Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sat Jun 1 2024 Valid 12Z Tue Jun 04 2024 - 12Z Sat Jun 08 2024 ...Excessive heat likely to continue over southern Texas, while heat builds in the West for the latter half of next week... ...Overview... As the medium range period begins Tuesday, expect an upper-level shortwave to track across the north-central U.S. along with a surface low pressure/frontal system, spreading rain and thunderstorms to much of the Mississippi Valley. This shortwave trough will meet a somewhat blocky pattern farther east and slow/deepen over the east-central U.S. late week into the weekend, with rain chances gradually shifting toward the eastern U.S. along and ahead of the cold front. Meanwhile an upper ridge atop Mexico and southern Texas Tuesday will expand to affect much of the western U.S. midweek and beyond, producing potentially hazardous heat. Then an upper trough may approach the West Coast late week but with uncertain timing. ...Guidance/Predictability Assessment... Recent model guidance is more agreeable than it was a day ago regarding the timing of the shortwave tracking near the U.S./Canadian border Tuesday-Wednesday. This feature is likely to form a closed upper low by Thursday near to northwest of the Upper Great Lakes. Deterministic and AI models are more variable with the placement of the upper low and the southern extent of the resultant trough. The newer 00Z GFS/CMC end up reaching farther south with the trough than the previous 12/18Z cycle, but the 00Z ECMWF elongates the upper low west-east strangely by next Saturday. Thus the details are still in question by late week/weekend with this feature. The relatively better agreement with the north-central to eastern U.S. trough also yields better model consensus regarding the western U.S. ridge, but uncertainties remain on the ridge's periphery. The evolution of the eastern trough/low will affect the ridge's extent north, with the greatest differences of ridging vs. troughing arising in the north-central U.S. by Friday-Saturday. Southern stream energy and possible closed upper low west of Mexico through much of the period could play a role in the ridge's shape on the southwest side. Then, there is model spread with timing of troughing in the eastern Pacific approaching the Northwest late week. The ECMWF is showing ridging strongly holding on in the Northwest with the Pacific trough well west all the way through Saturday, but EC-based AI/ML models indicate the trough farther east. However, the GFS runs are well on the eastern side of the envelope. A middle ground more like the ensemble means seemed best for timing of that approaching troughing. The WPC forecast utilized a multi-model deterministic blend early in the forecast period given good agreement in the large scale pattern at least. As the period progressed, transitioned to a blend incorporating the GEFS/EC ensemble means to half Day 6 and more Day 7, given the increasing spread and uncertainty. ...Weather/Hazards Highlights... A vigorous shortwave and frontal system (anchored by southern Canada low pressure) reaching the Dakotas into the Upper Midwest on Tuesday will provide forcing and a focus for rain and thunderstorms as instability abounds ahead of it. Smaller scale shortwaves farther south should also track eastward and promote scattered convection. A Marginal Risk is in place for the Day 4/Tuesday ERO for much of the Mississippi Valley and expanded a bit into the Mid-South based on recent model trends. Some locations within this area will be sensitive due to rainfall from prior days. The best focus for widespread moderate to heavy rain is likely across the Upper Midwest, but uncertainty in amounts and placement as well as the effects of the rainfall precludes a Slight Risk issuance at this time. Into Wednesday, convection is forecast to shift east ahead of the cold front into the Great Lakes region and Ohio and Tennessee Valleys, and a Marginal Risk is delineated for those areas. For both days, the forward motion of the storms may be a limiting factor for flooding to become too concerning. Steady progression of the front will take rain chances into the East for Thursday-Friday, with possible focus of heavier amounts from the northern Mid-Atlantic into the interior Northeast where the best forcing is. Some lingering precipitation is forecast in the Northwest on Tuesday before drying out midweek with the amplifying ridge aloft. Gusty winds are possible in the northern Rockies behind the strong southern Canada/northern tier U.S. surface low on Tuesday. Into late week, some return flow of moisture is possible across the southern/central Rockies and Plains, but with considerable uncertainty in coverage and amounts of rain. The same can be said of possible precipitation affecting the Northwest late week. 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 through Tuesday 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 run 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 near/over 110F in the Desert Southwest. Scattered daily record high minimum and maximum temperatures are possible. Per the Climate Prediction Center, this hot pattern could continue into early next week. 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