Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sat Apr 30 2022 Valid 12Z Tue May 03 2022 - 12Z Sat May 07 2022 ...Overview... A series of Pacific systems (each supported by an upper trough that may contain a closed low for a period of time) progressing across the lower 48 will produce active weather during the period. These systems will spread rain and high elevation snow across the Pacific Northwest and north-central Rockies/High Plains, and then mostly rain over a large part of the central/eastern U.S. There will be two primary systems of interest, one tracking from the south-central High Plains through the Great Lakes early in the week and the second entering the West by Monday and reaching near the Midwest by Friday. Some thunderstorms could be strong with SPC indicating a threat of severe weather over the southern Plains and vicinity into Tuesday with the first system and then again a couple days later ahead of the next one. Heavy rain will also be a threat with each system. WPC has now issued an experimental medium range "Slight Risk" for excessive rainfall next midweek over/near the eastern southern Plains, with runoff issues affected by additional short range rainfall. The midweek threat signal is stronger than normal as a closed upper system/lead upper diffluence combine with increasing inflow into a wavy front. Late in the period the guidance suggests an evolution toward larger scale features with mean troughing aloft reaching the eastern Pacific/West Coast while ridging builds over central Canada and a trough reaches eastern Canada/Northeast U.S. The pattern during the period 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 next week. ...Model Guidance Evaluation... Latest 18/00 UTC GFS and 12/00 UTC ECMWF/UKMET mass field and QPF solutions seem best clustered days 3-5 (Tue-Thu) and have good ensemble and continuity support in a period with seemingly above normal predictability. WPC product continuity is well maintained, bolstering forecast confidence. Forecast spread increases dramatically into days 6/7 as guidance struggles both with Pacific system track issues inland into the West and with closed system progression out from the Plains. Recent GFS runs curiously show much more robust height falls/system digging across the West Coast to the Southwest while in contrast the Canadian and especially the ECMWF instead focus into the Northwest. A favored GEFS mean and double weighted ECMWF ensemble mean blend shows only moderate system digging into the West along with downstream system progressions out from the central U.S. on the slower side of the full envelope of guidance solutions. This best takes into account the nature of patterns with closed lows/troughs in a split southern stream. ...Sensible Weather/Threats... Over the West, another system will spread moisture from the Pacific Northwest through the Rockies into early and middle part of next week. Depending on exact system details, this could produce some enhanced totals, including accumulating snows into the north-central Rockies and adjacent High Plains. Subsequently, an amplified upper trough and surface frontal system forecast to reach the West Coast next Thursday-Saturday should bring a renewed precipitation focus into the Pacific Northwest and then farther inland. However, the southern extent of precipitation will depend on the upper trough's precise amplitude becomes increasingly and wildly uncertain by next Friday. Over the central/eastern U.S., the Northeast may see lingering light precipitation on Monday with a weakening front. The trailing cold front will likely 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. There is decent potential for areas of moderate to heavy rainfall from portions of the central/southern Plains northeastward, including a Slight Risk area of excessive rainfall during Monday into early Tuesday over the south-central U.S. Some severe weather is also possible, with the Storm Prediction Center currently indicating the best potential for severe thunderstorms across the southern Plains in that same time frame. A localized area of snow may be possible over the north-central High Plains. The guidance signal is also growing that the next system in the series tracking out of the West may produce yet another area of significant rainfall over and east from the central U.S. mid-late next week. Confidence is slowly improving for this system's details at least through Thursday, with progression in the slower portion of prior spread adding somewhat to the heavy rainfall potential over some locations. Prior rainfall could make some areas sensitive to additional rainfall from this latter system. Fortunately the guidance continues to produce most if not all precipitation to the south of the far northern Plains where soil conditions are the wettest. Schichtel 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