Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Mon Nov 13 2023 Valid 12Z Thu Nov 16 2023 - 12Z Mon Nov 20 2023 ...Pattern Overview and Guidance Evaluation/Predictability Assessment... It remains the case that the upper pattern across the Lower 48 will feature a slowly-moving upper low off the coast of California that gradually works inland next weekend, outpaced by northern stream upper ridging moving into southwestern Canada. The 00/06Z guidance cycle continued to show some spread in timing of the upper low moving east and opening into a trough, with the CMC on the fast side, GFS runs on the slower side, and the ECMWF a seemingly good middle ground. The newer 12Z model runs are clustering slower in general. This trough reaches the central U.S. early next week, cooling temperatures after a mild late week-weekend period. A model/ensemble blend seemed to work as an agreeable position for that trough axis/depth. A northern tier trough and a cold front will move quickly eastward during the latter part of the week, while southern stream energy across the Southeast carries a surface low across Florida for some enhanced precipitation. These features look merge/interact around Saturday for increased precipitation in the Northeast. Models are reasonably agreeable with these features and their evolution until early next week when models have varying degrees of digging energy on the west side of the trough near the Great Lakes. GFS runs indicate more digging vorticity than the the ECMWF/CMC and the ensemble means, which also displaces a surface high farther west in the GFS compared to other guidance. A solution more along the lines of the ECMWF and ensemble means looked best. Overall, favored a multi-model deterministic blend of the 00/06Z ECMWF/GFS/CMC/UKMET for the early part of the forecast period, gradually increasing to a majority of ensemble means for the late period as spread increased while maintaining some ECMWF proportion especially. This generally showed good continuity with the previous forecast. ...Weather/Hazards Highlights... By the start of the period Thursday, the heaviest rain should be pulling away from the Florida Peninsula into the western Atlantic. But lingering rain from easterly flow and a slow relaxation of well above normal precipitable water could create some lingering flooding concerns across the eastern coast of Florida, so maintained a Marginal Risk over Atlantic coastal areas just in case. Urban areas would be most susceptible to excessive rainfall. Farther north, light precipitation moving across the Upper Great Lakes/Ohio Valley Thursday with a cold front could trend heavier late week across the Appalachians eastward as Atlantic moisture gets pulled in. Potential precipitation amounts and types continue to vary in the model guidance especially over the Northeast, dependent on the location of a coastal low and the front behind it. For the West/Southwest, the slow approach of the closed southern stream upper low will only gradually bring in bouts of light to moderate/terrain enhanced rain and mountain snow that expand eastward in time. The delay in the height falls likely lessens the stronger moisture flux into the area as the system fills before moving ashore, but most areas of California into the Southwest will see at least measurable rainfall (especially coastal areas) outside some of the shadowed deserts. Given the consistent slower and drier trend in the guidance, no excessive rainfall risk areas are in place. As the amplified upper system finally crosses the Divide, return flow out of the Gulf of Mexico will help increase moisture over the Plains then Mississippi Valley, with rain expanding in coverage late weekend into early next week. At the same time, another Pacific system, but in the northern stream, will progress inland from the Northwest to the north-central Rockies for the weekend along with a quick swath of rain and mountain snow before ejecting into the north-central states to lift limited moisture into the region. Above average temperatures should be widespread across much of the CONUS on Thursday, with the highest anomalies of 15-25F above normal stretching from the central/northern Plains to the Upper Midwest. Though temperatures look to moderate slightly there on Friday, above normal temperatures should be persistent into the weekend but start to cool on Monday under the incoming trough. Temperatures will be warmer than average in the East Thursday-Friday but moderate close to normal on Saturday, while temperatures in the West should generally be near normal. Tate/Schichtel 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