Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sat Feb 27 2021 Valid 12Z Tue Mar 02 2021 - 12Z Sat Mar 06 2021 ...Heavy rain threat shifting eastward into the Southeast on Tuesday before diminishing... ...Overview and Guidance Evaluation... Since the ECMWF shifted toward the general GFS/GEFS scenario of forming a closed low off California by the start of the period, global models and ensembles have been showing indications of converging toward a common solution on forecasting the pattern evolution across the eastern two-thirds of the country for the latter part of next week. There are still plenty of forecast uncertainties yet to be resolved. Nevertheless, it is encouraging to note that the GFS has now defaulted into the typically faster solution cluster versus the typically slower ECMWF. The better model agreement therefore has increased the confidence for the upper low to track across the Southwest on Wednesday into Thursday followed by an increasing chance for cyclogenesis to occur over the southern Plains. There is also a general model trend of tracking this system closer along the Gulf Coast later next week instead of the early GFS idea of a much more inland track. The latest (12Z) ECMWF now shows a faster forward motion of the upper low, which is another encouraging sign that the model solutions are converging. Meanwhile, the CMC offers a more suppressed solution which keeps the system more as an open wave while tracking east across the northern Gulf. Otherwise, for the first half of the period, the GFS cluster takes the rain faster eastward across the Deep South into the Southeast. The GEFS mean and EC mean show a better agreement. Along the West Coast, a deep trough originated from a deep upper low in the Gulf of Alaska is forecast to edge closer toward the coast by the end of next week. There is a trend for the trough to arrive earlier than previous forecasts. 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. The WPC medium-range forecast package was derived based on a multi-model consensus of the 00Z ECMWF/00Z EC mean and the 06Z GFS/GEFS with smaller contributions from the 00Z CMC/CMC mean and the 00Z NAEFS. Beginning on Day 5, more of the 06Z GEFS together with the 00Z EC mean were used to handle the increasing forecast uncertainty. ...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 but a general southward trend has been noted. 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 with a trend for earlier arrival time. 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 the East Coast most likely to see some highs 10-15F below normal, followed by a 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. Kong/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