Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Thu Mar 05 2020 Valid 12Z Sun Mar 08 2020 - 12Z Thu Mar 12 2020 ...Heavy Precipitation threat early-mid next week for southern California/Southwest... 19 UTC Update... Model and ensemble solutions are becoming increasingly well clustered in the day 3-5 timeframe (Sun-Tue), bolstering forecast confidence. While forecast spread increases quite rapidly over the central and eastern U.S. days 6/7 (Wed-next Thu), guidance differences are coming into better agreement upstream with an eastern Pacific storm now set to approach southern California and the Southwest. This storm offers a widespread threat for heavy precipitation over the region early-mid next week. Accordingly, the WPC medium range product suite was primarily derived from a composite blend of quite compatable model and ensemble guidance over the West through medium range time scales, and over the central and eastern U.S. days 3-5 before reverting to just the ensemble means. Schichtel ...Guidance/Predictability Assessment... As upper level ridging builds across the Eastern U.S., lead upper shortwave/energy will cross the West Coast this weekend. This occurs as additional energy upstream drops southward to amplify troughing aloft and act to develop a closed low well off the West Coast by Sunday. The models have come into much better agreement with the progression of the shortwave into the Plains on Monday, and eventually into the East next Tuesday. There is also good agreement between the GFS/ECMWF/UKMET through day 5 on the upper low lingering slowly drifting southward and settling off Southern California next Tuesday. After this, differences begin to emerge with whether the energy moves quickly east into California and the Southwest U.S. day 6-7 (GFS solution), or it lingers over Southern California/northern Baja Mexico (ECMWF solution). The ECENS and GEFS means would support a middle ground solution, which is a reasonable place to be at this point. Additional northern stream energy enters the Pacific Northwest next Tuesday, and moves steadily eastward across the Northern Plains on Wednesday. The GFS is the most robust of the deterministic models in showing this troughing becoming quite amplified as it moves across the Northeast with the potential for cyclogenesis off the Mid-Atlantic coast by next Thursday. The ECMWF is quite flat with this wave, however there is support from various ensemble members as well as the ensemble means for a solution close to the GFS so it's not totally unreasonable. The blend for this cycle of the WPC progs used a total deterministic blend (GFS/ECMWF/UKMET) for days 3-4, steadily increasing weighting of the ensemble means thereafter reaching a 50/50 deterministic/ensemble mean blend by day 7. This maintains good continuity with the previous shift for days 3-6. ...Weather/Hazard Highlights... Inland progression of a northern and southern stream shortwave trough energies over the Northwest and down into CA/Southwest/Rockies will support swaths of rain/terrain enhanced snowfall this weekend. Downstream propagation of the southern stream shortwave, approach of a northern stream frontal surge, and potential dryline development also lends into a signal for Plains surface low development by early next week. Emerging/lead return flow from the Gulf of Mexico will fuel an expanding rain/strong convection pattern with the best focus across the eastern Plains/MS Valley. Activity will then move out across the east-central U.S. as the main low/front low works downstream and a surface low potentially organizes off North Carolina. Cold air to the north of the low may also support some risk of heavy snows from the Upper Midwest into the Great Lakes then northern Northeast early to middle of next week. The West should overall dry out into Mon as the lead energy moves inland, but heavier rain/mountain snows may target portions of south-central CA/Southwest into next midweek with anticipation of the slow approach toward the coast of a main east Pacific closed low and leading frontal system. Temperature wise, much of the country will either be near or above normal through most of the period. The biggest max temp anomalies are expected across the Central Plains/Midwest (+20F anomalies possible) on Sunday. Above normal temperatures move into the Ohio Valley/Northeast early next week as well, though anomalies should moderate somewhat. Meanwhile, the central U.S. and Southern tier should remain slightly above normal through day 7 as upper level ridging holds following the passage of the southern stream shortwave this weekend. Santorelli Hazards: - Heavy precipitation across portions of the southern Sierra Nevada, Tue, Mar 10. - Heavy precipitation across portions of the northern Cascades, Wed, Mar 11. - Heavy rain across portions of the Olympic Peninsula, Thu, Mar 12. - Heavy rain from portions of the mid-Mississippi Valley, across the Tennessee Valley into the southern Appalachians, as well as portions of southern California, and central Arizona, Tue-Wed, Mar 10-Mar 11. - Flooding possible across portions of the Southeast. - Flooding occurring or imminent across portions of the Southeast and the lower Mississippi Valley. - Flooding likely across portions of the Northern Plains. - Much above normal temperatures across portions of the central to northern Plains into the upper Midwest, Sun, Mar 8. 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