Extended Forecast Discussion NWS Weather Prediction Center College Park MD 214 PM EDT Mon Jun 3 2024 Valid 12Z Thu Jun 06 2024 - 12Z Mon Jun 10 2024 ...Major HeatRisk expected for many areas in the West later this week with record high temperatures... ...Overview... Broadly, a western upper ridge and eastern upper trough pattern will set up later this week through the weekend, 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. Meanwhile light to modest rain is quite likely in the East Thursday into the weekend in the northeastern U.S. underneath the trough and a slowly-moving upper low, where temperature will be near to slightly below average. Additionally, 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. ...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 first concern is the depth of the trough/upper low extending south and east from the anchoring upper low near the Great Lakes. Dynamical and AI/machine learning (ML) models have differed in the low/trough position and depth; namely, many ML models indicated a farther south or southwest centroid of the upper low and thus the resultant trough's southern extent compared to the dynamical models. Aspects of the pattern affecting the upper low include ridging to the north and east over Hudson Bay/northern Ontario (a stronger ridge pushing the upper low farther south) and the ultimate evolution of a developing west-central Canada upper low that still shows considerable spread in the guidance (deterministic models were much slower to raise heights compared to the ensemble means). Continued to favor a slightly deeper low over the Northeast than the ensemble means, which was generally a blend of the GFS/ECMWF/UKMET/Canadian deterministic models. Spread and thus uncertainty continued to grow with what becomes of the trough into early next week -- i.e., how quickly will the troughing depart and will it be reinforced from the northwest. Trends seemed to favor the slower and less progressive solutions. Ridging in the West through late week is agreeable in the model guidance, though of course the eastern side will be dependent on the trough's shape. 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. Differences with this have plagued the forecast (especially timing), at least partially related to the complicated evolution of the upper pattern over the Gulf of Alaska as two upper lows exhibit a Fujiwara evolution. Overall, the WPC forecast favored a blend of the deterministic models early in the forecast, and only gradually increased the proportion of ensemble means to ~40% by the end of the period as the signal for the upper low in the Northeast was fairly week. ...Weather/Hazards Highlights... On Thursday, the upper trough and the surface cold front will shift into the Eastern Seaboard and lead to rain and thunderstorms there. Modest moisture influx coupled with potential for training in the slowly-evolving pattern yields a lower-end (Marginal) threat of excessive rainfall over parts of the Northeast (especially in the terrain of NY into western New England near the best forcing/moisture plume and lower flash flood guidance). Uncertainties persist regarding the precise evolution and track of the approaching upper low/trough which will affect the speed of the surface front and in turn intensity/duration of heaviest rainfall. Even behind the first cold front, there may be diurnally-enhanced episodes of rainfall in the northeastern U.S. into late week and next weekend given the persistence of upper troughing. Held off from issuing any Marginal Risk there for Day 5/Friday as rain amounts look to decrease compared to Thursday, but there is a non- zero risk of flooding concerns especially if heavy rain falls in similar areas as the previous day. Farther west, some return flow of moisture (including perhaps Pacific moisture advecting around the upper ridge) is possible across the southern/central Rockies and Plains, with one or more surface boundaries helping to focus showers and thunderstorms. Placement and track of this convection each day is rather uncertain based on the shape of the ridge, as reflected by the relatively low QPF. However, potential exists for much higher amounts as evidenced by the 90th-95th percentile QPF. Scattered showers and storms may begin Thursday but the signal for their existence and intensity is too low to delineate any ERO risk. By Day 5/Friday, storms should increase in coverage and magnitude in the central U.S., potentially producing heavy rain. Maintained a Marginal Risk centered over Kansas/Oklahoma and clipping surrounding states. This is somewhat broad due to the uncertainty in placement--not all areas within may see heavy rainfall--but it may be able to lessen in size in future forecasts if models converge. Additional rounds of rain are likely in south- central parts of the Plains and Mississippi Valley into the weekend, with rain chances also increasing for the southern/central Rockies and Front Range. Some scattered rainfall is possible late in the week over parts of the Interior West/northern Rockies, likely increasing in coverage early next week as troughing approaches. 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, with the experimental HeatRisk product showing 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 in the Southwest as the weakening upper low approaches for 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. Fracasso/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