Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Sun Feb 28 2021 Valid 12Z Wed Mar 03 2021 - 12Z Sun Mar 07 2021 ...Overview and Guidance Evaluation... At least through the 12Z/18Z cycles, guidance was showing some favorable converging trends with respect to the evolution of an upper low that forms offshore California during the short range and then progresses inland, along with how developing eastern U.S. upper troughing will eventually influence what remains of this feature later in the period. At the same time the models and means have consolidated fairly well for the upper trough amplifying off the West Coast into early Friday, but then continue to diverge with specifics of the leading shortwave as an upstream feature amplifies into the overall mean trough next weekend. Among data reviewed for this forecast the 18Z GFS and 12Z ECMWF/CMC ended up clustering fairly well for the upper low tracking through the Southwest and into the Plains. The 18Z GFS represented a slower trend from the 12Z run. The 12Z UKMET was the slow extreme. The 18Z GFS/12Z ECMWF and their means also provided a decent basis for the preferred blend for eastern U.S. upper trough that develops in response to the upstream pattern and ultimately suppresses the Southwest/Plains system toward the Gulf Coast before strengthening it over the western Atlantic next weekend. At the moment it looks like a close call regarding how much of an effect this system may have along central and northern parts of the East Coast but with greater likelihood of minimal influence. Specifics of this system by then will depend on exact details of northern stream trough energy which so far has differed among guidance and consecutive runs. Convergence for the eastern U.S. and vicinity has been by way of the GFS trending more amplified with the eastern trough and the ECMWF backing off from its prior depth somewhat. As upper troughing amplifies off the West Coast mid-late week, latest consensus represents an eastward trend in the ECMWF over the past couple days and slightly slower adjustment in the GFS. Then after early Friday the GFS/GEFS mean have consistently been ejecting the leading shortwave quickly inland as upstream energy drops into the mean trough, in contrast to the much slower and more amplified/sharper ECMWF. The 12Z CMC was a compromise though on the fast side with the upstream feature. Some aspects of each solution could have some merit. Typical GFS biases suggest it could be too fast but at that time frame there has been a modest faster trend with this feature in most guidance recently. Also the idea of a flatter shortwave ejecting into the downstream ridge could have some merit versus the ECMWF/ECMWF mean that bring more height falls into the ridge. Thus a blended approach seems best for the purposes of a single deterministic forecast. New 00Z model runs seem to continue the theme of volatility from recent days. The GFS/CMC/ECMWF have all trended westward with the upper low offshore California, joining the UKMET. The 00Z cluster remains slower than the 12Z ECMWF/18Z GFS as the system tracks through the Southwest but gradually catches up over the Plains on Friday. However this slower trend is enough to lead to a more suppressed surface pattern farther eastward as the eastern upper trough has more time to amplify. Meanwhile solutions have remained in their respective clusters near the West Coast during the latter half of the period. ...Weather/Threats Highlights... On Wednesday expect precipitation over the Southeast/southern Mid-Atlantic to taper off as a wave initially along the southeastern coast continues into the Atlantic. Some rainfall could still be locally heavy early in the day depending on timing of this system. The upper low closing off to the west of California during the short range period should move through the Southwest and into the Plains during the latter half of the week. This system will bring some rain into parts of southern California and rain/higher elevation snow to portions of the Southwest and southern Rockies. Then rainfall over the Plains (possibly moderate to heavy in places) should stay suppressed at least as far south as the southeastern quadrant of the lower 48 as the surface system drops toward the Gulf Coast or even into the Gulf, possibly continuing over the western Atlantic. Confidence is improving for the general path of the upper low through the Southwest into the Plains but timing is still a question. However confidence remains lower than desired for precipitation coverage and intensity over the eastern half of the country due to the combined uncertainties of the upper low's timing and relation to the upper trough that develops over the East. 00Z model trends toward slower upper low timing could allow for sufficient eastern U.S. trough amplitude to prevent most of the moisture from extending eastward from the Plains. Currently expect most effects to remain offshore the East Coast if low pressure develops over the western Atlantic next weekend. Precipitation ahead of the amplifying eastern Pacific trough aloft will initially focus primarily over Vancouver Island and into the Olympics, with some moderate to heavy totals possible. Expect rain and mountain snow of varying intensity to extend southward to the central West Coast by late week and the weekend. Some light precipitation will be possible at times during the period over the northern Rockies, and possibly over parts of the Interior West/central Rockies next weekend depending on the details of shortwave energy aloft. The forecast is very consistent in showing above normal temperatures over the Northern Plains and vicinity through the period, with decent coverage of plus 10-25F anomalies for highs and plus 10-20F anomalies for morning lows. Gradual amplification of the large scale pattern will bring a warming trend to the West. The warmest days should be around Friday-Saturday with the Great Basin the most likely to see readings up to 10-15F above normal. On the other hand areas near the East Coast will see a cooling trend that could bring highs down to 5-15F below normal by next weekend. Rausch 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, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml