Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM 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 struggles resolving the evolution of an upper level trough pattern over the eastern half of the country early next week. There's broad agreement regarding the evoloution of the Southeast/Mid-Atlantic system on day 3. Some variance develops on day 4 with respect to a shortwave trough descending into the Northern Plains/Upper Midwest as well as the condition of what will then be the East Coast trough. Interaction between the northern and southern stream systems with potential phasing as they exit into the Western Atlantic has introduced spread in the ensemble and deterministic guidance on day 5. The 06z GFS is notably slower than the rest of the guidance with respect to the positively tilted trough propagating through the central part of the country. Thus, a general model blend was utilized through day 5 with increasing ensemble means through day 7. ...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. Low temperatures in the low to mid 30s may lead to frost or freezes over portions of northern Florida Saturday and Sunday morning. 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. Kebede/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