Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Mon Nov 20 2023 Valid 12Z Thu Nov 23 2023 - 12Z Mon Nov 27 2023 ...Pattern Overview... Most guidance offers fairly similar ideas for the large scale pattern, with a mean ridge over the eastern Pacific into northwestern North America while a deep upper low over or near Hudson Bay anchors a broad mean trough covering much of Canada into at least the northern part of the lower 48. Positively tilted mean troughing should also extend over the western U.S., as favored by teleconnections with the upstream ridge. This regime will likely bring below normal high temperatures across a majority of the country (and coldest anomalies to the central U.S.) with an area of snow dropping southward over and near the northern-central Rockies and High Plains. However the forecast is much more uncertain regarding precipitation coverage and amounts across the South and East from Thanksgiving into the weekend, corresponding to shortwave differences that lead to considerable differences for surface evolution. ...Guidance Evaluation/Predictability Assessment... Leading into the start of the forecast period early Thanksgiving/Thursday, the exact character of the system forecast to be off the New England coast continues to be in doubt. A number of UKMET/GFS runs have been depicting a different evolution of shortwave energy in the short term versus other guidance, leading to a somewhat slower and/or stronger surface low pressure. The 18Z GFS still had a variation of this lower confidence detail aloft but was closer to the majority cluster than most other recent GFS runs. However the new 00Z ECMWF has switched to solution closer to the 00Z GFS, raising the possibility of this alternate scenario. Behind this system, differences in how an upper low ejects from the Rio Grande Valley plus varying influence from northern tier flow and energy emerging from the western U.S. lead to a wide variety of possibilities for potential low pressure between the Gulf of Mexico and East Coast/western Atlantic. Recent CMC runs have been the most pronounced but also consistent with this wave and its farther northwestward extent of moisture while ECMWF runs have been trending stronger with an East Coast track by Saturday. The GFS has taken longer to start reflecting this wave and the 18Z run had ended up just a little slower than the 12Z ECMWF. However the new 00Z GFS/ECMWF have lowered heights over New England, significantly suppressing the overall surface pattern. Reflecting the broad spread among individual members, the ensemble means lean weak and offshore with what surface reflection there may be by the weekend. Trends observed through the 12Z/18Z cycles seemed to favor somewhat better definition than the means but with weaker low pressure than the GFS/ECMWF/CMC were showing. However the 00Z GFS/ECMWF adjustments significantly lower what low-moderate confidence there had been in a certain evolution. Meanwhile the guidance is gradually coming into better agreement for the shape of western U.S. troughing into late week, with increasing suggestion of an upper low briefly closing off by around early Friday and then opening up as some of the trough energy continues eastward. Toward the end of the period the 12Z/18Z GFS showed weaker troughing left over the West versus most other models and ensemble means, while the 12Z CMC was a deep extreme with its upper low. 00Z runs of both the GFS and CMC favorably trended closer to the means (and teleconnection-favored pattern relative to the East Pacific ridge) by the start of next week. Cyclonic flow extending into the northern tier of the lower 48 will likely bring at least one cold front into the region by Saturday-Monday. Specifics will depend on lower predictability shortwave details, so the timing and southward extent of the front(s) may vary noticeably from run to run. Preferences toward a moderate/intermediate solution where differences existed led using 70 percent total weight of the 18Z GFS/12Z ECMWF and the rest 18Z GEFS/12Z ECMWF means for the first half of the period, followed by a rapid transition toward 50-60 percent ensemble weight by days 6-7 Sunday-Monday. ...Weather/Hazards Highlights... The relatively more confident part of the forecast is for an area of snow to drop southward over the northern-central Rockies and High Plains late this week, with some locally enhanced totals possible. Then the snow should trend lighter and more scattered as energy within upper trough (with possible embedded low) progresses onward. On the other hand, confidence is quite low for the coverage and amounts of precipitation over the South and East from late this week onward. Possibilities range between a fairly wet Gulf/East Coast surface system which could also produce snow in the northwestern periphery of the moisture shield, and much lighter/scattered precipitation aside from what activity would cross Florida with a more suppressed surface system. Continue to monitor forecasts for latest trends regarding the southern/eastern U.S. surface pattern and expected precipitation. Expect most of the country to see near to below normal high temperatures from Thanksgiving into the start of next week, with highs up to 10-20F below climatology most likely over the central-southern High Plains Friday through the weekend and becoming more confined to the southern Plains by Monday. Also the central-western Gulf Coast region may see highs of 10-15F below normal on Thanksgiving. One exception to the chilly theme may be locations along the central-northern West Coast where Pacific ridging aloft could be just close enough to keep highs a little above normal. Rausch 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