Extended Forecast Discussion NWS Weather Prediction Center College Park MD 303 AM EDT Wed Jun 07 2023 Valid 12Z Sat Jun 10 2023 - 12Z Wed Jun 14 2023 ...A multi-day thunderstorm and local heavy rain threat will continue into early next week from a very saturated Northern Rockies through portions of the Great Plains... ...Overview... A quite blocky flow pattern looks to persist through at least early next week as an anomalously strong upper ridge/high remains sandwiched between mean troughing/closed lows over the Southwest and the Northeast. This pattern continues to support diurnal showers and storms, some locally heavy, from the southern Plains through the northern Rockies and parts of the Great Basin. The next wave rounding the mid-continental ridge crosses Ontario and approaches the Northeast this weekend, with growing consensus on closing off a low over the Upper Great Lakes around Monday-Tuesday next week. This would prompt an additional and locally enhanced precipitation focus by later weekend into early next week out across the east-central U.S. and then the Eastern Seaboard. ...Guidance Evaluation/Predictability Assessment... There remains fairly good agreement on the overall synoptic scale pattern across the CONUS into early next week. The first half of the period was based on a blend of the latest deterministic solutions (and the National Blend of Models for QPF). Even into days 6 and 7 (Tuesday and Wednesday) though, models seem to be trending towards a fairly strong closed low over the Great Lakes, with some weakening/opening up as it pushes into the Northeast. The GFS solutions remain on the more progressive side which the ECMWF/CMC a little slower, which has more support from the ensembles as well. Meanwhile, out West, the models agree a closed low over California/the Southwest should open up and shift east and possibly finally break down the blocky pattern. Plenty of uncertainty and trends left to monitor though. For days 6 and 7, the WPC forecast leaned towards the ensemble means along with the deterministic ECMWF and CMC for some added details. This maintains good continuity with the previous WPC progs as well. ...Weather/Hazards Highlights... A daily risk for heavy showers and strong thunderstorms from the southern Plains through the northern Rockies and westward into the Great Basin is expected to be slow to dislodge over about the next week as tropical Pacific moisture remains funneled northward between a strong central U.S. ridge and an upper low/trough parked over California/the Southwest. A frontal boundary dropping south and eastward through the region should also help to enhance rainfall totals this weekend, with particular concerns over areas that have been very wet as of late (northern and central Rockies/western Great Plains). There was enough model support to add a slight risk on the day 4 Excessive Rainfall Outlook from central/eastern Montana into western Nebraska especially given high stream flows in that region. The dynamics support at least locally heavy convection and possible complexes farther south and east along the boundary into the south-central Plains/Mississippi Valley but there is still enough model uncertainty over this region which has also been much drier to preclude extending the slight risk that far east on day 4. It could be needed in future updates though as models begin to resolve the details more. For day 5, a slight risk is depicted in eastern Colorado and far southeast Wyoming mainly given the antecedent wetness and favorable conditions. Farther into the central U.S. and the Ohio Valley, strong thunderstorms should focus along the fairly progressive frontal boundary and a marginal risk remains expanded into this region for days 4 and 5 as models indicate the potential for at least locally heavy rainfall. The Great Lakes closed upper trough/low development and associated and wrapping surface low and frontal characteristics trailing over the Appalachians/East/Southeast into early next week may then set the stage for development of a pattern favoring additional widespread threats for heavy rain/thunderstorms to monitor. Ejecting energies out of the Southwest also looks to favor additional rounds of rainfall and convection again around mid-week next week for parts of the south-central Plains. Temperatures over the northern tier and back into the Pacific Northwest should moderate some this weekend, but may creep back up again early next week with anomalies generally +10-15F possible. Underneath possible closed low development over the Great Lakes/Midwest and back into the Southwest/California, temperatures should trend cooler than normal. Upper ridging moving into parts of the southern Plains early to middle of next week should allow for warmer than average values there as well. Santorelli 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