Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM 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... An anomalously deep upper level low over northern Baja California at the start of the medium range period (Tuesday) is expected to advance eastward toward the southern Plains and open up as it approaches the Deep South mid to late week. Pulling ample Gulf moisture northward and impinging it along a stationary/lingering surface boundary will be the focus for several episodes of rainfall, some of which could be heavy. The forecast confidence starts off average at best but quickly degrades to below average through the remainder of the medium range period. The progression of the aforementioned upper low and its potential phasing with northern stream energy mid to late week is the main driving factor for lower predictability. The GFS continued its fast bias, opening up the wave too quickly and allows for more phasing with the northern stream energy over the East Coast Thursday/Friday and overall faster progression. Meanwhile, the non-NCEP solutions, supported by their ensemble means, suggests a slower evolution of the upper level low and less interaction with shortwave energy in the northern stream until late in the week or early next weekend (Day 6-7 time frame). By Day 6/7, the differences between the GFS and non-NCEP guidance (ECMWF/CMC) are very significant, particularly across the central/eastern CONUS where the ECMWF offers deep troughing compared to the GFS version which has more zonal flow. Upstream across the eastern Pacific and West Coast, models show slightly better agreement for a deep closed low offshore, but differ with respect to the timing/speed with more typical biases seen (GFS fast, ECMWF slow). The updated WPC model blend preference leaned heavily on the deterministic ECMWF for Day 3-5 with some inclusion of the CMC/UKMET/ECENS mean. As predictability and forecast confidence lowered into Day 6-7, higher inclusion of the ECENS mean was incorporated. ...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 across the Southeast 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 from eastern Texas eastward toward the lower Mississippi River Valley Tuesday/Tuesday night. Winter precipitation and threat for heavy snowfall will be mainly confined to the interior portions of the Northeast and New England Day 6-7. 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/Taylor 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