Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Sat Feb 27 2021 Valid 12Z Tue Mar 02 2021 - 12Z Sat Mar 06 2021 ...Heavy rain threat over the South continuing into Tuesday-Tuesday night and then diminishing... ...Overview and Guidance Evaluation... Although the past day of guidance has shown a strong trend toward the general GFS/GEFS scenario of eastern Pacific shortwave energy forming a closed low off California by the start of the period, there are still plenty of forecast uncertainties yet to be resolved. As of the 12Z/18Z cycles there were still some differences in where this upper low closes off and how quickly it progresses through the southwestern U.S. (GFS fastest). While this system moves into/through the Southwest, guidance continues to advertise amplification of an upstream eastern Pacific trough--which should ultimately amplify downstream North American flow as well. By late in the period most guidance amplifies eastern U.S. troughing more than the GFS/GEFS mean runs. This leads to the ECMWF/CMC and their means becoming more suppressed with the surface pattern over the eastern half of the country compared to the GFS/GEFS mean. Teleconnections relative to a positive height anomaly center over the central Pacific as well as a negative anomaly center over eastern Siberia in the D+8 multi-day means support more troughing over eastern North America than forecast by the GFS/GEFS. It remains to be seen to what extent though. Otherwise, the aforementioned amplifying eastern Pacific trough the approaches the West late in the week continues to display some timing spread with the ECMWF/ECMWF mean somewhat slower than other solutions. Exact timing will become sensitive to progressive upstream flow late in the period. Over the eastern half of the country early in the period, the GFS is on its own in becoming much faster and more sheared than other solutions (including the GEFS mean to some degree) with a compact upper low which consensus shows over the south-central Plains as of early Tue and then opening up as it continues eastward. For the first half of the period a 12Z/18Z operational model blend represented the preferred non-GFS cluster for the initial Plains upper low early Tuesday and surface waviness that it may generate over the Southeast by early Wednesday as the opening upper feature continues onward. The blend also provided an intermediate timing for the upper low that forms offshore California and then moves into the Southwest. Teleconnection relations recommended tilting the forecast over the eastern half of the country somewhat more toward the ECMWF/ECMWF mean/CMC relative to the GFS/GEFS mean by days 6-7 Friday/Saturday, while a more even balance could be valid farther westward. New 00Z guidance thus far maintains support for the non-GFS cluster from the Plains into the East early in the period while the UKMET/CMC are consolidating close to the 12Z ECMWF for the upper low tracking into the Southwest in contrast to the GFS that has trended even faster than its 18Z runs. The new ECMWF becomes just a bit faster than its 12Z run. Later in the period over the East, the new CMC/ECMWF still support the established non-GFS cluster for the overall pattern though the ECMWF has hedged a bit less suppressed than the prior run. The new ECMWF also adjusted a fraction faster with the late period trough nearing the West Coast. ...Weather/Threats Highlights... The favored guidance consensus still shows the potential for another episode of locally heavy rainfall over parts of the South into Tuesday-Tuesday night. Expect the upper low forecast to be over the south-central Plains early Tuesday to open up as it continues eastward, interacting with a front near the Gulf Coast and supporting one or more frontal waves along with some inclusion of Gulf moisture. Lighter precipitation could reach as far north as the central Appalachians and Mid-Atlantic. Precise northward extent and intensity of precipitation remains fairly uncertain. The upper low closing off to the west of California should bring moderate amounts of moisture into the southwestern U.S. and Plains by mid-late week (some snow possible at high elevations over the Four Corners states), still with only moderate confidence for timing at best. Confidence trends even lower for the northward extent of precipitation over the eastern half of the country by late week/weekend, given the differences in how much eastern U.S. upper troughing develops in response to upstream amplification. To the east of the Rockies, with both systems there could be some wintry weather in the northern fringe/higher elevations of the precipitation shield if the moisture extends far enough north. Over the Northwest/West Coast, precipitation should initially be light to moderate and primarily over northern areas. The upper trough approaching the West Coast later in the week should bring an increase and southward expansion of precipitation but with continued uncertainty over the exact timing of this evolution. The above normal temperatures expected over the Northern Plains and vicinity through the period are one of the more consistent and confident aspects of the forecast. There should be decent coverage of plus 10-20F anomalies for highs/morning lows most days and some locations could see highs greater than 20F above average. Southern and eastern states will see a cool day Tuesday with East Coast areas most likely to see some highs 10-15F below normal, followed by rebound toward normal. Locations near the East Coast could see a cooling trend next weekend depending on how much upper troughing develops. The West will see a warming trend that should support highs reaching 5-15F above normal by late in the week. 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