Extended Forecast Discussion NWS Weather Prediction Center College Park MD 402 PM EDT Fri Apr 29 2022 Valid 12Z Mon May 02 2022 - 12Z Fri May 06 2022 ...Overview... Continuing from the short range time frame, 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 Monday 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 issued an experimental medium range Slight Risk for excessive rainfall early in the week over/near the eastern southern Plains. 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... The most contentious aspect of the forecast in recent days has generally been the system reaching the West on Monday and then continuing through the western-central U.S. for the rest of the week. After a slower adjustment in the ECMWF a couple days ago, there was a consistent guidance spread between faster GFS runs and slower ECMWF while the CMC/UKMET tended to be in the middle to slower part of the envelope. Over the past day the GFS and GEFS mean have made a fairly dramatic slower adjustment, leading to improved clustering within what had been the slower part of the spectrum. New 12Z models maintain continuity with this developing majority scenario through early day 6 Thursday so confidence is improving at least through that time. Some divergence still arises thereafter, with varying ideas for upstream flow entering the West and flow in the southwest part of the eastern Canada mean trough--with potential effects on the system reaching the central U.S. For both of those areas the 06Z/12Z GFS offer somewhat less confident solutions, bringing more amplified troughing into the West and showing more southwestward trough elongation into the Great Lakes versus prevailing consensus. Note that model/ensemble spread for the trough near the West Coast increases significantly after early Thursday, and the ECMWF/ECMWF mean have trended toward a less amplified trough over the past day. For the early week system tracking from the south-central High Plains through the Great Lakes, there are still some detail/strength differences with trends generally favoring an intermediate solution. Some differences are related to still-unresolved details of interaction with northern stream flow. CMC runs have been persistent with slower timing than other guidance to the east of the Plains. A 06Z/00Z operational model blend early followed by a model/ensemble mean mix represented the improving slower consensus for the western through central U.S. system while yielding the desired intermediate (and closer to continuity) forecast of the leading system early in the week. Upper troughing/leading surface front pushing into the West late in the week are fairly stable relative to previous forecast. ...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-Friday should bring a renewed precipitation focus into the Pacific Northwest and then farther inland. The southern extent of precipitation will depend on the upper trough's precise amplitude which becomes increasingly 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. The northern half of the Plains and Midwest will be on the chilly side next week. The most extreme anomalies should be over the north-central Plains on Monday (north of the first central U.S. system) with highs 15-25F below normal. Expect a trend closer to normal toward the end of the week. The West will see temperatures vary from west to east with system progression, with some highs 5-15F below normal crossing the region during the first half of the week to be followed by above normal highs of similar magnitude Wednesday-Thursday. The West Coast will trend back to near/slightly below normal late in the week. Most of the East will tend to see above normal temperatures next week with Tuesday-Wednesday likely producing the greatest coverage of plus 10F or greater anomalies. Rausch/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 Hazards: - Heavy rain across portions of the Central/Southern Plains, and the Middle/Lower Mississippi Valley, Mon, May 2 and Wed, May 4. - Severe weather across portions of the Central/Southern Plains, and the Middle/Lower Mississippi Valley, Mon, May 2. - Severe weather across portions of the Central/Southern Plains, Wed, May 4. - Flooding possible across portions of the Upper Mississippi Valley and the Northern Plains. - Flooding occurring or imminent across portions of the 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 Central/Southern Plains, the Central/Southern Rockies, the Central Great Basin, and the Southwest, Tue, May 3. - Enhanced wildfire risk across portions of the Southern Rockies, the Southern Plains, and the Southwest, Mon, May 2. - Flooding possible across portions of mainland Alaska, Mon-Fri, May 2-May 6. 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