Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Tue Dec 10 2024 Valid 12Z Fri Dec 13 2024 - 12Z Tue Dec 17 2024 ...Continuing to monitor a potentially significant storm system to affect the Pacific Northwest by this weekend... ...Overview... Multi-day means continue to depict nearly zonal flow across the lower 48 from late this week into the first half of next week, containing a sequence of shortwaves with varying amplitude and tending to be spaced a couple days or so apart. Each associated surface system should produce episodes of rain and higher elevation snow over portions of the West and then development of precipitation over the southern Plains and expanding across the east-central U.S. then continuing into portions of the East. Northern areas may see some wintry weather. While guidance depicts the general pattern well, particular models/ensembles have been very stubborn within their respective clusters which show significant differences for system details/timing. This is especially the case for the system forecast to be approaching the Pacific Northwest this weekend. Expect temperatures to be near or above normal across most of the country during the period aside from a chilly Friday over northern/eastern areas. ...Guidance/Predictability Assessment... Initially looking at the forecast from the big-picture perspective, the primary forecast problems involve the eventual details of Friday's Rockies/Plains shortwave as it moves into the East followed by the evolution/timing of the fairly strong system nearing the Northwest by Saturday (and trough progressing eastward thereafter) as well as the next system forecast to arrive around Monday-Tuesday. Continuing into the new 00Z cycle, ECMWF/ECens and CMC/CMCens runs remain significantly slower with the latter two systems versus the GFS/GEFS/UKMET while machine learning (ML) models still favor the faster side of the spread--with a small subset even faster than the GFS cluster for the Northwest system on Saturday. The updated forecast maintains relative continuity with a tilt more toward the GFS/GEFS given the consistency of the ML themes as well as the generally progressive nature of the large scale pattern. Focusing more closely on the system developing over the eastern Pacific Friday into the weekend, eventual differences in small- scale but very important details between the faster/northward GFS and slower/southward ECMWF clusters seem to start showing up within the initially flat North Pacific flow around late Wednesday, so there is a chance it may take another day or two to achieve better clustering. The timing differences have a significant impact on the precipitation totals along the central West Coast into the Pacific Northwest (much heavier in the slower ECMWF cluster). As mentioned above, ML models generally favor a progressive solution with their envelope even extending beyond the faster side of the dynamical spread. Such fast timing may be overdone given a fairly good emerging consensus among operational runs and a decent number of ensemble members for a depth in the 970s or 980s mb. As for other systems, the next shortwave reaching the West (supporting a less impressive surface system than its predecessor) has adjusted a little faster in the 00Z ECMWF by next Tuesday so it is now closer to the majority cluster. The new 00Z CMC has dramatically improved the depiction of this shortwave compared to the old 00Z run. The shortwave progressing into the East by Sunday is questionably slow/closed in the 00Z UKMET and then a lot of lower-predictability detail differences arise for how the shortwave energy may or may not split and the resulting surface reflection/precipitation. A blend approach minimizes emphasis on any particular detail that has low confidence. ...Weather/Hazards Highlights... A shortwave reaching the Rockies/Plains by Friday will continue eastward with time, with some details of its energy becoming increasingly uncertain toward the East Coast by Sunday-Monday. This feature and associated surface wave/frontal system may spread precipitation across the east-central U.S. by Friday-Saturday and then across the East Coast. Generally expect light to moderate amounts with this system. Locally heavier totals cannot be ruled out but overall guidance signals are too weak to suggest any risk areas in the Days 4-5 Excessive Rainfall Outlooks. Details will depend on finer scale shortwave details that have low predictability several days out in time. The northern part of the precipitation shield may contain some wintry weather at times, though probabilities for 0.25 inch liquid in the form of snow are currently no higher than 10-30 percent. The storm likely nearing the Pacific Northwest by Saturday will bring the potential for areas of significant rain and higher elevation snow to locations along the central and northern West Coast. Brisk to strong winds will also be possible. Guidance continues to show significant differences in evolution details and track/timing for this system (though with a little less spread for depth). These differences yield either considerable or less extreme precipitation over parts of western Oregon/northern California, though with this region sitting within the best overlap among solutions for heaviest activity. Thus the Day 4 ERO maintains a Marginal Risk close to continuity, with the Day 5 ERO introducing a Marginal Risk that expands a little to the south and east. Some of this West Coast system's moisture should continue across the West during the weekend, with the shortwave/surface system likely producing another area of precipitation over the east- central states by around the start of next week. Expect another Pacific system to produce additional rain and higher elevation snow across the West during the first part of next week. Friday will be on the chilly side across the northern tier and the East, with locations over the Upper Midwest near the Canadian border likely seeing the coldest anomalies of 10-20F or so below normal. Aside from lingering modest cool anomalies over the interior Southeast on Saturday, the rest of the lower 48 should see near to above normal temperatures from Saturday onward. Warmest anomalies for highs should be in the plus 10-15F range over parts of the Plains on Sunday-Monday. Expect broader coverage of some areas of plus 10-15F anomalies for morning lows. 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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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