Extended Forecast Discussion NWS Weather Prediction Center College Park MD 247 AM EDT Mon Mar 09 2020 Valid 12Z Thu Mar 12 2020 - 12Z Mon Mar 16 2020 ...Southwest heavy rain threat to emerge over the south-central U.S later this week... ...Guidance/Predictability Assessment... An interesting and active flow regime remains on tap for the lower 48 and vicinity during the medium range as a relatively progressive northern stream and less progressive/separated southern stream are forecast. Overall, model consensus has improved somewhat compared to recent days through much of the forecast period. An upper-level low is forecast to move across the southwestern U.S. during Thu-Fri (days 3-4), and emerge into the central U.S. Fri night-Sat. After that, the feature is expected to weaken as it quickly moves eastward. Model consensus on timing of this feature has continued to improve, although some variability remains, with solutions waffling a bit from run-to-run on the exact forward speed. The ECMWF continued to handle this system relatively well, and was heavily weighted in the initial WPC blend during days 3-5, along with smaller components of the GFS and ECENS/GEFS ensemble means. Farther north, models were relatively agreeable on a shortwave forecast to cross the Midwest/Great Lakes/northeast during the same time frame. A trailing cold front associated with this wave is forecast to linger across the Southern Plains Fri-Sat before moving south into the Gulf of Mexico by Sun-Mon as a relatively strong polar surface high moves from central Canada into the Great Lakes. By Fri-Sat, models continue to show an amplifying shortwave diving southward across western Canada toward the northwestern U.S. A complex potential interaction with additional northern stream energy across western Canada continues to result in model spread as to the specific evolution of this feature. The ECMWF has been fairly consistent in showing a relatively deep cutoff low developing across the Pacific Northwest by early Sat, which then slowly drifts southward along the West Coast through Sun and finally inland by Mon. The GFS is a bit different, delaying the development of a cutoff feature a bit, causing the feature to move farther inland across the West before cutting off across the Great Basin by Sun. The consensus of current ensembles favors something more along the lines of the ECMWF, which is reflected in the WPC forecast. The forecast during days 5-7 (Sat-Mon) was based on gradually increasing ensemble means (especially the ECENS) along with some continued use of the ECMWF. ...Weather/Hazard Highlights... Portions of the Southwest will see continued heavy precipitation potential on Thu as the upper-level low moves across the region. Farther east, moist southerly flow should focus showers and thunderstorms along a slow moving/wavy front stretching eastward across the south-central U.S. As the Pacific shortwave energy (former upper low) reaches the central U.S. by Fri/Sat, showers and thunderstorms, some strong, are expected to become more widespread from the Southern Plains northeastward to the Appalachians. Latest model guidance shows an increasing signal for potentially heavy rains from portions of eastern Oklahoma eastward toward the Tennessee Valley Thu-Fri, focused along a stationary frontal boundary. There remains some uncertainty with the main axis of any heaviest amounts and associated runoff risks, however. In the colder air north of the system, an area of winter weather is possible across Colorado and east into the adjacent High Plains Fri-Fri night. Meanwhile, the Northwest should see an increase in rain/mountain snow heading into the weekend as the aforementioned trough and frontal system affect the region with increasing vigor. Rain/snow with this system are forecast to spread inland across the West through the weekend. Ryan 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