Extended Forecast Discussion NWS Weather Prediction Center College Park MD 251 AM EDT Mon Oct 21 2024 Valid 12Z Thu Oct 24 2024 - 12Z Mon Oct 28 2024 ...General Overview... A cold front is forecast to exit the East Coast by Thursday morning and bring a return to more seasonable temperatures after the recent warm spell, with a large surface high settling in across the Eastern U.S. behind it. A second frontal boundary is also expected to drop south across the Midwest and Ohio Valley by the weekend, reinforcing the autumnal airmass and mainly dry weather. Out West, a Pacific cold front is likely to reach the Pacific Northwest coast late Friday or early Saturday, bringing a return to rain and mountain snow from northern California to Washington state. A more robust pattern change is on the distant horizon for the West as the upper level trough likely amplifies further going into early next week, and a strong cold front passing through the region. ...Guidance/Predictability Assessment... Compared to last night, there has been improvement in the level of model agreement with the 00Z guidance, with the split flow over the eastern Pacific becoming better resolved. At the time of fronts/pressures issuance, the 12Z CMC was an outlier solution with the building Western U.S. trough Sunday and into Monday, and did not have much ensemble support, so it was phased out of the forecast blend by late Saturday. However, the new 00Z run is more compatible with the 00Z ECMWF and closer to the ensemble means. The GFS is slower with arrival of the trough across the West by Monday, and a little slower than the GEFS mean. To start, a general blend of the deterministic guidance worked well as a starting point on Thursday, and ensemble means were gradually increased to about half by Monday. ...Weather/Hazards Highlights... Most of the country should be void of heavy rainfall as moisture will be limited despite a few frontal boundaries and troughs passing through. Warm air advection across the Midwest on Thursday around the backside of the surface high should support mainly light to moderate showers ahead of an approaching cold front from the Northern Plains, and this will likely weaken further as it reach the Northeast to close out the work week. Some showers are also likely for portions of the Florida Peninsula with northeasterly flow in place. The exception will be western Washington and Oregon where onshore flow and terrain enhancement will tend to increase rainfall prospects across this region, mainly next weekend as the moisture plume ahead of the Pacific front moves inland. A few inches of rainfall will be possible over the Friday night- Monday morning time period for the coastal ranges and the western slopes of the Cascades. Heavy snow is possible for the highest mountains. However, the rainfall totals should be limited enough during the Day 4 and 5 period (Thursday and Friday) to warrant keeping the excessive rainfall outlooks for those days blank for now. There is still some discrepancy in the guidance for the arrival of this rainfall event, so stay tuned to future forecasts. In terms of temperatures, most of the country should experience above average temperatures during this forecast period, and this especially holds true across the Plains and the Desert Southwest, where highs could be 10-20 degrees above average most days, with the greatest anomalies on Sunday-Monday across the central and northern Plains. This will equate to highs in the middle 60s to upper 70s for those regions. Readings closer to late October climatology are likely for the East Coast states in the wake of the cold fronts. Things will likely trend to cooler than usual for the Pacific Northwest by Monday as the upper trough builds and more clouds/precipitation develops. Hamrick 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