Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 PM EDT Wed Apr 01 2020 Valid 12Z Sat Apr 04 2020 - 12Z Wed Apr 08 2020 ...Overview and Guidance/Predictability Assessment... Model and ensemble guidance continues to show mean ridging aloft over the Gulf of Mexico and most of eastern North America, though with some cyclonic flow at times over New England and the Canadian Maritimes. Shortwave energy filtering through this overall ridge will bring a front into the East, likely stalling over the Mid-Atlantic, while the trailing part of the boundary returns northward as a warm front early next week. Meanwhile western North American upper troughing will transition from being anchored by a western Canada upper low to a digging trough along the West Coast--likely with an embedded low. By next Tue-Wed guidance diverges significantly (among each other and in some cases between consecutive runs) regarding the character of a ridge over the eastern Pacific and flow around this ridge. This divergence yields increasing uncertainty for how much eastward progress the West Coast upper low makes through midweek. On the positive side, so far there has been somewhat better clustering/continuity with a northern stream shortwave supporting a surface system that tracks from the northern/central Plains into the Upper Great Lakes/Ontario around Tue-Wed. From day 3 Sat into day 5 Mon, some timing differences persist for the compact feature heading into northern California on Sat and then weakening farther inland. GFS runs have tended to be on the faster side of guidance. Spread has narrowed for the energy that drops southward from just off British Columbia early Sat, most likely leading to a closed upper low over or near northern California by early Mon. This is near the middle of the envelope from 24-36 hours ago. Consensus shows only minor detail adjustments for the front reaching the eastern U.S. and its anchoring southern Canada low. A general blend of latest operational models provided a reasonable forecast early-mid period. As the differences/trends with the eastern Pacific upper ridge and surrounding flow become increasingly dramatic by days 6-7 Tue-Wed, the most prominent trend in the models and ensembles (GEFS/ECens, not yet CMCens) over the past day is toward a stronger Pacific ridge and less northern stream interaction. This leads to slower progression of the upper low tracking over or a little offshore California. The latest GEFS/ECMWF means provide a good starting point in principle, reflecting the change in continuity and with timing about two-thirds toward the slower 00Z ECMWF/CMC scenario versus the GFS that has tended to be on the faster side of the envelope. The 12Z GFS has trended slower than previous runs but remains faster than the means. Farther east the model/mean majority has held up fairly well for the Plains into Great Lakes/Ontario system. So far this feature has shown less sensitivity to the variability/spread in flow details upstream than might be expected, though the new 12Z ECMWF has made some changes. For both features the 00Z GFS was somewhat closer to preference than the 06Z run. Based on these considerations, the operational model blend began to transition 06Z GFS input with the 00Z GFS on Mon while days 6-7 increased to 40-50 percent total 06Z GEFS/00Z ECMWF means with the remaining weight consisting of slightly more 00Z ECMWF/CMC than 00Z GFS. ...Weather/Hazard Highlights... Some of the specifics remain uncertain but the pattern still looks active for rain/high elevation snow over parts of the West, especially California. A compact system will bring an area of precipitation into northern California early in the weekend. Then the upper trough/embedded upper low trailing immediately behind should maintain precip over California with some moisture extending into the Great Basin and Rockies. Best potential for highest five-day totals will be along the Sierra Nevada with significant snow accumulations possible over higher elevations. Meaningful precip will also be possible along coastal ranges of California, including the southern part of the state. Recent trends for a slower upper low would lead to a longer duration of precipitation. During this weekend mostly light rain (maybe a little light snow extremely far north) will accompany a front reaching the East while an area of enhanced rainfall may extend along parts of the Gulf Coast ahead of the trailing part of the front that stalls. Low level flow from the Gulf may contribute to a developing area of rainfall over the eastern half of the country during the first half of next week. Locations within an area from Texas into the Tennessee Valley still have the best potential for highest totals but confidence remains moderate at best. There may be localized areas of moderate-heavy rainfall elsewhere to the east of the Rockies. After the Plains region sees 5-15F below normal readings on Sat, the evolution of the upper pattern will lead to increasingly widespread warmth over the central/eastern U.S. and a multi-day period of below normal temperatures over the West. Some areas from the Plains into the East may see morning lows up to 20-25F above normal by next Tue-Wed while highs will likely be 10-20F above normal on one or more days. Some locations may see daily record values for highs/warm lows. Coolest temperatures versus normal over the West should be focused over California with some areas seeing highs 10-20F below normal while less extreme anomalies extend farther inland. Rausch 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 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