Extended Forecast Discussion NWS Weather Prediction Center College Park MD 208 PM EST Thu Jan 18 2024 Valid 12Z Sun Jan 21 2024 - 12Z Thu Jan 25 2024 ...Multiple rounds of potentially heavy precipitation will affect the West... ...Heavy rain and flooding potential increase across the south-central to Eastern U.S. next week... ...Overview... The large scale pattern next week will shift notably from this week's cold pattern, as an upper ridge axis shifts from the central U.S. into the East and mean troughing gets established in the western to central U.S. early to mid-next week. This will promote warm and moist inflow likely from both the Pacific and the Gulf, warming temperatures in the eastern half of the country after a chilly Sunday in some areas. Multiple days of heavy rain and potential for flash flooding are expected across the south-central U.S. next week amid the stalling upper flow pattern and increasing moisture. As shortwaves track through the western trough, multiple rounds of precipitation are likely in the West into next week, with heavy rain across California in particular. ...Guidance/Predictability Assessment... Model agreement on the synoptic scale remains reasonably good through the period, though with still some lingering differences in the details, which become more noticeable with time (as is typical). Lingering western Atlantic troughing Sunday will get shunted away as the upstream upper ridge moves into and sets up over the East during the workweek, likely reloading with time as the Atlantic subtropical ridge supports it. Models/means continue to agree well with these features. Farther west, mean troughing will persist over the western half of the lower 48 with relatively more model spread. The most notable differences arise the second half of next week as separate systems into the West work to amplify/reinforce troughing over the West-Central U.S.. This leads to different degrees of amplification and placement of the mean trough, though all well within the realm of solutions for a day 6 or 7 forecast. Also some question in the strength and position of upper level ridging into the West Coast next Thursday. With good initial model agreement, the WPC forecast used a blend of the latest deterministic guidance for the early part of the period. Incorporated the GEFS and EC ensemble means starting Day 5 and increased their proportion to 40 percent by Day 7 amid the increasing model differences. This served to smooth out individual model differences but maintain some strength in individual systems. The CMC was not preferred late period due to it being slightly faster/farther inland with ridging over the West. ...Weather/Hazards Highlights... The West Coast states can expect rounds of precipitation into next week. Early in the week, a likely atmospheric river is forecast to bring moisture to California and sink south gradually, while a potent shortwave and the left exit region of the jet stream will provide upper-level dynamical support. These should combine with some instability possibly sneaking onshore, leading to periods of high rain rates likely reaching well over half an inch per hour. Slight Risks of excessive rainfall are indicated for Sunday and Monday given this setup, with Sunday's ERO encompassing northern and central parts of California into far southwest Oregon and Monday's focused on southern California. How far south to extend the ERO on Sunday remains a challenge as model guidance varies with amounts. The Sierra Nevada will likely receive heavy snow with the precipitation activity early next week. Rain and snow are expected farther east into the Intermountain West/Southwest as well, as the Pacific Northwest looks to see periodic precipitation through the week. Amounts farther inland could add up over multiple days, but at this point no risk areas are indicated for flooding since rain rates do not look very high and the region can typically handle quite a bit of rain. Parts of the interior Northwest could have some possibly persistent freezing rain lasting into Sunday. Increasing moisture ahead of the western U.S. troughing will start to spread rain into the southern Plains on Sunday. The trend has been for heavy rain to start over south Texas late Sunday night, so a Marginal Risk area remains in place there on the Day 4 Excessive Rainfall Outlook. Rain totals are forecast to quickly ramp up on Monday and last through Tuesday-Wednesday for parts of the southern Plains into the Lower Mississippi Valley/Gulf Coast, and these heavy totals could produce flash flooding concerns. As a starting point, a Slight Risk is delineated for Day 5/Monday in the southeastern quadrant of Texas into far southwestern Louisiana where the best potential for instability to produce high rain rates is, with a Marginal Risk extending farther north in Oklahoma and the Lower Mississippi Valley. The precipitation shield will gradually spread northeastward on Monday and Tuesday with some wintry weather on its northern side. In some areas across the Mississippi Valley into the Midwest, precipitation may start as freezing rain or mixed precipitation but then transition to rain as warm air spreads farther north. The Great Lakes vicinity is most likely to see persistent wintry weather out of this precipitation, but even there it looks fairly light at this point. Precipitation may eventually reach the East around midweek after a dry period there, and given the warmer temperatures and snowmelt, areal and stream flooding may be a concern. Cold air from an Arctic high will linger into Sunday across portions of the Midwest, where low temperature anomalies of 15-25 degrees below average are possible before quickly warming to near normal for highs. The southern Plains to Mid-South and southeastern U.S. look to see below normal lows and highs Sunday, possibly persisting into Monday in some areas, but moderating closer to normal. The warming trend will continue further into next week across the eastern half of the U.S. underneath the upper ridge allowing for more southerly flow. By Wednesday-Thursday, the eastern half of the country should see above normal temperatures, around 15-30F above average for lows (locally higher in the Midwest) and 10-20F above for highs. From the Rockies westward, low temperatures around 10-15F above normal and high temperatures generally within a few degrees of normal are expected. Santorelli/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