Extended Forecast Discussion NWS Weather Prediction Center College Park MD 344 PM EDT Thu Apr 28 2022 Valid 12Z Sun May 01 2022 - 12Z Thu May 05 2022 ...Overview... Latest guidance for the Sunday-Thursday period maintains the theme of an active pattern with a series of Pacific systems (each supported by an upper trough that may contain a closed low for a period of time) entering and progressing across the lower 48. These systems will spread rain and high elevation snow across the Pacific Northwest and northern half of the Rockies, and then mostly rain over a large part of the central/eastern U.S. Some areas of precipitation may be heavy. This pattern will likely leave the southern half or third of the West, the northern Plains near the Canadian border, and the Gulf Coast as the relatively drier areas during the forecast period. As for individual features, a leading system will weaken as it tracks northeast from the Upper Mississippi Valley Sunday onward while another over the West by early Sunday should reach the Midwest/Great Lakes by Tuesday and the next one entering the West by Monday may reach as far east as the Mississippi Valley by next Thursday. A larger scale upper trough approaching the West Coast next Thursday may also bring some moisture into the Northwest. ...Model Guidance Evaluation... Models and ensemble means still key on the same individual systems but continue to exhibit a combination of meaningful spread and/or run-to-run variability/trending. Some of the difficulty in the forecast is that specifics will depend on not only the details of each system as it tracks over the lower 48 but also the character of central into eastern Canada mean troughing whose southern periphery may interact with at least the leading two features. The system forecast to be over the Upper Mississippi Valley as of early Sunday is still well clustered at that time but then uncertainties of southern Canada flow quickly come into play as it lifts through and away from the Great Lakes. By Tuesday the most notable trend is over the Northeast where a more persistent surface ridge leads to earlier dissipation of the front approaching from the west on Monday. For the next system that reaches the West just before the start of the period early Sunday, there has been decent clustering and continuity through about late Monday when the system reaches the Plains. From there guidance shows increasing divergence and some trending for how flow aloft near the U.S.-Canadian border may influence the system. In general the trend over the past day is for greater interaction which leads to a stronger surface low with farther northwest track. If anything, new 12Z guidance has increased the spread as the UKMET has become an amplified extreme with northern stream flow (yielding a deep/northern surface low) while the CMC shows little stream interaction (to produce a weaker and slower surface low). Through the 00Z/06Z cycles, not much has changed for the forecast of the third system which should reach the West on Monday. The 00Z ECMWF remains a southern extreme with the upper low tracking into the West and remains on the southwestern side of the envelope for the rest of the period. With time the GFS maintains its position in the faster part of the spread. GEFS mean runs have tended to be similar to or somewhat slower than the operational GFS while the UKMET/CMC have generally been between the two extremes--though the new 12Z CMC is about as slow as the 00Z ECMWF by day 7 Thursday. Prefer to maintain an intermediate solution while awaiting better convergence among the guidance. Most models and means have been fairly well behaved thus far for the upper trough nearing the West Coast next Thursday. GFS runs have been more inconsistent, occasionally pulling more energy off the southern periphery of the trough (most pronounced in the 00Z run). The updated forecast started with a composite of 00Z/06Z operational models for about the first half of the period and then incorporated varying weights of 06Z GEFS/00Z ECMWF means along with some input from the models and WPC continuity. This approach maintained a solution close to continuity where guidance was consistent or significant spread persists, while representing some trends where sufficiently agreeable. ...Sensible Weather/Threats... Over the Northwest and northern Rockies, one system should be producing rain/mountain snow over the Rockies on Sunday while another system right behind it will spread moisture from the Pacific Northwest through the Rockies during Sunday into Tuesday. Depending on exact system details, the second one could produce some enhanced totals over or near the northern Rockies. The upper trough approaching the West Coast by Thursday may start to bring light precipitation into the Pacific Northwest at that time. Over the central/eastern U.S., the leading surface low tracking out of the Upper Mississippi Valley on Sunday and associated fronts will spread precipitation across much of the eastern U.S. with generally light to moderate rain. Potential exists for pockets of locally heavier activity though. The trailing cold front will stall over the southern Plains while the next system emerges from the West, with this system currently expected to track through the Plains/Midwest early next week. Currently there appears to be decent potential for some areas of moderate to heavy rainfall from portions of the central/southern Plains northeastward, but it will take additional time to refine more precise threat areas within this area. Some severe weather is also possible, with the Storm Prediction Center currently indicating the best potential for severe thunderstorms across the southern Plains Sunday and Monday. The next system in the series may produce yet another area of significant rainfall over and east of the north-central through southern Plains. Even more with this system, confidence is fairly low for details of coverage and timing. A system on the deepest side of the spread could produce some snow in the northern periphery, but this has fairly low probability at the moment. Some areas over the central U.S. need to monitor forecasts of these systems given the potential to produce significant weather and sensitivity to additional rainfall. At least for the time being, the majority of forecasts produce most precipitation to the south of the far northern Plains where soil conditions are the wettest. The northern half of the Plains and Upper Midwest will see the most persistent below normal temperatures during the period. Some areas within this region may see highs at least 15-20F below normal on Sunday-Monday and then remain 10-15F below normal Tuesday through Thursday. System progression over the West will bring variable readings across the region late weekend into early next week. A ridge moving into and over the West will bring a broader area of plus 5-15F anomalies for highs next Wednesday-Thursday. The East will tend to see above normal temperatures, with morning lows tending to have more coverage of plus 10F or greater anomalies than daytime highs. Some locations in the Northeast could see slightly below normal temperatures early in the week. 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 Hazards: - Heavy rain across portions of the Middle/Lower Mississippi Valley, and the Southern Plains, Sun-Mon, May 1-May 2. - Heavy rain across portions of the Central/Southern Plains and the Middle/Lower Mississippi Valley, Wed, May 4. - Severe weather across portions of the Southern Plains, Sun, May 1. - Severe weather across portions of the Lower Mississippi Valley, and the Southern Plains, Mon, May 2. - Flooding possible across portions of the Upper Mississippi Valley and the Northern Plains. - Flooding occurring or imminent across portions of the Lower/Upper Mississippi Valley, and the Northern Plains. - Flooding likely across portions of the Upper Mississippi Valley and the Northern Plains. - Enhanced wildfire risk across portions of the Southern Rockies, the Southern Plains, and the Southwest, Sun, May 1. - Flooding possible across portions of mainland Alaska, Sun-Thu, May 1-May 5. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, 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/#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