Extended Forecast Discussion NWS Weather Prediction Center College Park MD 342 PM EDT Thu May 04 2023 Valid 12Z Sun May 07 2023 - 12Z Thu May 11 2023 ...Overview... The forecast flow pattern over North America next week looks to feature mean ridging over the central-eastern CONUS and mean troughing over the central-western CONUS. The ample yet uncertain downstream ejection of numerous upper-level impulses and associated weather focusing surface systems are expected to tap an influx of Gulf moisture to favor broad and widespread shower/storm chances from the Plains to the Mississippi Valley/Midwest and Southeast/East. Conditions will remain unseasonably cool in the West under the influence of mean troughing, with a daily precipitation focus for the northwest/northern tier of the region. ...Guidance/Predictability Assessment... Guidance still shows a generally similar and slowly evolving larger scale pattern through the weekend into Monday despite transitional blocky flow over North America and adjoining waters. Forecast spread has decreased a bit in this period, bolstering forecast confidence to near average levels overall. Even so, ample differences and run to run consistency issues remain with local weather focusing smaller scale systems embedded within the flow. A composite blend of reasonably compatible model guidance from the 06 UTC GFS and 00 UTC ECMWF/UKMET/Canadian seems to provide a good forecast basis that tends to mitigate smaller scale variance consistent with system predictability, but system continuity is less than stellar. This solution though does have good ensemble and National Blend of Models (NBM) support. Ensemble spaghetti and cluster plots however then show a more rapid increase in forecast spread/uncertainly for the rest of next week. Accordingly, the WPC medium range product suite was then primarily derived from a still compatible 06 UTC GEFS/00 UTC ECMWF ensemble means and the NBM. The newer 12 UTC guidance suite and trends have overall remained in line with this aforementioned forecast plan. ...Weather/Hazards Highlights... A steady supply of Gulf moisture over the central to eastern United States will favor a stormy and wet pattern next week. Embedded shortwave energy aloft as well as general front progression will dictate the chances for locally heavier rainfall. The experimental WPC medium range Excessive Rainfall Outlook (ERO) is set to issue a "marginal risk" threat area over the Midwest/southwestern Great Lake states, by Monday given a decent guidance signal for supporting shortwave and surface frontal wave approach to focus moisture/instability. While no other EROs have been introduced in the Day 4/5 timeframe, the environment would suggest there will likely eventually be additional Marginal Risk areas, if not higher. Model QPF has otherwise been rather nebulous, from the southern Plains and Mississippi Valley eastward across the east-central U.S. to the Mid-Atlantic/Carolinas. With the 12 UTC ECMWF trending favorably compared to the prior 00 UTC run, there is now a growing signal for more organized precipitation with strong convection and to shift focus back from the Rockies into the Plains next Wednesday/Thursday with slow approach of a main upper trough/height falls out from The West. High temperatures will be running above average broadly from the Plains to the Southeast/Mid-Atlantic under the influence of mean ridging aloft, with some unseasonably warm highs 15+ degrees above average centered on the Central Plains/Middle Mississippi Valley. Seasonable temperatures and low precipitation chances are expected across the Northeast with high pressure in the vicinity through the forecast period. Some temperature fluctuation as well as breezy winds may effect New England dependent on the proximity of an upper-level closed low over Canada and subsequent cold front passage. Energetic upper-level pattern with mean troughing aloft will favor periodic precipitation chances in the West next week, especially for the Pacific Northwest/Great Basin/Northern Rockies. Some snow showers will be possible for higher mountain elevations, but seasonably rising snow levels will keep these chances isolated. Temperatures will remain cool and below average, especially across the West Coast states. Putnam/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 Rockies, the Central Plains, and the Northern Plains, Wed-Thu, May 10-May 11. - Heavy rain across portions of the Great Lakes, and the Upper Mississippi Valley, Mon, May 8. - Flooding possible across portions of the Middle Mississippi Valley and the Northern Plains. - Flooding occurring or imminent across portions of the Northern Plains, the Northern Rockies, the Middle Mississippi Valley, the Mid-Atlantic, the Northern Great Basin, and the Upper Mississippi Valley. - Flooding likely across portions of the Pacific Northwest, the Northern Rockies, and the Northern Great Basin. - Heavy precipitation across portions of mainland Alaska, Tue-Wed, May 9-May 10. 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