Extended Forecast Discussion NWS Weather Prediction Center College Park MD 341 PM EST Mon Feb 13 2023 Valid 12Z Thu Feb 16 2023 - 12Z Mon Feb 20 2023 ...Midwest-Great Lakes snow and heavy rainfall/severe weather threat from the central Gulf Coast northeastward toward the Appalachians Thursday into Friday ... ...Pattern overview and Guidance/Predictability Assessment... An amplified and positive-tilt south-central U.S. upper trough and northern stream upper trough from central Canada Thursday will interact and work across the East Coast into the weekend. A series of upstream northern stream impulses will then uncertainly tend to carve out broader cyclonic flow across the U.S. northern tier in the wake of the lead systems. Meanwhile, a closed upper low/trough is slated to dig southward off the West Coast through later this week and may progress slowly toward the Southwest by early next week. The WPC medium range product suite was primarily derived from a composite blend of now more reasonably clustered latest guidance from the GFS/ECMWF/UKMET/Canadian and the compatible National Blend of Models. The composite blend tends to mitigate consistent with predictability mainly smaller scale system differences, to include some input from the GEFS/ECMWF ensemble means from the weekend into early next week to better address growing forecast spread. Overall, this all also acts to maintain good WPC product continuity. ...Weather/Hazard Highlights... A lead system will track northeastward from the mid-lower Mississippi Valley Thursday through the East Coast Friday. While there remains differences with the details and timing of the main waves, forecast clustering has improved with the latest 12 UTC guidance. The relatively greater potential for meaningful snow/ice still extends Thursday from the mid-Mississippi Valley through the Great Lakes and onward into Friday across the northern tier of the Northeast. Leading Gulf of Mexico inflow along/ahead of the system's trailing front should promote areas of heavy rainfall for some locations from the central Gulf Coast states northward across the Tennessee Valley to the Ohio Valley and south-central Appalachians as locally enhanced by cell training, wet soil conditions and terrain. The day 4 experimental Excessive Rainfall Outlook (to be issued soon after issuance of this discussion) will again depict a Slight Risk area. Rainfall along the East Coast should be overall somewhat lighter due to expected acceleration of the upper trough/leading cold front. The Storm Prediction Center now shows a threat for severe weather Thursday from the central Gulf Coast inland across the Tennessee and mid-Ohio Valleys. This system may be strong enough to produce some areas of brisk winds. Meanwhile upstream, cooling high pressure will settle in over the Interior West during the latter half of the week as this system crosses the East. The upper trough/low forecast to track offshore California late this week late this week through the weekend may bring a modest increase of precipitation to areas along the immediate coast. There has been enough spread and variability with the upper low track to limit confidence for determining how much moisture tracks along the coast versus staying just offshore, but the latest trend is offshore. Trends have been slower for the eventual arrival of the upper low into the Southwest, but eventually it should bring at least some moisture into parts of the Southwest/southern Rockies early next week, with precipitation possibly starting to develop over parts of the southern Plains/east-central U.S. in about a week. Meanwhile, northern stream flow rounding the upstream Pacific ridge should bring an increase of rain and mountain snow to the Northwest/northern Rockies, especially by Sunday-Monday. The Four Corners states and parts of the Plains will be quite chilly Thursday into Friday with highs 10-20F below normal. Temperatures will moderate thereafter, but the Interior West and Rockies may remain modestly below normal (mostly single digit anomalies) into early next week. Meanwhile expect unseasonably warm air across the East Thursday into early Friday ahead of the cold front advancing across the region. Some temperatures could reach up to 20-30F above normal, with widespread record warm lows, but record highs tending to be more confined to the Northeast and the Florida Peninsula. Frontal passage will bring a brief period of near to below normal temperatures Friday-Saturday (especially over the South) but then expect temperatures to rebound to above normal levels over the central-eastern U.S. by Sunday-Monday as supported by the large scale pattern evolution. Rausch/Schichtel Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy rain across portions of the Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, and the Ohio Valley, Thu, Feb 16. - Heavy snow across portions of the Pacific Northwest, the Northern Rockies, the Northern Great Basin, and the Northern Plains, Sun-Mon, Feb 19-Feb 20. - Heavy snow across portions of the Middle Mississippi Valley, the Great Lakes, and the Upper Mississippi Valley, Thu, Feb 16. - Severe weather across portions of the Lower Mississippi Valley, the Tennessee Valley, the Southern Appalachians, the Southeast, the Great Lakes, and the Ohio Valley, Thu, Feb 16. - Flooding possible across portions of the Southeast. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - Much below normal temperatures across portions of the Central Plains, the Central Rockies, the Central Great Basin, the Northern Great Basin, the Southern Rockies, the Southern Plains, and the Southwest, Thu-Fri, Feb 16-Feb 17. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml