Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sun Feb 28 2021 Valid 12Z Wed Mar 03 2021 - 12Z Sun Mar 07 2021 ...Overview and Guidance Evaluation... During the course of the past 12 to 24 hours or so, models and ensembles have made another significant adjustment pertaining to the synoptic pattern evolution through the medium-range period across U.S. mainland. The issue remains to be the timing of an upper low that is forecast to track across the Southwest later this week, together with its subsequent interaction with a broad trough that is forecast to dip into the eastern U.S. Recent global model runs have significantly delayed the time of arrival of the upper low into the Southwest. This delay in arrival time leads to less interaction with the broad upper trough dipping into the eastern U.S., which would lessen the potential for late-week cyclogenesis over the southern Plains. In fact, there is now general model consensus to suppress further development of the a frontal wave over the southern High Plains late this week due to amplification of the East Coast upper trough. The WPC medium-range forecasts followed this model trend and showed only a suppressed frontal wave over the southern High Plains late this week. Earlier in the forecast period, models generally agree that a developing frontal wave will move off the Southeast U.S. coast at a slightly faster pace on Wednesday. Meanwhile, as upper troughing amplifies off the West Coast mid-late week, latest model consensus continues to showing a trend of pushing the trough slightly faster toward land. The 12Z CMC was a compromise though on the fast side with the upstream feature. Some aspects of each solution could have some merit. The GFS is the fastest with respect to pushing the trough inland across the Pacific Northwest on Saturday versus the ECMWF's Sunday arrival. The typical GFS biases suggest it could be too fast but at that time frame there has been a modest faster trend with this feature in most guidance recently. Also the idea of a flatter shortwave ejecting into the downstream ridge could have some merit versus the ECMWF/ECMWF mean that bring more height falls into the ridge. Thus a blended approach seems best for the purposes of a single deterministic forecast. The WPC medium-range forecast package was derived based on a multi-model consensus of the 00Z ECMWF/00Z EC mean and the 06Z GFS/GEFS with smaller contributions from the 00Z CMC/CMC mean and the 00Z NAEFS. 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... On Wednesday expect precipitation over the Southeast/southern Mid-Atlantic to taper off as a wave initially along the southeastern coast moves off into the Atlantic. Some rainfall could still be locally heavy early in the day depending on timing of this system. The upper low closing off to the west of California during the short range period should move through the Southwest and into the Plains during the latter half of the week. This system will bring some rain into parts of southern California and rain/higher elevation snow to portions of the Southwest and southern Rockies. Then rainfall over the Plains should stay suppressed at least as far south as the southeastern quadrant of the lower 48 as the surface system drops toward the Gulf Coast and possibly dissipating before reaching the coast. Precipitation across the Southeast is now much suppressed late week into the weekend compared with previous forecasts. Expect most effects to remain offshore the East Coast if low pressure develops over the western Atlantic next weekend. Precipitation ahead of the amplifying eastern Pacific trough aloft will initially focus primarily over Vancouver Island and into the Olympics, with some moderate to heavy totals possible. Expect rain and mountain snow of varying intensity to extend southward to the central West Coast by late week and the weekend. Some light precipitation will be possible at times during the period over the northern Rockies, and possibly over parts of the Interior West/central Rockies next weekend depending on the details of shortwave energy aloft. The forecast is very consistent in showing above normal temperatures over the Northern Plains and vicinity through the period, with decent coverage of plus 10-25F anomalies for highs and plus 10-20F anomalies for morning lows. Gradual amplification of the large scale pattern will bring a warming trend to the West. The warmest days should be around Friday-Saturday with the Great Basin the most likely to see readings up to 10-15F above normal. On the other hand areas near the East Coast will see a cooling trend that could bring highs down to 5-15F below normal by next weekend. 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