Extended Forecast Discussion NWS Weather Prediction Center College Park MD 308 PM EST Tue Mar 02 2021 Valid 12Z Fri Mar 05 2021 - 12Z Tue Mar 09 2021 ...Overview and Guidance Evaluation... Initially the upper level pattern over the CONUS is expected to be characterized by troughing along both coasts with a compact shortwave trough over the Southern Plains. That system is forecast to quickly move toward the Southeast and become absorbed into the developing longwave trough over the East Coast this weekend while the West Coast energy traverses the Rockies and weakens as it opens up into the Plains Sunday. Reinforcing shortwave troughing will then settle over the West Coast early next week while downstream ridging flattens out east of the Rockies. From a sensible weather perspective, focus of precipitation will be along the Southeast/Florida early in the period while the West Coast, particularly the Pacific Northwest, remains unsettled to active. Temperature anomalies will be noteworthy for the Northern Plains. This cycle model guidance showed just average agreement through day 5 with the progression of the shortwave energies from the Plains to the Southeast and West Coast through the Rockies. The GFS was considerably faster than the consensus (UKMET met was equally as slow) and it showed a faster/flatter progression of shortwave energy coming into the Plains by day 5. The 12Z run showed a similar solution which is decidedly different from consensus and the other ensemble means. Its inclusion in the WPC blend was considerably less compared to the more favorable ECMWF/CMC which showed a slightly more amplified idea. This muddles the forecast for day 5-7 east of the Rockies where either more ridging or a flatter quasi-zonal flow takes shape. Meanwhile, for the West Coast, guidance shows fairly good agreement in another shortwave trough settling along the coast by late this weekend into next week. Overall, the WPC blend was initialized with mostly a ECMWF heavy component with inclusion of the CMC and GFS. By day 6-7, mostly ECMWF/ECENS was incorporated with some inclusion of the GEFS. ...Weather/Threats Highlights... The Southern Plains upper low will weaken/fill as it becomes absorbed into the longwave trough over the East Coast this weekend. Its associated surface low, compact and quick moving, will bring a period moderate to perhaps locally heavy rainfall to portions of Florida, particularly portions of the northern/central peninsula where the combination of daytime heating, available moisture, and lift interact. This wave will then deepen as it moves well offshore and cooler, Canadian high pressure builds across the entire East Coast. With troughing expected through most of the period across the Pacific Northwest, expect generally unsettled weather with frequent periods of precipitation. Overall, each individual system could produce some moderate to locally heavy precipitation and the cumulative effect over the period is for several inches of precipitation mainly confined to the extreme coastal ares of northern California, Oregon, and Washington. Much above normal to near record warm temperatures are forecast Friday, this weekend, and early next week for portions of the Northern Rockies and Northern Plains. Daytime highs in the 60s (nearing 70F in South Dakota) will be 20-30F above normal for this time of year. Nighttime lows may stay above freezing Saturday and Sunday and this would be 10-20F above normal. Some records could be broken for daytime highs and lows. These warm temperatures will gradually spread toward the Great Lakes, Northeast, and Mid-Atlantic early next week though will likely moderate some (anomalies 10-15F). Below normal readings are forecast for the West Coast late this weekend into next week. Taylor 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 precipitation across portions of the Pacific Northwest, Fri, Mar 5. - Heavy precipitation across portions of California and the Pacific Northwest, Fri, Mar 5 and Mon-Tue, Mar 8-Mar 9. - Heavy snow across portions of California, Mon-Tue, Mar 8-Mar 9. - Flooding possible across portions of the Southeast. - Flooding occurring or imminent across portions of the Southeast, the Lower Mississippi Valley, the Central Appalachians, and the Ohio Valley. - Flooding likely across portions of the Central Appalachians and the Ohio Valley. 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