Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 AM EDT Sat Oct 21 2023 Valid 12Z Tue Oct 24 2023 - 12Z Sat Oct 28 2023 ...A heavy to possibly excessive rainfall threat increasingly likely for parts of the southern and central U.S. early to mid next week... ...Overview... The medium range period begins Tuesday with an upper low diving through the Southwest into northern Mexico which may linger for a day or two before weakening and lifting through the central U.S. mid to 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 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., 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... As early as day 4/Wednesday, the guidance shows considerable uncertainty regarding both the Upper low over the Southwest/northern Mexico and energy near/into the Northwest. The GFS remains the quickest and most amplified to bring a compact closed low through the Northwest on Wednesday-Thursday which acts to kick the Southwest upper low through the Southern Plains much earlier than the rest of the guidance, eventually spinning up a nice looking cyclone over the north-central U.S. later in the week. The better consensus and latest model trends however suggest much weaker energy manifesting over British Columbia, eventually bringing some sort of shortwave/troughing through the Northwest but not until Friday, which causes the Southwest upper low to linger a couple of days before being lifted through the south-central U.S.. Eventually by day 7/Saturday, models sort of converge on broad troughing across the West/North with blocky ridging over the East. Ensemble guidance also shows significant spread in the details of these systems leaving this a very low confidence forecast. A look at the new 00z runs (available after forecast generation time) generally maintains status-quo with no major shifts in individual pieces of guidance. The forecast blend for tonight did use a fully deterministic blend days 3 and 4, but more heavily weighted towards the ECMWF. After this, the GFS was replaced with the ensemble mean guidance which made up the majority of the blend by days 6 and 7. This maintains good continuity with the previous WPC forecast as well which seems prudent at this time given the uncertainty and run to run variability in the guidance. ...Weather/Hazards Highlights... Favorable moisture from the remnants of Norma should begin spreading in earnest into the Southwest/Southern Plains by Tuesday ahead of the upper low parked over the Southwest/northern Mexico, interacting with a frontal boundary through the Southern Plains. Models are beginning to converge on an increasing threat for heavy rainfall and despite dry antecedent conditions, could pose a flash flood threat given the tropical moisture connection and potential for training along the the front. Given this, opted to introduce a slight risk to the Day 4/Tuesday Excessive Rainfall Outlook across parts of north Texas into west-central Oklahoma. Modest rainfall may also spread northward into the central Plains/Midwest ahead of a stronger cold front through the region and there is a marginal risk across this region on the ERO to cover this potential. Rainfall should linger across the Southwest/Southern Plains/Mid-Mississippi Valley into Wednesday as well, with some heavy rainfall potential, but uncertainty in amounts and location of the highest amounts is too uncertain for anything more than a marginal on the Day 5 ERO at this time. Rainfall may trend lighter and slightly eastward later in the week as the upper low begins to weaken across the south-central U.S.. Out West, probabilities for meaningful snow at higher elevations of the northern Rockies should start increasing by early in the week, with some potential for generally light but accumulating snowfall even extending into parts of the northern Plains later in the week, but highly dependent on the currently very uncertain upper level and surface pattern evolution. Well above normal temperatures will spread from the Midwest into the East Tuesday-Thursday with the greatest anomalies (15-20 degrees above normal) likely Tuesday across parts of the Midwest. In the Northeast, morning low temperatures on Tuesday may be cold enough to support a frost/freeze threat, but should moderate thereafter as upper ridging builds into the region. Temperatures will trend cooler Wednesday and beyond for the northern Rockies/Plains where daytime highs could be 10-20 degrees below normal behind the cold front. 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, 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