Extended Forecast Discussion NWS Weather Prediction Center College Park MD 131 PM 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... The main features of interest in the guidance at the start of the Medium Range period (Friday Feb 16) are a couple shortwaves in an energetic northern stream flow pattern over the northern tier of the eastern/central U.S., a southern stream shortwave over Mexico shifting eastward over the Gulf, and a highly amplified ridge over the West with a closed high well into Canada. The guidance is in relatively good agreement with the overall placement and evolution of the main features through the middle of the period. One notable difference is a trend in the GFS for a slower and stronger northern stream shortwave dropping southward across the Midwest and towards the East Coast/South which would help enhance precipitation chances along the Gulf Coast and into Florida Friday/Saturday in combination with the southern stream system. This also adds some uncertainty to wintry precipitation chances from the Great Lakes to New England, with much more widespread snowfall if the GFS were right, but it remains the outlier for now. Thus, opted to begin the updated forecast blend with a lean towards the 00Z ECMWF/CMC over the 06Z GFS/00Z UKMET. Attention then turns to the West Coast for the latter part of the period Sunday into early next week as another Atmospheric River event is on the horizon. All the guidance is in agreement on an initial shortwave passing through the West Coast Saturday/Sunday, with the UKMET weakest with this feature. Subsequent height rises immediately follow in its the wake as an influx of warm, moist air from the Pacific proceeds a closed upper-low approaching the coast. One notable trend in both the more recent GFS and ECMWF guidance, particularly in the ECMWF, is for a slower and more northerly approach of the closed low. Earlier runs had this system onshore by Tuesday whereas it now lingers along and off the coast. The CMC remains a more distinct outlier with a more northerly track lifting towards British Columbia, though a subsequent shortwave helps to reinforce troughing off the West Coast that better aligns with the mean axis in the other guidance. Shifted the forecast blend to a mix of the 06Z GFS/00Z ECMWF along with an increase in the 00Z ECens and GEFS means while removing the CMC for the latter half of the forecast period. While the slower trend does bring the GFS/ECMWF closer to the CMC, it still remains more of an outlier and this keeps better continuity with the prior forecast given the trend overall has still resulted in a bit of a shift in the track/evolution of the system. The greatest contribution comes from the 00Z ECens mean with a mean trough axis that provides the best compromise between the GFS and ECMWF solutions. Despite some of the shifts in the guidance, the GFS/ECMWF and GEFS/ECens all still indicate a multi-day heavy rain/snow event for California with smaller-scale timing/location 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 northern Florida. 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 and inland into upslope areas of the southern Cascades/northern Sierra 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 and a high chance for heavy rainfall gradually shifting southward Sunday into Monday, and potentially continuing into Tuesday. 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. Putnam/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