Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Sat Jan 16 2021 Valid 12Z Tue Jan 19 2021 - 12Z Sat Jan 23 2021 ***Heavy rainfall event likely for portions of the interior South next week and strong upper low for the Desert Southwest*** ...Weather Pattern Overview... A strong impulse sinking southward along the West Coast Monday night is expected to become a detached upper low southwest of southern California. There will be some spokes of shortwave energy that ejects eastward from this low across the southern tier states, whilst the northern stream flow is more progressive. Another trough will likely set in across the Pacific Northwest by the end of the week and herald a pattern change for the western U.S. ...Guidance/Predictability Assessment... The deterministic guidance is generally in good agreement on the split flow pattern across the continental U.S. through early-middle part of next week, and are strongly signaling an anomalous cut-off low developing near southern California and the adjacent offshore waters by Tuesday. The 00Z CMC maintains the upper low the longest before becoming absorbed, whilst the 00Z ECMWF is more progressive in ejecting the energy to the east, with the GFS between these solutions. By the end of the forecast period Saturday, the ECMWF is a bit more robust with the next trough crossing the Great Lakes region, and slightly slower with the arrival of the next trough along the West Coast. The WPC fronts and pressures forecast was primarily derived from a CMC/GFS/ECMWF blend through Wednesday night, and then transitioned to roughly half ensemble means and half CMC/GFS/ECMWF for Thursday through Saturday. ...Weather/Threats Highlights... The main weather story next week will be the potential for a multi-day heavy rainfall event, extending from eastern Texas to interior portions of the Deep South, mainly during the Wednesday-Thursday time period. A prolonged period of moist southerly flow from the Gulf and Caribbean ahead of a slow moving low pressure system over southern Texas, in combination with moisture convergence along a quasi-stationary front, may result in 1 to 3 inches of rainfall for parts of this region and some potential for flooding. There remains significant model differences regarding the exact placement of the QPF axis and magnitude, so changes in the forecast are quite likely closer to the event. Meanwhile, there appears to be an increasing chance of wintry precipitation spreading across the higher elevations of the southern Rockies, and rain over the lower elevations of southern Arizona and southwestern New Mexico. Moderate to heavy snow is likely again for portions of the central and northern Rockies late in the week as the next western trough arrives. By Thursday and Friday, the chance for snow returns for portions of the Northeast as the next storm system arrives. 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. 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