Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 PM EST Sat Dec 3 2022 Valid 12Z Tue Dec 6 2022 - 12Z Sat Dec 10 2022 ...Heavy rain becoming more likely from the South-Central Plains to the Mid-South/Tennessee Valley early-mid next week... 19Z Update: The 12Z model guidance suite has very good synoptic scale agreement through Wednesday evening with the broad cyclonic flow aloft over the north-central U.S., the trough moving across the Intermountain West, and the building upper ridge over the Gulf of Mexico. Forecast confidence has improved some for the end of the week, but there are still some noteworthy differences with the timing and degree of amplification with the next big trough developing over the West Coast and then the Intermountain West to close out the work week and beyond. 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 and into Saturday. The QPF forecast was derived from a roughly 75% NBM/12% GFS/12% ECMWF blend for most of the U.S., with some manual adjustments across California and also to slightly narrow the axis of heaviest QPF across the Mid-South. The potential exists for 3 to 5 inches of rainfall from eastern Oklahoma to western Tennessee as moisture is lifted over a quasi-stationary frontal boundary, and a Slight Risk of excessive rainfall was introduced for the Day 5 period across northeastern Oklahoma into north-central Arkansas. The previous forecast discussion is appended below for reference. /Hamrick ----------------- ...Guidance and Predictability Assessment... The latest WPC medium range product suite was mainly 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 Tuesday into Wednesday. The larger scale upper flow pattern evolution remains generally compatible in guidance for much of next week including the latest 00 UTC model runs. However, smaller scale embedded system focus and run to run continuity issues remain evident in this sensitive pattern with uncertain multi-stream system interactions. Accordingly, transitioned to usage of a blend of compatible GEFS/NAEFS/ECMWF ensemble means later Wednesday through next Saturday. There is a growing signal for heavy rains from the South-Central Plains to the Mid-South early-mid next week, but was hesitant to issue any experimental ERO "Slight" risk areas due to uncertainties with local main QPF focus. For QPF, targeted a blend to the 1 UTC NBM with limited incorporation of WPC continuity to soften run to run variance. ...Pattern overview and Weather/Hazard Highlights... It remains the case that 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 shows 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, followed by a potentially vigorous shortwave trough digging into the Pacific Northwest Thursday/Friday to renew precipitation into the region, with ample system energies working over an unsettled West into next weekend. 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 South-Central Plains to the Mid-South onward across 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 to the interior Northeast mid-later next week. Schichtel 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