Extended Forecast Discussion NWS Weather Prediction Center College Park MD 423 PM EDT Thu Jun 01 2023 Valid 12Z Sun Jun 04 2023 - 12Z Thu Jun 08 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... A strong/blocking upper ridge/high builds over the northern Plains/Manitoba this weekend and persists well into next week, supporting prolonged above normal temperatures from the northern Plains into middle/upper Mississippi Valley through at least next Thursday. Weakening shortwave/upper low energy lingering over the northern Rockies along with a following southern stream Pacific low that reaches southern California Monday will maintain periods of convection over the Rockies and parts of the central/southern Plains, Sierra Nevada and Great Basin/southern Cascades through much of next week. Above normal temperatures will prevail over the northwestern and north-central states through at least midweek. Confidence increases for troughing and closed lows to persist over the Northeast for much if not all of next week in this blocked pattern. This anomalous low and associated trough will bring cool conditions to the Eastern Seaboard through the period with wetter conditions for at least New England. ...Guidance Evaluation/Predictability Assessment... Confidence has increased on a pair of lows affecting the Northeast through the medium range forecast period. The first develops over the Canadian Maritimes Saturday and persists east of Maine through Sunday night when a reinforcing wave from Quebec closes into an upper low as it tracks southeast over the Northeastern states Monday before stalling/occluding (per the 00Z ECMWF/12Z GFS) and expanding over the Northeast into or through Thursday. The 12Z CMC produces a weaker western wave that fails to close as well over the northeast (it has an open wave over the Northeast on Tuesday for example). The 12Z GFS/GEFS is deeper and farther south with the upper low position than the 00Z ECMWF which is likely due to further reinforcing shortwave troughs from Canada which are understandably low confidence in position/strength by Day 5/6. Farther west a key feature is the low currently south of the Gulf of Alaska that merely drifts east to northern BC through early next week in the blocked pattern. The concern for the CONUS is how far south the associated trough gets over the Pacific Northwest and northern Rockies. 12Z guidance is in great agreement for the trough axis to dip south of Canada which should have little effect on the Pacific Northwest and be a factor in allowing sub-tropical moisture to be drawn north over the northern Rockies. 12Z guidance remains consistent/similar for the upper low reaching southern California Monday and lingering over California into the middle of next week. Decent agreement among guidance available this morning (00Z/06Z) allowed mostly deterministic sources through Day 5 with a focus toward the 00Z ECENS/06Z GEFS for Days 6/7. ...Weather/Hazards Highlights... A central continental ridge with weak troughing over the Intermountain West keeps a mainly diurnal risk for heavy showers and thunderstorms over much of the Rockies/High Plains through Texas and the Sierra Nevada/Great Basin into or through next week. Localized flash flood potential continues through at least Monday night given wet ground conditions from recent and soon to occur rainfall. The Days 4-5 Excessive Rainfall Outlooks (covering Sunday through Monday night) depict Marginal Risk areas spanning the Rockies/portions of the High Plains through much of the southern Plains where the combination of broad flow from the West Gulf of Mexico up the Great Plains along with some instability. Increasing tropical Pacific moisture from the upper low reaching southern California Monday allows diurnal heavy rain threats for the Sierra Nevada for Day 4, expanding over the Great Basin for Day 5. Embedded Slight Risk areas may become necessary, particularly if the heavy rain focus is over antecedent wet areas. The deepening upper trough/lows should favor diurnally favored pockets of rainfall over and near the Northeast through Thursday. 12Z guidance is in decent agreement for the NHC monitored feature in the eastern Gulf on it tracking south, though lingering troughing over the Florida peninsula should allow for wetter conditions to continue there into early next week. 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 10-15F anomalies for highs likely each day. Initial warmth over the Great Lakes will moderate early next week after a cold front pushes through from the northeast. The Northwest will be the other focus for very warm temperatures, with highs 10-20F or so above normal through at least midweek. Expect below normal temperatures over the Northeast and at times into the Mid-Atlantic as upper troughing becomes established. 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. for the early-middle part of next week. Jackson 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: - Flooding possible across portions of the Northern Plains. - Flooding occurring or imminent across portions of the Northern Rockies and the Northern Great Basin. - Flooding likely across portions of the Central Rockies, the Central Great Basin, and the Northern Great Basin. - Much above normal temperatures across portions of the Pacific Northwest and the Northern Great Basin, Mon-Wed, Jun 5-Jun 7. 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