Extended Forecast Discussion NWS Weather Prediction Center College Park MD 137 AM 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 central U.S. next 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 US, 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 confidence is below normal. ...Guidance/Predictability Assessment... The latest deterministic guidance showed fairly good agreement through most of the period with respect to the large scale pattern, particularly through day 6 across the western U.S. with the multiple systems dropping southward. The main areas for differences are in the northern stream split flow over southern Canada and by the end of the period with the ejection of the shortwave energy into the southern Plains and potential low pressure moving toward the MS Valley. The 00Z GFS was a faster solution, pushing ahead than any of the deterministic guidance while the CMC/ECMWF were on the slower end of the model spread. The slower trends were favored and the WPC blend by day 6/7 incorporated mainly the 12Z ECMWF/ECENS with lesser inclusion of the GFS/GEFS mean. ...Weather/Threats Highlights... Waves of low pressure interacting with a surface boundary over the lower MS River Valley and a steady stream of Gulf moisture will promote the development of a few rounds of moderate to locally rainfall over portions of the Ark-La-Tex and Deep South. Exactly where the frontal boundary drapes remains a bit uncertain and the heaviest axis of rainfall will be tied to that, so the overall forecast confidence is medium at best. But the potential is there for several inches during the period, while more is possible if the rounds overlap over the same area. 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. For temperatures, the greatest departures from normal are forecast to be across the TX/LA Gulf Coasts where the southerly moist flow holds up nighttime readings well above normal. Lows are expected to be 15 to near 20 F above normal. The most prolonged below normal temperature readings are forecast to be over the northern Rockies during the period, as much as 10 F colder than normal. 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 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