Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Sat Feb 29 2020 Valid 12Z Tue Mar 03 2020 - 12Z Sat Mar 07 2020 ...Heavy rainfall threat over portions of the Southern and Eastern U.S. Tuesday and Wednesday... ...Overview and Guidance/Predictability Assessment... The medium range period starts out Tuesday with a deep closed upper low over northern Baja California, which should swing east and then eventually northeast across the Southern Plains by Thursday. This kind of track continues to favor a heavy rainfall threat from eastern Texas to the Southern Appalachians Tuesday and Wednesday. Right off the bat, the GFS is a noticeable outlier with how fast it is with this system compared to the rest of the guidance. The ensemble means seem to favor a slower solution, and even though the GEFS mean is a hair faster than the ECENS mean, its not nearly as fast as the GFS. A non-GFS blend was preferred for this system. After Thursday, whatever energy is leftover from the above system looks to get absorbed into reinforced troughing across the Ohio Valley thanks to northern stream shortwave energy which enters the Pacific Northwest early Wednesday. As was noted last night, the CMC continues to be the most amplified/developed with this feature, while the GFS is fairly flat with the wave. The ECMWF and UKMET (which of course drops out after day 5) lie somewhere in the middle. However, model run to run continuity is quite poor with this feature. Amplified troughing dropping southward from the Gulf of Alaska should reach the Pacific Northwest by day 6/Friday. Models continue to struggle with the details and evolution of this feature. The ECMWF has been fairly consistent in showing the troughing lingering along/off the West Coast, while the GFS is consistent in pulling it inland across the Northwest by Day 7. The ensemble means seem to trend towards a middle ground solution. Based on the above assessment, tonights blend for the WPC medium range progs featured a majority deterministic model blend days 3 and 4 between the latest runs of the ECMWF and the UKMET, along with minor contributions from the ensemble means. After that, confidence in the details concerning the northern stream energy and also the troughing along the West Coast days 6-7 is low, and so a mainly ensemble mean blend was preferred (with some percentage of the ECMWF for a little definition). This fits well with previous shift continuity. ...Weather/Hazard Highlights... Confidence continues to increase in the pattern evolution at the surface and aloft supporting a potentially significant rainfall event from portions of eastern Texas to the southern Appalachians/southern Mid-Atlantic Tuesday and Wednesday. Strong low level flow of Gulf of Mexico moisture will develop ahead of this system, bringing anomalously high precipitable water values into the Southeast quadrant of the U.S.. The best axis of rainfall should focus along a slow moving frontal boundary across the Tennessee Valley/Southeast, with several pieces of guidance indicating multi-inch rainfall totals, possibly falling over already overly saturated regions which have seen above normal rainfall over the past 30 days. Therefore, an excessive rainfall threat is likely to accompany this system. In addition, the Storm Prediction Center continues to monitor severe weather potential, with the current outlook highlighting an area over and just west of the Lower Mississippi Valley for Tuesday-Tuesday night. Continue to expect any winter weather effects to be fairly limited and confined to the Northeast and higher elevations. The Pacific Northwest should see daily rounds of rain/mountain snows associated with energy moving through in the flow aloft next week. 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. Above normal temperatures are generally expected across the CONUS Days 3-7 with the exception right along the low/trough exiting the Desert Southwest early next week and crossing the Southeast late in the week. Minimum temperatures of 20 to 25 degrees above normal look to progress from Texas across the Southeast through 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