Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 AM EST Tue Feb 13 2024 Valid 12Z Fri Feb 16 2024 - 12Z Tue Feb 20 2024 ***Major atmospheric river event possible for portions of California Sunday into Monday with heavy rain and Sierra snow*** ...General Overview... Broad cyclonic flow aloft is expected across the north-central U.S. for the end of the week, and mainly zonal flow across much of the country east of the Rockies with a ridge axis over the Intermountain West. The trough over the Great Lakes amplifies further going into the weekend, and a southern stream upper trough is progged to enter the Gulf region by Saturday and induce a relatively weak surface low that should track across Florida and then offshore. Meanwhile, a big surface high from Canada will be settling southward across the Plains and then eventually the Deep South region by late Sunday with colder temperatures accompanying it. The West Coast will likely become quite unsettled going into the weekend and next Monday with increasing chances of a major atmospheric river event for portions of central and southern California. ...Guidance/Predictability Assessment... Overall, the 00Z model guidance suite is in better agreement across the continental U.S. compared to the same time frame yesterday, especially for the West Coast. There are some modest timing differences with the arrival of the upper trough across the Great Lakes, with the CMC faster and the GFS slower and more amplified. The CMC remains a weaker solution with the closed upper low near California early next week and centered well to the north of well clustered ECMWF and GFS guidance, but it has trended more in line with the model consensus with the Gulf Coast low. The fronts/pressures were primarily derived from a GFS/ECMWF/UKMET/some CMC Friday-Saturday, and then dropping the CMC after Saturday and gradually increasing the ensemble means, which accounted for about 40-50% of the forecast blend going into early next week due to the greater mesoscale differences. ...Weather/Hazards Highlights... The low pressure system developing over the Gulf Friday night has gradually trended south over the past few days of model runs, with consequential rainfall mainly confined to the Texas coastal region and the northern Florida Panhandle. Although rainfall totals are generally expected to be under two inches for both of these areas, there may be just enough higher rainfall rates to warrant a Marginal Risk area for southern Texas on Friday, and potential episodes of training convection across northern Florida on Saturday to justify a Marginal Risk there. For the West Coast region, most of the rainfall will likely remain offshore until the 6-12Z time period Saturday morning, but there are some model differences in arrival of the heavier rainfall, so the Marginal Risk area is maintained for the Day 4 period Friday-Friday night for coastal portions of northwestern California. A more substantial axis of heavier rainfall is becoming more likely across this region going into Saturday and Saturday night with some rainfall totals approaching two inches, and therefore greater confidence in having a Marginal Risk for the Day 5 period. Looking ahead to Sunday and Monday, a major atmospheric river event is becoming more evident in the latest model guidance as a slow moving Pacific low approaches California, with anomalous moisture flux directed towards the coastal terrain. Right now, it appears the highest rainfall totals late in the forecast period would be most favored for the Transverse Ranges where several inches of rainfall is a good possibility, so this will continue to be monitored closely. Heavy snow is also likely for the Sierra and northern California mountains during this time period. The surface high settling south from western Canada will likely be accompanied with temperatures running about 10-20 degrees below mid-February averages across much of the north-central U.S. on Friday behind the cold front. However, it appears that this airmass will not be as cold as the arctic air intrusion back in mid-January. This airmass then settles across the south-central U.S. and into the Ohio Valley going into Saturday with highs generally 5-15 degrees below average, but a warmer trend across the Northern Plains and Midwest as the airmass modifies by Sunday and into early next week. In fact, most of the nation should be be warmer than average by next Tuesday east of the Rockies, perhaps up to 20 degrees above across portions of the central and southern Plains, and slightly below average for the West Coast. 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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw