Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 PM EST Thu Jan 5 2023 Valid 12Z Sun Jan 8 2023 - 12Z Thu Jan 12 2023 ...Significant heavy precipitation threats for California to renew again later weekend through early next week... 19Z Update: The 12Z model guidance suite has come into better agreement on the synoptic scale pattern across the continental U.S. through the middle of next week, with above average confidence on the strong shortwave trough tracking from California to the south-central U.S. late Monday through Thursday. There is also less model spread with the main QPF axis across central California early next week, and in combination with the extremely saturated antecedent conditions, a Moderate Risk of excessive rainfall is now in effect for the Day 5 period (12Z Monday to 12Z Tuesday) for the central CA coast and the coastal mountain ranges, where a few inches of additional rainfall will only aggravate ongoing creek/stream flooding. Looking ahead to next Thursday, the prospects of heavy rain and strong thunderstorms from the ArkLaTex region to Alabama have increased since the previous forecast, with a well-defined surface low likely developing based on the latest model guidance. In terms of temperatures, no major arctic air intrusions are expected, with temperatures generally running 5 to 15 degrees above normal across much of the east-central U.S. for the Monday to Wednesday time period. The WPC fronts/pressures forecast was primarily derived from a multi-deterministic model blend through early Tuesday, with about 40% of the ensemble means included by next Thursday. The previous forecast discussion is appended below for reference. /Hamrick -------------------------- ...Guidance/Predictability Assessment... The weather pattern over much of the continental U.S. remains very active and more complicated than usual, with multiple uncertain embedded shortwaves aloft within relatively well defined northern and southern stream flows. However, forecast spread and run to run continuity in guidance has slowly improved over the past few guidance cycles to bolster forecast confidence and predictability. The eastern Pacific will continue to be extremely active with with a series of deepened low pressure systems for the West Coast with lead atmospheric river moisture to significantly amplify amounts and impacts. The WPC medium range product suite was primarily derived from a blend of best clustered guidance from the 18 UTC GFS/GEFS mean, 12 UTC ECMWF/UKMET and ECMWF ensemble mean, WPC continuity and the 01 UTC National Blend of Models (NBM). WPC preferred blend weighting shifts more in favor of the compatible ensemble means at longer time frames by next midweek in an effort to best mitigate growing smaller scale variances consistent with predictability. The latest 00 UTC guidance suite generally remains in line with this forecast strategy. ...Weather/Hazard Highlights... Lead upper-level troughing moving through the West is forecast to reach the Plains and help develop a modest surface frontal wave to subsequently track with some uncertainty eastward from the vicinity of the Lower Ohio Valley to the Mid-Atlantic coast Sunday into Monday. Compared to the models, the NBM still seems to be missing some modest precipitation potential that may produce some light wintry weather for parts of the Ohio Valley through the Appalachians as well as a trailing area of rainfall down trough the western Gulf Coast into Sunday and modestly to the Southeast into early next week. WPC QPF and weather grids were accordingly adjusted. However, the big weather story remains well upstream over California. WPC experimental Day 4/5 Excessive Rainfall Outlook (ERO) "slight" risk areas have been issued valid Sunday and Monday for northern and north-central California respectively. The threat is amplified as additional rounds of heavy rainfall with ample runoff issues are likely to persist through early next week in a dangerously wet pattern with multiple Atmospheric Rivers working inland ahead of a series of deepened eastern Pacific storms. Forecast rainfall amounts on Monday in particular look for multiple inches over a broad area of northern and central California. Many areas are likely to be sensitive to additional rainfall given the ample recent and shorter term forecast rainfall. High probabilities for Sierra snowfalls are also depicted in the WPC Winter Weather Outlook (WWO) Sunday through next midweek. Periods of strong, gusty winds will likely accompany each system across northern and central California and the adjacent Great Basin. Guidance also shows some favorable flow into the Northwest for especially southward facing terrain. There is also the potential for more modest shortwave/terrain enhanced snow to work into the Great Basin toward the Rockies, but perhaps especially by next midweek with a better maintained system pushing inland across the region with snow amounts trending upward in the recent forecasts. This system then has potential to spawn moderate central Plains cyclogenesis late period whose potential lead return Gulf inflow up through the MS Valley/vicinity may offer an opportunity for an expanding area of late period rainfall along with emerging snow/ice/freezing rain potential on the northern periphery of the precipitation area as moisture feeds into a cooled antecedent high pressure airmass that may dig in advance from Canada into the north-central through eastern U.S.. Schichtel/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