Extended Forecast Discussion NWS Weather Prediction Center College Park MD 241 PM 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. The greatest area of uncertainty remains in the eventual evolution of the Eastern trough, including a deep Great Lakes upper low and additional energy dropping in from central Canada. Some of the latest 00z guidance (available after forecast generation time) has trended stronger with the secondary energy acting to push the Great Lakes low east faster than previous runs. At least through the 00z/06z guidance (which is what today's WPC forecast was based on), there was some agreement between the GFS and ECMWF and ensemble means in the East, but the new 12z GFS is significant stronger with secondary energy forming another deep closed low into the Upper Midwest/Great Lakes early next week and actually indicating weak ridging building back in over the East. Run to run continuity in the reloading of the Eastern trough through the period is not great and so uncertainty in how the forecast across the East will evolve is very low. Out West there is also a lot of uncertainty in smaller scale shortwaves periodically interrupting the Western U.S. ridge and a larger trough moving towards the West coast late weekend/early next week. The strength of the ridge will help determine how much of that energy is able to penetrate into the West. The WPC forecast was able to use a general blend of the latest deterministic guidance for the early period (Friday-Saturday) amidst good model agreement. But by Sunday and especially beyond, greater issues and uncertainties arise as indicated above and so increased percentages of the ensemble means were used to help temper the details somewhat. Overall, this maintained good large scale agreement with previous shift WPC continuity, but some changes in frontal structure/placement were needed. ...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 remains uncertain, but there is potential for heavier amounts. Given some better consensus in the latest guidance, was able to tighten up the current Day 4/Friday marginal risk on the ERO to only include much of central/eastern Nebraska into southwest Missouri where models show the best potential for excessive QPF amounts. 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, continued a Slight Risk 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) risk. 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. Santorelli/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