Extended Forecast Discussion NWS Weather Prediction Center College Park MD 359 PM EDT Sat Sep 20 2025 Valid 12Z Tue Sep 23 2025 - 12Z Sat Sep 27 2025 ...Heavy rain threat for portions of the central U.S. to southern/central Appalachians next week... ...Overview... The main system of note during the medium range period will be a deepening shortwave into the Central U.S. on Tuesday with increasing confidence the energy will close off into a cutoff low and move rather slowly eastward across the Midwest and Ohio Valley for the latter half of the week. This should focus rain that could be heavy near a resultant surface low pressure/frontal system in the lower/mid-Mississippi Valley, Ohio Valley/Mid-South, and southern/central Appalachians. Meanwhile, another cutoff low off the California coast could lead to a round of rain over southern California into the Southwest as it lifts north and then east, while an upper ridge ahead of this low over the Southwest could lead to a period of triple digit heat early-mid week. ...Guidance/Predictability Assessment... An omega block pattern looks to set up over the CONUS next week, consisting of an upper low near/over California, a large cutoff low over the Central U.S. moving east, and an amplified ridge in between. Recent model agreement is better than a day or two ago with the evolution of the upper low forming near the central Rockies/High Plains by Tuesday and tracking east slowly through the mid-Mississippi Valley and reaching the Ohio Valley by late week. The associated surface low also shows reasonable consensus. A multi-model blend with some ensemble inclusion worked well for these features and maintained good continuity with the previous forecast. But there is still some model spread with the timing of a northern stream trough moving across the Northeast, affecting a cold frontal position ahead of it and precipitation chances. The 00/06Z ensemble means, especially the CMC/ECens, and the 00Z CMC tended to be farther west than the GFS and ECMWF runs that were east. Now with the 12Z runs, the ECMWF has slowed down but the CMC has sped up. So will continue to wait for better consensus in that region. There is general agreement for a smaller southern stream upper low to drift northward from the eastern Pacific into California and then move east through the period. Some timing and placement differences are seen, but typical for the medium range period. In the northern stream, model agreement has increased with showing low amplitude flow as a weak shortwave moves through British Columbia around midweek, and a northeastern Pacific trough perhaps edging just into Washington late week. This leads to the southern stream low potentially remaining a separate feature in the Intermountain West even into late week. The West could also be handled with the WPC forecast approach of a multi-model blend early in the period with increasing proportions of the ensemble means to half by the end of the period. ...Weather/Hazards Highlights... Widespread rain will focus near the upper low and associated frontal boundaries as it becomes more defined by Tuesday. Guidance continues to show increasing agreement on an area of heavy rainfall for eastern parts of the south-central Plains and the lower/mid- Mississippi Valley. Show a Slight Risk for these areas in the Day 4/Tuesday ERO with a broader Marginal Risk for surrounding areas. This is considered a higher-end Slight Risk atop parts of the Ozarks, where heavy rain rates are forecast to train over this typically flood-prone area. By Wednesday, the slow movement of the upper/surface low eastward should carry the heaviest rain chances east with it, into parts of the Tennessee Valley into the lower Ohio Valley. Uncertainty increases with the details by then, but this area seems to have enough consensus for a Slight Risk on Day 5/Wednesday, though its placement may move in future issuances. There is increasing potential for localized heavy rain in unstable air ahead of the low's cold front, so extended the Marginal Risk to the Gulf Coast, despite some model spread in placement. There is quite a bit of uncertainty in the northern side of the frontal system that could allow for more rainfall in the Great Lakes and/or Northeast Tuesday/Wednesday. The rain activity with the primary low will move slowly eastward with time after midweek into parts of the East, with a likely heavy rainfall focus in the south-central Appalachians. Farther south, some diurnally driven showers and storms are possible across the Florida Peninsula throughout the period. Meanwhile, an upper low lingering near California early-midweek should result in some rainfall over the state and into the Southwest as ample moisture advects ahead of it. Precipitable water anomalies could be 3-4 standard deviations above normal into California, though instability appears weaker in this event compared to the recent rain event. Will show Marginal Risks for parts of California and Nevada on Days 4 and 5 for isolated flooding potential with the much above normal moisture. Burn scars will be particularly vulnerable to flooding. There are some timing differences with the upper low and thus precipitation timing, so continue to monitor forecasts for refined details. Lighter precipitation is forecast to spread into parts of the Intermountain West as the energy moves north and east. The Pacific moisture could allow for convection across the Southwest as well, so delineated a Marginal Risk for portions of Arizona and New Mexico into far west Texas on Day 5/Wednesday. The Northwest should generally be dry until possibly late week ahead of a possible upper trough. A ridge axis will build from the Four Corners states into the West with some above normal temperatures across northern California and the northern tier states. The Desert Southwest may see temperatures reaching or exceeding 100F, mainly Tuesday-Thursday. Expect near to below normal temperatures for the Central U.S. under the large upper low. As the low drifts eastward as the week progresses, temperatures will cool a bit from above average across the East to near average. Tate/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