Extended Forecast Discussion NWS Weather Prediction Center College Park MD 152 AM EST Fri Dec 29 2023 Valid 12Z Mon Jan 01 2024 - 12Z Fri Jan 05 2024 ...Overview... The latest models and ensembles show a pattern transition for next week towards a series of Pacific shortwaves (some with an embedded upper low) to affect the West coast, and then track across the southern tier. In the East, a leading trough will pass through Monday-Tuesday, with secondary troughing to follow mid to latter next week as a ridge builds over western Canada. This pattern keeps the bulk of the precipitation confined to the West Coast and the Gulf Coast/Southeast with locally heavy rain possible in some spots. ...Guidance/Predictability Assessment... Overall, the guidance shows good agreement on the large scale pattern, but a lot of questions remain with the details and timing of individual systems. There was enough agreement early in the week between the 12z/18z (Dec 28) guidance to use a general model blend as the starting point for the overnight WPC medium range package. By Wednesday, and especially beyond, the greatest area of uncertainty surrounds the timing of the second shortwave/upper low dropping down the West Coast Wed-Thurs and into the Southwest on Friday. The last few runs of the GFS (including the new 00z run tonight) are notably faster with this system compared to the 12z ECMWF and CMC, but it should be noted that the new 00z run of the ECMWF this morning (available after forecast generation time) did trend significantly faster with the upper low and is in better agreement with the GFS runs. Ensemble mean plots suggest though there is still a lot of uncertainty. Downstream, there are also questions with amplitude/sharpness of shortwave energy into the Southeast and also details around additional troughing/energy across the Northeast. For the second half of the period, the WPC forecast for tonight relied closer to the ensemble means with more emphasis towards the slower Western U.S. evolution as advertised by the 12z (Dec 28) ECMWF and CMC deterministic models. ...Weather/Hazards Highlights... A couple of weak fronts through the East on Monday may produce mainly light precipitation across parts of the Lower Great Lakes/Central Appalachians as well as down across the Southeast/central Gulf Coast. The initial upper low/shortwave into the Southwest may bring some light precipitation to parts of the region on Monday, but should begin to tap Gulf of Mexico moisture to fuel more widespread showers across parts of the Southern Plains/Gulf Coast/Southeast Tuesday-Wednesday. The latest model guidance is indicating just modest moisture and instability availability, and not enough at this point to warrant any kind of risk area on the Day 5/Tuesday Excessive Rainfall Outlook. The second low forecast to approach the West Coast by Wednesday and move into the Southwest by Thursday may bring organized precipitation to the West Coast and some areas farther inland, with relatively higher totals most likely over favored terrain in portions of California. The greatest temperature anomalies during the period should focus across the far northern tier near the Canadian border with some locations seeing multiple days with readings 10-20F or so above normal, with morning lows tending to be farther above normal than daytime highs. Most areas from California eastward across the southern tier should see near to below normal high temperatures during the period as a series of upper level systems move through. 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