Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Sun Mar 3 2019 Valid 12Z Wed Mar 6 2019 - 12Z Sun Mar 10 2019 ...Weather Pattern Description... The flow across the CONUS will begin the medium range in a rather amplified setup, and then gradually trend more zonal and progressive by the mid to latter portion of the forecast period. Shortwave energy ejecting from a mean upper trough off the West Coast will cross the country every couple days. Meanwhile, a broad upper trough initially in place across the Great Lakes/Northeast should gradually lift out by the middle to end of the upcoming work week. ...Model Guidance Evaluation and Preferences... The medium range forecast period begins on Wednesday with a West Coast trough and broad cyclonic flow encompassing the eastern half of the nation, with a broad upper ridge over the Rockies. Noteworthy model differences appear as early as Day 4 (Thursday) with the break down of the ridge as Pacific energy lowers heights and a quasi-zonal flow pattern returns. The FV3, ECMWF, and UKMET were in reasonably good agreement on the passage of shortwave energy across the High Plains and Upper Midwest through Thursday and Friday, while the GFS was more amplified. Even greater model differences reside across the West Coast region for Friday and beyond, with the CMC much slower with the trough axis progression. This large scale trough and eventual evolution into a closed upper low will eventually spawn the development of a strong surface low across the central U.S. Although the chances are good that a strong storm system will develop, there are major differences in the placement and track of the low, so future forecasts are likely to change significantly in the coming days since confidence is below average in this portion of the forecast. Given these considerations, the forecast was initially based on a multi-model deterministic blend (12Z ECMWF/UKMET/FV3) during days 3-5 (Wednesday through Friday). For next weekend, the forecast was mainly derived from ensemble mean (ECENS and NAEFS) weighting, with some continued emphasis on the 12Z ECMWF/FV3 solutions as well. ...Sensible Weather and Potential Hazards... Although the coldest temperatures will be prior to the start of this forecast period, it will still be very cold by early March standards across much of the central and eastern U.S. on Wednesday and Thursday. A large arctic high will encompass much of the U.S. east of the Rockies, and temperatures only slowly moderate by the end of the week with anomalies of 15 to 30 degrees below normal through Thursday, and a second surge of cold temperatures is expected for the northern plains and Upper Midwest through next weekend, keeping readings about 20 degrees below normal. However, much of the southern and eastern U.S. warms up to near average levels by the weekend as a storm system gathers strength across the Plains. Heavy rain and mountain snow return to the western U.S. by Tuesday night as an atmospheric river event accompanies a strong Pacific low headed towards California. The Sierra Nevada will likely get an additional 1 to 3 feet of snow, thus adding to its extensive snow pack. Significant snow is forecast for the central and northern Rockies on Wednesday and Thursday as the moisture plume advects inland, albeit not to the same degree as California. With a large storm system possibly affecting the central U.S. next weekend, widespread rain is expected to develop from portions of the lower/mid-Mississippi Valley to the Ohio Valley and Appalachians as a rich supply of Gulf moisture surges northward. Depending on where the surface low develops, a band of heavy snow and windy conditions are possible across parts of the northern plains, Upper Midwest, and western Great Lakes region. D. Hamrick 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