Extended Forecast Discussion NWS Weather Prediction Center College Park MD 347 PM EST Fri Dec 02 2022 Valid 12Z Mon Dec 05 2022 - 12Z Fri Dec 09 2022 ...Heavy rain becoming more likely from the Central Plains to the Mid-South early-mid next week... 19Z Update: The 12Z model guidance suite has very good synoptic scale agreement through about Wednesday morning with the broad cyclonic flow aloft over the north-central U.S., the trough moving in across the West Coast, and the upper ridge over the Gulf of Mexico. However, forecast confidence drops off considerably by day 7 (Friday) as models struggle with timing and evolution of shortwave perturbations across the western U.S., and the potential for phasing between northern/southern stream flow late in the period. The WPC forecast was primarily derived from a multi-deterministic blend through Wednesday, and then increased use of the ensemble means along with some previous WPC continuity going into the end of the week. The QPF forecast was derived from a roughly 75% NBM/12% GFS/12% ECMWF blend, and QPF was slightly increased from the blend across portions of central and northern Montana to better account for light QPF in upslope flow behind a strong cold front. The potential exists for 2-4 inches of rainfall across portions of Arkansas and into Tennessee as moisture is lifted over a quasi-stationary frontal boundary. The previous forecast discussion is appended below for reference. /Hamrick -------------------------- ...Guidance and Predictability Assessment... The WPC medium range product suite was primarily derived from a composite of reasonably well clustered guidance from the 18 UTC GFS and 12 UTC ECMWF/UKMET/Canadian models along with WPC continuity and the 1 UTC National Blend of Models valid Monday. The larger scale upper flow pattern evolution remains generally compatible in guidance well into medium range time scales and offers some common trend for a slower southern stream solution in recent guidance including the latest 00 UTC runs. However, embedded system timing/surface feature focus and run to run continuity issues build in a sensitive pattern with uncertain multi-stream system interactions. The 12 UTC ECMWF ensembles were not available for this forecast package due to a technical issue, but the 12 UTC NAEFS mean that includes input from both GEFS and Canadian ensembles does seem to provide a decent forecast basis Tuesday-next Friday with and extending from a general composite of varied model guidance in a pattern with less than stellar continuity with local specifics. Accordingly, felt compelled to only broadly imply threat areas and targeted a blend of NBM QPF with some WPC continuity to soften run to run variance and local focus. ...Pattern overview and Weather/Hazard Highlights... Much of the lower 48 will experience progressive flow aloft to channel between a mean closed low/trough over central Canada and a ridge over the Gulf of Mexico next week. Northern stream shortwave around the Canadian vortex may favor some periods of snow from the Midwest to the Northeast. Guidance has trended towards a positively-tilted upper-level trough persisting over the eastern Pacific and western U.S. with only a gradual eastward drift during the early-mid next week prior to later week ejection into the flow. An upstream ridge should move into the Northwest U.S./western Canada by midweek or so, followed by a shortwave trough approaching the Pacific Northwest around Thursday to renew precipitation into the region. This pattern should promote below normal temperatures over most of the West and north-central states on most days, with rain and mountain snow affecting some parts of the West. The pattern also looks to offer a favorable setup for some heavier rainfall over portions of the east-central U.S. next week as multiple lead moisture sources uncertainly interact with a couple fronts, perhaps with a focus from the Tennessee Valley to the southern Appalachians. A mainly dominant northern stream flow and associated settling of colder air to the north of these fronts may also provide a possibility for an overrunning swath of snow/ice/freezing rain on the north side of the rain shields, perhaps from the south-central Plains through the Ohio Valley states mid-later next week. Schichtel - Heavy rain across portions of the Lower Mississippi Valley, and the Southern Plains, Wed-Thu, Dec 7-Dec 8. - Heavy rain across portions of the Central Appalachians, the Tennessee Valley, and the Ohio Valley, Mon-Tue, Dec 5-Dec 6. - Heavy rain across portions of the Northeast, Tue, Dec 6. - Heavy snow across portions of the Pacific Northwest, Thu, Dec 8. - Heavy snow across portions of the Central Rockies, and the Northern Rockies, Mon, Dec 5. - Much below normal temperatures across portions of California, and the Northern Great Basin, Wed-Thu, Dec 7-Dec 8. - Much below normal temperatures across portions of the Upper Mississippi Valley, and the Northern Plains, Mon-Wed, Dec 5-Dec 7. 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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