170 FXUS02 KWBC 250650 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 AM EDT Thu Sep 25 2025 Valid 12Z Sun Sep 28 2025 - 12Z Thu Oct 02 2025 ...Heavy rain threat continues through Sunday for the Southwest, with very uncertain tropical impacts across parts of the Southeast next week... ...Overview... An upper trough/low over the Southeast will be slow to dislodge through much of the period due to a large Bermuda High anchored over the Atlantic. Northern stream shortwaves near the Canadian border will be much more progressive. A persistent upper low near the Southwest will finally lift north and weaken Sunday and beyond in response to amplified troughing across the West. Heavy rainfall threats for the medium range period will initially focus across the Southwest and southern Mid-Atlantic, but increasing precipitation is expected by next week across the West, with some snow in the higher elevations. Latest guidance continues to show significant uncertainty regarding the tracks of newly formed Humberto and another likely tropical low to its west near the Bahamas and what impacts this will have on parts of the Southeast or Mid-Atlantic coasts. ...Guidance/Predictability Assessment... The latest guidance continues to show good agreement on the large scale synoptic set up through at least the first half of the period, but still with some uncertainty in the details. The biggest and most impactful area of uncertainty remains the Southeast and western Atlantic as newly formed Humberto and another likely tropical system to its west round the west side of the Bermuda High. Guidance shows good agreement that Humberto should curve away from the East Coast, but it's the second likely tropical that forms to its west near the Bahamas that needs to be closely monitored. With this system, guidance has shown significant uncertainty on how it does or does not interact with Humberto and how close it comes to the coast. There is a lot of run to run inconsistencies in the guidance ranging from a recurvature away from the coast to a landfall in the Carolinas. The WPC forecast tonight preferred to stay close to continuity until there is a better consensus on this system. Refer to the National Hurricane Center for the latest on both of these systems. Out West, the guidance shows good agreement that an upper low over the Southwest will weaken and lift to the northeast as it's absorbed into the larger scale troughing moving into the West Coast. There remains some timing uncertainties on how quickly this happens. The models show good agreement on an amplified trough moving into the West coast around Tuesday of next week, but a lot of uncertainty still on the timing and ejection of energy out of this trough into the interior West which has significant implications for distribution of QPF across the region. The WPC forecasts tonight were based on a non-UKMET blend (using yesterdays 12z/18z guidance) through day 5 (the UKMET was slower with the possible tropical north of the Bahamas). After day 5, increased weighting of the ensemble means to help with late period uncertainty across the West and also in the Southeast with the linger upper low/energy and interactions with the tropical systems. This overall maintains good continuity with the previous shift. ...Weather/Hazards Highlights... A frontal boundary lingering along the East Coast will support daily showers and storms especially for the Mid-Atlantic states into next week. Anomalous moisture with a possible tropical connection will maintain at least a localized heavy rainfall threat for the southern Mid-Atlantic/Carolinas and parts of the Appalachians. Marginal risk areas are maintained on the Days 4 and 5 (Sunday and Monday) Excessive Rainfall Outlooks for this region for now given significant uncertainty in QPF amounts. The threat may continue beyond Monday depending on the evolution of the tropical systems off the Southeast coast and there are pieces of guidance suggesting significant rainfall amounts so adjustments to the current QPF forecast will be needed. At the very least, much of the East Coast can expect an increased risk of Rip Currents and High Surf associated with these systems next week. Meanwhile, Pacific moisture ahead of an upper low over southern California will result in rainfall continuing into Sunday, though with lesser intensity than the short range. Anomalous moisture and sufficient instability will continue to support at least a localized flash flood threat Sunday into Monday, particularly for vulnerable burn scars. This region remains highlighted with a marginal risk for both Sunday and Monday EROs with a more targeted slight risk on Day 4 across central New Mexico where QPF amounts are greatest. Precipitation will increase by next week across the Northwest U.S. associated with amplified troughing over the region. Widespread rain is expected across the Pacific Northwest and into the northern Great Basin/Rockies. The heaviest precipitation is expected from western Washington to northern California and the Sierra Nevada. A marginal risk was added to the Day 5/Monday ERO tonight for northern California. Some snow may fall in the highest elevations of the West as well. Warmer than average temperatures will shift into the Northern Plains and maintain as upper ridging builds over the region next week. Daytime highs could be 10-20 degrees above normal for parts of the Upper Midwest. Below normal temperatures over the Southwest will expand northward along much of the West Coast by next week. 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 $$