Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Wed Jan 17 2024 Valid 12Z Sat Jan 20 2024 - 12Z Wed Jan 24 2024 ...Another Arctic airmass comes into the central U.S. this weekend, but not as extreme as the first one... ...Multiple rounds of potentially heavy precipitation will affect the West... ...Heavy rain potential increases across the south-central U.S. into next week... ...Overview... The broad and deep upper troughing that has dominated the pattern over the lower 48 this week should finally get shunted east this weekend, as an upper ridge axis moves from the Intermountain West Saturday into the central U.S. Sunday and toward the East early next week. Northwesterly flow ahead of the ridge will bring in another Arctic surface high, producing much below normal temperatures across the central and eastern U.S. this weekend before moderating. While temperatures are forecast to be generally be 15-30F below average, this second round of cold temperatures should be less extreme than the recent cold wave at most locations. Then upper troughing with multiple embedded shortwaves will be atop the western half of the U.S., leading to multiple rounds of precipitation in the West this weekend into next week. Additionally, for the early part of next week, return flow from the Gulf of Mexico could lead to heavy rain across the south-central U.S., with some wintry weather possible on the northern side of the precipitation shield around the Great Lakes even as temperatures warm. ...Guidance/Predictability Assessment... Model guidance continues to show good agreement on the large scale pattern with expected differences in small scale features. Models indicate an upper trough shifting from the East into the western Atlantic this weekend with cold surface high pressure on its backside. An upper ridge tracks eastward behind it, with its axis over the Rockies Saturday migrating to the Mississippi Valley early Sunday and into the East next week, where it persists. Models are quite agreeable on these features. Then mean troughing is forecast to set up in the West during the period. There are some differences in shortwaves within the trough this week into early next week, but they fall within typical spread expected in the medium range period. These will take additional time to resolve especially since they are tracking from the data-poor Pacific that causes models to struggle. By Tuesday-Wednesday energy may consolidate to develop an upper low in the Southwest possibly splitting away from the trough into the southern stream. This will continue to be monitored as the details are still nebulous. Given the good general agreement in the model guidance, the WPC forecast began with a multi-model deterministic blend and was able to maintain a blend of primarily deterministic runs throughout the whole period. Did incorporate some GEFS and EC ensemble means through the latter half of the period to smooth out model differences but maintain some strength in individual systems. ...Weather/Hazards Highlights... The West Coast states can expect rounds of precipitation this weekend into early next week. Multiple days of rain and periods of rain rates over half an inch per hour are possible as instability may sneak onshore, along with good dynamical support for heavy rain from strong shortwaves aloft. These could cause flooding concerns especially into Sunday-Sunday night. Rounds of precipitation should continue into early next week from the West Coast and expand farther into the Intermountain West. Some locally moderate/heavy totals are possible over favored terrain, depending on upper level and surface details that will be better resolved in coming days. Parts of the interior Northwest could have some possibly persistent freezing rain this weekend. Increasing moisture ahead of the western U.S. troughing will start to spread rain to the southern Plains on Sunday. Rain totals are forecast to quickly ramp up on Monday and Tuesday for parts of the southern Plains into the Lower Mississippi Valley, and these heavy totals could produce flash flooding concerns, though beyond the Excessive Rainfall Outlook period. The precipitation shield gradually spreading northeastward on Monday and Tuesday could spread some wintry weather on its northern side. The Great Lakes vicinity is most likely to see wintry weather out of this precipitation, but freezing rain or mixed precipitation cannot be ruled out farther south, but with low confidence in the details. Elsewhere, the East will generally be dry through the period after some lingering lake effect snow into Saturday. Arctic high pressure will provide a reinforcing surge of cold air that should be well into the central and eastern U.S. by Saturday. Temperatures 15-30 degrees below normal will be widespread on Saturday. The highest potential for hazardous cold and dangerous wind chills will be across the north-central Plains into parts of the Upper Midwest. Localized anomalies more than 30F below normal are possible right underneath the high in the north-central Plains/Mississippi Valley, with low temperatures near or below -20F possibly setting records. At least winds should be weak right in the vicinity of the high to limit wind chills. Overall this cold episode will still be notable even if not quite as extreme as the current/recent cold event for most areas. Temperatures should still be chilly on Sunday but starting to moderate toward normal. Then temperatures look to continue to warm Monday-Wednesday as the surface high shifts into the Atlantic and the upper ridge allows for more southerly flow. By midweek the eastern half of the country has above normal temperatures forecast, around 15-30F above average for lows and 10-20F above for highs. From the Rockies westward, near normal temperatures for highs and low temperatures around 10-15F above normal are expected. 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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw