Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1247 PM EST Sun Jan 8 2023 Valid 12Z Wed Jan 11 2023 - 12Z Sun Jan 15 2023 ...California remains active into next weekend as a series of Atmospheric Rivers brings heavy rain and mountain snow... 18Z Update: Overall the 12Z model guidance has a good synoptic scale depiction for the middle to end of the week, with the best model agreement noted in the southern stream flow across the southern tier states. However, the GFS and the GEFS mean are slower with the progression of the storm system across the south-central U.S., and this was the case yesterday as well. 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, a small Slight Risk area was added for the Day 4 period across coastal portions of northern California between San Francisco and the Oregon border associated with the late week atmospheric river, but this is expected to be lighter than the event expected tomorrow. The overall QPF across the eastern U.S. for the end of the week is slightly lower compared to the previous forecast owing to a faster trend in the latest model guidance, except for the slower GFS. In terms of the WPC fronts/pressures, a nearly deterministic model blend along with some previous WPC continuity was used for Wednesday and Thursday, and then a slight increase of the ensemble means going into Friday and the weekend 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 U.S. should remain progressive but active, and trending more amplified, through at least next weekend. A very active east Pacific will continue to target an already very saturated California with more heavy rain and mountain snow potential as troughing builds offshore later this week with another shortwave moving inland next weekend. A shortwave initially over the Four Corners region on Wednesday will shift east, with guidance continuing to suggest a compact closed upper low likely over the Mississippi Valley by Thursday. Northern stream energy across the Great Lakes may combine with this system to amplify the trough over the East, eventually allowing for low pressure development off the Northeast Coast next weekend. The latest guidance continues to agree on the overall synoptic pattern, but there are a lot of uncertainties in the timing and details of especially the closed low/amplified trough in the East and any sort of possible nor'easter off the Northeast Coast the second half of the period. The latest 00z GFS (available after forecast generation time) is significantly faster with this feature through the Midwest, a favorable trend given the rest of the guidance, but does differ quite a bit from its previous run with the evolution over the Eastern U.S. next weekend. The rest of the guidance shows a lot of uncertainty in the interaction between this system and weaker northern stream energy late period resulting in a lot of model run to run variability. Given there are more questions than answers here in the East late period, it seems prudent to lean on the ensemble means, which at least present a reasonably blended and middle ground solution. Out West, there is again good large scale agreement off the West Coast, but timing differences exist as a couple of shortwaves cross into California next weekend. Enough agreement on days 3 and 4 allowed for a simple blend of the latest deterministic guidance for tonight's WPC forecast. After that, leaned more heavily on the ensemble means to help mitigate the differences, especially in the East. Despite the run to run variability in the models, this approach did help maintain good continuity with the previous shift. ...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. The next in the series looks to begin Wednesday, though should not be quite as strong or impactful as the one forecast during the short range period. Even still, there is potential for at least locally heavy rains mainly across northern California, where many areas are already very sensitive to additional rainfall. There may be a brief break late next week, but the next AR to impact the region should come in around next weekend and shift southward along the Coast bringing again more heavy rainfall, flooding, and mountain snow threats to the state. To the north, rain and mountain snows are also likely across the Pacific Northwest with additional shortwave energy. A shortwave through the Four Corners on Wednesday should spawn moderate central Plains cyclogenesis that tracks east-northeast through the Midwest into Friday. 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. Guidance shows potential for some snow and ice 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 and also as additional low pressure develops off the East Coast. Some in the form of snow mainly well inland and north, but low confidence on amounts and potential impacts. Although, with a lack of favorable upper level blocking features and an appreciably cold air mass in place, this event looks to be more of a rain than snow event for many in the Northeast. The warmest temperature anomalies will feature across the Southern Plains into the middle Mississippi Valley on 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 while rain and clouds keep most of the West Coast near normal. 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