Extended Forecast Discussion NWS Weather Prediction Center College Park MD 340 PM EDT Wed Jun 28 2023 Valid 12Z Sat Jul 01 2023 - 12Z Wed Jul 05 2023 ...Major heatwave expands from Texas through the mid-lower Mississippi Valley and Southeast... ...Heatwave develops over Desert Southwest... ...Excessive rainfall threat focus from the Ohio Valley through the Northeast this weekend... ...Pattern Overview and Guidance Evaluation/Predictability Assessment... The medium range period (Sat July 1 - Wed July 5) begins in the middle of an overall pattern change away from the blocky flow that has been in place for much of June. The stubborn upper-level ridge/closed high over northern Mexico/the south-central CONUS is forecast to shift eastward over the Southeast and weaken to start July. An upper-level wave will follow over the Plains as a ridge builds over California and the Great Basin. The wave is expected to lift northeastward towards the East Coast as the overall flow transitions a bit more zonal, especially across the southern tier, as more aggressive upper-level activity lifts northward over southern Canada and the northern tier in a more summer-like pattern. An upper-level trough looks to drop southeastward across western Canada towards the northern tier and will help to drive another system southward over the Midwest/Plains in the mid- to late period. Some southern stream energy may linger over the south-central CONUS/Southeast as well as another weak wave moving over California. The model guidance is in generally good agreement with decent run-to-run continuity shown between the ECMWF/GFS. The most notable differences are related to the timing/strength of the discussed waves, though the more subtle nature of the flow makes predictability at the longer ranges more difficult, regardless. In terms of trends, the wave over California has been shifting weaker in the latest runs of the ECMWF/GFS and the latter system dropping south from Canada has slowed. Given the good agreement in the guidance, a general deterministic model blend of the 00Z ECMWF/CMC/UKMET and 06Z GFS works well for the WPC medium range product suite during the early to mid-forecast period, and follows continuity from the prior forecast. A shift towards a contribution from the 00Z GEFS/ECens means is used for the end of the period as the forecast-time limited UKMET contribution ends and uncertainty with respect to the details in latter waves grows. Despite general run-to-run continuity in the guidance on the large-scale, there does remain some notable uncertainty with areal coverage of QPF early in the period. ...Weather/Hazards Highlights... The main ejection of upper trough energies out from the Rockies/central Plains into the holiday weekend is expected to focus showers and strong to severe storm chances along a frontal system from the central Plains through the mid-Mississippi and Ohio/Tennessee Valleys to the Appalachians/northern Mid-Atlantic/Northeast along with a threat for locally heavy rainfall and runoff issues. "Marginal Risks" of excessive rainfall are in effect for the Day 4/5 Excessive Rainfall Outlooks given instability/moisture pooling and potential for repeat/training of cells along the frontal system and ahead of the deepening wave. Antecedent rainfall and additional influence from northern stream energy are also in the mix. A "Slight Risk" area is outlined for portions of the Middle Mississippi to Lower Ohio Valley for Day 4/Saturday given high pooled precipitable water values and overlap of higher QPF totals/ensemble probabilities. A noted bit of uncertainty remains with the timing/location/progression of potentially organized convection which may lead to additional areal adjustments of the risks. The overall convection and heavy rain focus should shift southeast and become less organized into early next week with wave and trailing front progression and eventual stalling. Precipitation chances will increase across the central/northern Plains and the Upper Midwest Tuesday/Wednesday ahead of the next frontal system dropping south from Canada. Excessive heat to start the weekend from southern/southeast Texas into the Lower Mississippi Valley and Southeast/Florida will briefly expand northeastward into the Carolinas through Monday before a general downward trend towards Summer averages outside of south Texas. The heat will be oppressive with widespread high temperatures in the mid-90s to low 100s which, when combined with the humidity, will lead to heat indices in the 110-115 degree range, locally even a bit higher. Excessive Summer heat will also overspread portions of the West this weekend, with the central California Valleys seeing highs of 105-110 degrees, upwards of 10-15 degrees above average. The Desert Southwest will experience a notable heating this weekend as well with max temps 5 to 10 degrees above average which means low to mid 110s for these hot environs south from Las Vegas and west from Phoenix. Elsewhere, above average high temperatures in the upper 80s to mid 90s along the northern tier of the central U.S. this weekend will shift eastward into the Upper Midwest/Great Lakes region ahead of a cold front, which will bring much cooler temperatures in the 70s to portions of the central/northern High Plains for the July 4 holiday. Putnam/Schichtel 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 Hazards: - Flooding possible across portions of the Northern Plains. - Hazardous heat across portions of California, Fri-Sun, Jun 30-Jul 2. - Hazardous heat across portions of the Lower Mississippi Valley, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, and the Southern Plains, Fri-Tue, Jun 30-Jul 4. - Hazardous heat across portions of the Central and Southern Plains, the Lower and Middle Mississippi Valley, the Tennessee Valley, the Southern Appalachians, and the Ohio Valley, Fri, Jun 30. - Hazardous heat across portions of the Lower and Middle Mississippi Valley, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, the Southern Plains, and the Ohio Valley, Fri-Sat, Jun 30-Jul 1. - Heavy rain across portions of mainland Alaska, Sun-Mon, Jul 2-Jul 3. - Heavy rain across portions of mainland Alaska, Fri, Jun 30. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: 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