Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 AM EDT Thu Apr 22 2021 Valid 12Z Sun Apr 25 2021 - 12Z Thu Apr 29 2021 ...Heavy snow possible for the Sierra Sunday... ...Heavy rain threat for parts of the southern Plains into the South next week... ...Overview... Progressive yet modestly amplified/blocky upper level flow will favor a couple robust systems across the lower 48 next week. The first will be through the West where an incoming longwave trough and surface cold front will deliver enough moisture into the Sierra for appreciable to perhaps significant snow to higher elevations with light to modest rain for the lowlands. A healthy amount of moisture should expand across the Great Basin as well. As this trough reaches the Plains, southerly flow out of the Gulf may add moisture to an unstable environment where severe weather is possible, which may lead to local areas of heavy rain. This system will track toward the Great Lakes with a continued threat of modest to perhaps locally heavy rain across the lower Mississippi Valley next Wed-Thu. Temperatures will be variable during the period, modulated by frontal passages. ...Guidance/Predictability Assessment and Preferences... Through the 12Z/21 and 18Z/21 guidance cycles, the deterministic models offered a reasonably well-clustered consensus for the exiting system off New England and the incoming western trough. However, as in previous days, the GFS/GEFS were quicker than the ECMWF/Canadian (and their ensembles) as well as the UKMET and continuity as it moves through the Great Basin/Southwest. This may be tied to a lack of definition of trailing energy out of western Canada/eastern Alaska on the east side of an upper high that would act to slow the trough progression. Again favored the slower solutions near the ECMWF for the Tue-Thu period, though there was actually a bit closer agreement by next Thu as the system lifts toward the Great Lakes. Increased the ensemble weighting of the ECMWF ensemble mean as the UKMET then Canadian were dropped from the blend and incorporated a small minority weighting of the GEFS mean since even though the ECMWF ensemble mean has been quite consistent, the pattern does allow for a somewhat quicker pace. ...Weather/Hazards Highlights... Exiting system in the Northeast will wrap around showers (some snow in higher elevations) on Sunday into Monday. Some enhanced rainfall is possible over New England with windy conditions region-wide as the low deepens. Temperatures will be cool behind the front before warming up Tue-Thu. In the West, the incoming Pacific upper trough/low and surface front will push through California Sunday and the Great Basin Monday. This system will spread lower elevation rain/mountain snow focused over southwestern Oregon down into northern and central California, especially in the Sierra. Modest rain/snow will also fall across the Great Basin/central Rockies with lowering snow levels behind the front. The highest totals with this system will be into the Sierra where over a foot is possible. Once the system moves to the Plains by Tuesday, potential will increase for severe weather (see SPC for details) and the potential for locally heavy rainfall. This system will continue eastward where a continued supply of Gulf moisture ahead of the boundary will sustain the chance for modest to locally heavy rain through the lower Mississippi Valley into the Tennessee and perhaps Ohio Valley as well. An area of above normal temperatures over parts of the Rockies and into the Plains by early next week will shift into the northeastern 1/4 of the CONUS by midweek. Temperatures may rise well into the 70s and low 80s. Trailing the front that crosses the West, temperatures over the region will decline to 5-15F colder than normal by next Mon-Tue and the back to near and slightly above normal Wed-Thu. 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