Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sun Jan 17 2021 Valid 12Z Wed Jan 20 2021 - 12Z Sun Jan 24 2021 ...Multiple days of rain expected for the Lower Mississippi Valley to Southeast... ...Colder temperatures and increasing precipitation in the West could spread into the central U.S. next weekend... ...Overview... A split upper-level flow pattern will be in place through much of this forecast period, with a well-defined and detached upper low just south of southern California for the middle of the week, followed by another amplifying trough along the West Coast by next weekend. Precipitation is forecast to spread into the Desert Southwest while moderate to heavy rain is possible for southeastern portions of the U.S. for the latter half of the week. The second trough will lead to cold temperatures and precipitation across much of the West, with multiple days of snow across higher elevations, and there are indications that snow could spread into northern/central parts of the Plains by next weekend. Meanwhile, negative NAO (high pressure block over Greenland) and a negative EPO (ridge over Alaska) should be in place, and this will indirectly lead to quasi-zonal and broad cyclonic flow over the north-central CONUS. ...Guidance/Predictability Assessment... Good deterministic model agreement persisted through the 00/06Z model cycle with the overall pattern regarding the anomalous upper low near the Desert Southwest and and the broad upper trough over the northeastern U.S. Wed. Some differences remain in the timing of the low/trough ejecting eastward and getting absorbed into the southern stream flow (likely Thu), with the 00Z UKMET still being a fast outlier and not preferred. By the Friday to Sunday time period, while there is good consensus on a second trough/possibly closed low dropping southward along the West Coast, its timing and depth remain in question. GFS runs are inconsistent with the degree of the energy closing off at periods, and the 00Z CMC remained slower than other deterministic guidance with the trough's movement. Thus, the WPC fronts/pressures forecast incorporated some 06Z GEFS mean and 00Z EC mean into the blend for the latter part of the period to minimize individual model differences, but the 00Z/06Z GFS and 00Z ECMWF were still reasonable enough to be used throughout. ...Weather/Threats Highlights... Multiple days of rainfall remain possible for the latter half of the week across the ArkLaTex into parts of the Lower Mississippi Valley and Southeast, as Gulf moisture streams into the region and interacts with frontal boundaries. While recent model guidance has backed off somewhat on rain amounts, perhaps due to a quicker moving front pushing southward through the region, 2-3 inches of rain could still occur and potentially cause flooding issues where convection repeats over the same areas. Farther west, moderate rain can be expected for southern Arizona and southwestern New Mexico. Then as the trough swings into the West, it will help create precipitation, mostly in the form of moderate to heavy snow in higher elevations. The most notable snow amounts are likely to be in the Wasatch and Central Rockies. Then by next weekend, wintry weather could spread across the Northern/Central Plains toward the Upper Midwest ahead of a surface low pressure system, while farther south the Lower Mississippi Valley could get another soaking of rain. Precipitation amounts and positioning of heaviest totals for next weekend are uncertain at this time. Light precipitation is possible for the northeastern U.S. around Wed-Fri as a frontal system moves through, with areas downwind of Lake Erie and Lake Ontario seeing the most snow accumulation due to lake enhancement. In terms of temperatures, readings are expected to be well above normal across the Northern Plains Wed with highs 15 to potentially 25 degrees above normal. This will likely transition to slightly below average readings by Fri across this region, as below normal temperatures also expand in coverage across the western U.S. for late week as the upper-level trough heralds a change in the weather pattern. Tate/Hamrick Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook 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