Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Mon Apr 05 2021 Valid 12Z Thu Apr 08 2021 - 12Z Mon Apr 12 2021 ...Overview and Guidance Evaluation/Predictability Assessment... Model guidance this morning keeps the general theme of bringing relatively progressive upper troughs and embedded shortwaves through the Northwest to interact with a low pressure system that is forecast to slow down across the upper Midwest as an omega block sets up over eastern Canada. The ensemble means from the GEFS, ECMWF, and CMC show good agreement on this overall pattern evolution through the medium-range period although the EC mean shows a more distinct deepening of an upper trough over the northern Plains on Day 7. A broad swath of rainfall with embedded heavy rains will likely edge eastward into the eastern U.S. on Thursday with good model agreement on its timing. The rain will extend northwest across the Great Lakes and warp around the low pressure system moving into the upper Midwest. Some of the rain across the upper Midwest could change over to a period of light snow as a cold front arrives from the west. Meanwhile, models continue to show decent signal for heavy rainfall to continue across the Deep South to the Southeast on Thursday. By late Friday into Saturday, there is good consensus for another episode of heavy rain to develop over the lower Mississippi Valley. Models have generally trended wetter regarding this episode. Meanwhile, a good chance of rain will likely to continue into the weekend across the Mid-Atlantic with a lingering front. In light of the above considerations, the WPC medium-range forecast package was based on the consensus of the 00Z ECMWF, 06Z GFS together with a smaller contribution from the 00Z CMC. More of their ensemble means were used for Days 5 - 7 to handle the uncertainty. ...Sensible Weather and Potential Hazards... Combination of the initial Midwest upper low/surface system and upstream energy that replaces it will produce a period of precipitation of varying intensity over the Midwest/Great Lakes. Most of this activity will be rain with a slight chance for the northwestern fringe of the moisture shield to contain a little snow. The front approaching from the western U.S. toward the leading system could provide a focus for enhancement. A fairly broad area of rainfall will also extend across many other portions of the East along/ahead of fronts associated with the northern systems. Heaviest rainfall is most likely to be over portions of the Southeast into Friday. By late Friday into the weekend, shortwave energy traversing the southern Plains could support another episode of heavy rainfall across the Deep South toward the eastern Gulf Coast. Over the Northwest/northern Rockies expect an episode of fairly light rain/mountain snow around Thursday followed by another event late week into next Monday. Highest totals with the latter should be over the Olympics and Washington Cascades. Clouds and precipitation will tend to support above normal morning lows over the eastern U.S. for most of the period. Some locations in the Midwest/Great Lakes may see record warm lows on Thursday, within an area of plus 15-25F anomalies. High temperatures will be more moderate but the Great Lakes and vicinity could still see daytime readings exceed 10F above normal late this week into Saturday. Meanwhile expect a localized area of well above normal temperatures over southern Texas, especially late this week when highs could reach at least 15F above normal, with a moderating trend to follow. Actual high temperatures on Friday are forecast to exceed the century mark along the Rio Grande Valley. The southern half to two-thirds of the West will generally be above normal through the period with plus 5-12F anomalies for highs. Farther north expect a couple periods of modestly below normal highs behind progressive cold fronts. Kong/Rausch 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