Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Sat Apr 24 2021 Valid 12Z Tue Apr 27 2021 - 12Z Sat May 01 2021 ...Heavy rain threat from the Southern Plains into the Middle Mississippi Valley next week... ...Guidance/Predictability Assessment and Preferences... Although guidance has differed with important aspects of the forecast recently, the past day of model/ensemble runs has provided a dramatic increase of spread for flow details downstream from a strong ridge that builds into the West mid-late week. This spread arises from a combination of timing differences for the upper trough/embedded low forecast to emerge from the West/southern Rockies as well as various ideas for the timing and amplitude of northern tier U.S. and southern Canada flow. The most long-standing issue has been the timing of the initial western trough/embedded upper low. Recently the UKMET/CMC runs have tended to be slowest with it, though the 00Z UKMET adjusted faster toward the typically more progressive GFS/GEFS idea in spite of the UKMET being closer to the ECMWF cluster for the strength of the ridge building into the West. The ECMWF/ECMWF mean had been moderately progressive and phased but not to the degree of the GFS/GEFS mean. However 00Z runs of the ECMWF/CMC and their means made a major change in the northern stream, becoming faster with the shortwave energy so that it amplifies into a northeastern U.S. trough instead of having any chance to interact with the southern stream feature. This leaves the southern stream upper low anywhere between the southern Plains and northwestern Mexico into late week. Illustrative of the forecast uncertainty, the UKMET reverted from its 00Z run back to the ECMWF/CMC cluster for the southwestern low and joins those models in principle for a more progressive northeastern trough by Friday. The new 12Z ECMWF is still slow with the southern stream and trended even faster with the northern stream. In contrast, models and means are more agreeable in principle for an eastern Pacific upper trough that should arrive at the West Coast early in the weekend. There are typical differences with precise timing and amplitude but these pale in comparison to the spread seen farther east. The updated forecast blend incorporated elements from the broad spectrum of forecast guidance available through the 06Z cycle, as multi-day trends favored some incorporation of the slower scenario for the western trough/upper low while the 00Z UKMET provided a signal at the time that a more phased GFS/GEFS-type solution could not be fully ruled out. By late in the period this resulted in a partial trend toward faster frontal progression over the East and slower waviness hanging back near the Gulf Coast. As expected, continuity is better over the West although the Pacific front arriving by Saturday is a bit faster. ...Weather/Hazards Highlights... The system affecting the West early in the week will bring some lower elevation rain/mountain snow with the best focus over favored terrain in Arizona through the Rockies of Colorado and Wyoming on Tuesday. Farther east, 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 about Tuesday night 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. The Storm Prediction Center continues to monitor the threat for severe weather with this event as well so check the latest outlooks for further information. 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 a wavy front extending from the Plains into Great Lakes and New England will contain temperatures up to 10-20F above normal from parts of the central Plains into the Ohio Valley/Great Lakes and Mid-Atlantic/Northeast through midweek. Some or most of New England may remain north of this front and thus be considerably cooler. Increasing pattern uncertainty over the East after Thursday leads to low confidence in the temperature forecast over some areas. Meanwhile locations from the Southwest and Four Corners into southern High Plains should see a day or so of cool highs (up to 10-15F below normal) through midweek. The cooling trend farther east will likely be more modest. Continue to expect very warm to hot conditions to build over the West after midweek as a strong upper ridge builds over the region. Expect highs to 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. Rausch/Santorelli 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