Extended Forecast Discussion NWS Weather Prediction Center College Park MD 129 AM EST Thu Jan 23 2025 Valid 12Z Sun Jan 26 2025 - 12Z Thu Jan 30 2025 ...Overview... The start of the period on Sunday will feature an upper level closed low diving southward along the California coast, and only very slowly moving eastward with time next week. This should bring beneficial precipitation to parts of southern California that have been dealing with devastating wildfires. Organized rains/terrain snows should make their way downstream into the Southwest as well. Meanwhile, northern stream mean troughing, and reinforcing shortwaves will cross from the northern Plains into the Great Lakes and Northeast into next week. Moderate to locally heavy rainfall will develop to the south across the Lower Mississippi Valley Sunday and Monday as a slow moving frontal boundary moves through the region. Additional frontal wave genesis over the Southern Plains into the middle of next week given Southwest low slow approach may again increase lead return flow and emerging rainfall to monitor. ...Guidance/Predictability Assessment... On the large scale, the guidance continues to show reasonable agreement, but there remains plenty of uncertainty in the details and timing of individual systems. Through early next week, models show good clustering of the low as it slowly moves south through California. After Tuesday though, there is some uncertainty in timing into the Southwest/Four Corners and also with less predictable energy that may shear off from the low. Ensemble mean guidance provides a good middle ground for this feature in the late period. To the north, mean northern stream troughing over the Northern Plains- Great Lakes-Northeast continues to show uncertainty in the amplitude of individual shortwaves dropping through, especially next Wednesday and Thursday. Accordingly, WPC guidance used a general model blend for the first half of the period. For days 6 and 7, transitioned to a 60/40 split between the ensemble means and the deterministic GFS and ECMWF which seemed to provide the best starting point amidst the uncertainties. ...Weather/Hazards Highlights... The slow moving upper low through California on Sunday will continue to allow for much needed rain to southern California. Even though QPF amounts are not too high, given sensitivity especially in fresh burn scar locations, it may not take much to cause flooding concerns. For this reason, a small marginal risk was added to parts of Southern California on the Day 4 Excessive Rainfall Outlook. Modest snows are possible across the terrain of southern California, with rain and mountain snow into parts of the Southwest as well. A tightened gradient behind the cold front dropping south may produce some gusty winds along parts of the Rockies late this week and California and Nevada on Saturday with offshore flow and modestly favorable fire weather conditions possible again behind the upper low next week to keep an eye on. Farther east, Gulf moisture will interact with a front approaching the Lower Mississippi Valley to Tennessee and Ohio Valleys by this coming weekend/early next week. Latest guidance continues to show heavy rainfall totals over parts of the southern U.S., with the potential for heavy rain and thunderstorms on Sunday across the Lower Mississippi Valley. A marginal risk was carried into the new Day 4/Sunday Excessive Rainfall Outlook from eastern Texas into Mississippi where storms may train along a slow moving frontal boundary. Snow or a wintry mix is possible on the northern side of this precipitation shield mainly over the Ohio Valley, south- central Appalachians, and possibly the Tennessee Valley and vicinity. Precipitation may spread towards the East Coast early next week as the frontal system progresses with some rainfall redevelopment across the Southern Plains and lower Mississippi Valley as well. This signal is increasing with development of an organized frontal wave with Southwest upper system approach. Much of the East should finally be back up towards normal temperatures early next week, though readings a few degrees below normal are possible across the Southeast. Anomalously cold temperatures will also develop in the High Plains, Rockies, and much of the West under the influence of upper troughing. The coldest anomalies will be in the High Plains and Rockies to Southern High Plains where high temperatures could be 10-20 degrees below normal. Late period ridging over the northern tier could allow temperatures to rise above normal next week, gradually shifting south and east with time. Santorelli 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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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