Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Fri Feb 26 2021 Valid 12Z Mon Mar 01 2021 - 12Z Fri Mar 05 2021 ...Heavy rain threat over the South continuing into early next week and then gradually diminishing... ...Overview and Guidance Evaluation... The medium-range period continues with the theme of uncertainty that has been plaguing the late-week forecast lately. At issue is whether a compact upper low will detach from the northern stream trough well off the Pacific Northwest Coast early next week before eventually emerging into the Plains late next week. In the mean time, there is good model consensus on the threat of heavy rainfall across the Deep South early next week. The rain should slide eastward and could bring areas of heavy rain across the interior Southeast on Tuesday. The GFS has been quite consistent in forecasting the aforementioned trough fracture off the Pacific Northwest early next week. This flow separation issue ultimately leads to high uncertainty in the synoptic pattern evolution over the eastern Pacific and then eastward across much of the lower 48. Meanwhile, the ECMWF has largely avoided significant trough fracture but has been keeping a digging trough down into the West Coast. This would result in a much more progressive pattern propagating downstream across the northern tier states in stark contrast with the GFS which eventually brings the detached upper low into the Plains with potential for cyclogenesis late next week. On the other hand, the latest (12Z) ECMWF did indicate a switch toward the GFS' trough fracture scenario. It appears that the amplitude of the upper trough currently in the northern Pacific is sharp enough to allow such trough fracture to occur further downstream. Given past performances of the ECMWF and the GFS in recent months across the northeastern Pacific, the morning medium-range package adopted an intermediate solution which is comparable to the CMC. This solution allows a low pressure wave to form over the southern High Plains during the middle of next week before showing some amplification over the central/southern Plains late next week. The WPC medium-range fronts and pressures were derived based on the 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 due to the above normal forecast uncertainty. ...Weather/Threats Highlights... The upper low tracking out of the southern Rockies and through the Plains early in the week should interact with a front near the Gulf Coast, supporting one or more waves along the front and a northward flow of Gulf moisture. This evolution will favor another episode of locally heavy rainfall across parts of the South during the first part of the week, with lighter precipitation extending farther north over the southern half of the Plains and an increasing portion of the East. There may be some snow in the northern part of the precipitation shield. The rest of the forecast has lower than average confidence for specifics. One scenario--currently somewhat more likely--would have a period of moderate rain/mountain snow over the Northwest and vicinity at the start of the week with mostly light activity then spreading into the Interior West/Rockies followed by precipitation of varying intensity spreading across the East in association with one or more surface lows/fronts (which may eventually consolidate). The other possibility will be for a much lighter event over the Northwest into the Rockies early in the week and less moisture extending downstream, while a system off California moves into the Southwest by midweek or so with moisture continuing eastward into late week. Either way some moisture may reach the West Coast late in the week depending on the exact timing/strength of the upper trough amplifying over the eastern Pacific. The upper low emerging from the southern Rockies early in the week will keep highs up to 10-20F below normal over the southern High Plains on Monday followed by a rapid rebound to near normal as the feature continues onward. Then cool high pressure building over the East will lead to highs 5-15F below normal near the East Coast on Tuesday, again followed by a quick recovery. The Northern Plains region will likely be consistently warmer than average next week with most days featuring some plus 10-20F anomalies. A majority of the West should see near to slightly below normal readings early-mid week followed by a warming trend as upper ridging builds over the region. As with precipitation, some aspects of the temperature forecast over the lower 48 will depend closely on the uncertain details of individual systems. 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