Extended Forecast Discussion NWS Weather Prediction Center College Park MD 235 PM 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 late Monday-Tuesday, with the risk for some heavier rainfall particularly on Tuesday, and across southern portions of the Mississippi Valley by Wednesday. Mixed precipitation including ice could be threats in the transition zone between rain and snow. 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. As has been the case for several days/model cycles, many GEFS members and GFS runs tend to be a bit faster with the eastward progression of this closed low Tuesday-Wednesday compared to EC/CMC guidance. This is particularly notable by Tuesday-Wednesday as the wave moves eastward through northwest Mexico and the Southwest U.S. as it is becoming incorporated into the mean flow. Fortunately the past couple of CMC runs have sped up to a better middle ground position. So despite the GFS (especially the 00Z) being a little faster, a multi-model deterministic blend of the 06Z GFS and 00Z ECMWF/CMC/UKMET were able to be used for the early part of the medium range forecast. Unfortunately this does not resolve some variations between models in frontal placement and QPF placement and amounts in the east-central U.S. around the Ohio/Tennessee Valleys and vicinity. The GEFS mean and GFS runs continue to be more bullish than other guidance with this QPF. Then model guidance continues to show good agreement at least on the large scale for energy to dig and create a broad trough in the West by midweek and likely amplifying for the latter half of the week as additional energy comes in. By later in the week, earlier GFS runs indicated an upper low closing off in the eastern Pacific from the positively tilted trough. In fact the 00Z GFS showed 500mb heights pulled westward compared to any ensemble member from any suite, with the 06Z GFS a little less extreme. So the GFS solutions were not preferred. But the newer 12Z GFS seems more in line with the other recent guidance, with a slow track of the trough eastward--though a closed low within certainly cannot be ruled out. By the latter part of the period, a fair amount of deterministic ECMWF and CMC were able to be maintained while increasing the proportions of the EC and GEFS ensemble means. ...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 could also increase snow chances for the High Plains. Chances will diminish Wednesday to the north as the focus shifts farther 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 are forecast to be around 10-15 degrees below normal for most locations. Much colder temperatures are expected farther 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 is then forecast to 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 farther east into the Northern/Central Plains and Upper Midwest continues to increase, which could spread to the interior Northeast by late week. 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 for Day 5 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 Lower Mississippi Valley on Wednesday could 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 may still continue for the West Coast into Friday. Putnam/Tate 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