Extended Forecast Discussion NWS Weather Prediction Center College Park MD 215 AM EDT Thu Oct 26 2023 Valid 12Z Sun Oct 29 2023 - 12Z Thu Nov 02 2023 ...Arctic Surge through the Central U.S into next week.... ...Overview... Upper troughing looks to be sliding through the West as the period begins on Sunday, with shortwaves working to maintain and renew the troughing as it moves through the Midwest and eventually into the East next week. This allows for a much below normal airmass to spill into the Central U.S., including as far south as Texas and the Gulf Coast, sliding east with time. Heavy rain looks to be a threat Sunday along a frontal boundary through the south-central Plains and Midwest, but conditions should dry out thereafter as surface high pressure envelopes much of the nation. Upper ridging should build over the West again by next midweek as the next trough works into the East Pacific/West Coast. ...Guidance Evaluation/Predictability Assessment... The main forecast issues during the medium range are with strength and timing of a shortwave from the Northern Plains to the Northeast Tuesday-Thursday. The ECMWF has consistently been much more amplified with this than the rest of the guidance, with yesterdays 12z run even showing a closed low over the Upper Midwest/Ohio Valley. Through yesterdays 12z/18z model runs, the GFS barely shows this feature at all/is much weaker and the CMC was a nice middle ground but much faster. However, the new 00z guidance for tonight from the GFS and CMC has trended stronger/more amplified, though still not as strong as the ECMWF. Otherwise, models show good agreement for an initial shortwave through the northern tier on Sunday-Monday and upper ridging building back in over the West. There are some greater differences with the next trough towards the West Coast late in the period. The WPC forecast for tonight used a blend of the latest operational guidance days 3-5. Trended more towards the ensemble means (with minor contributions from the deterministic solutions) for the latter half of the period which seemed to provide a nice middle ground for the shortwave differences through Great Lakes/Northeast and maintained acceptable continuity with the previous WPC forecast. ...Weather/Hazards Highlights... Instability and anomalous moisture along a slow moving front into Sunday will continue to provide focus for heavy rainfall from parts of the south-central Plains into the Midwest/Ohio Valley with potential for repeat/training of storms. Some lingering uncertainty on exactly where the heaviest totals set up, and dry antecedent conditions could limit a widespread flash flood threat, but latest model QPF amounts combined with the potential for additional rains over the next few days leading into the medium range forecast period do seem to still support a slight risk area on the day 4/Sunday ERO across the Mid-Mississippi Valley, with a marginal risk extending northeast and southwestward along the span of the frontal boundary. Some potential for lingering snow across the central Rockies and adjacent High Plains on Sunday on the north side of the precipitation shield, with transition zone freezing rain/sleet just south of this area. Beyond that into next week, much of the country should be mainly dry as high pressure dominates the CONUS. There should be modest lake effect rain/snow showers downwind of the Great Lakes. Of note is also an uncertain lifting of a low up from the Caribbean to the southwestern Atlantic in some guidance in advance of a main cold front that is now being monitored by NHC. Meanwhile, the approach of an amplified eastern Pacific upper trough/height falls and surface system energies does overall favor a moderately wetter pattern in about a week for the Northwest. Much below normal temperatures across the West into the northern/central Plains will continue to expand south and east on Sunday bringing the first real cold snap of the season to many across the central U.S., including as far south as the Southern Plains/Texas and coinciding with outdoor Halloween festivities next Tuesday. Daytime highs 20-30 degrees below normal are likely with a few daily records possible within this cold air mass. Meanwhile, well above normal temperatures will continue Sunday and Monday for the Southeast/East underneath a retreating upper ridge. Expect a broad area of highs 10-20 degrees above normal with some morning lows 20-25 degrees above normal. Daily records are possible, with record warm lows likely to be more numerous than record highs. The chilly central U.S. airmass should push eastward with time next week (with some moderation) with much of the country east of the Rockies below normal next Tuesday-Thursday. After Monday, temperatures across the West should be within a few degrees of normal. 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