Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Sat Apr 30 2022 Valid 12Z Tue May 03 2022 - 12Z Sat May 07 2022 ...Overview... A series of systems will produce episodes of precipitation from the Northwest U.S. and northern-central Rockies (rain and high elevation snow) through the central-southern Plains and much of the East. A system tracking out of the Midwest early Tuesday onward and another one tracking out of the West should be the last in the series during the currently established pattern consisting of discrete troughs/closed lows, while mean troughing aloft setting up near the West Coast late week into the weekend will likely bring a couple frontal systems and associated precipitation focus into the West. The most active weather should be with the system emerging from the West by midweek with the Storm Prediction Center indicating a threat of severe weather over the southern Plains and vicinity and WPC showing an experimental medium range "Slight Risk" for excessive rainfall over parts of the region as well, with runoff issues potentially enhanced due to wet ground from rainfall expected in the short range time frame. The midweek threat signal is stronger than average as a closed upper system/lead upper diffluence combine with increasing inflow of Gulf moisture into a wavy front. Rainfall will continue spreading into/through the East later in the week. ...Model Guidance Evaluation... Guidance through the 00Z/06Z cycles offered significant forecast challenges in multiple respects. Every day or so the models keep changing the specifics of flow aloft over eastern Canada into the eastern U.S., leading to oscillations in strength/track of low pressure tracking out of the Midwest early Tuesday onward. Latest clustering suggests somewhat less interaction between Canada troughing and the Midwest shortwave, generally yielding a weaker/southward surface low and then by early Thursday a wave offshore from the Northeast. Meanwhile the pronounced spread continues for the upper low tracking out of the West, with the GFS/GEFS mean on the fast side and the ECMWF slowest. From Thursday onward the 00Z ECMWF mean and CMC/CMC mean offered a compromise closest to what has historically been the best starting point in this type of timing discrepancy, especially involving a closed low, which is a two-thirds lean toward the slower side of the envelope. General preferences ultimately leaned closest to the 00Z CMC/CMC mean and ECMWF mean. With the mean trough settling near the West Coast, there is a general theme of a leading shortwave reaching the coast Thursday and then another feature arriving around Saturday. However models/ensembles have varied wildly for amplitude of the leading shortwave. Some GFS runs/GEFS ensembles have been quite amplified while ECMWF runs/ECMWF ensembles have been trending noticeably flatter (with the CMC/CMC mean a compromise). Compared to the very amplified 00Z run, the 06Z and then 12Z GFS runs have made a pronounced flatter trend closer to the consensus. Guidance preferences led to starting with a 06Z/00Z operational model composite for the first half of the period, employing slightly less than typical weight of the GFS component. Then the forecast transitioned to a model/ensemble mean blend including more CMC than ECMWF along with remaining input consisting of the 00Z ECMWF/CMC means and 06Z GEFS mean. New 12Z model trends (including a faster adjustment in the ECMWF) remain supportive of slower timing than the GFS for the system emerging from the West, but faster than the 00Z ECMWF. ...Sensible Weather/Threats... Over the West, the upper low forecast to track from near southern Idaho through the central Rockies Tuesday-Wednesday and associated surface reflection will spread rain and higher elevation snow across the northern-central Rockies and High Plains during that time. This system could produce some locally enhanced totals depending on exact details. Then an amplified upper trough and surface frontal system forecast to reach the West Coast and progress inland next Thursday-Saturday should bring a renewed precipitation focus into the Pacific Northwest and then farther eastward. The southern extent of precipitation will depend on the upper trough's precise amplitude which has been quite uncertain so far, but latest trend suggest that most moisture should be confined to the northern half of the West. The upper dynamics/surface front could reach far enough east to start producing at least scattered rainfall over the northern High Plains by next weekend. The Pacific Northwest may see additional precipitation by next Saturday with another front. Over the central/eastern U.S., the initial rainfall focus will be with a system tracking out of the Midwest early Tuesday onward. This low and associated fronts should produce the highest totals over the lower Great Lakes and Ohio Valley with somewhat lighter amounts extending eastward through the East Coast. The next system in the series will likely emerge over the Plains during the day Wednesday and lead to another episode of potentially heavy rainfall, with the WPC experimental Excessive Rainfall Outlook showing a Slight Risk over/near parts of the south-central Plains for Wednesday into early Thursday. The Storm Prediction Center is also monitoring severe threats over and just east of the southern Plains Wednesday-Thursday. Rain should continue pushing eastward from the Plains through the rest of the week with some pockets of moderate to heavy amounts. System details by late in the week are still fairly uncertain but currently the most likely axis of highest rainfall totals extends from the Midwest through the Mid-Atlantic. Over the central/east-central U.S., rain that falls prior to this final system could increase sensitivity to added rainfall. Fortunately the guidance is consistent in keeping this system's moisture south of the far northern Plains where soil conditions are the wettest. The system initially crossing the West will support highs 10-15F below normal over the northern Rockies into Great Basin on Tuesday. Then this system's progression will reinforce the chilly pattern over the central Plains/Midwest (highs up to 10-15F below normal Tuesday-Thursday) before temperatures may finally moderate toward the end of the week. Warmer readings will cross the West mid-late week with best potential for highs 10-15F above normal on Wednesday-Thursday. A gradual increase of clouds/precipitation will bring highs down to moderately below normal levels over the Northwest Friday-Saturday. The northern two-thirds of the East will see above normal temperatures into midweek, followed by a gradual cooling trend that will expand coverage of near to somewhat below normal highs late 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 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