Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Fri Mar 19 2021 Valid 12Z Mon Mar 22 2021 - 12Z Fri Mar 26 2021 ...Overview... Guidance continues to show a broad mean trough aloft settling over the lower 48, between a strong eastern Pacific ridge and western Atlantic mean ridge that will become anchored by a strengthening upper high over Cuba and the Bahamas by mid-late week. Ahead of the mean trough expect an active weather pattern over the central and eastern U.S. with multiple areas of heavy rainfall potential over the course of next week. Pacific features digging into the western side of the mean trough will bring periods of rain and mountain snow to locations from the Pacific Northwest into a majority of the Rockies. Finally, the upper trough with possible embedded low forecast to be over the East Coast to start the week should gradually progress to the east/northeast, pushing along associated western Atlantic low pressure. Models and ensembles continue to agree on the large scale pattern while differing on some meaningful timing and detail issues. ...Model Guidance/Predictability Assessment... The system initially off the East Coast continues to provide a challenge for the guidance and predictability is still tempered by the fact that the upper support essentially becomes embedded within the larger scale western Atlantic mean ridge. GFS runs are persistent in being more offshore than other guidance due to extending the upper trough farther eastward, while remaining solutions still vary considerably. At least the new 12Z UKMET has adjusted southeastward toward the general consensus in contrast to its previous two northward runs that greater influence on New England. Preference remains close to continuity as yielded by a multi-model compromise. The next system of note is a vigorous shortwave ejecting out of the Rockies on Monday and reaching into the Upper Midwest by midweek, supporting surface low pressure that tracks northeast from the southern High Plains. Trends over the past day or so have been toward a more concentrated evolution by day 4 Tuesday and then toward the middle of recent spread by day 5 Wednesday (by way of the deep GFS trending a little weaker and earlier ECMWF runs becoming stronger). There is still a fair amount of timing spread with the GFS/GEFS consistently on the fast side and the UKMET lagging. Guidance seems to be in a gradual process of trying to converge for the next system digging through the West. The most common theme is that the energy will form a closed low just south of the Four Corners by early Wednesday and then open up as it ejects to the east and northeast, encouraging a surface wave to lift northeastward from the western Gulf Coast late in the week. Among solutions through the 06Z cycle, the primary cluster appeared to center over the 00Z ECMWF mean, an average of the past two ECMWF runs (00Z faster, 12Z/18 run slower), and 00Z CMC/06Z parallel GFS. Operational GFS runs have been faster with somewhat more stream separation than indicated by consensus. New 12Z GFS/CMC runs are now close to each other by 12Z next Friday and a bit faster than the aforementioned majority/preferred cluster. The new 12Z ECMWF nearly duplicates its 00Z run. Upstream Pacific energy should dig into the West during the latter half of next week, with a general model/mean blend providing a reasonable starting point and good continuity. Stream separation that occurs over the North Pacific early in the period provides some lingering uncertainty over specifics (especially in the 12Z UKMET and to a lesser degree in the CMC). Guidance preferences led to an early-period blend that leaned mostly away from the GFS for the western Atlantic and Plains/Upper Midwest systems, followed by a transition to an average among the past two ECMWF runs, 00Z CMC/06Z GFSP, and more 00Z ECMWF mean than 06Z GEFS mean. ...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 Monday-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. The upper dynamics ejecting from the Rockies will support a central Plains through Upper Midwest system early-mid week. This system may produce a focused area of enhanced snowfall over or near the Colorado Rockies (the majority likely to be just before the medium range period early Monday). Lighter snow is possible on the northwest fringe of the precipitation shield from the central High Plains northeastward on the cold side of the surface low track. Farther south, improving Gulf inflow should promote areas of heavy rainfall from the central Plains/Midwest into the Lower Mississippi Valley and Gulf Coast with lighter rainfall extending northeastward. Recent guidance has been highlighting the potential for highest rainfall totals near the central/east-central Gulf Coast around midweek as the front trailing from the Midwest system decelerates/stalls while awaiting the next western U.S. shortwave. This latter ejecting western energy should generate a late week system that lifts northeastward over the east-central U.S., spreading areas of enhanced rainfall over a greater portion of the East. Latest trends for the western Atlantic system have lowered the rainfall amounts for some locations along the East Coast from Monday onward. Some areas may still see a period of brisk winds as supported by the gradient between the system and high pressure to the north. 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. High temperature anomalies of plus 5-10F over Florida later in the week are less impressive in absolute terms but readings could be warm enough to approach daily record values for some sites with relatively shorter periods of record. 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, 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