Extended Forecast Discussion NWS Weather Prediction Center College Park MD 405 PM EDT Fri May 07 2021 Valid 12Z Mon May 10 2021 - 12Z Fri May 14 2021 ...Heavy rain possible over parts of the South early-mid week... ...Well below normal temperatures over the central High Plains with some snow in the central Rockies early next week... ...Overview... Between mean ridging aloft expected to prevail over the northeastern Pacific and northwestern Caribbean, a blocky pattern that develops over the western U.S./Canada during the weekend should become more progressive next week. Early next week an upper trough emerging from the West combined with low level upslope flow will produce chilly temperatures and enhanced rain/higher elevation snow over the central Rockies/High Plains. Southern stream energy and then this upper trough will interact with a front that settles along the Gulf Coast for a time, bringing the potential for heavy rain/thunderstorms over areas from the southern Plains into the Southeast. Other parts of the East Coast may see meaningful precipitation as well. The West should see a warming trend as an upper ridge moves into/across the region. By Friday a modest upper trough nearing/reaching the West Coast may reverse that trend over the Northwest. ...Guidance Evaluation and Preferences... Through the 00Z/06Z cycle, the GFS/ECMWF/Canadian/UKMET offered reasonable clustering within the ensemble spread to use as a starting point. After next Wednesday, the GFS/ECMWF/Canadian had some more favorable-than-not aspects over the CONUS, but each was a bit removed from the good ensemble consensus to warrant a lessening of their inputs. By next Fri, relied on an ensemble blend along with the previous forecast which still fit well with the ongoing thinking in a quasi-zonal/progressive pattern. Trend has been toward a more defined upper system in the Northeast (southeast Canada) next Tue on the heels of the departing sfc wave through the northern Mid-Atlantic/Southern New England Mon. Ensemble guidance has been mostly stable with the longwave pattern by the end of next week, suggesting a coastal low off the NC coast next Thu, but east-west differences in recent ECMWF/GEFS ensemble runs persist. ...Weather Highlights/Hazards... Early next week expect a period of enhanced precipitation over the central Rockies/High Plains from the combination of the upper trough over the West and strong Plains high pressure leading to low level upslope flow. Accumulating snow is likely in the higher terrain of Colorado and Wyoming while cold rain should fall in lower elevations. Otherwise much of the West should be fairly dry through the period. Moisture and southern stream shortwave energy along a front settling toward the Gulf Coast may already support some areas of moderate to heavy rainfall across the South to start the week. Then the progression of the western upper trough may produce some waviness along the front and intensify rainfall from parts of the southern Plains through the Southeast. Some areas across the southern tier may be sensitive to this additional rainfall in light of significant totals received over the past week, though at least late this week into the weekend should be fairly dry over the South. Some of the southern tier moisture may extend farther northward over the eastern U.S. mid-late week as potential low pressure develops. During Monday-Tuesday the temperatures over the central portions of the High Plains/eastern slopes of the Rockies should be more reminiscent of March, with highs up to 20-25F below early May averages. This region will see a steady warming trend after Tuesday, possibly to the point of seeing highs 5-10F above normal by next Friday. Much of the central and eastern U.S. should see multiple days with highs 5-15F below normal due to the influence of an upper trough over the northeastern quadrant of the lower 48 during the first half of the week and Canadian high pressure reaching areas to the east of the Rockies. Meanwhile above average temperatures will expand across the West as an upper ridge progresses into and across the region. The best potential of plus 10-20F anomalies for highs will be from parts of California and Oregon into the Great Basin. The Pacific Northwest may see a cooling trend late next week with the arrival of an upper trough. Fracasso/Rausch Hazards: - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, and the Southern Plains, Mon-Wed, May 10-May 12. - Heavy rain across portions of the Southeast, Wed, May 12. - Heavy rain across portions of the Mid-Atlantic, Thu, May 13. - Heavy rain across portions of the New England, Fri, May 14. - Flooding occurring or imminent across portions of the Ohio Valley. 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