Extended Forecast Discussion NWS Weather Prediction Center College Park MD 405 PM EDT Fri Nov 03 2023 Valid 12Z Mon Nov 06 2023 - 12Z Fri Nov 10 2023 ...Overview with Guidance Evaluation/Predictability Assessment... A synoptic-scale split flow pattern is forecast to prevail across the North American continent through the medium-range period. A west-southwesterly flow across the southwestern U.S. and a northwesterly flow across central Canada will tend to converge near the Great Lakes and tend to create a quasi-zonal upper flow pattern across the contiguous U.S. However, the merging of these two air streams has resulted in issues on exactly where the stream interaction will occur over the mid-section of the country, where a pair of low pressure waves are forecast to interact midweek next week. There has been a general trend for the models to develop a more consolidated low pressure wave farther and farther to the north over the central Plains toward midweek next week while another low over the northern Plains will tend to weaken and be absorbed by the southern low. This general model trend has also resulted in (1) a slower southward progression of colder air down the western U.S., (2) a more amplified upper trough from the eastern Pacific to reach the southwestern U.S., and (3) more uncertainties to the evolution of the flow pattern downstream late next week, particularly from the Mid-Atlantic northward to New England. Despite the notable model adjustments discussed above, model solutions have been very agreeable with each other in the latest model cycle. The WPC forecast was based on a blend of the latest deterministic and ensemble means from the 06Z GFS/GEFS, 00Z ECMWF/EC mean, and the 00Z CMC/CMC mean, transitioning to mostly a blend of the ensemble means by Day 7. The 06Z GFS downplays the development of the low pressure over the Ohio Valley/Great Lakes on Day 6, which is not supported by the GEFS mean as well the ECMWF and CMC. Therefore, a higher percentage of the 06Z GEFS was given to this time frame. ...Weather/Hazards Highlights... The western U.S. will see active weather into Monday with the possibility of a weak atmospheric river event into the Pacific Northwest. Onshore flow will continue into the coastal ranges and into the Cascades in the Pacific Northwest, with shortwave energy providing upper-level dynamical support and some instability for slightly enhanced rainfall rates. Locally heavier rainfall on the west facing slopes may be enough to cause some minor flooding concerns, particularly for urban areas and burn scars, and especially as rainfall from the prior days brings wetter antecedent conditions. Thus a Marginal Risk is planned in the Excessive Rainfall Outlook for Monday. Precipitation should reduce in magnitude by Tuesday, allowing for no heavy rainfall or flash flooding risk areas. But some areas of heavier snow are possible for higher elevations of the Cascades and into the northern Rockies for Monday-Tuesday. Expect precipitation from the Pacific Northwest into the Rockies to generally trend lighter by the middle of next week as upper-level ridging briefly builds back in, before precipitation chances increase again late week. Farther east, the Midwest/Great Lakes region to Northeast should see rounds of precipitation next week as low pressure and frontal systems track across. Amounts and north-south extent of the precipitation remains somewhat uncertain though. Locations near the Canadian border and the higher elevations of northern New England may also see some snow or mixed precipitation during the period. Rain should be the prevailing precipitation type elsewhere through midweek, though snow levels may lower by Thursday for some snow chances in the interior Northeast, but confidence is low at this point. Temperatures will be generally above average across the country as the period begins Monday, with the central U.S. seeing the largest anomalies at 10-20F above normal. On Tuesday and Wednesday the south-central U.S. can expect highs around 15-20F above normal, potentially setting some record highs as temperatures could rise into the mid-80s. Cold fronts tracking south and east should gradually limit the area of above normal temperatures across the CONUS, cooling temperatures across the northern tier, then the central U.S. to Ohio Valley, and finally across the Southeast by around Friday. Areas behind the front should see near to below normal temperatures, spreading progressively from the western U.S. into the south-central U.S. and toward the Ohio Valley, Great Lakes, and the Northeast late week. Kong/Tate 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 California, and the Pacific Northwest, Mon, Nov 6. 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