Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 PM EST Wed Jan 31 2024 Valid 12Z Sat Feb 03 2024 - 12Z Wed Feb 07 2024 ...System emerging from the West to spread significant precipitation across the northern-central Rockies plus the Plains and Gulf Coast/Southeast through the weekend... ...Another strong weather system increasingly likely to impact California by early next week... ...Overview... Latest guidance continues to show an active weather pattern for the West for the first half of next week. Initial upper troughing near the Pacific Northwest should interact with an incoming southern stream system to produce a large scale positively tilted mean trough that will promote a strong southwesterly flow of moisture into the West over a multi-day time frame, with heaviest rain/mountain snow likely to focus over California. Meanwhile an upper low tracking from the southern half of the Rockies through the Southeast during the weekend and early next week will support a strong surface system and a heavy rainfall threat across the southern tier. Additionally, a strong upper ridge centered over the Upper Midwest/Great Lakes from the weekend into Monday should merge with another ridge building in across the Southwest to yield a southern Plains into Northeast ridge by next Wednesday. These ridges will support well above normal temperatures over much of the central U.S. The ridge that eventually reaches the Northeast by Wednesday will displace a Canadian Maritimes upper trough/low that will promote northwesterly flow aloft over the Northeast from the weekend into early next week. ...Guidance/Predictability Assessment... Model guidance continues to show reasonable agreement in the large scale pattern described above, while differences in the details persist. The southern tier upper low and associated surface lows showed typical levels of spread through early next week. Generally favored the 06Z GFS and 00Z ECMWF that were well clustered, while the 00Z CMC was displaced a little west/slow. By Tuesday-Wednesday the low track becomes more questionable as the deterministic ECMWF runs linger farther southwest in the western Atlantic compared to other guidance. The 00Z EC ensemble mean (and the similar placed CMC mean) may be a good middle ground between the slower deterministic EC and the faster GFS/GEFS solutions. Some of the typically higher performing EC-based AI models are somewhat similar to the EC mean as well. Farther west, the upper troughing and embedded upper low and surface lows are a concern across the eastern Pacific to West Coast. The 12Z CMC has finally aligned better with other guidance aloft after several runs of showing less phasing between the Pacific energy dropping south and Pacific energy tracking east early next week, displacing the centroid of the upper low to the southwest. However, the 12Z CMC surface low is farther west of consensus and is still slow to bring precipitation into California, so a GFS/ECMWF and ensemble mean blend is still preferred. By Tuesday-Wednesday, northern stream energy arising from the far northern Pacific/Alaska shows model spread and could impact positioning of the larger trough/low. The differences aloft lead to surface possibilities from a fairly progressive leading low after Sunday-Monday to persistence of this low off the West Coast (whether meandering in place or even dropping southward) through Wednesday. Any frontal waves offer additional uncertainty. Given these considerations, the WPC forecast used a deterministic blend of guidance favoring the 06Z GFS/00Z ECMWF early. Incorporated some of the GEFS and EC ensemble means by Day 5 and ramped up their percentages fairly quickly given the increasing spread by late period. This maintained reasonable continuity from the previous shift. ...Weather/Hazards Highlights... Upper trough energy over the West/Rockies to begin the weekend will produce a broad area of moderate to heavy snow over the northern and central Rockies on Saturday with lighter activity lingering into Sunday. Precipitation over the High Plains may fall as rain and/or snow depending on location, with relatively better potential for some meaningful snow over parts of the northern and central High Plains. The upper low breaking off from the southern part of the trough along with the associated surface system may lead to a period of easterly precipitation enhancement along the central Rockies as well. Rain north and thunderstorms farther south should already be in progress from the central Plains south-southeast to the Gulf Coast as of early Saturday, with eastward progression of the system over the weekend spreading some heavy rainfall across the Gulf Coast states and Southeast. The Day 4 Excessive Rainfall Outlook valid Saturday-Saturday night maintains a Slight Risk area along the central Gulf Coast where the best instability and ensemble probabilities of highest rainfall totals overlap, with operational model runs showing heavy rainfall potential as well, likely in the form of a squall line marching eastward with high rainfall rates. Model trends for the past couple of cycles have shown lingering convection back into Southeast Texas, necessitating some westward expansion of the Slight Risk. The surrounding Marginal Risk area extends north-northwest into southwestern Missouri and eastern Kansas, for somewhat lower rainfall totals but with ground conditions starting out on the damp side. We continue to monitor a possible secondary maximum of heavy rainfall around Texarkana, where a tongue of instability stretches and the front/surface low track could be slower moving for multiple hours of convection, which could produce flooding issues considering those wet antecedent conditions. Models continue to waffle with placement and overall the model rainfall amounts have trended somewhat down, so continued to hold off on a northern expansion of the Slight Risk. Heavy rainfall potential should continue eastward into the Southeast/Carolinas by Sunday-Monday. The Day 5 ERO continues to depict a broad Marginal Risk area over the Southeast, with current signals somewhat diffuse for where best rainfall focus will be, precluding introduction of any embedded Slight Risk. One region of potentially greater interest may be over parts of Georgia/South Carolina which will be just to the north of one or more fronts that could help to focus heavier rainfall, with at least the ECMWF showing some instability crossing this area as well. Farther south, expanded this Marginal Risk to include southern Florida as some guidance has a focus for convection there over some sensitive urban areas. Some heavy rainfall could linger into Monday over or near the Carolinas, while departure of the surface system may lead to a period of strong winds along/offshore the Southeast coast. While specifics are still not unanimous in the guidance, the majority of guidance suggests steadily increasing confidence in a multi-day atmospheric river event affecting California from around Sunday into the next workweek. A potentially strong system that may produce gusty winds should track to a position just offshore northern California by early Monday, bringing the leading surge of moisture. Slow movement of the supporting upper trough may produce a continued southwesterly flow of moisture into the region through at least Tuesday (with precipitation enhancement possibly generated by one or more trailing frontal waves) while the leading edge of the moisture begins to push into the Southwest/Great Basin. Following the guidance majority that brings in moisture earlier than CMC runs, the Day 5 ERO indicates a Slight Risk across much of coastal California. The Slight was expanded northward from the previous issuance, as model guidance generally has moderate to heavy rain arriving in northern California first for a longer duration of rain Sunday there, though perhaps with weaker rain rates than farther south. The Marginal Risk expands farther inland, but of course not to higher elevation areas of the Sierra Nevada where heavy snow is likely. Precipitation should persist especially for southern portions of California well into the workweek. This event still appears likely to produce greater low elevation rain/mountain snow totals than the preceding event due to a longer duration of southwesterly flow aloft and a more direct connection with lower latitude moisture. Continue to monitor forecasts for this time frame as specifics gradually become better resolved. The upper ridge initially over and near the Upper Midwest and then becoming aligned from from the southern Plains through Great Lakes will support a broad area of unseasonably warm temperatures over the central U.S. through the period. Continue to expect the greatest anomalies to be over/near the Upper Midwest where morning lows may be as warm as 25-35F above normal and exceeding daily records for multiple days, while daytime highs should be a little less extreme, up to 15-25F above normal. In contrast, the Southwest U.S./California should see cool daytime highs through the period with some localized areas up to 10-15F below normal. Clouds/precipitation over the West should keep morning lows near to above normal. Florida should see a couple cool days with highs 5-10F below normal by Tuesday-Wednesday after passage of the Southeast system. Tate/Rausch 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