Extended Forecast Discussion NWS Weather Prediction Center College Park MD 242 AM EDT Mon Jun 05 2023 Valid 12Z Thu Jun 08 2023 - 12Z Mon Jun 12 2023 ...A broad multi-day thunderstorm and local heavy rain threat will continue over the next week from the Northern Rockies to the Great Plains... ...Overview... The upper level pattern during the medium range period will remain very amplified and blocky over the CONUS as an anomalously strong upper ridge is sandwiched between mean troughing along the West Coast and also in the Northeast. This pattern supports daily showers and storms, some locally heavy, from the southern Plains into the northern Rockies and parts of the Great Basin, an area which has been quite wet as of late. There are some indications the pattern may finally either break down or shift eastward around next weekend but there is a lot of uncertainty in the specifics. ...Guidance Evaluation/Predictability Assessment... A general model blend of the deterministic solutions worked well for days 3-5 amidst good model agreement. By days 6 and 7, there is growing spread regarding a shortwave dropping in towards the Upper Great Lakes. The ECMWF continues to be weaker/more progressive with this feature into the Northeast next Monday, likely due to being more aggressive to break down the central U.S. ridge. The better consensus (including the GFS and CMC) maintain the omega block over the CONUS allowing this troughing to dig deeper into the Midwest. Out West, an initial upper low over California on Thursday should weaken and lift north as a second piece of energy cuts off from Eastern Pacific troughing to close off a deep low over southern California again by next weekend. Models seem to be trending towards a rather strong low, but there remains some uncertainty in the details of its evolution. The WPC blend favored the better agreeable ensemble means late period given this issues described above. ...Weather/Hazards Highlights... A daily diurnal risk for heavy showers and thunderstorms will be maintained through much of the period from the southern Plains to the northern Rockies and westward into the Great Basin as tropical Pacific moisture is funneled northward in between a strong central U.S. ridge and the upper low parked over California. The days 4 and 5 Excessive Rainfall Outlooks continue to depict a large marginal risk across this entire region. An embedded slight was placed across parts of Montana as anomalous moisture focuses along a frontal boundary over an area that has been very wet lately. Additional embedded slight risks are also possible within the larger marginal risk area, especially if heavy rainfall falls over antecedent wet areas. Elsewhere, a front moving southward through the Upper Midwest and Ohio Valley later this week into the weekend should focus showers and thunderstorms along the boundary. A leading cold front settling over the Southeast should be a secondary focus for scattered convection, with possibly more widespread/heavy showers across southern Florida later this week along a weakening front. Anomalous ridging will generally maintain above normal temperatures most days from the Upper Midwest/northern Plains into the Northwest. The warmest day looks to be Thursday, where daytime highs could be 10 to 20 degrees above normal, especially for parts of central and western Washington. Temperatures should moderate some across the northern tier states into next weekend. Expect near to below normal temperatures over the Northeast and at times into the Mid-Atlantic beneath upper troughing, mainly early in the period. Clouds and periods of rainfall should keep highs 5-10F or so below normal over the southern half of the Rockies/High Plains for most of the period and the second upper low reaching southern California next weekend will bring a reinforcing shot of much cooler temperatures 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