Extended Forecast Discussion NWS Weather Prediction Center College Park MD 359 PM EDT Fri Oct 20 2023 Valid 12Z Mon Oct 23 2023 - 12Z Fri Oct 27 2023 ...Moderate to heavy rainfall increasingly likely for parts of the southern and central U.S. early to mid next week... ...Overview... The medium range period begins Monday with troughing exiting the East Coast to be replaced by a developing upper ridge lasting much of the week. Out West, an upper low is likely to dive into the Southwest/northern Mexico before weakening and lifting into the Southern U.S. later next week. Additional energy looks to move into the Northwest around mid-week, but model guidance continues to show considerable uncertainty in the track and strength of this energy and any potential interactions with the upper low over the Southwest. Regardless, it seems likely remnant energy/moisture from what is currently Hurricane Norma could stream northward to spread potentially heavy rainfall into parts of the southern and central U.S. ahead of a strong cold front, while on the backside, snow chances increase in the Northern Rockies and perhaps even into parts of the Northern Plains late next week. ...Guidance Evaluation/Predictability Assessment... The latest guidance generally agree that a cutoff low will dive into the Southwest U.S./northwestern Mexico and influence sensible weather impacts over the Southern/Central Plains next week. However, the areas of uncertainty remain around whether this upper low will stall out or not and to what extent Hurricane Norma will phase with or influence it. The 00z EC/CMC/UKMET were well clustered on the slower side of the trough's propagation, whereas the last several runs of the GFS have not only been inconsistent with one another but also weaker and much more progressive with the trough. The GFS is especially faster with the trough after day 5 when it kicks it out into the Mississippi Valley while the other guidance keeps it slowly moving through the Southern Plains. Meanwhile, a northern stream trough set to enter the Northwest early this week continues to carry a substantial amount of uncertainty with it, particularly after day 4. The 06z GEFS and 00z CMCE tend to be faster than the 00z ECE with respect to this trough, but each ensemble suite still develops a lot of spread throughout the evolution of this system. Run-to-run, the operational GFS appears to be more consistent than the EC, with several ECE members developing a ridge over the Pac NW. This solution appears to cluster well with some Canadian members as well. Still though, a solution featuring a trough of some sort is favored over the Northwest as this will likely produce heavy snow over the Northern Rockies. ...Weather/Hazards Highlights... The better consensus suggests Monday should be relatively quiet across the country, though recent runs of the GFS have been rather persistent in depicting the onset of locally heavy rainfall over the Southern Plains as early as Monday, so this is something to watch. Still, given the uncertainty, opted to keep the Day 4 Excessive Rainfall Outlook blank, but a risk area may need to be introduced in future updates if other pieces of guidance trend faster like the GFS. By Tuesday, favorable moisture and instability from Norma remnants should begin spreading in earnest into the Southern Plains, with at least locally heavy rainfall likely, possibly spreading into parts of the Midwest along the strong frontal boundary. A broad marginal risk for excessive rainfall was introduced on the Day 5/Tuesday ERO, stretching from eastern New Mexico northward into the Upper Midwest. It is possible an embedded slight risk may be needed somewhere, but at least the southern half of this region has been very dry lately and there is still a considerable amount of uncertainty on timing and rainfall amounts. Rainfall may linger or spread very slowly eastward later next week, but should trend lighter as the better moisture weakens and the upper low opens up and moves east. Out West, probabilities for meaningful snow at higher elevations of the northern Rockies should start increasing by Monday/Tuesday, with some potential for generally lighter snowfall amounts even extending into parts of the northern Plains later in the week, but highly dependent on the currently very uncertain upper level pattern evolution. Well above normal temperatures will spread from the Plains into the Midwest/Northeast Monday-Wednesday with the greatest anomalies (15-20 degrees above normal) likely Monday across the Central Plains. To the East, morning low temperatures on Monday may be cold enough from the Great Lakes into the Appalachians to support a generally frost threat for many, but should moderate thereafter as upper ridging builds into the region. Temperatures will trend cooler Wednesday and beyond for much of the west, but especially for the northern Rockies/Plains where daytime highs could be 10-20 degrees below normal behind the cold front. Kebede/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 Hazards: - Heavy rain across portions of the Central Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, the Southern Plains, and the Upper Mississippi Valley, Tue-Thu, Oct 24-Oct 26. - Heavy snow across portions of the Northern Plains and the Northern Rockies, Tue-Wed, Oct 24-Oct 25. - Frost/freeze across portions of the Northeast, the Central Appalachians, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, the Great Lakes, and the Ohio Valley, Mon-Tue, Oct 23-Oct 24. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw