Extended Forecast Discussion NWS Weather Prediction Center College Park MD 329 PM EDT Tue Apr 14 2020 Valid 12Z Fri Apr 17 2020 - 12Z Tue Apr 21 2020 ...Overview and Guidance/Predictability Assessment... The medium range begins Friday with separation of the northeast CONUS trough as it pushes offshore and the positively tilted trough across the northern Plains to the north-central Rockies along with a closed low off the CA coast as a vortex consolidates north of Hudson Bay. Differences in the progression of the northern Rockies trough/CA low develop among guidance on Day 3. The 00Z ECMWF is the flattest solution which is fastest with shifting the surface feature the East Coast while the more amplified trough from the GFS/UKMET/CMC generally results in a slower surface low. However, the 12Z GFS came in even more progressive which makes for a similar progression of the surface low to the 00Z ECMWF, but they are similar for different reasons. Guidance is consistent with the CA low ejecting east across southern CA Friday, but that is where consistency ends. There are considerable differences among deterministic guidance beginning on Day 3. The greatest deviations are 00Z UKMET which is the farthest south (into northwest Mexico) and the 00Z CMC which is drawn into a remnant of the trough over the northern Rockies. Based on these deviations of the ejection of the West Coast low in the 00Z CMC/UKMET and repercussions of these deviations through Day 5 led to both models being less preferred starting on Day 4. The flatter 00Z ECMWF is more progressive than the GFS with the West Coast low starting on Day 4. The more progressive ECMWF also allows a more meridional eastern CONUS trough from the Hudson Bay low Day 5 into 6 as a strong western CONUS/Canadian ridge develops. Because of these deterministic differences, ensemble guidance is preferred starting on Day 4 with the 00Z ECENS more heavily weighted in WPC blend than the 06Z GEFS Days 5 to 7. As a result of favoring the ECMWF/ECENS the WPC forecast is generally a little more progressive than the previous. A cutoff low of the West Coast looks to get drawn east under this ridge in the Day 6/7 time frame though there is little confidence in this given the differences among the ensemble members. ...Weather/Hazard Highlights... As the low from CA crosses the Four Corners Friday night/Saturday and the northern stream trough crosses the northern tier/Great Lakes will promote a return of Gulf of Mexico moisture to fuel showers and thunderstorms across the south. Heavy rainfall is possible with this system, mainly from the lower Mississippi Valley into the Southeast late Saturday into Sunday. Model variability remains, so confidence in specifics remains low at this time. There is uncertainty for precipitation in the Northeast Saturday/Sunday (Days 4/5) given the potential for offshore low pressure development. The deterministic GFS continues to rapidly develop a low off the Northeast. While the more progressive ECMWF/ECENS is more preferred there are some similar 06Z GEFS ensemble solutions, so this will need to be monitored. Below normal temperatures are expected with the northern stream trough over the northern Rockies, progressing across the northern Plains, Great Lakes through the weekend, then continuing for the eastern CONUS in the redeveloping trough. Meanwhile the developing western CONUS trough makes for above normal temperatures across The West Sunday into next week. Jackson 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 Lower Mississippi Valley, the Southern Appalachians, and the Tennessee Valley, Sat-Sun, Apr 18-Apr 19. - Flooding occurring or imminent across portions of the Lower Mississippi Valley, the Northern Plains, the Tennessee Valley, the Middle Mississippi Valley, the Mid-Atlantic, the Southern Appalachians, the Upper Mississippi Valley, and the Southeast. - Much below normal temperatures across portions of the Central Plains, the Central Great Basin, the Northern Plains, the Northern Rockies, the Central Rockies, the Middle Mississippi Valley, the Northern Great Basin, the Upper Mississippi Valley, the Great Lakes, and the Ohio Valley, Fri-Sat, Apr 17-Apr 18. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Fri-Sun, Apr 17-Apr 19. - High winds across portions of mainland Alaska and the Aleutians, Fri, Apr 17. - Much above normal temperatures across portions of mainland Alaska, Fri-Mon, Apr 17-Apr 20. 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