Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Mon Dec 25 2023 Valid 12Z Thu Dec 28 2023 - 12Z Mon Jan 01 2024 ...Locally heavy precipitation possible for Northern California late week... ...Overview... It generally remains the case that upper ridging over western Canada, as well as the western U.S. through at least Saturday, will support a mean trough over the eastern U.S. during the period. Within the eastern trough a leading upper low should track out of the Lower Ohio Valley with continued uncertainty over how northern stream flow may interact. Another shortwave/surface front may drop into the mean trough Saturday onward. Meanwhile weakening shortwave energy will quickly brush the Northwest early Thursday and then a sharpening/negatively tilting trough will likely reach the West Coast by Friday night/early Saturday, bringing a brief episode of enhanced precipitation to the central West Coast. After early Saturday the details become very unclear for what will happen to this shortwave energy as well as for potential upstream energy. Expect chilly temperatures for most of the period over the South and near to above normal readings over most other areas of the lower 48. ...Guidance/Predictability Assessment... Models continue to trend into better agreement with the mid-larger scale system and pattern evolution for Thursday and Friday, albeit with lingering smaller scale differences that seem best mitigated by a composite blend of the 06 UTC GFS and 00 UTC ECMWF/UKMET/Canadian that seems quite compatible with the 13 UTC NBM, ensemble means as well as WPC product continuity, bolstering forecast confidence. Cycle to cycle model continuity issues and forecast spread increase over the extended New Year's holiday weekend into 2024, lowering forecast confidence. This remains the case with the latest 12 UTC model cycle. Accordingly, a blend of the more reasonably compatible and cycle consistent GEFS/ECMWF ensemble means seems to provide a better forecast basis some manual adjustments were applied to maintain some surface system detail consistent with support and guidance signal. WPC medium range surface pressure/frontal progs were simplified to blend best with the split flow pattern over the nation and predictability. ...Weather/Hazards Highlights... A leading departing wave should bring some precipitation to the Northeast on Thursday, with snow over far northern New England and rain to the south. The upper low tracking out of the Lower Ohio Valley should support an area of rain/snow along its path late this week, with best potential for highest snow accumulations over favored terrain in the Appalachians, but pocketed cold air and deformation to the north of closed low passage may support additional spillover/areal coverage as per the latest WPC Day 4/WWO centered on Friday. Interior Northeast locations could see another episode of snow depending on details of eastern Canada trough energy by the weekend, while some mostly light precipitation may be possible with a wave/front over the central/eastern U.S. from Saturday onward. Rainfall could be somewhat heavier in the event of a less probable slower upper trough in that time frame. Over the West, a late week Pacific shortwave/weakening frontal system will provide the best focus for meaningful precipitation along the central West Coast. The Day 5/Friday Excessive Rainfall Outlook proposes a Marginal Risk area covering portions of northern California and far southwest corner of Oregon. Model QPFs vary over this region but there is an indication of some instability reaching the California coast and the system's orientation could allow for some training bands of enhanced rainfall. Another event a couple days earlier should lead to damp antecedent conditions as well. Some activity may linger into Saturday, while snow is possible at high elevations. Confidence remains low regarding coverage of any lighter precipitation that could extend farther into the West. The Pacific Northwest should also see some precipitation from this event but with lower totals than forecast along the central West Coast. The forecast pattern will favor multiple days of chilly weather across the South, with temperatures 5-15F below normal. Florida should see the best potential for double-digit anomalies. The Southeast should begin to moderate by Sunday-Monday but with Florida likely staying below normal through that time. On the other hand, parts of the far northern Plains and Upper Midwest will be consistently above normal by 10F or more. Morning lows from the Upper Midwest into Northeast may be at least 20F above normal on Thursday, which could lead to some daily records for warm lows in the Northeast if they hold for the calendar day. Much of the West will see moderately above normal readings during the period but with a slight cooling trend if upper ridging weakens a little as currently forecast. Rausch/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