Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Thu Feb 25 2021 Valid 12Z Sun Feb 28 2021 - 12Z Thu Mar 04 2021 ...Heavy rain threat gradually diminishes across the interior Deep South early next week... ...Overview and Guidance Evaluation... An ongoing heavy rain event should extend across the interior Deep South towards the southern Appalachians as the forecast period begins on Sunday. There has been a gradual southward shift in the heavy rain axis as additional upper level energy is forecast to eject from the upper level trough dipping into the southwestern U.S. On the other hand, models are trending slower with respect to the eastward motion of the upper low itself. This could result in keeping the heavy rain threat a bit longer in the Deep South on Monday. Thereafter, there is a general consensus among model guidance that the bulk of the moisture will gradually move eastward across the Southeast ahead of the upper trough. However, uncertainty remains quite high regarding the subsequent developments of this system. A blend of the 00Z ECMWF mean, 06Z GEFS and the 00Z CMC mean yielded a developing low pressure wave moving of the Southeast U.S. coast by the middle of next week. Meanwhile, deterministic models and ensembles continue to struggle with both the timing and evolution of the next trough approaching the West Coast from the Gulf of Alaska/North Pacific region as early as Sunday night. The GFS and to a lesser extent the GEFS mean continue to indicate the development of a closed upper level low that lingers well offshore, while the ECMWF keeps the upper trough largely over land as it digs down the West Coast early next week. The latest (12Z) ECMWF is even more progressive than the 00Z run. The CMC is somewhere in between. These differences have implications for the southern tier states for the latter part of next week. This leads to below average forecast confidence from Day 5 onward for this part of the country. The WPC medium-range fronts and pressures was derived based on the consensus of the 00Z ECMWF/00Z EC mean and the 06Z GFS/GEFS for Days 3 to 4. More of the EC mean and 06Z GEFS together with a larger contribution from the 00Z CMC mean were used due to the above normal forecast uncertainty beginning on Day 5. ...Weather/Threats Highlights... The upper level trough across the southern Rockies and then the southern Plains will allow Gulf moisture to lift northward as multiple waves of low pressure develop along a slow-moving front. This setup will favor a few rounds of locally heavy rainfall late this weekend into early next week from the Arklatex to the southern Appalachians. The heavy rain threat should gradually diminish by Tuesday as the bulk of moisture should slide eastward across and then off the Southeast afterwards. Meanwhile, uncertainty is quite high regarding the specific details across the Southwest to the Plains. Mountain snows are possible for the central/southern Rockies as the upper trough moves through. Some precipitation could be moving into the central Plains afterwards. High temperatures will likely be about 5 to 15 degrees above average across the eastern U.S. on Sunday ahead of the front, and overnight lows will likely be even more above average across portions of the Deep South owing to southerly flow from the Gulf of Mexico and widespread cloud cover. These readings should return closer to late February/early March averages as the frontal boundary settles south of the Gulf Coast by the middle of the week. Meanwhile, readings should be slightly above average for much of the central and northern Plains for much of this forecast period. Kong/Hamrick 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