Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Sun Mar 01 2020 Valid 12Z Wed Mar 04 2020 - 12Z Sun Mar 08 2020 ...Heavy rainfall threat over much of the Southeast U.S. on Wednesday... ...Overview and Guidance/Predictability Assessment... An anomalously deep upper level low over northern Mexico by Wednesday will advance eastward into the Southern Plains and open up as it approaches the Deep South. Ample Gulf of Mexico moisture will be pulled ahead of the trough, continuing to favor an ongoing heavy rainfall threat across much of the Southeastern states to start the medium range period. Right off the bat, the GFS remains faster than the non-NCEP (and ensemble mean) solutions, opening the wave up too quickly over the Southern Plains. A northern stream shortwave drops into the north-central U.S. by Thursday, eventually providing reinforced troughing over the Eastern U.S. after the initial wave passes. Again, the GFS is much flatter/weaker with this wave and eventual surface low off the East Coast than the remainder of the solutions, which are also supported by the ensemble means. Out West, guidance agreement has gotten a bit better with respect to a closed low/shortwave dropping southward out of the Gulf of Alaska late in the week, though the GFS does continue to be much too quick to pull the first wave of energy inland, while the rest of the guidance, supported by the ensemble means, is almost a full day behind. The WPC model blend preference for medium range continued to lean pretty heavily on the deterministic ECMWF for days 3-5, with some contribution from the CMC and the GEFS/ECENS means. Thereafter, as confidence and predictability with the details increases, weighting of the forecast was towards the agreeable ensemble means, with some small percentage of the ECMWF to maintain some definition. This maintains good continuity with previous WPC shift for days 3-6. ...Weather/Hazard Highlights... Heavy rainfall will be ongoing across much of the Southeast, and models continue to indicate slight southward shifts with the heavy rain axis from recent days. Thus, confidence is high in an event, but still average at best in the exact location of heaviest rainfall. The current forecast shows multi-inch rainfall totals Wednesday into Thursday from central Mississippi eastward to South Carolina. Some of these same areas have seen above normal rainfall over the past 30 days, and therefore, an excessive rainfall threat is likely to accompany this system. Winter precipitation and the threat for heavy snows will stay mainly confined to the interior portions of the Northeast and New England. The Pacific Northwest should see daily rounds of rain/mountain snows associated with energy moving through in the flow aloft. The heaviest precipitation for mainly coastal locations and mountain ranges should be ahead of the deep low exiting the Gulf of Alaska Thursday into Friday, eventually dropping southward into parts of California by next weekend. Above normal temperatures are generally expected across the CONUS Days 3-7. Minimum temperatures around 20 degrees above normal will progress into the Southeast on Wednesday in the warm/moist flow ahead of the main storm system. The interior West ridge late next week brings above to much above normal maximum temperatures from the Great Basin to the central/northern Plains for Thursday/Friday, with max temp anomalies 15 to 20+ above normal entering the Great Plains by next weekend. Santorelli 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