Extended Forecast Discussion NWS Weather Prediction Center College Park MD 416 AM EDT Tue Jun 4 2024 Valid 12Z Fri Jun 07 2024 - 12Z Tue Jun 11 2024 ...Major HeatRisk expected for many areas in the West late week with record high temperatures... ...Overview... Broadly, a western upper ridge and eastern upper trough pattern will set up late this week, though weak troughing may approach the West Coast early next week. The ridge will produce potentially hazardous heat across much of the West through the end of the week. The forecast pattern should support increasing coverage and intensity of showers/storms over the central or south-central Rockies and Plains as systems ride the ridge through much of the forecast period. Meanwhile, periods of light to modest rain are likely in the northeastern U.S. underneath the slow-moving trough/low. ...Guidance/Predictability Assessment... While model guidance is generally agreeable in showing the western ridge/eastern trough pattern in some fashion, differences in the details will impact sensible weather across the Lower 48. Most models have fortunately converged somewhat regarding the placement of the Great Lakes upper low and the depth of the trough extending south through Friday-Saturday, though the 12Z CMC seemed too fast to move it eastward. Into Sunday and beyond though, the low/trough's movement eastward (or lack thereof) becomes questionable. One concern with the evolution of this upper low is the possible interaction with another upper low to its northwest over Canada. Overall the 12Z guidance indicated this low over Manitoba or so on Sunday and moving southeastward, serving to kick the Great Lakes low east early next week. The 18Z GFS was generally alone in keeping this low well northwest of the other guidance where the two lows do not interact much. So leaned away from the 18Z GFS...but the newer 00Z model solutions came in more like the 18Z GFS, with the Canada upper low farther removed northwest and the Great Lakes upper low stalling and keeping the trough deeper across the East early next week. So there will likely need to be some changes to the next forecast to show this trend. In the West, ridging through late week is agreeable in the model guidance. Models show a southern stream upper low near/west of Baja California opening up as it approaches the Southwest this weekend, with some expected detail differences but will at least help decrease the heat. Much farther north, upper low development in the northeast Pacific Friday-Saturday will extend troughing south that approaches the West Coast early next week. The upper pattern over the Gulf of Alaska is complicated as two upper lows exhibit a Fujiwara evolution, but models generally show weakening troughing approaching the Northwest by Sunday. This trough should track east Monday-Tuesday but with the details dependent on the pattern downstream. The WPC forecast used a blend of deterministic guidance favoring the ECMWF and GFS early in the forecast, lessening the incorporation of the deterministic models (particularly the 18Z GFS, possibly incorrectly) as the period progressed in favor of the GEFS and EC ensemble means. The means reached 60 percent of the blend by Days 6-7. Additional changes to the forecast are expected, particularly given how the trends in the Great Lakes upper low/trough will impact fronts and temperatures and QPF. ...Weather/Hazards Highlights... Some return flow of moisture (including Pacific moisture advecting around the upper ridge) is likely across the southern/central Rockies and Plains, with one or more surface boundaries helping to focus multiple rounds of showers and thunderstorms. Placement of this convection is uncertain, as reflected by the relatively low QPF, but potential for much higher amounts exists. A Marginal Risk is delineated on the Day 4/Friday ERO across Kansas and Oklahoma and clipping surrounding states. By Day 5/Saturday, moisture transport is forecast to increase even more and the instability and overall setup will likely cause heavy rain rates. Recent model guidance is notably agreeable at least for now in a heavy rainfall axis of 3-5 inches near the Oklahoma/Kansas/Missouri/Arkansas intersection. Given these factors, have a Slight Risk delineated for that area on Saturday within a broader Marginal covering south- central parts of the Plains and Mississippi Valley. Though the exact placement may have to shift around in future cycles, the ingredients will certainly be sufficient for flash flooding concerns. Additional rounds of rain are likely in the Plains and south-central parts of the Mississippi Valley early next week. Rain chances should also gradually increase across the southern and central Rockies late week through the weekend and remain into early next week. The threat for heavy rainfall to cause flash flooding there seems sub-Marginal at this point given recent drier than average conditions. Scattered light rainfall is possible late in the week over parts of the Interior West/northern Rockies, likely increasing in coverage and amounts early next week as upper troughing approaches. Rounds of rain and possible thunderstorms are expected across the Lower Great Lakes to the Northeast underneath the upper trough. The heaviest rain amounts during the medium range period are forecast on Friday, but since the initial front will be coming through and amounts look to decrease compared to the short range period, do not have any Marginal Risk in the ERO for Day 4/Friday. However, there is a non-zero risk of flooding concerns especially if heavy rain falls in similar areas as the previous days. The western ridge will promote notably/record hot temperatures across the West especially for the latter part of this week. Highs broadly will run 10-20 degrees above average and locally 20+ degrees above average in the Great Basin. High temperatures above 110F are forecast for the Desert Southwest (including Las Vegas), with 90s and 100s stretching into the central California valleys to the Great Basin to much of Texas. Daily record high minimum and maximum temperatures are possible. Through Friday, the experimental HeatRisk product shows a large area of Major (level 3 of 4) and some Extreme (level 4 of 4) outline. The higher temperature anomalies look to shift into the Northwest for the weekend with some minor moderation of the temperatures and HeatRisk in the Southwest as the weakening upper low approaches from northwestern Mexico. Meanwhile, most areas east of the Rockies should trend to slightly below normal behind the cold front. Exceptions to this trend will be Texas (especially southern areas), and Florida where highs several degrees above normal may lead to some daily records. Tate 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 forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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 https://www.wpc.ncep.noaa.gov/#page=ovw