Extended Forecast Discussion NWS Weather Prediction Center College Park MD 222 AM EDT Sat Mar 20 2021 Valid 12Z Tue Mar 23 2021 - 12Z Sat Mar 27 2021 ...Overview... The guidance continues to show a broad mean trough aloft settling over the Central U.S. through much of the medium range period with several embedded quick moving shortwaves. Ahead of this mean trough, an active weather pattern is expected with heavy rainfall potential from the Gulf Coast into the East. Shortwave energy digging into the western side of the trough should also bring periods of rain and mountain snow across parts of the West. Models and ensembles continue to agree on the overall large scale pattern, but differ on some meaningful timing and detail issues. ...Model Guidance/Predictability Assessment... A vigorous shortwave ejecting out into the Upper Midwest on Tuesday, will support surface low pressure that tracks northeast from the Central Plains. Models have come into better agreement regarding strength and timing of this system so a multi-model blend should suffice. Meanwhile, another shortwave will dig through the West with a possible closed low over the Four Corners region early Wednesday, before opening back up as it lifts to the northeast thereafter. The GFS is a notable outlier, now lagging well behind the majority of the guidance. A blend away from the GFS is preferred. This does result in a faster progression of the cold front across the Eastern U.S. compared to previous continuity, but this is supported by recent trends in both the ECMWF and the CMC (as well as ensemble means). Finally, amplifying shortwave energy into the Southwest U.S. late week continues to offer some degree of uncertainty. Interestingly enough, the GFS is quite a bit faster/weaker than the ECMWF/CMC and means so again, a blend away from the GFS is a good starting point. Overall, guidance preferences led to an early-period blend leaning mostly on the deterministic guidance (away from the GFS). After day 5, increasing weighting of the ensemble means was used to mitigate the late period timing and detail differences, while still including some deterministic ECMWF/CMC just for better feature definition. ...Weather/Threats Highlights... The developing mean trough aloft with periodic reinforcement will bring below normal temperatures over much of the West into the Rockies during most of next week. Expect coolest temperatures to be Tuesday-Wednesday with highs 10-15F below normal over parts of the Great Basin and central-southern Rockies. Many areas will see a moderating trend after midweek but another cold front may reinforce minus 5-10F anomalies for highs from the Northwest into central Rockies. Incoming shortwaves/leading surface fronts will bring periods of rain and mountain snow to the Pacific Northwest and extending eastward/southeastward through the West into the Rockies as the shortwaves amplify. Most of this activity should be of light to moderate intensity but some localized heavier areas will be possible, especially across parts of the Colorado Rockies Tuesday into Wednesday. The upper dynamics ejecting from the Rockies will support an organized system lifting through the Upper Midwest early to mid-week dragging a cold front eastward. North and west of the surface low, some light snowfall could fall in the sufficiently colder airmass. To the south, improving Gulf of Mexico inflow should promote areas of heavy rainfall from the Lower Mississippi Valley/Gulf Coast with lighter rainfall extending northeastward. Recent guidance continues to highlight the potential for highest rainfall totals near the central/east-central Gulf Coast around midweek as the front trailing from the Upper Midwest system decelerates/stalls while awaiting the next western U.S. shortwave. This latter ejecting western energy should generate another late week system that lifts northeastward over the east-central U.S., spreading areas of enhanced rainfall into a greater portion of the East. Warmest temperature anomalies will progress from the Midwest/western Great Lakes into the Northeast over the course of next week. These areas should see one or more days with highs of 10-20F above normal. Morning lows will be somewhat more above normal with greater southward extent of plus 10F and greater anomalies over the eastern half of the country, aided by the moist flow from the Gulf. Santorelli/Rausch 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, winter weather outlook probabilities and heat indices 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml