Extended Forecast Discussion NWS Weather Prediction Center College Park MD 241 PM EST Fri Feb 28 2020 Valid 12Z Mon Mar 02 2020 - 12Z Fri Mar 06 2020 ...Heavy rainfall threat over portions of the Southeast U.S. Monday night into Thursday... ...Overview... A deep closed upper low off southern California Sunday night will slowly fill as it swings southeast across the central Baja Peninsula through Monday night before shifting east and opening into a trough as it crosses northern Mexico through Tuesday night. This trough then rejoins the northern stream as it shifts east-northeast across the southeastern CONUS to the Mid-Atlantic into Thursday. This track continues to favor heavy rainfall from parts of east Texas across the Lower Mississippi Valley to the southern Appalachians before shifting offshore. Meanwhile, shortwave energy shifts southeast across BC Sunday night as the first in a series of impulses that direct Pacific moisture into the Pacific Northwest ahead of deep low pressure is shifting south from the Gulf of Alaska through the middle of next week. This maintains a wet pattern for the Pacific northwest next week starting Sunday night. Upper ridging amplifies over the interior West late next week ahead of the Gulf of Alaska low. ...Guidance/Predictability Assessment... The main forecast challenge is the progression of the low initially off southern CA. The GFS remains the fastest to eject this system east and open it into a trough. The 06Z GFS was so much faster than the 00Z ECMWF/CMC/UKMET that the GFS was not included in the preferred model blend of pressures/fronts starting on Day 4/Tuesday. The 12Z GFS is slower than prior runs, but remains a fast outlier, particularly for when it crosses into TX Tuesday night and beyond. A preference is given to the slower and in alignment 00Z ECMWF/CMC through Day 5/Wednesday. The 06Z GEFS mean is not as progressive, so some inclusion is used through the forecast period with ensemble means (favoring the 00Z ECENS) reaching a majority by Day 5/Wednesday. ...Weather/Hazard Highlights... Confidence continues to increase in the pattern evolution at the surface and aloft supporting a significant rainfall event over much of the Southeast Days 4-6. The slower, more southern 12Z GFS aides this confidence a bit. Strong low level flow of Gulf of Mexico moisture will develop ahead of this system and bring anomalously high precipitable water values to the Southeast ahead of a slow moving cold front Monday into Thursday. The leading surface trough looks to develop and allow intermittent rain Sunday night into Tuesday over the lower Mississippi Valley before the approaching wave provides better forcing and focused heavy rain along the front roughly from east Texas to the Carolinas into Thursday. A consensus of the 00Z ECMWF/ECENS/06Z GEFS indicates a slowly sagging front through the Southeast, a little south than the previous guidance suite and close or over areas of the Deep South that have had the highest rain in the past 30 days. Therefore, an excessive rain threat continues for this region. 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. Refer to latest SPC products for updated information. Continue to expect any winter weather effects to be fairly limited for such a system this time of year and confined to the Northeast and higher elevations. Out west, the Northwest should see daily rounds of rain/mountain snows associated with energy moving through in the flow aloft Monday through Friday. The heaviest precipitation for the Pacific Northwest looks to be immediately ahead of the deep low exiting the Gulf of Alaska in the Thursday night time frame. Rather dry conditions can be expected the ridge shifting over the western to central CONUS late next week. 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 Monday through Wednesday in the warm/moist flow ahead of the main storm system. The interior West ridge late next week brings much above normal maximum temperatures from the Great Basin to the central/northern Plains for Thursday/Friday. Jackson 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 Hazards: - Heavy precipitation across portions of the Pacific Northwest, Thu-Fri, Mar 5-Mar 6. - Heavy rain across portions of the Southern Plains, Lower/Middle Mississippi Valley, the Tennessee Valley, the Ohio Valley, the Southern/Central Appalachians, the Southeast, and the Mid-Atlantic, Mon-Thu, Mar 2-Mar 5. - Heavy snow across portions of the Pacific Northwest and the Northern Great Basin, Mon-Fri, Mar 2-Mar 6. - Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Mon-Wed, Mar 2-Mar 4. - Severe weather across portions of the Southeast, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Tue, Mar 3. - Flooding occurring or imminent across portions of the Southeast, the Middle/Lower Mississippi Valley, the Ohio Valley, and the Tennessee Valley. 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