Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Tue Dec 26 2023 Valid 12Z Fri Dec 29 2023 - 12Z Tue Jan 02 2024 ...Locally heavy precipitation possible for Northern California late this week... ...Overview... Western U.S. into Canada mean ridging aloft that should be most pronounced late this week into the weekend will likely support mean troughing over eastern North America during the period. The primary features within the mean trough will be a deep upper low tracking out of Kentucky/Tennessee as of Friday, eastern Canada energy that may dig across New England, and then an upstream shortwave expected to drop into the northern tier by Saturday and cross the East around early next week. Meanwhile guidance expects a couple Pacific shortwaves to push into the long-term mean ridge, with some continued uncertainty in the details. The leading one arriving by Saturday may bring a brief episode of enhanced precipitation to the central West Coast. The South and especially Florida will see below normal temperatures late this week into the weekend while most other areas of the lower 48 should see near to above normal readings. ...Guidance/Predictability Assessment... Guidance is continuing to have a difficult time resolving various details within the eastern U.S. mean trough. Already as of days 3-4/Friday-Saturday there is significant divergence over the evolution of eastern Canada energy which per latest spread could track anywhere between the Great Lakes/Upper Ohio Valley (00Z UKMET) or have the resulting closed low stay north of Maine (18Z/00Z GFS) The preferred model average would have an upper low tracking over New England, which would be slightly weaker and northward of the 12Z ECMWF. Likely due in part to the lack of northern stream interaction, the 00Z GFS has trended noticeably slower and southward for its upper low ejecting from Tennessee. Surface details along the East Coast will be sensitive to these differences aloft. For the upper trough dropping into the Upper Midwest by Saturday and continuing into the East thereafter, there have at least been some favorable trends away from more extreme components of the prior spread. The 12Z ECMWF reduced the southwestward elongation that earlier runs had exhibited, while by day 7 Tuesday the new 00Z GFS trends very close to the 12Z ECMWF versus 18Z/12Z runs that were slower and had a farther south closed low track. By late in the period the past couple days of ECMWF-initialized machine learning models have been advertising a fairly progressive trough, perhaps even faster than the current majority cluster of the ensemble means and 00Z GFS/12Z ECMWF. The new 00Z CMC is one such solution from the operational model realm. A tendency toward greater progression would reduce the potential for western Atlantic low pressure by next Tuesday to have a significant effect on the East Coast. Over the West, guidance is maintaining good relative agreement for the negatively tilted shortwave forecast up to the point that it reaches the West Coast around Saturday. There has been a lot of divergence for the details after that point, but latest runs seem to be consolidating toward the sensible idea of having the southern part of the trough quickly undercut the core of the western ridge centered over/north of the northern Rockies while the northern energy shearing out as it pushes into/around the remaining ridge. The next approaching area of shortwave energy is more diffuse with some recent timing differences. In particular the GFS/GEFS mean have been on the slower side with what should be the best defined troughing that reaches the southwestern U.S. and northwestern Mexico, while ECMWF-initialized machine learning models have generally been running on the fast side of most guidance. The new 00Z GFS has trended faster to match the 00Z ECMWF mean. Based on overall guidance comparisons/preferences based on 12Z/18Z guidance, the updated forecast started with a 12Z/18Z model composite early in the period and then trended toward 60 percent total ensemble mean input (equally divided among the 18Z GEFS, 12Z ECens, and 12Z CMCens) with the GFS contribution eliminated by day 7 Tuesday (leaving the 12Z ECMWF/CMC). ...Weather/Hazards Highlights... The combination of the upper low tracking out of Kentucky/Tennessee from early Friday onward and potential digging eastern Canada energy (with associated surface features) may produce areas of precipitation from the Ohio/Tennessee Valleys into the Northeast late this week into the weekend. The relatively better chance for some snow should be over and west/southwest of the central Appalachians, near the initial upper low, and northern New England depending on details of Canadian dynamics. Specific details of coverage and amounts have lower than average confidence at this time. The weak system tracking through the Upper Midwest/Great Lakes during the weekend may produce some mostly light snow near its path while some rain may fall farther south along the trailing cold front. Southern rainfall totals are uncertain at this point, most likely light to moderate but with heavier activity possible if the minority scenario of stronger southern stream energy and/or interaction with slower northern stream dynamics were to occur. A late week Pacific shortwave/weakening frontal system will provide the best focus for meaningful precipitation along the central West Coast. The Day 4/Friday Excessive Rainfall Outlook maintains a Marginal Risk area covering portions of northern California with no change from yesterday's Day 5 update. In general the forecast scenario has not changed much, with some variance in model QPFs but with an indication of some instability reaching the California coast and the system's orientation potentially allowing for some south-to-north training bands of enhanced rainfall. Another event a couple days earlier should lead to damp antecedent conditions as well. Some of the latest model runs are hinting at the possibility of a slightly southward focus and lower spot maxima but the ensembles maintain support for keeping the Marginal Risk area as-is for the time being. Some activity should linger into Saturday with the best moisture reaching the Sierra Nevada. Some snow will be possible at high elevations. This event will bring lighter totals to the Pacific Northwest. Any precipitation extending farther east into the western U.S. should be light and scattered as the upper dynamics weaken, while an upper trough arriving early next week should produce relatively light precipitation. Expect chilly temperatures across the Gulf Coast/Southeast late this week and into the weekend, with Florida seeing the best potential for some readings at least 10F below normal. A moderating trend should confine negative anomalies just to Florida by Sunday. A frontal passage early next week may lower temperatures again over the Southeast by next Tuesday. In contrast, parts of the far northern Plains and Upper Midwest (excluding north-central Plains locations that may still have snow cover) will be consistently above normal by 10F or more and morning lows should stay above normal over the Northeast. Much of the West will see moderately above normal readings during the period but with a gradual cooling trend as the western U.S. mean ridge aloft weakens at least temporarily. The southern Plains will see some variability but should be above normal overall with the weekend offering the warmest temperatures. 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