Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Sat Jan 16 2021 Valid 12Z Tue Jan 19 2021 - 12Z Sat Jan 23 2021 ...Multi-day rain event for the southeastern U.S. next week, while a strong upper low causes precipitation in the Desert Southwest... ...Weather Pattern Overview... A potent upper low is forecast to detach from progressive northern stream flow and sit near southern California Tue/Wed before ejecting eastward Thu. Precipitation is expected for the Desert Southwest with this pattern. Farther east, Gulf of Mexico moisture overrunning frontal boundaries should lead to multiple days of rain for the southeast quadrant of the CONUS. By the latter part of the week, another upper trough will likely set into the Pacific Northwest and create a pattern change for the West, with cool temperatures and increasing precipitation chances. ...Guidance/Predictability Assessment... Deterministic guidance continues to show good agreement especially through the first part of the period with the aforementioned upper low cutting off near southern California/Pacific Ocean/far northwestern Mexico, with some slight differences in depth and centroid position at times. A multi-model deterministic blend was able to be used for Tue/Wed for this system as well as rounds of low amplitude troughing centered over the Great Lakes. By Thu, some differences arise with how quickly the upper low energy ejects eastward and becomes reabsorbed with the main flow. The 00Z CMC was particularly slow with the feature and was not preferred from Thu onward, but other deterministic models are not in total agreement either. Then by Fri/Sat, while guidance is agreeable on troughing dropping southward into the West, its timing and depth remain in question. The 00Z ECMWF was weaker than its previous runs, and GFS runs have varied in intensity as well but appear a little faster than the EC. Toward the end of the week, increased the weighting the 00Z EC and 06Z GEFS ensemble means to reduce individual model influence, but left some incorporation of the 00Z ECMWF and 06Z GFS. ...Weather/Threats Highlights... Next week, the main concern weather-wise is with the potential for a multi-day rainfall event across the southeastern portion of the U.S, from the Southern Plains toward the Lower Mississippi/Tennessee Valleys and Southeast. Gulf of Mexico and Caribbean moisture will stream into the region and interact with a slow-moving surface low near the Texas/Mexico border and its associated quasi-stationary front, as well as another front approaching from the north. This should lead to rainfall totals of 1-3 inches around Wed-Fri. The current axis of heaviest rain is forecast from the ArkLaTex into parts of the Lower Mississippi Valley, but this is subject to change as model spread still remains. Farther west, precipitation is forecast for the Southwest ahead of the upper low, with wintry precipitation forecast for higher elevations of the Southern Rockies and rain over the lower elevations of southern Arizona and southwestern New Mexico. As troughing comes into the West for the latter part of next week, moderate to locally heavy snow is possible particularly across higher elevations of the Rockies. Light wintry precipitation is also possible for the Great Lakes and Northeast toward Thu-Fri of next week. In terms of temperatures, readings should be near to slightly above normal for most areas to begin the week, and some 10-20 degree positive anomalies are possible across the northern Plains by Wednesday ahead of the next front. By the end of the week, there are some hints of an Arctic airmass affecting the northwestern quadrant of the U.S., although the core of the Arctic high should remain north of the Canadian border. Tate 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