Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Mon Sep 19 2022 Valid 12Z Thu Sep 22 2022 - 12Z Mon Sep 26 2022 ...Heavy rainfall threat lingers for the Four Corners/Rockies into Thursday... ...Guidance Evaluation/Predictability Assessment... Latest model and ensemble solutions seem pretty well clustered through medium range time scales over the lower 48, bolstering forecast confidence despite some lingering local embedded system timing/strength differences. The WPC medium range product suite was primarily derived from a composite of the 06 UTC GFS/GEFS, 00 UTC ECMWF/EC mean, and the 00Z CMC/CMC mean. The results agree with the 13 UTC National Blend of Models (NBM) well except from Day 5 onward when the model blend provided a larger QPF footprint than the NBM. Given the relatively good model agreement out to day 7, more of the deterministic solutions were incorporated into the blend for Days 6 and 7. Guidance differences seem well mitigated by this blending process consistent with predictability. WPC product continuity is well maintained in this manner and newer 12 UTC ECMWF generally remains in line. ...Pattern Overview and Weather/Hazards Highlights... On Thursday, enhanced rainfall should be exiting New England as a cold front moves steadily into eastern Canada. Meanwhile, an amplified upper trough/low and frontal system slowing lifting through the north-central Great Basin/Rockies Thursday into Friday will favor cooler temperatures that may lead to some snow for high elevations in northwestern Wyoming Thursday night. Maximum daily temperatures are forecast to temporarily dip 10 to 15 degrees F below average. Meanwhile, anomalous moisture carried across the Four Corners region associated with the southern jet stream is forecast to extend into the beginning of the medium-range period on Thursday before waning by Friday. Moderate to locally heavier rainfall/thunderstorms appear to be focused from the Four Corners to areas immediately to the east, where a slight risk of excessive rainfall is indicated on the Day 4 experimental WPC Excessive Rainfall Outlook. Farther north, an area of enhanced rainfall closer to the surface low center will work across the northern Great Basin/Rockies into the northern High Plains on Thursday with some heavy rain possible. By Friday, the main activities should move across the northern Plains into the upper Midwest. By late next week into the weekend, the system should continue to work its way east across the Great Lakes reaching the northeast by Monday. Best chance of showers and some thunderstorms will be ahead of this system as well as what appears to be a well-defined cold front trailing across the Appalachians and through the South. A surge of cool Canadian air should usher in fall-like weather through the northern tier through the weekend and then toward the East Coast by next Monday. An eroding lead upper ridge over the South will support lingering 10-15F warmer than average temperatures into Thursday. This will be in advance of the aforementioned cold front. Heat will quickly get suppressed with this frontal passage as quite cooled Canadian post-frontal high pressure settles into the central to eastern U.S. The lingering front may focus some showers across Florida through the weekend. Kong/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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices are at: 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