Extended Forecast Discussion NWS Weather Prediction Center College Park MD 206 PM EST Thu Feb 01 2024 Valid 12Z Sun Feb 04 2024 - 12Z Thu Feb 08 2024 ...Southern tier system likely to spread heavy rain across parts of the Southeast through at least Sunday... ...Another strong weather system increasingly likely to impact California with heavy rain by early next week and eventually spread across more of the West... ...Overview... With some persistent detail differences, guidance continues to show an active weather pattern for the West at least through midweek. Expect a large scale positively tilted mean trough to 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. With time some of this moisture should extend into parts of the Intermountain West and Rockies as the upper trough drifts eastward and broadens. Meanwhile an upper low tracking from the southern Plains through the Southeast during Sunday-Monday 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 Great Lakes ridge by midweek. These ridges will support well above normal temperatures over much of the central U.S. However the flow aloft over the Northeast has become more uncertain in latest model runs, with a wide potential spread between the upper ridge moving in from the west or reinforcement of upper troughing over or just east of the region. ...Guidance/Predictability Assessment... The latest runs of guidance maintain a high confidence for an active wet period for the West Coast/parts of the Southwest with a gradual eastward drift with time and eventual broadening later in the period. Multiple cycles have struggled with resolving individual upper features (initial energy that may consolidate around Sunday-Monday plus one or more northern stream features that may drop into the upper trough) that have lead to variability of where precipitation will occur and its magnitude. The CMC has been persistently lagging among the solutions while the UKMET continues to be the furthest east/fastest solution. Although there is some timing and QPF axis differences, the best picture for the medium range evolution remains to be a combo for the ECWMF and GFS. Guidance for the system that tracks across the Southeast has average confidence, with the UKMET consistently on the southern side of the spread. Over the past day a new complication has developed regarding the ultimate evolution/track of this system over the western Atlantic, as the GFS/CMC now are much more aggressive to dig trough energy south through New England and beyond by Tuesday onward. Those solutions result in a farther north track and greater wind impacts (if not even some precipitation) along the northern East Coast. Teleconnections relative to positive height anomalies forecast over southern Canada, as well as the 12Z ECMWF-initialized ML models, do not favor upper troughing as close to the East Coast as what the GFS/GEFS suggest--but also upper ridging perhaps not reaching quite as close to New England as in the ECMWF/ECMWF mean. Thus a compromise looks reasonable at this time. The WPC blend followed continuity by starting with the 00Z/06Z GFS/00Z ECWMF and then including/increasing the EC ensemble means/GEFS means for the middle and later periods. ...Weather/Hazards Highlights... There is a strong signal for another plume of high moisture to move onshore over a multi-day period supporting heavy rain to central/southern stretch of the Golden Coast and points inland along with moderate to heavy snow in the areas of higher terrain. Soils across this part of the country will already be pre-conditioned thanks to an atmospheric river. This second AR event is depicted to yield daily accumulations of 3 to 7+ inches Sunday and Monday across central and southern portions of California. The 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. Heavy snow favoring southward-facing terrain should also spread into the Rockies during the early-mid week time frame. Given the increasingly anomalously wet period and dynamic support, Moderate Risks were raised for the Transverse Ranges and adjacent locations. The Slight and Marginal Risk areas that were already in effect were expanded further east/southeast across southern California, 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 through at least midweek but with gradually decreasing totals. 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. By next Wednesday-Thursday, leading height falls aloft and associated low pressure may develop precipitation over the northern Plains. A fairly deep surface low pressure system will track across the Southern Plains and Southeast Sunday through Monday ushering in a swath of heavy rainfall across the Gulf states and Southeast. A a broad Marginal Risk for excessive rainfall was maintained for the Day 4 period. One region of potentially greater interest continues to 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. Guidance expectations for instability over this area do not appear particularly impressive though. Some heavy rainfall could linger into Monday over or near the coast of the Carolinas. Current guidance is not showing a coherent enough of a signal to recommend a Marginal Risk over this localized area in the new Day 5 ERO but will continue to monitor for improved clustering in upcoming model runs. Once the surface system moves offshore, expect a period of strong winds along/offshore the Southeast coast and possibly farther north as well. The upper ridge initially over and near the Upper Midwest and then becoming aligned 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 exceed daily records for multiple days, while daytime highs should be a little less extreme, up to 15-25F or so above normal. In contrast, the Southwest U.S./California should see cool daytime highs through the period with a few locations up to 10-15F below normal on one or more days. 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. Temperatures over the Northeast from Tuesday onward have greater uncertainty than most other areas. The most likely scenario would yield near to above normal temperatures as upper ridging approaches, but there is lesser potential for a nearby upper trough/low to keep temperatures colder. Campbell/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