Extended Forecast Discussion NWS Weather Prediction Center College Park MD 207 PM EST Sat Feb 18 2023 Valid 12Z Tue Feb 21 2023 - 12Z Sat Feb 25 2023 ...Heavy snow likely across higher elevations of the West and the Northern Tier of the U.S. next week... ...Record cold possible in the West while record warmth is possible in the Southeast... ...Overview... Much of the attention next week will be on a significant winter storm moving through the Northern Plains and Upper Midwest, eventually into the Interior Northeast. Heavy higher elevation snow will also continue over a large portion of the West, focusing farther southward later in the week. Rain chances are expected to increase for the Ohio and Tennessee Valleys Tuesday, with a more expansive area of rain possible from the south-central U.S. northeastward on Wednesday. Mixed precipitation including ice could be a threat across the rain/snow transition zone draped across portions of the Midwest/Lower Great Lakes. An increasingly amplified pattern will lead to significant temperature anomalies with above average max/min temperatures over the East and below average max/min temperatures for the West. ...Guidance/Predictability Assessment... After some considerable discrepancies over the past several days regarding the west-east placement of a southern stream upper trough/low moving across the Southwest/northern Mexico Tuesday, with the GFS suite tending to show a faster eastward track/merge with the northern stream compared to the slower ECMWF and at times much slower CMC, model guidance is finally coming into better agreement toward a middle ground solution for timing. Though there are still some small differences like the deterministic ECMWF on the slower side, and whether or not a closed low remains on Tuesday within the low, this is within normal spread for the forecast lead time. This feature moves quickly through the mean flow as a potent shortwave through the Southern Plains Wednesday to atop the Mid-Atlantic early Thursday. This shows fairly good agreement in principle but even the small differences could impact the axes of east-central U.S. rainfall, which will likely take longer to resolve. Additional energy is likely to dig into the trough in the West by Thursday, with some model divergence in what happens within this trough by late week. The bulk of recent models/ensemble members seem to be pulling a phased trough eastward, but some older runs of the GFS and ECMWF, as well as the most recent 12Z CMC, have been spilling energy southwestward toward the Pacific atop southern California or so and forming a closed southern stream low. While this is possible, at this point, favored the majority of the solutions with more phasing, but some separation in northern/southern streams by Saturday. Given the overall reasonably good agreement among the model guidance, the WPC forecast utilized a multi-model deterministic blend of the 06Z GFS and 00Z ECMWF/CMC/UKMET early in the period, trusting the blending process to smooth out any smaller-scale disagreements. Some GEFS and EC ensemble mean guidance was incorporated by the latter part of the period amid increasing uncertainty in the details. ...Weather/Hazard Highlights... The combination of the deepening, energetic upper-level trough and a couple of frontal systems will create widespread heavy snow chances and cold temperatures in the West. Heavy snow chances are forecast to cover most of the West Coast ranges and Interior West Tuesday, spreading into the Southwest on Wednesday and lingering from higher elevations of California and the Southwest while the Northwest quiets down. Multiple feet of snow are likely in some Western higher elevations through the week. Precipitation looks likely to return to the Pacific Northwest/Northern Rockies by late week. Attention will quickly shift farther east as an embedded shortwave helps to initiate a low pressure system forecast to track eastward across the Central Plains and into the Midwest. The chance for likely significant heavy snowfall Wednesday-Thursday along and to the north of the surface low track across the Northern Plains and Upper Midwest continues to be rather high considering the forecast lead time. Snow chances have also come up across the Interior Northeast/New England Thursday into Friday as the low pressure system departs the East Coast. Farther south, ample warm advection ahead of the low pressure system's cold front with Gulf moisture streaming in could lead to increasingly widespread showers Tuesday especially across the Ohio/Tennessee Valleys, expanding to include southern parts of the Plains and Mississippi Valley Wednesday as southern stream upper-level energy moves in. Wet antecedent conditions over portions of the Central/Southern Appalachians and Ohio and Tennessee Valleys may lead to an isolated flash flood risk Tuesday and Wednesday, while instability could be sufficient for higher rain rates farther southwest across the eastern Southern Plains to Lower Mississippi Valley, prompting an extension of a Marginal Risk in this issuance of the experimental Day 5 Excessive Rainfall Outlook. However, confidence in heavier convective rains remains low overall in terms of the axis/placement. Additionally, an axis of mixed precipitation including freezing rain is likely to set up across the Midwest/Lower Great Lakes and into the Interior Northeast along the rain/snow transition. Another threat with this system will be for some very strong, gusty winds moving through southern California, the Southwest, and into the Southern High Plains around midweek. Temperature-wise, there will be quite a contrast across the country given the amplified trough/ridge pattern. Cold temperatures of 20 to as much as 40 degrees below normal, especially in terms of highs, are likely across the northern half of the Rockies and High Plains Wednesday-Friday in particular. The western U.S. should see temperatures more around 10-20F below average, but this could be enough to set or tie some record lows especially in lower elevations of the Pacific Northwest by Thursday-Friday. Record cold high temperatures could expand farther into the Interior West. Meanwhile, widespread milder temperatures ranging from 10-30F above average are likely from the south-central into much of the east-central U.S. and much of the Eastern Seaboard other than the Northeast. This is likely to set dozens of warm low and high temperature records, with the biggest areal coverage currently expected on Thursday. The Southeast should be the most persistently warm, and monthly record highs cannot be ruled out given the the 594dm upper-level high centered just east of Florida. Cold fronts passing through the Southern Plains to Ohio Valley and Mid-Atlantic will moderate temperatures closer to normal in those areas by late week. Tate/Putnam 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 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