Area Forecast Discussion
Issued by NWS Buffalo, NY

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655 FXUS61 KBUF 091117 AFDBUF Area Forecast Discussion National Weather Service Buffalo NY 617 AM EST Mon Dec 9 2019 .SYNOPSIS... Low pressure will track across the central Great Lakes today with above normal temperatures and rain showers, followed by colder air and lake effect snow showers for mid week. High pressure will move across the region and bring a return to dry weather for Thursday night and Friday. && .NEAR TERM /THROUGH TONIGHT/... Regional radars showing reflectivities increasing along the north shore of Lake Ontario up through the Saint Lawrence Valley early this morning. Upstream ground truth showing just light rain with temperatures in the upper 30s to lower 40s. This activity will continue to push slowly south through the morning hours and will tend to weaken as it encounters some drier air in place across the area. Temperatures have warmed enough above freezing to end the threat for freezing rain across Lewis County. An upper level trough will dig across the Great Lakes region through tonight, while a surface low strengthens as it tracks across the central Great Lakes today and into southwestern Quebec by tonight. A south-southwest low level jet of 45-50 knots will provide warm air and moisture advection and favorable jet dynamics will support a round of widespread rain. Rainfall amounts heavily influenced by upsloping, with higher resolution guidance showing localized amounts exceeding an inch, while the majority of the area will get between a half inch and an inch of rain. The steadiest rain will be this afternoon/evening, then will taper off from west to east tonight. Temperatures will be well above normal during the period with high temperatures actually being met during this evening in the 40s across most areas, and even some lower 50s near the Lake Erie shoreline where downslope warming will be most pronounced. The cold front will enter western New York late tonight with southwesterly winds gusting to 35-45 mph. This front may also produce an additional round of rain showers. && .SHORT TERM /TUESDAY THROUGH THURSDAY/... A still deepening surface low will move across central and northern Quebec on Tuesday, with a trailing cold front crossing the eastern Great Lakes early in the morning. The majority of the large scale forcing and moisture transport will be associated with the warm advection regime well ahead of the cold front. The actual cold front will only produce a few brief showers as it moves east across the area early in the day. Developing upslope flow will allow showers to linger a little longer across the higher terrain east of Lakes Erie and Ontario. Precipitation will change over to wet snow in these upslope areas, but any accumulation will be minimal. Temperatures will still be in the 40s before daybreak, but will drop through the 30s during the day with moderately strong cold advection in the wake of the cold front. Wind gusts will reach around 30 knots in the morning northeast of the lakes in the wake of the cold front. A weak secondary cold front will move south into the area late Tuesday afternoon and evening. This feature may produce an uptick in snow showers along the south shore of Lake Ontario as it captures a weak lake convergence band and moves it onshore. This may produce some light accumulations southeast and east of Lake Ontario. Following this secondary front, shear and dry air will likely force Lake Ontario lake effect to end completely for a time overnight Tuesday night. Off Lake Erie, the close proximity of surface high pressure will continue to introduce dry air and shear in the boundary layer, greatly limiting lake effect potential despite deepening cold air. Expect nothing more than a few snow showers Tuesday night across the western Southern Tier. Accumulations will be light Tuesday night, with no more than an inch or two southeast of the lakes. Wednesday a mid level trough and associated clipper low will pass well north of the area, with the clipper cold front crossing the eastern Great Lakes during the mid to late afternoon. The cold front itself will do little, with nothing more than a few light snow showers. It will introduce a more favorable lake effect environment however. Convergence along the advancing cold front will merge with pre-existing lake induced convergence over both lakes, resulting in intensifying bands of lake effect snow. Lake induced equilibrium levels will briefly rise to 10-12K feet with the passage of the clipper as moisture improves and a pool of cold air aloft crosses the eastern Great Lakes. A favorably deep mixed phase layer will be located within the cloud bearing layer, yielding dendritic growth. Off Lake Erie... Expect a band of relatively light lake effect snow showers to move north out of the Southern Tier and across Buffalo in the morning, ending up north of Buffalo across the Northtowns and Niagara County by late morning. This band will begin to move back south and east across the area by early to mid afternoon as the cold front reaches eastern Lake Erie. The band will intensify as it crosses the Buffalo area from a boost of synoptic support from the cold front, and increasing lake induced instability. The strong band will then move onshore from Buffalo all the way down to Chautauqua County and push inland through the Southern Tier through the late afternoon. The band of snow will continue for a few hours across the western Southern Tier Wednesday evening before quickly weakening as inversion heights lower, shear increases, and moisture decreases with high pressure building over Lake Erie. The fast pace of the southward band drift will greatly limit accumulations, even though the band of snow may become quite heavy. Early estimates suggest 2-4 inches from the Buffalo Metro area into portions of Genesee County, and 3-5 inches across the higher terrain of Southern Erie/Wyoming counties into the Chautauqua Ridge. Even though amounts will be limited, if the brief burst of heavy snow materializes it would produce very difficult travel for a few hours. Winds will also gust 30-35 knots around this time, producing blowing and drifting snow. Off Lake Ontario... Expect a similar trend just a few hours later. Tuesday morning disorganized lake effect snow showers over the lake will push to the