Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 PM EST Sat Jan 7 2023 Valid 12Z Tue Jan 10 2023 - 12Z Sat Jan 14 2023 ...Additional atmospheric rivers events with heavy rain and mountain snow for much of California next week... 19Z Update: Overall the 12Z model guidance has a very good synoptic scale depiction for next week and even into next Saturday, with the best model agreement noted in the southern stream flow across the southern tier states. However, the GFS becomes more amplified with a shortwave/closed low over New England and eastern Canada by Thursday, and it is also slower with the progression of the storm system across the south-central U.S. as early as mid-week. With the lack of a Greenland blocking high or 50/50 low near Newfoundland, the flow pattern is expected to remain fairly progressive across the continental U.S. with airmasses generally of Pacific origin and no major arctic air intrusions expected. In terms of excessive rainfall potential, the Slight Risk area on Day 4 was expanded some northward across the foothills of the west-central Sierra, and also farther south along the Laguna mountains east of San Diego. In terms of the WPC fronts/pressures, a nearly deterministic model blend was used for Tuesday and Wednesday, and then less of the GFS and more of the ensemble means going into the end of the week to account for some of the uncertainty. The previous discussion is appended below for reference. /Hamrick ---------------------------- ...Overview and Guidance/Predictability Assessment... The weather pattern over much of the continental U.S. should remain quite active through next week, with multiple embedded shortwaves crossing the West Coast and moving eastward through the south-central U.S. and eventually the East. There is very good agreement in the guidance that the pattern should trend more amplified with time as upper ridging builds over the central U.S. sandwiched between amplified troughs along both the West and East coasts. The highlight this forecast period continues to be a very active eastern Pacific sending multiple shortwaves/atmospheric rivers into California which will only exacerbate ongoing flooding concerns in that region. There is moderate to high confidence in the overall synoptic pattern, but low confidence in the details of individual shortwaves. The first shortwave through the West Tuesday-Wednesday shows some minor timing differences, but this feature becomes more uncertain as it closes off an upper low over the Central Plains around Thursday with surface cyclogenesis likely across the south-central U.S. Details are important for snow/ice potential on the north side of this system but especially across parts of the Northeast as there is support for a possible Nor'easter late next week into the weekend. Again, confidence is very low on any sort of significant system but its worth keeping an eye on the upcoming model trends. The WPC forecast used a general deterministic model blend for days 3-5 when model agreement was better. By 6 and 7, incorporated more of the ensemble means to help smooth out the late period detail differences (both in the East and the West with the next trough towards the coast). Did continue to include modest amounts of the deterministic guidance (GFS, ECMWF, and CMC) for some added system definition. ...Weather/Hazard Highlights... The big weather story through next week will remain out West as yet more atmospheric rivers take aim at an already over saturated California. One event will be ongoing as the period begins Tuesday, although the worst of it on Monday has now moved into the short range. Even still, the WPC experimental Day 4 Excessive Rainfall Outlook (ERO) continues to highlight a slight risk along the Transverse Ranges in southern California, where rainfall amounts of several inches are possible. Lingering rains are likely across central to northern California too, locally heavy in spots, where many areas are already very sensitive to additional rainfall given the recent series of atmospheric rivers. Significant snows will continue across the Sierras into Tuesday with snow extending into the Great Basin and Rockies too as the upper system shifts inland. After this, the next system looks to impact California again by Wednesday-Thursday with additional and potentially heavy rainfall and higher elevation snow. A leading shortwave into the Southeast on Tuesday into Wednesday may bring some light and scattered rain. Then the initial system in California should track east during the middle of next week with potential to spawn moderate central Plains cyclogenesis later next week. Lead return Gulf inflow into the MS Valley and vicinity may offer an opportunity for an expanding area of mid to late week rainfall across portions of the Deep South/Tennessee Valley. Confidence is increasing for some snow and ice potential on the northern periphery of the precipitation shield as the moisture feeds into a cooled antecedent high pressure airmass, but high uncertainty on the details on amounts and locations. Modest rainfall may spread into the Eastern U.S. ahead of the attendant cold front. Some in the form of snow mainly inland and north, but low confidence on amounts and potential impacts. The warmest temperature anomalies will feature across the Southern Plains into the middle Mississippi Valley Tuesday-Wednesday where daytime highs could be 15 to 20 degrees above normal. Warmer than average conditions shift into the eastern third of the nation by Thursday, but should moderate again late week across the Southeast to Mid-Atlantic with the next approaching system. Upper ridging building over the central U.S. will allow for another period of moderately above average readings from the High Plains/Plains into the Midwest and Great Lakes into next weekend. Santorelli/Hamrick 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