Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sat May 08 2021 Valid 12Z Tue May 11 2021 - 12Z Sat May 15 2021 ...Heavy rain possible over parts of the South early-mid week... ...Well below normal temperatures over the central-southern High Plains with some snow in the central Rockies early next week... ...Overview... Expect a moderately progressive upper pattern over the lower 48 during the period. A Great Lakes/Northeast upper trough will depart after midweek, followed by a trough emerging from the Rockies and likely reaching near the East Coast by next Saturday. A trailing upper ridge will cross the West mid-late week. By early next weekend the ridge is likely to be over the Plains while an amplifying trough arrives at the West Coast. Central Rockies/High Plains precipitation associated with the initial upper trough and low level upslope flow will be trending lighter Tuesday but chilly temperatures will remain before a moderating trend. Progression of the upper trough and interaction with a front over or just south of the Gulf Coast region should promote rain/thunderstorms over the South with some heavy activity possible. Some of this moisture may extend farther north/northeast after midweek. The trailing upper ridge will bring a brief period of warmth to the West followed by a return back toward normal readings as the next upper trough reaches the West Coast. ...Guidance Evaluation and Preferences... Most guidance has been finding a way to achieve a similar trough-ridge-trough configuration for the upper pattern by the end of the period next Saturday but the embedded details vary and there is now more divergence for the eastern U.S./western Atlantic surface pattern after midweek. From the Rockies eastward, combined details of the departing Northeast trough and specifics of the upstream trough emerging from the Rockies differ enough to produce widely different evolutions at the surface. Latest Canadian runs have held onto more southwestward elongation of the Northeast trough while the 12Z ECMWF significantly changed from its continuity in adjusting to a faster/more sheared depiction of the Rockies shortwave. In both of these cases the result is much greater dominance of surface high pressure over the eastern U.S./western Atlantic after midweek--in contrast to recent ensemble mean consensus for a weak wave to track near the Mid-Atlantic/New England coast. On the other side of the spectrum recent GFS runs have been the strongest with this development and the new 00Z GFS/GEFS mean have adjusted a little slower than the previous run and most other guidance. Near the West Coast, there is still a period of time mid-late week when models/ensembles suggest a very low-predictability regime with small-scale shortwaves progressing from the Pacific into western North America before upper troughing amplifies near the coast. Recent GFS runs have trended favorably slower toward consensus for the late-period trough, and while guidance is split over whether a compact upper low near the Alaska Peninsula as of early Tuesday ultimately shears apart (GFS/UKMET) or remains defined and becomes a component of the evolving trough (ECMWF/CMC), the agreement of models/means for the trough in principle provided some degree of confidence. This is before arrival of the new 00Z ECMWF which completely changes the pattern by separating eastern Pacific flow. The first priority of the forecast blend was to maintain some reflection of an East Coast wave late in the week per continuity and the means, so the late-period part of the blend incorporated some 18Z GFS and old 00Z/07 ECMWF along with the 18Z GEFS/12Z ECMWF means. This provided a reasonable depiction of consensus over the West while downplaying lower confidence specifics. An operational model composite (more weight of the last two ECMWF runs and 18Z GFS relative to the 12Z UKMET/CMC) was reasonable early in the period. ...Weather Highlights/Hazards... Precipitation over the central Rockies and High Plains should be tapering off on Tuesday, with some high elevation snow still possible. Most of the West will be dry during the period though the Northwest may begin to see scattered precipitation toward the end of the week as the upper trough moves in. Farther east, parts of the South may see some heavy rainfall as the upper trough progressing out of the Rockies interacts with a wavy front that should be over or just south of the Gulf Coast region. Compared to yesterday there seems to be a little more uncertainty over exactly where the highest rainfall totals will be. The most likely scenario of a frontal wave ultimately tracking near the East Coast would spread rain of varying intensity northeastward over the eastern U.S., but confidence in this evolution has decreased somewhat. Early in the period the upper trough and surface system affecting the Northeast should produce mainly light precipitation with a little snow possible in the highest elevations. Unseasonably cool temperatures will persist over the central Rockies and parts of the central-southern Plains into Tuesday with highs up to 20-25F below normal. The southern half of the Plains may still see highs 10-20F below normal on Wednesday but a quick warm-up should bring highs up to 5-10F above normal over central parts of the Plains Friday-Saturday. Many other portions of the central/eastern U.S. will likely see one or more days with temperatures 5-15F below normal through Wednesday or Thursday, followed by a moderating trend that should bring readings closer to normal by Saturday. Meanwhile continue to expect the main emphasis for 10-20F above normal highs to progress from parts of California and Oregon into the Great Basin during Tuesday-Friday. The upper trough nearing the West Coast should bring readings back toward normal over the West Coast states by Saturday. 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