Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Sat Apr 03 2021 Valid 12Z Tue Apr 06 2021 - 12Z Sat Apr 10 2021 ...Overview and Guidance Evaluation/Predictability Assessment... Some aspects of the medium range forecast continue to be challenging. After midweek the models and ensembles still show significant divergence for shortwave details within a larger scale mean trough expected to settle over northwestern North America and portions of the western U.S. coast. In addition to affecting timing/coverage/amounts of precipitation over the West, some of the energy emerging from this mean trough could ultimately affect the upper low forecast to track from the north-central Rockies into the Midwest during Tuesday-Thursday. Another factor at play for this upper low by late week into the weekend will be the fairly blocky pattern over the Atlantic and eastern Canada. This system will bring the potential for significant precipitation from the north-central Plains and Midwest through portions of the East. The models are gradually consolidating for their depiction of the upper low currently expected to track from near eastern Idaho into the Midwest during Tuesday-Thursday. Among the 12Z/18Z solutions for this time frame, the UKMET was the one that had to be excluded due to being a fast extreme from the start of the period. The new 00Z run looks more reasonable. Recent trends have been gravitating toward the stronger half of the spread. By Friday-Saturday the ultimate evolution of the upper low becomes increasingly uncertain due to the possible influence of upstream shortwave energy as well as models/ensembles showing a variety of possibilities for flow details over and south of eastern Canada (with eastern Canada seeing rising heights/ridging in general but potential for embedded shortwave/closed low energy). The 12Z and old 00Z ECMWF runs represent the two extremes, lifting northward or dropping southeastward. The new 00Z run has reverted to a more southern track late in the period. The GEFS/ECMWF means provide the most reasonable intermediate solution with the upper low center drifting into the Great Lakes. The forecast of details within the mean trough near the West Coast is quite uncertain as well. The GFS/UKMET/CMC are all suggesting in varying ways that leading energy reaching the trough axis by midweek should eject much farther eastward than ECMWF runs have shown thus far. GEFS/CMC means show a more subdued hint at such energy. As has been the case recently, the GFS digs upstream energy farther westward than most other solutions. In a favorable trend the GEFS mean has been adjusting its somewhat westward axis closer to the ECMWF mean. The past two ECMWF runs were somewhat opposite near the West Coast late in the period as well, with the 12Z run a lot flatter/broader with the trough than the prior 00Z run. The new run is sharper but strays a bit eastward by Saturday. Again a blend of the GEFS/ECMWF means provide the best intermediate starting point for a single deterministic forecast. Aforementioned comparisons/preferences led to a starting blend that consisted of the 18Z GFS and 12Z ECMWF/CMC for about the first half of the period and then quickly trended toward majority weight of the 18Z GEFS/12Z ECMWF means. Enough minority weights of the 18Z GFS and 00Z/02 ECMWF were included late in the period to enhance detail over the East slightly but not enough to have an adverse impact on preferences farther West. ...Sensible Weather and Potential Hazards... The upper low emerging from the north-central Rockies along with developing Plains/Midwest low pressure should promote an axis of potentially heavy rainfall over parts of the north-central Plains and Midwest during Tuesday or Tuesday night through at least Wednesday. Some snow will be possible over the north-central Rockies and High Plains. With time expect this system to spread moisture into the East with some areas of locally moderate to heavy rainfall possible. Increasing uncertainty over system details after Thursday decreases confidence for determining highest rainfall totals and their location at that time, though existing signals suggest they would be within an area from the Ohio Valley into Tennessee Valley through parts of the Mid-Atlantic/Southeast. The Northwest should see one or more periods of rain and mountain snow during the period but uncertainty over shortwave details will keep confidence low for specifics of coverage/timing/amounts. From Tuesday into early Wednesday well above normal temperatures (at least plus 10-20F anomalies) will prevail from the southern Rockies/Plains into the Great Lakes. The greatest anomalies should be over the central Plains and Midwest where morning lows may be 20-25F above normal. This warmth will moderate somewhat as it continues eastward ahead of the Plains system. The East should generally seeing highs of 5-15F above normal mid-late week while clouds and rainfall will lead to warmer anomalies of plus 10-20F for morning lows. Farther west, parts of the north-central Rockies/Plains may see modestly below normal highs into midweek while the mean trough aloft nearby keeps the Pacific Northwest somewhat below normal for highs. Most of the Southwest and Great Basin will likely see highs 5-10F above normal from midweek onward. 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