Extended Forecast Discussion NWS Weather Prediction Center College Park MD 251 AM EDT Mon Apr 05 2021 Valid 12Z Thu Apr 08 2021 - 12Z Mon Apr 12 2021 ...Overview and Guidance Evaluation/Predictability Assessment... From the height anomaly perspective, the most dominant feature over North America will be a strengthening eastern Canada mean ridge. The associated core of positive height anomalies to the east of Hudson Bay in latest D+8 means teleconnect to mean troughing over the Upper Midwest/western Great Lakes and a modest split flow regime over the western part of the continent (some ridging over western Canada and troughing over the southwestern U.S. through Baja California). Thus far the guidance has maintained the improved clustering that emerged in the 00Z cycle from 24 hours ago for the evolution of the initial Midwest upper low and upstream shortwave energy emerging from western Canada and northwestern U.S. Over the course of Thursday-Saturday the current consensus would have the initial upper low energy lifting northward as the upstream energy forms another upper low over the Upper Midwest/western Great Lakes. The upstream frontal system would also essentially replace the leading one as this evolution occurs. While differences have existed over this area over recent days, there has been decent continuity for a wavy front extending through the southern Mid-Atlantic. During the latter half of the period the 18Z GFS provided the most pronounced discrepancy versus other guidance for the second Midwest/Great Lakes upper low, tracking it into the Northeast instead of holding it farther west. Differences still exist for upstream flow from the northeastern Pacific eastward. To varying degrees the majority of guidance suggests that shortwave energy arriving into the Northwest by day 5 Saturday will be followed fairly quickly by a modest ridge that eventually reaches western Canada, versus the GFS/GEFS mean which have additional shortwave energy. The ECMWF cluster seems to provide a better lead into the teleconnection-favored pattern. The 12Z CMC may be overdone with how much shortwave energy digs into the West (more moderate in the new 00Z run) while the CMC mean is on the fast side with the shortwave. In light of the above considerations, the operational model blend early in the period trended toward greater weight of recent ECMWF runs and 12Z ECMWF mean relative to the 12Z GFS and 18Z GEFS mean by days 6-7 Sunday-Monday. ...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 but there is a slight chance that the northern fringe of the moisture shield could contain a little snow. The front approaching from the west 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 South. Late-period shortwave energy could support another episode of rainfall over parts of the Plains by Sunday or Monday. Over the Northwest/northern Rockies expect an episode of fairly light rain/mountain snow around Thursday followed by another event late week into the weekend. 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 East 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. 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. 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