Extended Forecast Discussion NWS Weather Prediction Center College Park MD 227 AM 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... ...Overview... Much of the attention next week will be on a potentially 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 further southward later in the week. Rain chances will increase for the Ohio and Tennessee Valleys Tuesday, with a more expensive area of rain possible across the Midwest and Southeast 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... The pattern during the forecast period begins with a southern stream trough moving through the Southwest/northern Mexico and into the eastern U.S. as much broader, large-scale troughing develops over the western U.S., with additional associated embedded shortwave features to consider. Though it may be easy to overlook given the active weather pattern across the country, a rather impressive upper-level high for this time of year sets in over the Florida Peninsula. The initial challenge of the period is handling the evolution of the southern stream trough/closed low near Southern California/Baja Mexico that eventually ejects quickly to the northeast as a more compact shortwave. As has been the case the last couple of days, the GFS has been on the faster end of the deterministic guidance overall, and shows much more variability in the solutions run-to-run. A number of GEFS members are also generally further east than the ECens ensemble members. Another consideration is that the closed low has trended weaker, with the GFS and CMC no longer depicting a closed low in the more recent runs. The next concern is the handling of an embedded shortwave that quickly develops and shifts northeastward over the Plains as the pattern amplifies with the trough developing over the West. This feature will likely dictate more of the surface pattern and subsequent weather impacts, but is also harder to capture at this time frame. Finally, there is very high variability both between the individual guidance and run-to-run for each model with the eventual evolution of the western trough late in the period. This is confirmed by a review of the individual GEFS/ECens members whose solutions significantly diverge with time. Some of the past runs of the guidance indicates the development of a deep closed low off the West Coast, though both the 12 Z ECMWF and 12/18 Z GFS runs shifted away from this scenario. The one main takeaway is that all of the guidance agrees that the trough will build to the southwest as well as become more positively tilted with time. The WPC medium-range forecast blend initially began with a multi-model ensemble with a lower contribution from the GFS given the noted concerns about it's variability and handling of the southern stream trough. The 18 Z GFS was chosen over the 12 Z given how this run was more similar to the other guidance with the timing of this feature. A small contribution from both the 12 Z GEFS/ECens means was introduced during the middle of the period to help capture the overall pattern in the West. However, retained an emphasis on the deterministic blend a bit longer in order to retain at least a subtle representation of the evolution of both the southern stream wave and second shortwave to the north over the Plains. Fortunately, the feature has become pronounced enough across the individual GEFS and ECens members to be apparent in the pattern in their respective means. Shifted to more of an emphasis on the means by the end of the period given the rapidly increasing uncertainty in the forecast, especially considering the trend away from a closed low off the West Coast in the deterministic guidance which has not been showing up in the ensemble means. This provides at least a bit of confidence during this forecast cycle that the overall pattern is well captured as both the deterministic and ensemble guidance agree on this evolution. Used a combination of the GEFS and ECens means given a noted more eastward position of the trough axis in the CMC mean. ...Weather/Hazard Highlights... A deepening upper-level trough over the West as well as embedded shortwave energy pushing eastward over the Plains will result in widespread heavy snow chances across both the West and the northern Tier of the U.S. A cold front will push southward through the Pacific Northwest/Rockies as anomalously high moisture streams in from the Pacific. Heavy snow chances will cover most of the West Coast ranges and Interior West Tuesday, spreading into the Southwest on Wednesday. The focus for snow chances will shift southward to the Sierra/Southern California ranges and Southwest as another system from the Pacific moves across the region Friday/Saturday. Upslope flow east of the Rockies will also lead to increased moderate to heavy snow chances for the High Plains Tuesday into Wednesday. Very cold temperatures 25-35 degrees below average are expected for portions of the Northern Rockies and adjacent Northern/Central High Plains following the passage of the cold front Wednesday through Friday. High temperature anomalies overall across the West Wednesday-Saturday are forecast to be around 10-15 degrees below average. While not as cold as across the Northern Rockies/High Plains, some record tying/breaking low maximum and minimum temperatures are possible along the West Coast and portions of the Northern Great Basin. Winds will be breezy along the front of the Rockies early in the week, with an additional risk for some very strong, gusty winds moving through southern California, the Southwest, and into the Southern High Plains Wednesday as a frontal system passes through. Attention will quickly shift further 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 potentially significant heavy snowfall Wednesday-Thursday along and to the north of the surface low track across the Northern Plains and Upper Midwest continues to increase. Snow chances have also come up across the Interior Northeast/New England Thursday into Friday as the low pressure system departs the East Coast. Meanwhile, a meandering frontal boundary over the Ohio/Tennessee Valleys will lead to increasingly widespread showers Tuesday, with rain chances expanding to include the Mississippi Valley Wednesday as the southern stream energy moves over the region. 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, though confidence in heavier convective rains remains low overall. An axis of mixed precipitation including freezing rain will likely set up across the Midwest/Lower Great Lakes and into the Interior Northeast along the rain/snow transition. Above average temperatures will be common for much of the Southern Tier of the country to start the period, with even higher anomalies expected for the eastern U.S. as anomalous high pressure aloft settles in over the Southeast. Highs on Wednesday and Thursday in particular will be running 15 to 25 degrees above average across portions of the Midwest, Mid-Atlantic, and Southeast, with widespread record-tying/breaking highs possible. 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