Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 PM EST Wed Feb 14 2024 Valid 12Z Sat Feb 17 2024 - 12Z Wed Feb 21 2024 ***Multiple atmospheric river events possible for portions of California with heavy rain and Sierra snow*** ...General Overview... Upper troughing to the east of the Rockies to start the weekend should steadily retreat northward into Canada while an incoming Pacific shortwave passes through an Intermountain West mean ridge and continues eastward across the lower 48. Meanwhile a southern stream shortwave progressing from Mexico into the Gulf will support one or more weak frontal waves that should track across or just south of Florida and then offshore. The combination of upper troughing east of the Rockies and broad surface high pressure originally from Canada settling over the southern tier this weekend will bring chilly conditions, with a warmer trend next week. The West Coast will likely become quite unsettled going into the weekend and especially into early next week with increasing chances of multiple atmospheric river events for portions of central and southern California. Some areas of lighter precipitation may spread across other parts of the West as well. ...Guidance/Predictability Assessment... Across the eastern Pacific/western U.S., guidance agrees reasonably well for the leading system affecting the region but some differences arise for the stronger trailing Pacific system. The most prominent difference involves latest GFS runs straying slower with the upper trough and farther southwest than consensus with the embedded upper low by Tuesday-Wednesday. This represents a flip in the GFS/ECMWF comparison versus some earlier runs, but the ensemble means strongly support faster progression than the GFS by the middle of next week and in fact the 00Z ECMWF-initialized machine learning models suggest timing could even be a little faster than the means. Thus preferences lean more toward the ECMWF and ensemble means when timing spread develops. Some CMC runs have been on the slow side as well but with a questionable evolution over the northeastern Pacific/northwestern North America. Also worth noting, the ML models suggest that the upper low and associated surface system could ultimately track farther north than even the ensemble means and most 00Z/06Z models. The new 12Z ECMWF has in fact adjusted that way, bringing the weakening upper/surface low to the Pacific Northwest instead of California, while also accelerating the upper trough. Elsewhere, there is decent agreement in principle for an Upper Great Lakes/New England wave around Sunday-Monday but continued detail differences. The 06Z GFS deviated the most from other guidance by early Monday with a slow/deep solution. Behind this system the guidance generally shows a mean frontal boundary draped across the northern tier but with poor agreement/continuity for specifics given sensitivity of surface waves to low-predictability shortwave details. A model composite looks reasonable for the surface wave(s) crossing the Gulf of Mexico/Florida during the weekend. Based on the above considerations, the updated forecast incorporated the 00Z ECMWF/00Z-06Z GFS plus smaller weights of the 00Z UKMET/CMC early-mid period and then quickly transitioned to about half 06Z GEFS/00Z ECens means with remaining ECMWF/GFS input by day 7 next Wednesday. ...Weather/Hazards Highlights... Guidance continues to show one or more frontal waves developing over the Gulf by Saturday morning and then tracking across or just south of the southern Florida Peninsula during the rest of the weekend. The most consequential rainfall should be mainly confined to northern Florida on Saturday and then most likely South Florida (but with considerable spread for northward extent) on Sunday. Although rainfall totals are generally expected to be on the order of 1 to 2.5 inches, there may be potential episodes of training convection across northern Florida on Saturday to justify a Marginal Risk there in the Day 4 Excessive Rainfall Outlook. Some slower moving cells may develop across the metro areas of Palm Beach, Broward, and Miami-Dade Counties, and this may locally produce enough heavy rainfall on Sunday to warrant a Marginal Risk area for Day 5 outlook. The aforementioned spread for Sunday's rainfall coverage tempers confidence in the details but there is better model/ensemble agreement that the greatest moisture anomalies will align over southern Florida, supporting the existing Marginal Risk area. A more substantial axis of heavier rainfall is becoming more likely across the northern half of California going into Saturday and extending inland across upslope areas of the northern Sierra, with some rainfall totals potentially reaching at least two inches. The axis of highest PW anomalies will likely be oriented towards the higher terrain of the coastal mountain ranges between San Francisco and Eureka, where a Slight Risk of excessive rainfall is depicted for the Day 4 period. There are still detail differences that require monitoring, but for now there is enough support to maintain the existing Slight Risk and surrounding Marginal Risk area until more specific trends become evident. Looking ahead to Sunday, expect another atmospheric river event to arrive ahead of a slow moving Pacific low approaching California, with anomalous moisture flux directed towards the coastal terrain of central California. While the best moisture anomalies cluster over south-central California by the end of the Day 5 period early Monday, model/ensemble signals for heaviest rainfall are less defined. This favors keeping the existing Marginal Risk area while waiting for a more coherent QPF focus in the guidance. Beyond early Monday, highest rainfall totals over California should be during the first half of the week and over the Transverse Ranges where several inches of rainfall will be possible, so this will event continue to be monitored closely. Heavy snow is also likely for the Sierra and northern California mountains during this time period. The two separate systems affecting California should spread some moisture with lighter precipitation totals across portions of the Intermountain West and Rockies. The upper/eastern Great Lakes into New England may see some areas of lake effect/terrain enhanced snowfall during the weekend in association with upper troughing and a surface system crossing the region around Sunday-Sunday night. Then a front and embedded waves may produce some areas of light rain/snow over the Great Lakes/Northeast and vicinity by Tuesday or Wednesday, but with low confidence in specifics at this time. The surface high settling over the southern tier states this weekend behind a cold front will likely promote temperatures running about 5-15 degrees below mid-February averages from Texas to the Ohio Valley on Saturday with some below normal readings persisting over parts of the southern tier (especially near the Gulf Coast/Florida) into Sunday or Monday. The Plains and Midwest will trend warmer from Sunday onward as mean flow aloft become more westerly, with some locations over the central Plains/Midwest seeing the best potential for highs reaching 20 degrees or more above normal by next Tuesday-Wednesday. Remaining areas east of the Rockies should trend above average as well. Clouds and precipitation will favor above normal lows and near to slightly below normal highs across the West. Rausch/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