Extended Forecast Discussion NWS Weather Prediction Center College Park MD 223 AM EDT Sun Apr 25 2021 Valid 12Z Wed Apr 28 2021 - 12Z Sun May 02 2021 ...Heavy rain threat from the Southern Plains into the Middle Mississippi Valley next week... ...Guidance/Predictability Assessment and Preferences... The medium range period (Wed-Sun) features amplified troughing (or a closed low) which drifts eastward with time from the Four Corners region, possibly reaching the Southeast/Tennessee Valley or East Coast by the end of the weekend. Upstream, strong ridging will build across the Western U.S. the first half of the period before it also gets kicked eastward by additional shortwave energy reaching the West Coast day 6-7. The largest forecast concern continues to be evolution and timing of the initial western troughing as it moves through the Southern Plains and points east. Although guidance has always struggled some with the timing of this feature, the past day of model/ensemble runs has provided a dramatic increase in spread. This arises from a combination of timing differences for the upper trough/embedded low, as well as various ideas for timing/amplitude/interaction with northern stream flow. Both the ECMWF/CMC have made the biggest shifts in the past day or so with a faster progression of northern stream energy across the northern tier into the Northeastern U.S., which leaves the southern stream trough/embedded upper low hanging back and lingering in the vicinity of the Southern Plains (with slow eastern movement) for several days. The ECMWF does eventually kick the energy eastward (more so than the CMC) but is still not nearly as fast at the GFS. The GFS (and off and on runs of the UKMET) support more phasing between the northern/southern stream energy resulting in a much faster progression of the trough, bringing it into the Eastern U.S. by next weekend. Although there has been a clear trend in all of the guidance for slower movement, prefer to land somewhere in the middle of the two extremes as this point (close to the ensemble means). WPC favored a quick transition to majority ensemble mean guidance by mid to late period, although with about a third contribution from the ECMWF for trough definition (which obviously gets washed out in the ensembles) and because the ECMWF is the deterministic piece of guidance closest to that of the ensemble means. This approach provides fairly good continuity with the previous WPC forecast and also works well for more agreeable Western U.S. troughing entering the picture by the last half of the period. ...Weather/Hazards Highlights... Interaction of Gulf moisture with the leading wavy front from the Great Lakes into southern High Plains will promote increasing coverage and intensity of rain/thunderstorms from just before the medium range period begins and onward, with heaviest rainfall expected from portions of Texas and Oklahoma into the Middle Mississippi Valley. Slow movement of the wavy front for a time could allow for training of convection and an increased flood risk. Dependent on trough evolution, there could be some threat for severe weather as well (see the Storm Prediction Center for additional details). Significant divergence in the guidance for the pattern east of the Rockies after early Thursday lowers confidence in the coverage and intensity of precipitation from late week into the weekend. Emerging trends suggest potential for rainfall to become progressive as it reaches the East while it lingers for a longer time over the southern tier and/or southern half of the Plains. Flow around the upper ridge building into the West and ahead of the next shortwave may bring some moisture into the Pacific Northwest and possibly extreme northern Rockies. Heaviest precipitation should remain over western British Columbia/Vancouver island. It is uncertain how far south into the Northwest any precipitation may reach through midweek or so, but approach of the late-period shortwave should increase the coverage over the Northwest and northern Rockies by Saturday. The warm sector ahead of the main front extending from the Plains into Great Lakes and New England will contain temperatures up to 10-20F above normal from mainly parts of the Tennessee Valley into the Mid-Atlantic/Northeast through midweek. Temperatures should moderate back closer to normal after Thursday for the East, but increasing pattern uncertainty over the East leads to low confidence in the temperature forecast over some areas. Meanwhile, continue to expect very warm to hot conditions to build over the West after midweek as a strong upper ridge builds over the region. Daily highs may reach 10-20F above normal over a majority of the West by Thursday and/or Friday with some daily record highs and warm lows possible over the Southwest and possibly other locations. The upper trough approaching from the eastern Pacific will begin a cooling trend over northern areas by Saturday, with plus 10-15F anomalies most likely to extend from parts of California and the Southwest into the north-central Plains at that time. Santorelli/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