Extended Forecast Discussion NWS Weather Prediction Center College Park MD 152 PM EST Wed Jan 17 2024 Valid 12Z Sat Jan 20 2024 - 12Z Wed Jan 24 2024 ...Another Arctic airmass pushes 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. 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. 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. Meanwhile, 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 agreement on the upper level pattern is pretty good this afternoon. The upper trough in the east will shift into the Atlantic while a ridge follows in its wake. Mean troughing will persist over the West Coast with several embedded shortwaves moving onshore troughout the period. There are some differences in the handling of upstream energy in the Pacific/Gulf of Alaska that will have some affect on the timing and position of these shortwaves, but these differences fall within typical spread expected in the medium range period. 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 good general agreement of 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. The GFS was favored slightly more in the blend early in the period because it has shown higher run to run consistency. Some GEFS and EC ensemble means were incorporated 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. Heavy rain may cause localized flooding issues for lower elevation portions of northern California on Saturday, so a Marginal Risk is in place in the Excessive Rainfall Outlook. Sunday looks to be a bigger day for heavy rainfall with a potent shortwave providing upper level dynamical support, periods of high rain rates (possibly reaching well over half an inch per hour), and some instability sneaking onshore. Additionally, a wet period late this week could prime soils ahead of this event. Thus a Slight Risk is in place in the ERO for Sunday-Sunday night for portions of northern California. The Slight area was expanded a bit southwards this afternoon to account for a southward trend in model guidance. Models vary on how much precipitation reaches southern California--the GFS/GEFSmean/UKMET keep precipitation farther north while the ECMWF/CMC runs have plenty farther south. Part of coastal southern California is in a Marginal Risk due to the relatively lower probabilities given the model disagreements. The Pacific Northwest will also see periodic precipitation, and parts of the interior Northwest could have some possibly persistent freezing rain this weekend. The Sierra Nevada will likely receive heavy snow with the precipitation activity this weekend into early next week. Some snow is also likely farther into the Intermountain West. Increasing moisture ahead of the western U.S. troughing will start to spread rain into the southern Plains on Sunday. Heavy rain may start over south Texas late Sunday night, so a Marginal Risk area has been added for this area on the Day 5 Excessive Rainfall Outlook. 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 will gradually spread northeastward on Monday and Tuesday and could cause 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, and 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. Winds will at least 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 should see above normal temperatures, 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. Dolan/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