Extended Forecast Discussion NWS Weather Prediction Center College Park MD 543 PM EST Mon Jan 18 2021 Valid 12Z Thu Jan 21 2021 - 12Z Mon Jan 25 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 north-central U.S. over the weekend... ...Overview... A split upper-level flow pattern across the CONUS at the start of the medium range period is expected to transition to primarily troughing over the interior West and southwest flow over the central U.S., which will likely result in a parade of storm systems toward the latter half of the week and next weekend. Rounds of moderate to locally heavy rainfall will be possible across portions of the south-central to southeast U.S. while a secondary trough ushers in much colder air for the northern tier and could bring snow from the Rockies eastward toward the Upper Midwest by next weekend, though snow amounts remain uncertain. ...Guidance/Predictability Assessment... Deterministic model guidance continues to show fairly good agreement through much of the period regarding the large scale pattern, with an upper low near southern California Thu getting quickly absorbed as it tracks eastward, and as troughing/energy drop southward along North America's West Coast Thu-Fri and then east into the West over the weekend. The 00Z CMC was on the stronger side with the West upper trough around Sat, while the 00Z UKMET was on the weaker side, but they balanced well. A multi-model deterministic blend sufficed for the first part of the period for the fronts/pressures forecast. Larger differences arise with the evolution of the trough early next week, in particular with the degree of separation from the northern stream and how troughing and energy interact with an upper low persistently spinning over central Canada. There are large run-to-run and model-to-model inconsistencies with this, and it affects surface low positions as well. At this point, the 06Z GFS appeared different enough with its pattern and too fast/northeast with its surface low to exclude, so used a component of the 00Z GFS by day 7 (along with the 00Z ECMWF), but generally favored the 00Z EC and 06Z GEFS means by the end of the period, which were reasonably well aligned. ...Weather/Threats Highlights... Waves of low pressure interacting with a surface boundary over the Lower Mississippi Valley and a steady stream of Gulf moisture will promote the development of a couple of rounds of moderate to heavy rainfall. The first is likely around Thu-Fri this week and should focus from the Lower Mississippi Valley into the Deep South. Then, as another frontal system ejects into the Plains early next week, more rain is expected for the region and for portions of the Tennessee and Ohio Valleys. The axis of heaviest rainfall remains uncertain especially for this second system. But the potential is there for a few inches of rain during the period, while more is possible if the rounds overlap over the same area, and could cause flooding concerns. As an upper trough digs over the Rockies/West, plenty of moisture and upslope flow will lead to some significant snow potential for the Central Rockies and Wasatch mountains. Some of this winter precipitation could spread across the Central Plains and Upper Midwest by next weekend, but details remain uncertain with respect to the development of low pressure and frontal interactions. In terms of temperatures, the greatest departures from normal are forecast to be across the TX/LA Gulf Coasts, where the southerly moist flow holds up nighttime lows well above normal, by 15-20 F. The most prolonged below normal temperature readings are forecast to be over the Northern Rockies during the period, around 5-10 F below normal for lows and 10-15 F below normal for highs. Tate/Taylor 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 Hazards: - Heavy rain across portions of the Middle/Lower Mississippi Valley, the Central/Southern Appalachians, the Tennessee Valley, the Southern Plains, and the Ohio Valley, Mon, Jan 25. - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, and the Tennessee Valley, Thu-Fri, Jan 21-Jan 22. - Heavy snow across portions of California, the Pacific Northwest, and the Northern Great Basin, Sun, Jan 24. - Heavy snow across portions of the Southern Rockies, the Central Rockies, the Great Basin, and the Southwest, Fri-Sat, Jan 22-Jan 23. - Heavy snow across portions of the Northern/Central Plains, the Great Lakes, and the Upper Mississippi Valley, Sat-Sun, Jan 23-Jan 24. - Flooding occurring or imminent across portions of the Southeast. 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