northeast end to near Kingston Ontario, possibly clipping Cape Vincent in northern Jefferson County. The band of snow will then intensify during the mid to late afternoon as the cold front approaches and synoptic support/instability improve. This intensifying band of snow will then move southeast across Jefferson County, crossing the Tug Hill region during the evening. The band will reach maximum intensity during this time frame as the best lake parameters line up with a period of upslope flow across the Tug Hill. The band will continue to march steadily southeast overnight, with a WNW to ESE oriented band of weakening lake snow moving into northern Cayuga/Wayne counties. A few snow showers may also clip the south shore farther west overnight from eastern Niagara to Monroe counties as boundary layer flow veers. Similar to Lake Erie, the short residence time of the band in any one location will limit accumulations. Expect 2-4 or 3-5 inches across the lower elevations of the eastern Lake Ontario region, with up to 6 inches across the Tug Hill Plateau. Wayne and northern Cayuga counties may see a few inches overnight. While amounts are not overly impressive, this band may produce difficult travel for a few hours with heavy snow and 30-35 knot wind gusts producing blowing and drifting. Thursday a few lake effect snow showers may linger in the morning southeast of Lake Ontario with minor additional accumulations. High pressure will then build east across the eastern Great Lakes, ending the lake effect snow and bringing some partial clearing. && .LONG TERM /THURSDAY NIGHT THROUGH SUNDAY/... High pressure will move east off the east coast Thursday night. Warm advection in its wake will bring an increase in clouds across the eastern Great Lakes, but any precipitation will stay well north of the region. Warm advection continues Friday as a mid/upper level ridge builds into the northeast states. The warm advection will produce some cloud cover, but dry weather will continue. After a chilly start, temperatures will warm into the upper 30s to lower 40s in most areas. A complex system will then cross our region this weekend. A southern stream trough will support low pressure developing over the Gulf coast Friday. The resulting low will move almost due north, with one low center moving up the west side of the Appalachians and into the eastern Great Lakes by Saturday, with a second weak lee side low moving up the eastern seaboard. This southern stream trough will phase with a northern stream trough moving towards the western lakes, allowing an expansive area of deep layer ascent to move north across our area late Friday night and Saturday. This will produce another round of rain areawide. There is a small chance the rain may start as a wintry mix across the North Country if precipitation begins early enough, otherwise this looks to be an all rain event as temperatures warm into the 40s on Saturday. The northern stream portion of this phasing system will then cross the eastern Great Lakes Saturday night and Sunday, producing additional chances of rain and wet snow showers as colder air moves back into the region. Highs Sunday will be in the mid to upper 30s. && .AVIATION /11Z MONDAY THROUGH FRIDAY/... Ongoing warm air advection will support increasing low level moisture reaching the western Southern Tier with MVFR cigs developing by 12z, then over the rest of Western and Central NY before 18z. Rain showers, possibly starting as mixed precipitation over inland cooler valleys of the North Country, will accompany the low level moisture early this morning. KART will be all light rain. Rain will then spread across the rest of Western New York this morning, then spread east by afternoon. Expect the southerly downslope flow to keep all TAF locations in the MVFR flight category with the exception of KJHW which does not benefit from southerly downsloping. A 40-50 knot LLJ will largely remain through this morning, which will result in LLWS. Outlook... Tonight...Areas of MVFR/IFR in rain. Tuesday...MVFR with rain showers ending from west to east. Snow showers possible late. Wednesday...IFR within lake effect snow east of the lakes. Thursday...VFR with localized IFR in lake effect snow showers east of the lakes. Friday...VFR. && .MARINE... Winds will stay generally out of the south today at 10 to 15 knots, but could be a bit higher east of Rochester this morning. A windy period is expected through much of the week as a strong cold front crosses the area Tuesday, followed by moderate westerlies Tuesday night through Thursday night. Marine headlines are outlined below. && .HYDROLOGY... A period of warmer temperatures will cause snow melt which will add to run-off from rain on today through Tuesday. For most basins, rainfall amounts will be less than an inch and the snow pack in place is limited, resulting in little risk for flooding. There is a bit more snow pack in place in the Black River basin, with Snow Water Equivalent (SWE) values in the 1-2 inch range. Model guidance shows a bit more QPF here, with 1-2 inches across localized favorable upslope regions. Latest RFC forecast has Boonville reaching action stage by late Tuesday, with Watertown poised to reach it just beyond the forecast period. This is in line with latest MMEFS ensembles which show a fairly high likelihood that these points will reach action stage but very low probabilities of flooding. Based on this, have added a mention of this risk to the HWO. The most likely outcome remains action stage, but the situation bears watching. The greatest risk is if temperatures are warmer than expected. If this happens then the additional snow melt would be a concern, but for now there is not enough confidence to issue any flood headlines. && .BUF WATCHES/WARNINGS/ADVISORIES... NY...None. MARINE...Small Craft Advisory from 10 PM this evening to 4 AM EST Thursday for LEZ040-041. Small Craft Advisory from 4 AM Tuesday to 1 AM EST Wednesday for LOZ043-044. Small Craft Advisory from 1 AM to 10 PM EST Tuesday for LOZ042. Small Craft Advisory until 10 AM EST this morning for LOZ045. && $$ SYNOPSIS...Apffel/TMA NEAR TERM...Apffel/TMA/Zaff SHORT TERM...Hitchcock LONG TERM...Hitchcock AVIATION...Apffel/TMA/Zaff MARINE...Apffel/TMA HYDROLOGY...Apffel is the U.S. government's official web portal to all federal, state and local government web resources and services.