Extended Forecast Discussion NWS Weather Prediction Center College Park MD 307 AM EDT Mon Mar 11 2024 Valid 12Z Thu Mar 14 2024 - 12Z Mon Mar 18 2024 ...Heavy snows possible across parts of the central-southern Rockies on Thursday into Friday... ...Heavy rainfall potential returns to the South late this week... ...Overview... The general flow over the CONUS will trend much more amplified during the medium range period as troughing over the West digs southward and closes off a deep upper low over the Southwest which meanders in place for much of the period. This would promote a possibly significant winter storm across the Colorado Rockies on Thursday, and a multi-day period of heavy rainfall potential across parts of the Southern U.S. late this week and into the weekend. Elsewhere, northern stream energy will amplify across the Upper Midwest/Great Lakes next weekend, as a strong ridge shifts over the West Coast. ...Guidance/Predictability Assessment... There remains good agreement on the overall large scale very amplified pattern in the latest guidance/ensembles, but still some considerable differences in the details with implications on sensible weather impacts, especially concerning QPF amounts and coverage across the South. There is lots of uncertainty regarding multiple streams of energy through the Upper Midwest/Great Lakes Friday-Monday. For Friday-Saturday, the ECMWF, GFS, and ensembles seem reasonably clustered, but the CMC was notably faster/different with one shortwave as it dropped into the Upper Midwest. Recent runs of the ECMWF (including tonight's new 00z run) suggests a more digging shortwave next Sunday-Monday which may be enough to pick up the Southwest low and begin to bring that energy eastward by early Monday. At this point, there is better support (from the ensembles and ECMWF-based AI models) for more separation of the two streams allowing the upper low to linger in place over the Southwest/far northern Mexico through at least Monday. However, some prior runs of the GFS have shown a similar evolution to the current ECMWF so there is still a great deal of uncertainty. Given the better support for some lingering energy/low to hold back, the WPC progs for days 6 and 7 trended away from the ECMWF late period and more towards the ensemble means with the GFS. The early periods used a blend of the ECMWF, CMC, and GFS (before the above noted issues with the CMC). The UKMET was not used in the blend tonight as it was much stronger with initial energy ejecting into the Plains on Thursday. Overall, good continuity was maintained with the previous WPC forecast. ...Weather/Hazards Highlights... A long-duration winter storm will be ongoing across parts of the central-southern Rockies as the period begins Thursday, with possibly significant snowfall in the highest terrain of Colorado, and more uncertain potential for notable snows across the Front Range urban corridor as well. Snows should shift southward into New Mexico by Friday. Meanwhile, a front draped across the Midwest will support some increasing precipitation on Thursday and although there will be ample moisture and instability present near the front for efficient rainfall to develop, dry antecedent conditions should keep the overall excessive rainfall risk to sub-marginal levels. To the South, moisture streaming ahead of the stagnant low over the Southwest will support a likely multi-day heavy rainfall threat across the South. There remains plenty of uncertainty in the exact placement of the highest QPF, but there is support in the dynamics for fairly broad Slight Risk areas on the Days 4 and 5 Excessive Rainfall outlooks. For Day 4, the slight extends from northeast Texas/southeast Oklahoma and covering much of Arkansas, with a shift to the south and east on Day 5 more into Louisiana and the central Gulf Coast states. Severe weather is also a threat with this convective activity per the Storm Prediction Center with a severe weather area delineated for parts of the southern Plains on Thursday. Some heavy rainfall potential may extend northward along the boundary into the southern/central Appalachians and the Mid-Atlantic and this potential is covered with a Marginal Risk on Day 5/Friday. Heavy rainfall should continue into parts of the Southeast this weekend as well. Mean ridging aloft over the eastern U.S. will support an expansive area of above normal temperatures to the east of the Rockies. The core of warmest anomalies should cover from the Midwest into the Northeast, with highs potentially reaching 15-20+ degrees above normal. Temperatures across the East will trend cooler with time after Thursday, but still generally above normal through the weekend. Over the West, moderately below average highs and near/above average lows will shift into the Great Basin/Four Corners Thursday and beyond. Temperatures along the immediate West Coast should increase by Friday and through the weekend as strong ridging builds in aloft. Below average temperatures will also spread from the north-central U.S. into the Mississippi Valley and eventually the Midwest this weekend/early next week as northern stream troughing moves through the region. Santorelli 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