Extended Forecast Discussion NWS Weather Prediction Center College Park MD 511 PM EDT Mon Apr 20 2020 Valid 12Z Thu Apr 23 2020 - 12Z Mon Apr 27 2020 ...Excessive Rainfall/Severe Weather Threat over the South Mid-Late Week... ...Overview... The synoptic pattern is expected to transition toward a western U.S. ridge / eastern U.S. trough pattern this weekend and into early next week. Toward the end of the period, incoming eastern Pacific energy will work to erode portions of the western ridge while troughing remains anchored in place over the east for day 6/7. While the large scale pattern exhibits average to slightly above average model agreement and consistency, there are some uncertainties in the smaller scale features and individual shortwave troughs. For the system early in the period across the Mid-MS River Valley to Mid-Atlantic, guidance still shows some latitudinal spread in the low track but have clustered in recent model runs. A severe weather threat including heavy rainfall still looks likely at the beginning of the medium range period across portions of the Southeast U.S. while widespread meaningful rainfall extends farther north Thursday into early Friday. ...Guidance/Predictability Assessment... For the system affecting the eastern U.S. later this week, the 00Z/06Z continued to trend a bit faster than earlier guidance and perhaps a bit to the north. The 06Z GFS shifted the furthest north with its low position at 12Z Thursday compared to the 00Z GFS run and other 00Z guidance. While plausible and within the ensemble spread, the bigger shift was discounted for this cycle. The CMC/UKMET continue to show some lag compared to the ECMWF and GFS for day 3/4. For the model blend preference day 3/4, heavier weights of the 00Z ECMWF and 00Z GFS were incorporated, which fits nicely in the model consensus approach. Beyond that system, there remains considerable spread for how the eventual troughing develops over the eastern U.S. and any associated surface low that moves through the flow late in the weekend. Given the higher model spread and less than desirable run to run consistency, more of the 00Z ECENS and 00Z GEFS were incorporated, especially toward day 6-7. Looking at the models and ensemble means as a whole, best probability for low pressure should translate from within the Ohio Valley to East Coast region early day 6 Sun to near or just offshore the New England coast by early day 7 Mon. Confidence in the specific deterministic model solutions is low but incorporating their ideas with the latest GEFS/ECMWF means best represented the most common aspects of the guidance while accounting for detail uncertainty. ...Weather/Hazard Highlights... The system affecting the eastern U.S. late this week will spread a broad shield of rainfall to the east of the Mississippi River. Expect the threat of heavy rainfall and potential for severe weather over the South to continue into Thu. Monitor Storm Prediction Center outlooks for the latest information regarding severe threats. Meaningful rainfall should reach farther north Thu-Fri, through the Ohio Valley/Mid-Atlantic and possibly parts of New England. Guidance spread for strength/track and timing of this system is still great enough to lower confidence for determining the coverage and intensity of rainfall over the northern Mid-Atlantic and New England in particular. The shortwave energy crossing the West late this week will bring a period of rain and higher elevation snow to the northern-central Rockies and vicinity. As this energy continues eastward and amplifies late week through the weekend there may be some areas of rainfall over the Plains with a broader shield of moisture spreading over the eastern states. There is still enough uncertainty over details at the surface and aloft to keep confidence fairly low in determining location and magnitude of the highest totals. Best ensemble probabilities for significant precip currently highlight an area encompassing the Mid-Atlantic/Appalachians/Northeast with a somewhat lesser signal for parts of the Ohio Valley/Great Lakes. Some operational model runs have other ideas though. Depending on exact evolution at the surface and aloft, there could be enough cold air present for some higher elevation snow near the Canadian border. A succession of shortwaves rounding the western U.S. upper ridge may bring periods of rain and possibly high elevation snow to parts of the Pacific Northwest into the extreme northern Rockies from the weekend into early next week. Guidance varies considerably on the timing and amplitude of individual shortwaves, leading to a wide range of possible outcomes for precipitation totals and southward extent of moisture. Above normal temperatures focused over and near California and the Southwest late this week should expand across most of the southern 2/3 or 3/4 of the West by Sat-Mon as ridging aloft builds over the region. Within this broadening area of warmth expect some highs to reach 10-20F above normal. Readings may be the warmest of the season so far and could approach daily records at a few locations. The Desert Southwest will likely see highs near or slightly over 100F. The eastern U.S. will tend to be on the near to below average side of the spectrum, with the system crossing the East late this week bringing clouds/rain followed by another push of cool air behind potential eastern U.S./western Atlantic surface development. Best potential for some highs 10-15F below normal will be on Sun-Mon. Rausch/Taylor Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy rain across portions of the Southeast, the Central/Southern Appalachians, the Mid-Atlantic, and the Tennessee Valley, Thu, Apr 23. - Flooding possible across portions of the Southeast, the Southern Appalachians, and the Tennessee Valley. - Flooding occurring or imminent across portions of the Mississippi Valley, and the Northern Plains. - Flooding likely across portions of the Middle Mississippi Valley and the Northern Plains. - Much above normal temperatures across portions of the Central/Southern Plains, the Rockies, the Northern/Central Great Basin, California, the Pacific Northwest, and the Southwest, Sat-Mon, Apr 25-Apr 27. - Much below normal temperatures across portions of the Great Lakes, the Mid-Atlantic, the Northeast, the Central Appalachians, and the Ohio Valley, Sun-Mon, Apr 26-Apr 27. - Enhanced wildfire risk across portions of the the Southern Rockies, the Southern Plains, and the Southwest, Thu, Apr 23. 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