Extended Forecast Discussion NWS Weather Prediction Center College Park MD 310 AM EDT Fri May 31 2024 Valid 12Z Mon Jun 03 2024 - 12Z Fri Jun 07 2024 ...Excessive heat likely to continue over southern Texas, while heat builds in the West for the latter half of next week... ...Overview... The early part of next week will feature progressive shortwaves and frontal systems in the northern two-thirds or so of the lower 48, leading to precipitation chances in the Northwest on Monday and possibly multiple rounds of convection in the central and eastern U.S. through midweek. Meanwhile very hot and humid conditions should continue across southern Texas under the northern periphery of an upper ridge anchored over Mexico, to the south of the storm track. By the latter half of the week, a more amplified and stagnating pattern develops, as upper ridging expands from Texas into the Interior West (expanding the coverage of potentially hazardous heat), while an upper trough axis develops downstream in the East. ...Guidance/Predictability Assessment... Model guidance is reasonably agreeable as the forecast period begins Monday, with a shortwave over the north-central U.S. that lifts northeast by Tuesday while additional energy approaches the Northwest. This latter feature will be a concern for model diagnostics and for sensible weather though, showing more model spread. Deterministic models all develop a closed upper low with this system by Tuesday, but differ considerably with its positioning/how progressive it is, especially by midweek. The 12Z ECMWF is the slowest/farthest southwest of the deterministic guidance, with GFS runs faster and the CMC in between. The ECMWF's slow track seemed unlikely considering that the EC-based AI/machine learning models were all faster/farther northeast than the operational ECMWF. Thus preferred the 18Z GFS/12Z CMC upper low track. The 12Z GFS seemed agreeable through Wednesday-Thursday but then dove energy southeast to form a closed low atop the southern Appalachians Friday, which was an outlier. With the incoming 00Z guidance, the models are generally faster than their previous runs (though the 00Z CMC seems a little rogue with diving an upper low toward the Mid-South Friday). The new 00Z ECMWF now looks more like the older 18Z GFS, while the 00Z GFS is even farther east/faster than its previous runs. So choosing the faster models may have been good for this forecast with only the 12/18Z guidance cycle available at the time, but overall there is still uncertainty with the timing yet to be resolved. Models are agreeable in indicating an upper ridge building into the Southwest for midweek and beyond. However, there are uncertainties with southern stream troughing breaking off in the east Pacific west of Baja California, and with the northeastern extent of the ridge in the central U.S. dependent on where the east-central U.S. trough ends up setting up. By Friday models go out of phase with the flow pattern nearing the Northwest, with GFS runs showing troughing while EC/CMC runs have ridging. Considering these factors, the WPC forecast was based on a blend of deterministic guidance early on, reducing the proportion particularly of the 12Z ECMWF and eventually the 12Z GFS in favor of the GEFS and EC ensemble means, with the means reaching 60 percent of the blend by Day 7. ...Weather/Hazards Highlights... An upper-level wave and atmospheric river coming into the Northwest will bring heavy rainfall continuing into Monday. Marginal Risks are in place for flooding concerns with the heavy rain. Snow levels may be lowering some by Monday in the Cascades and northern Rockies as the shortwave moves overhead, but there is still some concern for rain at elevations that still have a snowpack, possibly increasing the flooding threat. Precipitation should then exit Tuesday-Wednesday, with dry conditions across the West as ridging builds. An initial low pressure/frontal system in the central U.S. and the next one coming in from the West will lead to rounds of showers and thunderstorms broadly across central and eastern portions of the country. The focus for potentially heavier rain is rather uncertain for Monday as instability abounds in widespread areas, but there still seems to be a broad model signal for Missouri/Arkansas and surrounding states to maintain a Marginal Risk there for the Day 4 ERO. By Day 5/Tuesday, the focus for convection is a little more clear as instability pools ahead of the low pressure system tracking near the U.S./Canadian border and its cold front extending south. A Marginal Risk is in place across northern and central parts of the Mississippi Valley Tuesday. As the cold front tracks east, rain should shift toward the Ohio Valley and Appalachians by Wednesday-Thursday. Additional weak upper-level energies from the Southern Plains to the Southeast will also keep up the potential for additional storms. Next week will remain hot across 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+. Temperatures nearing or exceeding 100F may stretch across the southern High Plains early in the week but should moderate somewhat by midweek behind a frontal passage. Some above normal temperatures are possible across the northern tier at times. But the next big concern will be increasing heat across the West as upper ridging builds in. 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 of the Great Basin. Temperatures should reach over 100F across the central California valleys, and reaching over 110F in the Desert Southwest. Per the Climate Prediction Center, this hot pattern could continue at least into next weekend. 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