Extended Forecast Discussion NWS Weather Prediction Center College Park MD 201 PM EDT Sun Sep 20 2020 Valid 12Z Wed Sep 23 2020 - 12Z Sun Sep 27 2020 ...Rainfall Threat from Beta Remnants Shifts East Across The Southeast Late in the Week... ...Overview... A powerful trough rounding a deep Gulf of Alaska low will shift into the Pacific Northwest Wednesday night which will be followed by reinforcing shortwaves into the Pacific Northwest into Friday which will amplify the trough in the lee of the northern Rockies Friday into Saturday. Beta remnants are forecast to further weaken as they shift northeast from southeastern Texas across Louisiana and the lower Mississippi Valley as upper level low pressure to the north forms. There is a continued localized heavy rainfall threat from the Beta remnants for the rest of the work week. Upper ridging will return to the Southwest under the Pacific Northwest trough in the middle of the week and expand up the West Coast in the wake of the trough next weekend. Ridging over Florida will expand starting Thursday in the wake of the Beta remnants. ...Guidance/Predictability Assessment... By Wednesday, the 00Z/06Z guidance consensus is to have the dominant mid-level feature in the southern US well north of the low level Beta remnants as a shortwave impulse from northwest Texas pushing into Missouri. This northern feature should draw the Beta remnants northeast and farther inland over the Lower MS Valley across the TN Valley and to the southern Appalachians by Friday as noted by a manually slowed solution of the 00Z ECMWF/UKMET to match up better with 15Z NHC track. The 06Z/12Z GFS remain the slowest with the Beta track, lingering along the southwest LA coast. An active northern stream pattern over the northern Tier continues in the wake of Post-tropical Teddy shifting over eastern Atlantic Canada, and active and amplifying trough from the Gulf of Alaska low over Northwest CONUS with a shortwave shifting east across the Great Lakes Wed/Thur in between. The 00Z CMC is the first to diverge from consensus with a much more amplified Great Lakes trough Thursday to Northeast by Friday while the 00Z ECMWF/UKMET/06Z GFS are farther north into eastern Canada. The succession of shortwaves into the Northwest CONUS have understandable differences among guidance. The 00Z ECMWF/ECENS is preferred with its steady approach to the trough amplifying as it shifts east of the northern Rockies. This is reinforced by the 12Z GFS coming in quite similar to the 00Z ECMWF by being faster and more amplified than the 06Z GFS. ...Weather/Hazard Highlights... Beta remnants are forecast to take a slow path northeastward across southern MS Valley and across the Southeast to the southern Appalachians Wednesday through Friday which would spread locally heavy rain along and out ahead of its track. Uncertainty with rainfall remains high with the details but a gradual winding down of the system is expected late in the week. A cold front and trough from the Gulf of Alaska, followed by reinforcing/amplifying shortwaves will bring several days of rainfall to western WA/OR Wednesday through Friday. Showers will continue behind the front with the shortwave troughs over the northern Rockies. Precip looks to develop over the northern Great Plains ahead of the amplifying trough this weekend. Temperatures will be above to much above normal across the north-central U.S. ahead of the Pacific cold front. Warm conditions will spread through the Great Lakes next weekend and into the Northeast. By contrast, clouds and rain will keep max temperatures cooler than normal over the lower Mississippi Valley Wed-Fri as Beta weakens. Cooler max temperatures are expected for WA/OR in the wake of the cold front. Overall above normal min temperatures (without frost/freeze) are expected across the CONUS outside of western terrain. 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 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