Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM 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 that 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... Even early in the forecast period, model guidance from the 00Z/06Z cycle differed quite a bit with the evolution/timing of the fairly strong system nearing the Northwest by Saturday and trough progressing eastward thereafter. There has been a consistent signal for ECMWF/ECens and CMC/CMCens runs to be significantly slower/farther west compared to the GFS/GEFS/UKMET. But, EC-based ML models have uniformly been on the faster side of the spread, unlike the rest of the EC suite. This created low confidence in the EC/CMC-type slower solutions, and the most recent WPC forecast continued to favor the faster guidance cluster. Fortunately, the incoming 12Z ECMWF and CMC have switched to faster tracking of the features toward the coast and inland. Thus the 12Z suite is yielding higher confidence in this faster progression of the upper/surface lows into the Northwest, though the 12Z EC/CMC are still slower by early next week as the trough progresses into the central U.S. Elsewhere, another feature to consider is shortwave troughing traversing from around the central Rockies Friday into the Midwest over the weekend along with a gradually weakening surface low. The main model outlier with this was the slower 00Z UKMET, and otherwise a multi-model blend seemed reasonable despite some differences. The next shortwave or closed low approaching the West from the eastern Pacific into early next week has reasonable model spread with its timing. The WPC forecast favored the 00Z/06Z GFS, 00Z UKMET, and GEFS mean early in the period given the aforementioned EC/CMC discrepancies. Gradually increased the proportion of the GEFS mean and a bit of EC mean as the period progressed. Hopefully future forecasts can incorporate more EC/CMC guidance since their 12Z models are showing somewhat better alignment. ...Weather/Hazards Highlights... The storm likely nearing the Pacific Northwest Friday-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 may be possible at least offshore. While there are still some model differences, recent guidance is suggesting a relatively faster movement of the associated atmospheric river for less extreme rainfall amounts than some older models suggested. There is still certainly enough QPF through Friday-Saturday to maintain Marginal Risks for far southwest Oregon into northern California, with just minor tweaks to continuity. Some moisture should continue across the West into the weekend and heavy snow is possible in the higher elevations of the Cascades, Sierra Nevada, and northern Rockies. 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 in the Mid/Upper Mississippi Valley late week into the Northeast over the weekend, though probabilities for 0.25 inch liquid in the form of snow are currently no higher than 10-30 percent. Another round of precipitation is likely to develop across the east-central U.S. early next week as the initially western system pushes eastward and pulls in moisture. This currently looks to be a warmer system for mostly rain, though wintry weather may be possible in the far northern tier. Expect another Pacific system to produce additional rain and higher elevation snow across the West during the first part of next week as well. 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. Tate/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