Extended Forecast Discussion NWS Weather Prediction Center College Park MD 226 PM EDT Mon Oct 21 2024 Valid 12Z Thu Oct 24 2024 - 12Z Mon Oct 28 2024 ...General Overview... A leading cold front is forecast to exit the East Coast by Thursday morning and bring a return to more seasonable temperatures after a 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 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... The WPC medium range product suite was primarily derived from a composite blend of reasonably clustered guidance from the 06 UTC GFS and 00 UTC ECMWF/Canadian/UKMET along with compatible guidance of the 13 UTC NBM for Thursday into Saturday. In this blend, the 06 UTC GFS trended less amplified over the West late period, but fell within the envelope of possibilities, albeit less likely. Opted to blend into this composite compatible guidance of the GEFS/ECMWF/Canadian ensemble means at longer time frames for the rest of next weekend/Monday amid slowly growing forecast spread as forecast confidence lowers from just above normal to near normal levels. A similar guidance plan seems even better clustered with latest 12 UTC guidance. In particular, the 12 UTC GFS has fallen back into better agreement with most other guidance in showing a more amplified upper trough holding over the West early next week. ...Weather/Hazards Highlights... It overall remains the case that 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 for the coastal ranges and the western slopes of the Cascades. Heavy snow is possible for the highest mountains inland through the Northwest. 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. Hamrick/Schichtel 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