Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Mon Jun 3 2024 Valid 12Z Thu Jun 06 2024 - 12Z Mon Jun 10 2024 ...Hot temperatures across the West may be hazardous for the latter half of the week... ...Overview... Broadly, a western upper ridge and eastern upper trough pattern will set up later this week through the weekend, though 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 some rain is possible in the East on Thursday and perhaps continuing through the weekend in the northeastern U.S. underneath the trough. 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 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 with many ML models indicating a farther south centroid of the upper low and thus the resultant trough's southern extent compared to 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. Continued to favor a slightly deeper low than some of the more shallow guidance like the UKMET, but not to the southern extent of the ML models. For this forecast, that favored the 12Z GFS as well as the ensemble means. Spread and thus uncertainty continue to grow with what becomes of the trough into early next week, but at least the newer 00Z GFS and ECMWF seem somewhat in line with each other (00Z CMC farther east with the upper low). 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, with some expected detail differences. 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, some with timing of the eastward shift of the trough but especially as models vary with dropping energy southward to form upper lows somewhere in the eastern Pacific. The new 00Z model suite generally shows a faster trend of the trough approaching the Northwest and less chance for diving energies until perhaps Monday, but still show some variations. The WPC forecast favored a blend of the deterministic models early in the forecast, with the 12Z GFS heavier weighting than the 18Z, and gradually increased the proportion of ensemble means to half by the end of the period. ...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. Most guidance is still not too enthusiastic about heavy rainfall potential, but available moisture/instability ahead of a unseasonably strong upper low may be enough to produce some fairly intense rain rates. Thus the Day 4/Thursday ERO depicts a Marginal Risk area for the interior Northeast near the best forcing and lower flash flood guidance. Removed the northern Mid-Atlantic from the Marginal Risk as rain totals have sufficiently lowered there. 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 may be a nonzero 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. 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. For a first take at the Day 5 ERO, have a Marginal Risk delineated for 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 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, with 90s and 100s stretching into the central California valleys to the Great Basin to much of Texas. Scattered daily record high minimum and maximum temperatures are possible. The higher anomalies look to shift into the Northwest for the weekend with some minor moderation of the temperatures in the Southwest. 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