Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM 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 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... Overall the 12Z ECMWF/GFS/UKMET provided the best guidance cluster early in the period, followed by a transition toward emphasis on the ECMWF and 12Z NAEFS/ECMWF means. The forecast maintained a small minority of 12Z GFS input late in the period for its relative agreement over the East but not enough to influence the forecast over the West where it compared more poorly to the majority of guidance. Guidance has been having some difficulty resolving the specifics of waviness expected to affect the Northeast early next week, with even a consensus blend waffling somewhat from cycle to cycle for position and structure. Behind this system, the upper trough emerging from the West was slowest in the 18Z GFS/GEFS mean. The evolution toward a more progressive pattern may not favor the slow side of the envelope, though the ECMWF mean has trended a bit slower over the past couple days. The new 00Z GFS has nudged a little faster to provide a better comparison than the 18Z version. Majority cluster of recent ECMWF runs, ECMWF/CMC means, and some GFS runs would have this trough supporting surface development near the Mid-Atlantic coast by early day 7 Friday. The 12Z CMC was very out of phase with important features but the new 00Z run also shows this wave. Finally, multiple GFS runs become more progressive than most other guidance with flow from the North Pacific into North America. As a result the 12Z/18Z GFS runs were much less eager to develop an upper trough near the West Coast by next Friday. The new 00Z run shows a shortwave but still may be underdone with the reflection of a compact upper low that travels from near the eastern Aleutians eastward/southeastward. The new 00Z ECMWF has added a new wrinkle in the forecast by trending slower than model/ensemble consensus as the trough approaches the Northwest. By late in the period the Canadian ensemble contribution to the NAEFS provided a more compatible companion to the ECMWF/ECMWF mean for both the West Coast trough and the system forecast near the East Coast. ...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. Rausch/Fracasso 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