Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1256 AM EST Fri Mar 12 2021 Valid 12Z Mon Mar 15 2021 - 12Z Fri Mar 19 2021 ...Heavy rainfall threat in the Tennessee Valley early next week... ...Overview and Model Guidance/Predictability Assessment... Unwinding closed low over the Central Plains will lift into the Great Lakes, spreading rainfall east of the Mississippi on Monday, while another system enters the West. That system is forecast to track eastward through the middle of the lower 48 as another Pacific system comes into Washington and Oregon late in the week next week. The models and ensembles were drastically different in the short range over the eastern Pacific, hinging on how to evolve sharp troughing between Alaska and Hawai'i. The 12Z ECMWF/UKMET detached a southern upper low near 33N/145W Sunday while the 12Z (and 18Z) GFS/Canadian did not, leaving differences in the northern stream flow into the Pacific Northwest; namely, the ECMWF/UKMET were much quicker into/through WA/OR than the GFS/Canadian. The ensemble members tied to each parent model mostly followed along with a corresponding quicker or slower solution. Trends over the past few days showed a more consistent GEFS than ECMWF EPS and favored the less split solution shown by the 18Z GFS, though with a great deal of uncertainty. By next Tue into Wed, the GFS/ECMWF came mostly back into agreement with the track of and northwestern trough or upper low through the Four Corners and onto the Plains. Thereafter, differences in Canada ultimately led to the 12Z ECMWF falling out of favor in the East, likely too far south/deep with an upper low (on the edge or outside its ensemble members' spread). 18Z GFS simulated a reasonable solution as the upper trough may reach the Appalachians around the end of next week, supporting surface low pressure into the Tennessee Valley next Thursday. The 18Z GFS did appear too strong with a trailing upper/sfc low over the Northeast Pacific and perhaps too quick with the lead front around next Thursday into WA/OR. Trended toward the ensemble means as a consensus hedge given the lead-in uncertainty. ...Weather/Threats Highlights... Decaying Plains system will still support wrap-around snow on its northwest/north side in the cold sector (Corn Belt/Upper Midwest/Dakotas) on Monday with overrunning snow to the north/northeast ahead of the warm front as the surface high retreats into Atlantic Canada. This may support a transition to rain in some areas as milder air is drawn into the region. In the warm sector, modest to perhaps locally heavy rain, perhaps driven by convection, will blossom and expand into/through the mid-South to the Ohio River Valley. Focus will shift back to the west out of the southern Rockies as the next system may again tap the Gulf of Mexico for moisture into the lower Mississippi Valley Wed-Thu, and perhaps to the East Coast by next Friday. Temperatures will be mild ahead of the Plains system through the Southeast, but colder than normal in the Northeast until the high moves offshore. Much cooler than normal temperatures will follow behind the lead and subsequent Plains and Northwest systems, respectively. Temperatures may be variable across the High Plains but within about 5-15 degrees of normal for the middle of March. Fracasso 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