Extended Forecast Discussion NWS Weather Prediction Center College Park MD 122 PM EST Fri Mar 12 2021 Valid 12Z Mon Mar 15 2021 - 12Z Fri Mar 19 2021 ...A couple of rounds of heavy rainfall are possible across the southeastern quadrant of the CONUS next week... ...Overview... As a surface and upper-level low initially over the south-central U.S. track northeast and weaken, another upper trough is forecast to move into the Northwest Monday and track east through the week, likely spawning another low pressure/frontal system. As periods of Gulf moisture flow in ahead of these systems, heavy rainfall is possible Monday-Tuesday in the Tennessee Valley/Southeast and again Wednesday-Thursday in the Lower Mississippi/Tennessee Valleys, Southern Appalachians, and Southeast. ...Model Guidance/Predictability Assessment... The closed upper low over the central U.S. Monday appears initially well-handled by model guidance, but some differences remain with its track as it weakens and lifts out ahead of the next trough serving as its kicker. 00Z/06Z guidance produced generally quicker weakening of the related surface low over the Ohio Valley by Tuesday compared to previous runs. After a few model cycles with disagreements on the location and potency of troughing/energy coming into the Northwest for the early medium range period on Monday, the 00Z/06Z guidance tended to have a better consensus with this feature, given better alignment in the short range as that trough tracks over the Pacific compared to yesterday's 12Z runs. Guidance indicates this upper trough/low should drop southward into the Four Corners region and then track eastward across the Plains midweek and toward the Ohio Valley. This model cycle tended to show more separation between this feature remaining the southern stream and the northern stream, potentially through Thursday and beyond. The strength of that upper low remains in question, with the 00Z CMC/ECMWF stronger than GFS runs. However, its placement is not terribly different among the deterministic guidance. The surface low track associated with this feature aligns pretty well over the south-central Plains Wednesday, but spread in its position increases as it moves over the eastern U.S. and into the Atlantic late in the workweek. The 00Z ECMWF seems to be a northern outlier with its surface low position, though the 00Z CMC is north as well, while the 06Z GFS clusters the best with the ensemble means, so leaned toward the latter cluster. This surface low spread also affects the axis and precipitation type of the associated QPF from the Middle Mississippi Valley to the Northeast. All considered, model agreement for this cycle appeared generally okay, but run-to-run inconsistencies show that confidence is still not high. The WPC forecast transitioned from a mainly deterministic blend early in the period and trended toward the ensemble means later given the uncertainties arising. ...Weather/Threats Highlights... The initial low system will still support wrap-around snow on its northwest/north side in the cold sector, with best chances across the Corn Belt/Upper Midwest Monday. Additionally, overrunning atop a cold surface high could lead to some light snow in higher elevations of the Central Appalachians and into the Northeast Monday-Tuesday. Farther south, modest to potentially heavy rain, perhaps driven by convection, is likely across the Tennessee and Ohio Valleys and Southeast. Confidence then continues to increase regarding another round of rain near and ahead of the next low pressure system, with modest to possibly heavy amounts from the Lower Mississippi/Tennessee Valleys toward the Southern Appalachians and Southeast Wednesday-Thursday and perhaps the East Coast by next Friday. Precipitation is possible farther north as well, but as mentioned, details are uncertain at this point and vary depending on the low track. Underneath the West trough, some precipitation is possible over the Intermountain West toward the Rockies early in the workweek. Snow is once again likely for the Central Rockies and some could spread into the Central Plains Tuesday-Wednesday, but should not be as intense as the system this weekend. Renewed precipitation is expected for the Pacific Northwest and northern California for the latter part of the week as another upper low spins over the northeastern Pacific. Temperatures will be mild ahead of the Plains system through the Southeast, but colder than normal in the Northeast until the high moves offshore. Somewhat cooler than average 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. Tate 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