Extended Forecast Discussion NWS Weather Prediction Center College Park MD 217 AM 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. Meanwhile 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 reach 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... Preferences based on 12Z/18Z guidance led to a starting blend early in the period consisting of an operational model composite (somewhat more GFS/ECMWF than UKMET/CMC), followed by a trend toward half 18Z GEFS/12Z ECens means and more lingering 12Z ECMWF weight than GFS/CMC. Primary large scale considerations were that GFS handling of energy dropping into the West Coast upper trough late in the period became increasingly unsupported by other guidance, while the 12Z CMC strayed north with the Southeast system once reaching the western Atlantic. In the new 00Z runs, the CMC shows less phasing aloft near the West Coast than most other solutions--keeping the surface system farther offshore than model/ensemble consensus--and now the 00Z GFS strays northward with the western Atlantic system late. The specifics for evolution within the West Coast upper trough and resulting surface reflection look particularly uncertain by the latter half of the period. 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. ...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 much of the northern-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 central/north-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. The only adjustment from continuity was to expand the western end of the Slight Risk area per latest guidance trends. 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. There is some increasing concern for a secondary maximum of heavy rainfall around Arkansas, into which a tongue of instability stretches and the front/surface low track should be slower moving for many hours of convection, which could produce flooding issues considering those wet antecedent conditions. This signal persists but not agreeably enough yet to expand the Slight Risk into this region. Thus will continue to monitor going forward to see if this expansion may ultimately be needed. Heavy rainfall potential should continue eastward into the Southeast/Carolinas by Sunday-Monday. The new Day 5 ERO depicts a broad Marginal Risk area over the Southeast, with current signals somewhat diffuse for where best rainfall focus will be. 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. Some heavy rainfall could lingering 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, would bring 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 the 00Z CMC, the new Day 5 ERO plans to introduce a Slight Risk area along coastal California from near Monterey southward to/near the Transverse Ranges. The surrounding Marginal Risk extends into parts of northern California. 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. Rausch/Tate 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