Extended Forecast Discussion NWS Weather Prediction Center College Park MD 222 AM EDT Thu Mar 18 2021 Valid 12Z Sun Mar 21 2021 - 12Z Thu Mar 25 2021 ...Overview... Deep reloading western CONUS troughing lifting out through the Plains will be the main driver(s) to the forecast during the medium range. Multiple rain areas (some snow out of the Rockies to the western Plains) are favored around the lead area of low pressure into the Upper Midwest early in the week before focusing on Texas and the lower Mississippi Valley. A slow-moving ocean storm off the Southeast coast will lift north-northeastward just offshore before pulling out to sea. This system could bring some rain and brisk winds to coastal areas from the Southeast to southern New England. ...Model Guidance/Predictability Assessment... Through the 12Z/18Z guidance suites, reasonable clustering of the models led to a deterministic blend to start the forecast Sunday-Monday (12Z/18Z GFS and 12Z ECMWF/UKMET/Canadian), despite differences with the handling of the western trough and Southeast coastal low (middle ground solutions were preferred). For days the guidance had been trending weaker and farther offshore but since the 00Z runs last night reverted back north with less separation from the northern stream flow which now gets it tugged back northward. In the West, the ECMWF continued to be a laggard in the guidance but had support from several ensemble members. Nevertheless, preferred a solution nearer to its ensemble mean as the ECMWF has been too deep/slow with some recent troughs. By late Mon into Tue, GFS/GEFS were quicker with the height falls into the Mississippi Valley and points eastward whereas the ECMWF ensembles were slower, due in part to a stronger second upper trough into the West with which the GFS/GEFS were weaker. With the Canadian even slower than the ECMWF, opted to rely more on the ECMWF ensemble mean for timing of the troughs out of the West at a pace slower than the GEFS mean. Parallel GFS continued to be on the quicker side as well. ...Weather/Threats Highlights... Cold front into the Pacific Northwest Sunday will bring light to moderate amounts of coastal/lowland rain and mountain snow, followed by more showery precipitation and generally lighter amounts until about midweek (Wednesday). That system may bring another round of light to modest rain/snow to coastal areas and the Cascades as well as northern Idaho. Temperatures will remain cooler than normal through the period. The Rockies to the central states will see larger effects from the lead and trailing areas of low pressure along the progressing frontal boundary during the period. Precipitation will expand on Monday out of the Rockies and onto the central Plains where some locally heavier amounts may fall. Temperatures will be mild enough to support mostly rain except for the northwest side of the front and surface low. As the first low lifts into the Great Lakes, precipitation will expand to the northeast as well but may lessen in intensity. By then, trailing upper trough through the West will start to round the base in the Southwest and start to develop another surface low along the front over Texas. This may promote another area of perhaps locally heavier rain nearer to the Gulf coast as moisture will be quite sufficient by then. Temperatures will be near to above normal east of the frontal boundary, especially through the northern tier where highs may reach into the 60s to the Canadian border. 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 Hazards: - Heavy rain across portions of the Northern/Central Plains, and the Upper/Middle Mississippi Valley, Mon-Tue, Mar 22-Mar 23. - Heavy snow across portions of the Pacific Northwest and the Northern Great Basin, Sun-Mon, Mar 21-Mar 22. - Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Mon, Mar 22. - Heavy snow across portions of the Northern Plains and the Northern Rockies, Sat, Mar 20. - Heavy snow across portions of the Central Rockies and the Central Plains, Sun, Mar 21. - Flooding possible across portions of the Central Plains, the Tennessee Valley, the Upper/Middle Mississippi Valley, the Great Lakes, and the Ohio Valley. - Flooding occurring or imminent across portions of the Central/Southern Plains, and the Middle Mississippi Valley. - Flooding likely across portions of the Central Plains and the Middle Mississippi Valley. - High winds across portions of the Southeast and the Mid-Atlantic, Sat, Mar 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