Extended Forecast Discussion NWS Weather Prediction Center College Park MD 235 AM EST Fri Feb 17 2023 Valid 12Z Mon Feb 20 2023 - 12Z Fri Feb 24 2023 ...Heavy snow likely across higher elevations of the West next week with the chance of significant snowfall continuing eastward across the Northern Tier of the U.S. increasing... ...Overview... A lot of attention for the medium-range period will be on snow chances for the West and the increasing chance for significant snowfall over the Northern Tier of the country next week. Heavy higher elevation snow will spread southward over the Pacific Northwest and Northern/Central Rockies Monday into the Sierra, Great Basin, and Southwest Tuesday into Wednesday. There is a stronger signal for more significant snowfall spreading eastward over the High Plains into the Northern/Central Plains and Upper Midwest by Wednesday and Thursday. Rain chances will also expand over the Ohio and Tennessee Valley Tuesday and the Mississippi Valley Wednesday, with the risk for some heavier rainfall particularly on Tuesday. An increasingly amplified pattern will lead to significant temperature anomalies with above average temperatures over the East and below average temperatures for the West. ...Guidance/Predictability Assessment... Both the ensemble and deterministic guidance capture a similar pattern at the start of the forecast period with a southern stream trough/closed low off the coast of Southern California and low-amplitude mean troughing in the northern stream across the CONUS with accompanied embedded shortwaves. One trend that looks to be following the pattern of the prior forecast is how the GFS and GEFS mean are a bit faster with the eastward progression of this closed low. This difference is particularly notable in the middle of the forecast period with the ECMWF trending slower with the evolution of the wave as it moves eastward through northwest Mexico and the Southwest U.S. The latest 12 Z ECMWF as well as the 12 Z UKMET/CMC have a similar, westward position, with the CMC continuing to be the furthest west of all the guidance. The last few runs of the GFS have been following a similar trend, with a slightly more westward position by the 12 Z run. However, the latest 18 Z GFS shifted back eastward, significantly different than the rest of the guidance. Unfortunately, due to the closed low transitioning into a weaker, open wave, the GEFS/ECENS/CMC ensemble means do not capture the feature well. The energy becomes embedded within broader scale troughing developing over the western CONUS with the northern stream. There are still timing differences towards the end of the forecast period but the now embedded shortwave nature of the feature makes it hard to track within the deterministic models, let alone the means, and differences that late in the period will have to be resolved in the future. Otherwise, as noted, the other main feature in the overall pattern is for a broad trough to develop over the western CONUS as the pattern becomes more amplified, with ridging to the east. The phasing and axis of this feature, too, shows differences amongst the deterministic guidance, especially past runs of the ECMWF which were slower to amplify. The pattern becomes highly amplified by the end of the period, with a deep, positively tiled trough retrograding slightly over the West Coast, and notable differences in the deterministic guidance on whether a closed low forms. The model blend for the WPC medium range product suite began with a general even blend of the deterministic guidance given the relative similarity. Chose to go with the 12 Z GFS as opposed to the 18 Z given the notable shift in the evolution of the southern stream closed low/wave, diverging from the other deterministic solutions. Began to quickly reduce the influence of the 12 Z GFS because of this uncertainty in the forecast with additional weight given to the other deterministic guidance. Initially began to introduce the 12 Z ECENS mean over the GEFS mean given differences with the southern stream wave during the mid-range period. The contribution of the means to the blend starts low to help keep more of the structure of the wave from the deterministic guidance. Retained a small contribution from the 12 Z GFS given how it captured the overall pattern as the focus turned to the broader-scale trough over the West in the mid- to late period, which was more similar to the ECMWF and GEFS/ECENS means. This led to trending away from the CMC as it was a bit further east compared to the GFS and ECMWF with the trough as the other models began to build westward. Reliance on the means loses some of the clarity on whether a closed low develops with the trough in the West but given the lack of a consensus on this evolution from the guidance, the means still capture the overall pattern well. ...Weather/Hazard Highlights... A relatively tranquil pattern for most this upcoming weekend will quickly change entering next week as precipitation chances will increase across the country. Much of the focus will be on snow chances expanding across the West and eventually into the Northern Plains and Midwest. A cold front will push southward through the Pacific Northwest/Rockies as moisture begins to stream in from the Pacific. Heavier snow chances will rapidly increase and expand Monday into Tuesday from more isolated, higher elevation ranges of the Cascades and Northern/Central Rockies across more of the interior valleys and into the Great Basin/Sierra. Upslope flow east of the Rockies will also increase snow chances for the High Plains. Chances will diminish Wednesday to the north as the focus shifts further south over the Sierra, Central Rockies, and portions of the Southwest. Initially average temperatures will trend more below average later in the week as broad upper-level troughing builds over the West. High temperature anomalies Wednesday and Thursday will be around 10-15 degrees below normal for most locations. Much colder temperatures are expected further north following the passage of a cold front, with highs 25-35 degrees below normal for portions of the Northern Rockies and adjacent Northern/Central High Plains. 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 the Southwest and into the Southern High Plains Wednesday as a frontal system passes through. A low pressure system will develop east of the Rockies and move into the Midwest during the middle of the week. While there is still some uncertainty, the chance for more significant snow further east into the Northern/Central Plains and Upper Midwest has been increasing. Meanwhile, a meandering frontal boundary over the Ohio Valley/Southeast will lead to increasingly widespread showers over portions of the Ohio and Tennessee Valleys, particularly on Tuesday. A Marginal Risk has been introduced in the Experimental Excessive Rainfall Outlook as the potential for more convective, heavier rain rates over wet antecedent conditions may lead to the risk for some isolated instances of flash flooding. Another frontal system moving through the Mississippi Valley and into the Midwest on Wednesday will continue the chance for rain across the region. 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 mean upper-level ridging begins to build over the region. Highs on Wednesday and Thursday in particular will be running 15 to 25 degrees above average from the Mississippi Valley east to the East Coast, with record-tying/breaking highs possible over the Southeast and Florida. High pressure is forecast to settle in the center of the country by the end of the period, bringing an end to precipitation chances for most. Chilly, below average temperatures will reach as far south as Texas. Coastal/lower elevation rain and higher elevation snow chances will still continue for the West Coast into Friday. 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