vcnty of 1.5 inches. This will produce a 3-5 hour swath of fairly steady rain... some of which could be heavy at times...especially if there is any embedded convection. The precipitation will be found within a deep southerly flow...one that will allow for some training while also retarding the eastward progression of the main swath of rain. This will encourage a soaking rainfall across the Southern Tier...but the resulting downsloping will eat away at rainfall amounts across the lake plains. As the occlusion pushes east during the course of the night...a distinct dry slot will start to work across the region. This will bring an end to the widespread rain over the far western counties while the steady pcpn will be in the process of working across the North Country. The other potential issue for Monday night will be the risk for some strong gusty winds. A 50-60 kt low level jet will precede the sfc occlusion and likely will precede the steadier rain. A portion of these strong winds could be mixed to the sfc...with the greatest risk area being found along the Lake Erie shoreline where downsloping could come into play. Am not particularly impressed with the strength of the forecast inversion and direction of the flow at inversion level to do anything more than to continue the mention of strong gusts in the HWO product. On Tuesday...all of the forcing associated with the initial storm system will exit across the St Lawrence Valley while a wealth of mid level dry air will spread across the entirety of our forecast area. This will bring an end to the remaining steady rain that will be found over eastern sections and should limit any possible lake effect rain that tries to get going north/northeast of Lake Erie. H85 temps will drop to around 4c over Lake Erie by the end of the day...so this should be just short of establishing any real response off Lake Erie. The cold advection will lead to a cooler day though...as max temps will range from the upper 50s over parts of the Southern Tier to the upper 60s across the North Country. Interestingly enough...these readings will STILL be above normal late October values. As a strong H25 jet digs into an already deep longwave trough over the Lower Mississippi Valley Tuesday night...a narrow shortwave ridge downstream will push across our forecast area. While this will keep the bulk of our region precipitation free...continued cold advection within a south to southwest flow could direct some nuisance lake effect rain showers over parts of the Niagara Frontier. Will continue to use low chc pops for that area only. As the aforementioned jet rounds the base of the longwave trough on Wednesday...it will become negatively tilted while making its way across the Ohio Valley and parts of the Lower Great Lakes. Negligible height falls with the trough and a lack of any strong jet dynamics will limit the potential for showers...although continued lake effect showers should still be in the place east-northeast of Lake Erie. On Thursday...the axis of the negatively tilted trough will push away from our region via the St Lawrence Valley. While the -2c air at H85 will keep some very minor lake effect rain showers in place east of both lakes...rising heights and increased subsidence will support fair dry weather elsewhere. && .LONG TERM /FRIDAY THROUGH SUNDAY/... The longwave trough over the center of the country will `reload` Friday...as the next vigorous mid level disturbance will dive southwards across the northern plains. This will re-establish the longwave trough while the next (second) shot of colder air will pour southwards into the Lower 48. The leading edge of this next cold shot will follow a cold front that will extend south form a sub-1000mb low over the Upper Great Lakes. This front will make its way across the Lower Great Lakes on Saturday...then is forecast to stall over our forecast area as it will become parallel to the mean H925-70 flow. Following a nice day of weather on Friday...this scenario will favor scattered showers for both days of the weekend. Looking further ahead towards the following week...a third shot of cold air will charge southwards...and this one will have the best chance of supporting our first flakes of the season across our forecast area. && .AVIATION /06Z MONDAY THROUGH FRIDAY/... VFR flight conditions to start the 06Z TAFS will remain across the region through at least 18Z. A deepening area of low pressure will track from the Ohio Valley through the Central Great Lakes today, while swinging a cold front across the region tonight. This front will bring a steady area of showers, with flight conditions deteriorating to MVFR (possible So. Tier IFR) late this afternoon and through tonight. Rain will taper off between 05Z to the west, and near 12Z to the east. In addition a strong LLJ up to 50 knots at 2K feet may produce some areas of LLWS across WNY tonight. Also the deepening area of low pressure will bring some gusty winds to the region, upwards to 30 knots. Outlook... Tuesday...MVFR in occasional rain early east of Lake Ontario. Otherwise MVFR/VFR. Wednesday and Thursday...Areas of MVFR with a chance of showers. Friday...VFR. && .MARINE... Modest southeasterlies late this evening will veer to southerly and increase to moderately brisk levels overnight and Monday out in advance of an approaching cold front...though given the offshore direction and stable temperature profiles...conditions across the New York waters of both lakes should remain below advisory levels. Monday night and Tuesday the cold front will cross the region from west to east...with periods of potentially advisory-worthy conditions possible at the east ends of both Lakes Erie and Ontario. && .BUF WATCHES/WARNINGS/ADVISORIES... NY...None. MARINE...None. && $$ SYNOPSIS...HITCHCOCK NEAR TERM...HITCHCOCK/JJR SHORT TERM...RSH LONG TERM...RSH AVIATION...THOMAS MARINE...JJR

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