Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sun Jun 04 2023 Valid 12Z Wed Jun 07 2023 - 12Z Sun Jun 11 2023 ...Broad multi-day thunderstorm and local heavy rain threat from the Northern Rockies to the Southern Great Plains continues well into next week... ...Overview... The upper level pattern during the medium range period next week will remain stagnant and blocky as an anomalously strong upper ridge is sandwiched between troughing along the West Coast and also in the Northeast. This pattern supports daily showers and storms from the southern Plains into the northern Rockies and parts of the Great Basin, an area which has been quite wet as of late. This keeps above normal temperatures from the Northwest into the central U.S. while the Southwest and Northeast remain cool. ...Guidance Evaluation/Predictability Assessment... The latest guidance remains in good agreement regarding the overall blockiness of the synoptic pattern during the medium range period. There remains some differences in the details of both the Western and Northeast troughs, but models and ensembles unanimously suggest the central ridge could hold strong through at least next weekend. In the Northeast, waves of energy should help reinforce troughing across the Northeast through Friday, but a shortwave dropping in towards the Upper Great Lakes from Canada could try to push the New England upper low up to the north and new troughing becoming established next weekend across the Great Lakes and Midwest. The GFS is a lot more aggressive with this scenario bringing a deep closed low to just north of Lake Superior next Sunday/day 7. The ECMWF shows an open shortwave, but spread in the ensembles suggest either solution is plausible and worth watching. The WPC day 7 forecast leaned more on the ensemble means which is in best agreement with the ECMWF (albeit a bit slower). The CMC continues to be alone in suggesting a dipole approach to the upper low over New England rather than one consolidated low like the rest of the guidance shows so it was not included in the blend after day 4. The upper low over California continues to exhibit some minor differences in strength and how quickly it becomes eroded by troughing to the north, but a general model blend seems sufficient. The WPC blend for tonight used a general deterministic model blend for days 3 and 4, transitioning to mostly ensemble means by day 7. Favored the ECMWF late period over the GFS given the GFS issues over the Great Lakes and its better run-to-run consistency out west. This approach also maintained good continuity with the previous WPC forecast. ...Weather/Hazards Highlights... A daily diurnal risk for heavy showers and thunderstorms will be maintained through 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 an upper low parked over California. For now, the day 4 Excessive Rainfall Outlook continues to depict just one large Marginal risk area for this region, however embedded Slight risk areas may become necessary if heavy rains focus over antecedent wet areas. By day 5, the marginal risk region is largely the same as day 4, but did add a slight risk for parts of central/eastern Montana given more favorable moisture and dynamics and considering antecedent conditions in this region. The risk may extend past day 5 as well as a frontal boundary becomes established over the region. Elsewhere, a front through the Great Lakes/Upper Midwest should focus showers and storms ahead of it across the Northern Plains/Upper Mississippi Valley early to mid week. In the Northeast, the deepening upper trough/lows should favor pockets of rainfall into Friday as moisture gets wrapped back into especially northern Maine from a lingering low pressure system near the coast. In Florida, a lingering weak front should support unsettled conditions over the Sunshine State with better heavy rain chances in southern Florida late week as deeper moisture gets pulled north. Central U.S. ridging aloft will maintain an axis of above normal temperatures from the northern Plains south-southeast to the Mid-South, with plus 5-10F anomalies for highs possible most days. Moderation back towards normal is expected next weekend as lowering heights moves into the region. The Northwest will be the other focus for very warm temperatures, with highs 10-20F or so above normal Wednesday and Thursday. Expect near to below normal temperatures over the Northeast and at times into the Mid-Atlantic beneath upper troughing. 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 upper low reaching southern California Monday will also bring a cooling trend into the southwestern U.S. next week. 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