Extended Forecast Discussion NWS Weather Prediction Center College Park MD 153 PM EST Sun Jan 21 2024 Valid 12Z Wed Jan 24 2024 - 12Z Sun Jan 28 2024 ...Multi-day Excessive Rainfall/Flooding Threat from Southeast Texas to the Lower Mississippi/Tennessee Valleys and Gulf Coast states this week... ...Overview... Guidance still overall agrees that the large scale pattern aloft this week will shift notably from the ongoing frigid pattern, as a ridge shifts from the central U.S. into the East Coast and troughing becomes established in the western/central U.S.. This will channel moisture return from the Gulf of Mexico, with warming temperatures in the eastern half of the country. A series of deepened Pacific systems will periodically focus lead flow/precipitation inland into the Pacific Northwest before digging into a mean western U.S. trough in unsettled/cooled flow. Meanwhile downstream, a significant rainfall event remains looming as multiple days of heavy rain and runoff threats are expected amid the stalling upper flow mean pattern, with primary focus from southeast Texas through the Lower Mississippi/Tennessee Valleys and Gulf Coast states. Favorable upper jet/frontal waves support will act to increase and focus moisture and instability return as ejecting impulses ride/lift on the western periphery of a slow to dislodge eastern U.S. ridge. Expect widespread activity to also spread to the Midwest/Great Lakes and the Southeast/East through later week, with snow/ice potential on the north side of the precipitation shield. ...Guidance/Predictability Assessment... Guidance continues to show very good agreement with respect to the overall synoptic pattern featuring central/western U.S. longwave trough digging into northern Mexico at the start of the period mid work-week (Wednesday Jan 24). There has been a trend in both some of the latest GFS/ECMWF runs for shortwave energy embedded within the longwave trough to lift northward over the Midwest Thursday (Jan 25) as the axis of the mean longwave trough begins to shift slightly more towards the central U.S. and the pattern becomes a bit less amplified. The 00Z CMC solution is a bit slower and more aggressive with this embedded energy, lingering into Friday (Jan 26). A trend noted in the prior forecast for upstream energy over the northeastern Pacific to translate quickly southward and dig into the Southwest to help reinforce the mean trough position back towards the western U.S. by Friday remains in the latest 00Z ECMWF/CMC runs, with the CMC on the more aggressive side, and the 00Z/06Z GFS remaining a bit less aggressive. The corresponding ensemble mean guidance follows a similar evolution favoring the trends of their deterministic counter-parts, with the 00Z ECens mean residing generally in the middle of the envelope of solutions. A notable trend difference compared to the prior forecast arises during the mid- to late period, as the more aggressive upstream energy digging into the Southwest in prior runs of the ECMWF and CMC continued to hold the position of the longwave trough to the west while the GFS showed a more progressive, faster evolution eastward into the central U.S. into the weekend (Saturday Jan 27). However, the last few runs of the ECMWF culminating in the most recent 00Z run have begun to progress the trough eastward faster. On the other hand, by the end of the forecast period Sunday (Jan 28), the most recent 06Z run of the GFS shows a dramatic shift towards a more enhanced positively tilted orientation of the trough with lower heights remaining further to the southwest of the 00Z ECMWF run, which continues an eastward progression of the trough as upstream ridge amplification over the western U.S. is stronger. The combined most recent GFS/ECMWF runs are thus in much better agreement with the overall phasing of the trough, even if their positions have shifted past each other. The GEFS mean continues to follow the prior trend seen in the GFS for the trough to progress downstream faster than any of the other solutions, while the ECens is generally similar to the ECMWF with respect to the position/phasing of the trough and western ridge. The CMC has become more of an outlier to the other solutions with embedded energy digging well southwest of the other guidance into northern Mexico. One final notable trend compared to the prior forecast is for stronger embedded shortwave energy lifting northeastward from the Plains through the Midwest at the end of the period in both the 06Z GFS and 00Z ECMWF, which looks to increase precipitation chances in the Plains, as well as a slower progression of a frontal system through the Southeast next weekend. The updated WPC medium range forecast begins with a composite blend of the deterministic guidance, with an edge towards the 06Z GFS and 00Z ECMWF and a contribution from the ECens means which falls the closest to the middle ground of the solutions. Contributions from the CMC and UKMET are reduced by mid-period due to the noted differences between the CMC and other deterministic/ensemble mean guidance with respect to the initial shortwave lifting through the Midwest and more aggressive energies digging southwest into northern Mexico late in the period. A greater reliance on the ECens mean towards the end of the period helps account for increasing differences with embedded shortwave energies, specifically with embedded energy in the western ridge approaching California in the GFS, an outlier compared to the other guidance. The GEFS mean is not included as the solution remains similar to prior runs of the deterministic GFS with respect to a more progressive downstream evolution of the central/western U.S. longwave trough late in the period. ...Weather/Hazards Highlights... The Pacific Northwest/northern California will see periods of moderate rains into mid-later week with multiple Pacific system approaches. Digging energies into a mean upper trough over the West in this period should allow for precipitation to work inland to the north-central Intermountain West/Rockies to include some enhanced terrain/mountain snows. Upper ridge building up/inland through the West Coast heading into the weekend should increasingly limit activity with a more northward to offshore focus. Downstream, increasing moisture ahead of the western U.S. upper troughing will increasingly spread rain into the south-central U.S. early week. Heaviest rain totals remain forecast to slowly spread through Texas to the Lower Mississippi/Tennessee Valleys and Gulf Coast states and Southeast into mid/later week, and these will fuel growing flooding concerns given significant repeat/training potential and a signal in the guidance for locally very heavy daily totals of 3-4"+. WPC Day 4/Wednesday and Day 5/Thursday Excessive Rainfall Outlooks have Slight Risk and surrounding Marginal Risk areas from Southeast Texas to the Lower Mississippi/Tennessee Valleys, Gulf Coast states, and southern Appalachians given slow pattern translation with favorable upper support, deep moisture return, and repeat activity signature. There remains quite a bit of spread in the exact axis of heavier QPF among both the deterministic and ensemble guidance, particularly on Day 4/Wednesday, with the GFS/GEFS mean favoring areas further north and the ECMWF/ECens mean favoring areas along the Gulf Coast. However, as confidence increases in both the axis of heaviest rainfall as well as potential overlap with rainfall on Day 3/Tuesday leading to wetter antecedent conditions, a higher risk threshold may be needed. Precipitation will also start spreading more earnestly across the Midwest/Great Lakes and the East into mid-later week after a dry period, and given much warmer temperatures and snowmelt, areal and stream flooding may become a concern. There is also quite a bit of uncertainty with later week individual lifting frontal waves within the larger scale flow and the extent and focus of potential coastal waves next weekend to monitor. Meanwhile, there remains some probabilities for freezing rain across parts of the Midwest/Great Lakes into mid-later week. Wintry precip should transition to rain as warm air spreads farther north. Lingering cooled air from the Great Lakes/Appalachians to the northern Northeast seem most likely to lead to wintry weather later week and into the next weekend but again, still uncertain northern progression of warmer air may limit the duration/impact. Ridging over the east with troughing over the western/central U.S. will continue to bring above average temperatures to much of the eastern U.S. through next weekend, with the greatest warm anomalies of 15-25 degrees centered over portions of the Ohio Valley/Mid-Atlantic/Carolinas later this week. Increasing ridge amplification over the western U.S, and stretching into the north-central U.S., will bring a warming trend with high temperatures rising above average over the region next weekend. Putnam/Schichtel 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