Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM 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 be settling into a much more amplified regime during the medium range period. Already at the start of the forecast on Thursday, a sharp/amplified upper ridge off the West Coast will support a western trough and embedded deep Southwest upper low that will likely meander in place for much of the period. This feature would promote a possibly significant winter storm across the Colorado Rockies on Thursday with some snow persisting over the southern Rockies into Friday, along with a multi-day period of heavy rainfall potential across parts of the Southern U.S. late this week and into the weekend. A leading Plains through Northeast system may also produce some enhanced rainfall near its track late this week. The East Pacific into western Canada upper ridge will help to amplify downstream troughing into the eastern half to two-thirds of the lower 48 by this coming weekend into early next week, with a colder trend behind a northern tier to East Coast front. ...Guidance/Predictability Assessment... Within the fairly agreeable large scale forecast, the two greatest uncertainties involve the wave tracking from the central Plains/Midwest through the Northeast late this week and whether the upper low over the Southwest ultimately begins to phase with the amplifying upper trough to the east of the Rockies by late weekend/early next week. For the wave late this week, the consensus of 00Z/06Z guidance including the ECMWF-initialized machine learning (ML) models suggested that the GFS from those cycles was too fast while the 00Z ECMWF could be a tad slow (mainly on Thursday). The 00Z UKMET/CMC were closest to the majority scenario. The main priority for this part of the forecast was to downplay 00Z/06Z GFS influence. A slower trend in the 12Z GFS has improved model agreement in the new cycle. Latest clustering suggests this could become a fairly vigorous (but progressive) wave off the New England coast by early Friday. The 00Z ML models still showed quite a bit of spread for strength and latitude of the surface wave at that time though. This overall feature seems to have below average predictability, given a combination of upper dynamic support streaming from the Southwest upper low and approaching northern stream energy. For the Southwest upper low, the 00Z ML models are remarkably similar in keeping the feature separate from troughing to the northeast into next Monday, in a somewhat better defined fashion than the GEFS/ECens/CMCens means that also keep the two separate. Dynamical models are split, with latest CMC runs and 00Z GFS separate but 06Z/12Z GFS and latest ECMWF runs trending toward more phasing (albeit with the 12Z ECMWF delaying this just a tad). Current preference leaned toward the more separate solution favored by the means and corresponding operational model runs. The updated forecast started with a composite of the 00Z/06Z operational models during the first half of the period, with somewhat less than typical GFS weight due to aforementioned considerations for the northern stream wave. Then the ensemble mean and 00Z GFS/CMC preference for the overall pattern favored a rapid transition to emphasizing the means and those model runs late in the period. This solution kept GFS weight low enough not to place too much weight on that model's possible over-dampening of the western Canada upper ridge next Monday. ...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 while possibly lingering over southern Colorado as well. Farther east, a central Plains/Midwest through Northeast wave and frontal system will support some enhanced precipitation near its path during Thursday-Friday. On Day 4/Thursday, latest trends toward the middle/slower part of the envelope for this wave plus favorable moisture/instability in the warm sector, a subset of operational models with locally significant QPF, and shorter-term rainfall possibly tempering dry antecedent conditions, seem to favor expanding the Marginal Risk area as far north as Illinois and vicinity. Then by Day 5/Friday there is increased clustering toward an axis of potentially significant rainfall extending into the Northeast (possibly with some instability), with the northern part of the overall Marginal Risk area incorporating this majority scenario. Some moisture reaching far northern New England could produce a little snow. Meanwhile the moisture streaming northward ahead of the stagnant upper low over the Southwest and interacting with the front trailing from the aforementioned northern wave will support what should be a 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 Risk extends from northeast Texas/southeast Oklahoma through much of Arkansas and into nearby parts of Louisiana/Mississippi. The Slight Risk area shifts to the south and east on Day 5 more into far eastern Texas through 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. Heavy rainfall may continue into parts of the Southeast this weekend as well. The system tracking through southeastern Canada Saturday-Monday and trailing front may produce areas of rain and perhaps cold sector snow over parts of the Great Lakes/Northeast. 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 extend from the Midwest into the Northeast late this week, with highs potentially reaching 15-20+ degrees above normal. Temperatures across the East will trend cooler with time after Thursday, but should generally remain 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. 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. Temperatures along the immediate West Coast should increase by Friday and through the weekend as strong ridging builds in aloft, with the Northwest seeing the best potential for some highs reaching 10-15 degrees above normal. Rausch/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