Extended Forecast Discussion NWS Weather Prediction Center College Park MD 153 PM EST Thu Mar 04 2021 Valid 12Z Sun Mar 07 2021 - 12Z Thu Mar 11 2021 ...Overview and Guidance Evaluation... Models and ensembles this morning show decent agreement that slow-moving upper troughing will dominate the western U.S. as upper ridging builds across the Plains into the eastern U.S. during the medium-range forecast period. Much of the uncertainty will be focused across the northern Plains as upper-level energy is forecast to lift across the central Plains and then interact with the northern stream by midweek, but the amount of model spread is quite reasonable for a Day 6 and 7 forecast. The feature of interest will be an amplified eastern Pacific mean trough aloft that gradually makes its way into the western U.S. from Sunday through most of next week. This trough will support a broad area of rain and higher elevation snow across the West along with below normal temperatures. Meanwhile by Tuesday-Wednesday, a fairly vigorous shortwave expected to eject from the trough is likely to produce a significant northern Plains storm system that should spread a broad area of precipitation across the central/east-central U.S. Ahead of this evolution, an amplified East Coast upper trough will depart after Sunday and eastern U.S. heights will continue to rise through the period as ridging moves in from the central U.S. and an upper high builds from Mexico into the Gulf around midweek. There is decent consensus for the overall evolution within the East Pacific into western U.S. upper trough. The most likely scenario has an upper low drifting southward off the Pacific Northwest while energy flowing around the western/southern side of this low reaches the West by Tuesday and then continues northeastward to provide the dynamic support for the midweek Northern Plains storm. The primary forecast challenges involve the timing and evolution of the ejecting energy as well as the overall amplitude of flow to the southeast--affecting specifics of the surface system and how far southeastward the trailing front extends. Overall preference is to stay closest to the ensemble means for timing later in the period, given reasonable continuity thus far and intermediate position between faster GFS runs and slower ECMWF runs. The 00Z CMC offers a faster solution than the GFS but its ensemble mean compares well with the GEFS and the EC mean. Meanwhile, energy from the upper low off the Northwest should drop southeastward and possibly combine with upstream flow to yield an upper trough that reaches near the Great Basin/Southwest by day 7 Thursday. The WPC medium-range forecast package was derived based on multi-model consensus of the 00Z ECMWF/00Z EC mean and the 06Z GFS/GEFS with smaller contributions from the 00Z CMC/CMC mean. Beginning on Day 5, more of the 06Z GEFS together with the 00Z EC mean were used to account for the increasing forecast uncertainty. ...Weather/Threats Highlights... Expect a large portion of the West to see at least some rain/higher elevation snow during the period. Heavier precipitation should extend from southwestern Oregon through favored terrain in California, with highest amounts most likely along the Sierra Nevada. Meaningful though more moderate precipitation is likely across the Interior West and Rockies. Energy ejecting into the Plains should produce a fairly strong storm system that tracks across northern tier locations Tuesday onward, with a broad area of precipitation extending across the northern/eastern Plains and into the east-central U.S. Some wintry weather may be possible over extreme northern locations depending on exactly how the storm evolves. Farther south there may be locally moderate to heavy rainfall with the mid-Mississippi Valley and lower Ohio Valley currently the most likely areas to see highest rainfall totals. Check future outlooks as the system's details come into clearer focus. Around the end of the period and beyond, the cold front anchored by the surface low could stall with continued Gulf inflow helping to add to the rainfall potential. There is currently enough spread to temper confidence in this scenario somewhat but it requires monitoring. Much above normal temperatures will persist over the northern Plains/Midwest and expand into the Great Lakes through the first part of next week with readings generally 10-25F above normal. Daily records should be more numerous for warm lows than daytime highs. Development/progression of the northern Plains low pressure system and trailing front Tuesday onward will finally push this warmth more into the eastern half of the country by mid-late week. At that time, morning lows should remain as warm as 15-25F above normal while highs will likely be somewhat less extreme. The West will see a cooling trend spreading across the region from west to east during the period with highs tending to drop to 5-10F or so below normal. The East Coast states will start out chilly with similar negative anomalies on Sunday followed by a steady warming trend. Kong/Rausch 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