Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Mon Dec 25 2023 Valid 12Z Thu Dec 28 2023 - 12Z Mon Jan 01 2024 ...Locally heavy precipitation possible for the central West Coast late this week... ...Overview... 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... Multi-day means and teleconnections relative to the strong positive height anomalies over central and then western Canada offer continued support for the general mean trough over the eastern U.S. but embedded details continue to be an issue. By the latter half of the period there is also a wide array of possibilities for how incoming Pacific shortwave energy on Saturday may ultimately pass through the long term western U.S. mean ridge and for depictions of potential energy behind it. The 12Z/18Z runs and new 00Z solutions are finally starting to cluster better for the track of the upper low forecast to be over the Lower Ohio Valley as of early Thursday. The latest consensus falls within the southern half of the prior envelope, with GEFS mean runs and one or more GFS runs appearing to provide the earliest lead on such a path. Some ECMWF-initialized machine learning models also advertised a fairly southward track and in a greater proportion relative to recent operational model/ensemble spread. Guidance arrives at the current cluster by way of delayed interaction of northern stream flow, but with the forecast from Friday onward still plagued by a wide spread for how eastern Canada trough energy will evolve--affecting the ejecting eastern U.S. upper low and then details over the Northeast behind it (12Z ECMWF and 00Z UKMET/CMC much deeper with Northeast troughing relative to the GFS/GEFS by Saturday). At the surface, expect a leading wave to depart from New England early and then a new system to form near the Mid-Atlantic in response to the upper low, with a track most likely far enough offshore not to have a significant effect on New England. Farther upstream, guidance has rapidly become better defined with a shortwave dropping into the Upper Midwest by Saturday. Then solutions diverge for what becomes of this trough. The current majority suggests less southwestward elongation than depicted in the past couple of ECMWF runs or 12Z GFS, while 12Z ECMWF-initialized machine learning models favor a fairly progressive trough near the East Coast by early next week per the new 00Z GFS. The 18Z GFS and latest CMC runs are a compromise while the intermediate ensemble means with broad troughing have support from teleconnections. Over the West, some recent runs have been nudging a little faster with the negatively tilted shortwave trough forecast to reach the West Coast by Saturday. On the other hand, the new 00Z CMC has strayed to the slow and amplified side of the spread. As is typically the case with shortwave energy pushing into a mean ridge, guidance varies considerably for what becomes of this shortwave after its arrival. A slight majority scenario has been for some sort of split while the 12Z ECMWF-initialized machine learning models generally maintained a better defined shortwave pushing into the Interior West by Sunday, closest to the 12Z ECens mean (plus new 00Z UKMET through the end of its run) with the 18Z GFS/12Z CMC the closest in principle albeit with more splitting of the energy. The GEFS mean washes out the energy quicker than the other means or operational runs. By next Monday/New Year's Day, the most common theme is for another area of troughing to approach the West Coast. The 12Z machine learning models generally favored a southern stream trough reaching near California. Based on 12Z/18Z guidance, preferences led to an early period blend consisting of more 12Z ECMWF and 12Z/18Z GFS relative to the UKMET/CMC and a mid-period avoidance of the CMC. Later in the period the forecast increased 18Z GEFS/12Z ECens mean weight toward 50 percent while reducing 12Z ECWMF input, maintaining the GFS, and returning some CMC into the blend. ...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. 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 as well as the central California coast 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 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