Extended Forecast Discussion NWS Weather Prediction Center College Park MD 326 AM EDT Thu May 30 2024 Valid 12Z Sun Jun 02 2024 - 12Z Thu Jun 06 2024 ...General Overview... An active flow pattern over both central/eastern portions of the country and from the Pacific across the northern tier U.S. and southern Canada (with multiple frontal systems) should favor multiple episodes of showers and thunderstorms from the weekend into next week. The Southwest should remain mostly dry, while southern Texas should see persistent hot and humid conditions under the northern periphery of an upper ridge anchored over Mexico. A developing ridge over the southern half of the West by mid-week will bring widespread above average temperatures and potentially hazardous heat. ...Guidance/Predictability Assessment... Initial uncertainty into the medium-range period not surprisingly revolves around the timing of multiple shortwaves passing from the Plains/Mississippi Valley eastward towards the Midwest/Ohio Valley before shearing off towards the East Coast. The 12Z ECMWF/UKMET, supported by the 12Z ICON and suite of EC machine learning (ML) models, lie in the middle of the envelope of all solutions showing an initial wave lifting into the Ohio Valley with the 12Z CMC a bit faster and the 18Z GFS the most different, racing ahead the other solutions in eastward position/movement and with a secondary wave to its southwest. Timing is a bit more evenly distributed with another rather progressive wave following in its wake, with the cluster of solutions likely providing a good compromise. Began with a multi-model blend slightly favored towards the ECMWF/UKMET/CMC over the GFS given the stronger initial differences. Attention then turns west as shortwave energy ejects from a deep closed low south of Alaska, traversing an eastward path close to the international boarder, before a broad longwave ridge builds over the West. The deterministic guidance falls into three camps with respect to both features, with the ridge in the ECMWF taking a more western position anchored towards the West Coast, the GFS further east centered more over the Intermountain West, and the CMC in between, a bit closer to the ECMWF but less amplified with respect to both the ECMWF and GFS with the ridge in general. Further east, the GFS and ECMWF are more aggressive with the lead shortwave energy digging southward over the Midwest/Great Lakes while the CMC is similarly less amplified further east with a more northerly wave track. The 12Z ECens/GEFS means are similar to their respective deterministic counterparts with both the ridge and northern wave, though with a lesser degree of amplification. The 12Z ICON was more similar to the 12Z ECMWF, with the suite of EC ML models once again providing a range of solutions encompassing the deterministic/ensemble mean solutions. Given this range of solutions, opted for an even blend of the deterministic and ensemble mean guidance for this latter period until there is more movement towards one of the solutions. The overall pattern, specifically the notable ridging in the West, is still relatively well agreed upon, with a heat wave expected over the region but with differences on extent with respect to time and coverage. ...Weather/Hazards Highlights... Periods of showers and thunderstorms are expected broadly across central and eastern portions of the country heading into next week with multiple upper-level waves and accompanying surface frontal systems encountering a humid airmass. Both the uncertainty at this range with respect to the timing of the waves and their generally progressive nature make it a more difficult task to narrow down areas of potentially locally heavier rainfall and flash flood concerns. The updated day 4 and 5 Excessive Rainfall Outlooks (EROs) contain multiple Marginal Risks where guidance shows the best overlap of higher QPF values. For day 4 (Sunday), this includes portions of the Southern Plains and Lower Mississippi Valley as well as the Upper Midwest in the wake of an initial shortwave lifting through the Ohio Valley and a second approaching wave/frontal system passing from the Plains towards the Mississippi Valley. Some better agreement in the deterministic guidance for potentially more widespread heavier rainfall worthy of a Slight Risk was noted from Minnesota into Wisconsin, however the subtle and progressive nature of the upper-level wave still casts some doubt on the timing and extent of rainfall, backed up by low ensemble probabilities. Some moderate to heavy rainfall will also be possible in the Ohio Valley as the lead wave lifts through. An additional round of storms is expected day 5 (Monday) ahead of the frontal system pushing further into the Great Lakes and southward into the Middle Mississippi Valley and Central/Southern Plains, with another Marginal Risk introduced for the latter region. Another upper-level system is expected to traverse the international border and eventually dig southeastward into the Midwest. This may bring areas of moderate to heavy rainfall to the Upper Midwest Tuesday and Ohio Valley/Lower Great Lakes by Wednesday, though with greater uncertainty. Additional weak upper- level energies from the Southern Plains to the Southeast will also keep up the potential for additional storms. An upper-level wave and accompanying plume of Pacific moisture will bring heavy rainfall to the Pacific Northwest on Sunday. A Marginal Risk is included on the day 4 ERO along the Washington/Oregon coasts and inland to the Cascades. The warm moist air should keep snow limited to only the highest mountain peaks. The warmer temperatures may also help enhance snowmelt locally, adding to the risk for some isolated flooding concerns. While streamflows are low locally, another Slight Risk may be necessary here if the forecast QPF amounts continue to increase. The precipitation will spread inland over the northern Rockies Monday with the chance for locally heavy amounts over 1 inch, and a Marginal Risk introduced. Precipitation chances will linger into midweek for the Pacific Northwest, though should decrease each day as ridging builds in more broadly over the West. Forecast high temperatures will start above average by 10-15 degrees late this weekend into early next week over portions of the High Plains as well as New England, with generally near-average temperatures in between with daily bouts of storms for the Plains/Mississippi Valley/Southeast. One exception will be south Texas westward through the Rio Grande Valley, where the combination of hot temperatures into the 90s and 100s as well as humidity will keep heat indices in the upper 100s, locally 110+. Highs will initially remain below average over the Pacific Northwest as the Pacific system passes through the region. The big story temperature-wise will begin to develop by mid-week as ridging builds over the West. Highs broadly will be running 10-15 degrees above average, with even warmer anomalies of 15-20+ degrees for the central California valleys and portions the Great Basin. Forecast highs are into the 100s from west Texas into the Desert Southwest and central California valleys. Unfortunately, the Climate Prediction Center has highlighted much of this region with at least a Slight Risk of Excessive Heat with the expectation this heat wave will continue beyond the current forecast period into at least next weekend. Putnam 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