Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Mon Aug 07 2023 Valid 12Z Thu Aug 10 2023 - 12Z Mon Aug 14 2023 ...Heat wave to persist over the South into the Southwest... ...Excessive rain/flash flood threat over the Tennessee Valley/southern Appalachians Thursday... ...Overview... As of Thursday expect an elongating upper ridge axis to extend from the Rio Grande Valley eastward through the Florida Peninsula. By the weekend and early next week a building upper-level high should become stronger/more dominant farther westward, focusing over Texas and the southern Rockies. This ridge will maintain the ongoing southern tier heat wave with daily record high temperatures possible every day of the forecast period, with the most anomalous temperatures likely to be over Texas late this week. Farther north, low amplitude flow aloft will initially hint at a mean trough axis over the east-central U.S. The mean trough should gain better definition by Sunday-Monday as a building Northeast Pacific upper ridge may extend its influence into the Pacific Northwest and far southwestern Canada. The upper ridge may start to bring very warm to hot temperatures to the Pacific Northwest by the start of next week. Shortwaves within the cyclonic downstream flow will produce a series of central-eastern U.S. waves/frontal systems that are likely to produce episodes of locally heavy rainfall, but with some uncertain specifics due to the lower predictability of important details. The National Hurricane Center is issuing advisories for Tropical Storm Eugene in the Pacific, with dissipation of the surface reflection expected before early Thursday. However the system's remaining upper level energy and possibly some moisture could get incorporated into an upper trough/low forecast to evolve near the California coast. ...Guidance Evaluation/Predictability Assessment... Forecast guidance indicates the medium-range period will begin during a brief weakening phase in the stagnant upper-level high that has been in place across at least portions of the southern tier of the country for the last few weeks. Energetic upper-level flow with embedded shortwaves will be in place across the northern 2/3rds of the eastern half of the U.S., with a bit more upper-level ridging shifting northwestward keeping stronger flow aloft over northern portions of the West. The upper-level high begins to build back in, anchored over the south-central U.S., into the middle of the period, potentially expanding and combining with an upper-level high over the western Atlantic. The pattern also looks to amplify a bit as well, with mean long-wave troughing over the Plains/Midwest/Northeast as upper-level ridging builds in even more so over the West. The guidance remains in good agreement on the large-scale pattern through the middle of the period, with the typical increase in uncertainty on smaller-scale shortwave features, before a more significant difference emerges upstream over the northeastern Pacific in the mid- to late period. The 00Z ECMWF and CMC show a much more expansive ridge over the West, anchored over the northeastern Pacific, with an upper-level high developing over the northeastern Pacific. In contrast, the more recent 00Z and 06Z runs of the GFS have shown a deep upper-level low over the Gulf of Alaska transitioning into an open wave moving southeast into southwest Canada and the Pacific Northwest. Another feature of note has been the evolution of an upper-level low along the coast of California related to the remnants of Tropical Storm Eugene over the eastern Pacific. Recent runs of the GFS through 00Z have been trending towards a more expansive weakness in the flow, combining with the wave to the north, as opposed to a cutoff upper-low embedded south of the ridge in the ECMWF, and to a lesser extent the CMC. The 06Z GFS trended slightly back towards the 00Z ECMWF solution with respect to these features as well as the overall pattern across the West and northeastern Pacific, though still shows significant differences. Also, the 00Z CMC differs from the 00Z ECMWF and 06Z GFS by developing a more potent upper-level wave dropping south from northwest Canada into the north-central U.S. towards the end of the forecast period compared to the mean long-wave troughing further east over the northern tier in the other guidance. The 00Z ECens mean favors the upper-level high over the northeastern Pacific and mean long-wave troughing to the east, with a similar but slightly less amplified solution in the 00Z GEFS mean. The updated WPC forecast uses a general model blend of the 00Z deterministic guidance (06Z for the GFS) with a bit more emphasis on the ECMWF given the overall trend through the forecast period. The 00Z ECMWF solution is increasingly favored in the mid- to late forecast period as both the 00Z ECens and GEFS means support a similar pattern evolution. Contributions from these ensemble means are increased as smaller-scale differences grow overall in the deterministic guidance with increasing forecast lead time, including the additional noted differences in the evolution over the northern tier of the country between the CMC and ECMWF/GFS. This blend remains consistent to the prior WPC forecast which favored the ECMWF/ECens solutions and thus results in a similar forecast with respect to broad upper-level ridging over the northeastern Pacific/western Canada and U.S. ...Weather/Hazards Highlights... The potentially heavy convective event expected over parts of the Mississippi/Tennessee Valley region on Wednesday (now in the short range time frame) should extend eastward toward the southern Appalachians and vicinity by Thursday, favoring maintenance of the Slight Risk area depicted in that region for the new Day 4 Excessive Rainfall Outlook. Similar to yesterday, guidance offers some conflicting ideas for surrounding details from the Mississippi and Ohio rivers through the central Appalachians within an overall signal for some locally heavy rainfall, with a Marginal Risk maintained for the region. During Days 4-5 Thursday-Friday, the outlooks show a Marginal Risk area from the northern Plains into the central Great Lakes with the system approaching/crossing the region. Some locally intense rainfall may occur but antecedent ground conditions are on the drier side and moderate progression may temper totals somewhat overall. For Day 5 from the east-central Plains into the Southeast, there are some diffuse guidance signals for locally heavy rainfall within an environment of moderately above normal precipitable water values and decent instability. In addition one or more waves/fronts may provide some synoptic focus for convection. This scenario appears to support a Marginal Risk area at this time, with better definition perhaps arising in a few runs. From the weekend into early next week, expect some showers and thunderstorms to continue into the East with progression of the Great Lakes system and trailing front, with another round of some lighter, scattered showers possible across the northern tier with another system Sunday-Monday. Expect Florida to see an increase of rainfall this weekend as an area of enhanced moisture moves in from the east. The Four Corners/Southeast/California should see diurnally favored episodes of convection, though model/ensemble differences in the distribution/magnitude of moisture anomalies (including what may happen with any upper level moisture from Eugene) keep confidence lower than desired for specifics. One or more fronts settling into the central/south-central Plains look provide a focus for heavier showers and thunderstorms over that region this weekend, with some of the moisture over the West possibly drifting east to contribute. The ongoing heat wave over the southern tier should continue through the weekend and early next week, with greatest emphasis over the southern High Plains where highs could reach 10-15F above normal especially late this week. Temperatures may reach or exceed daily record highs from the central/western Gulf Coast into the southern Rockies Thursday-Friday, with the coverage of possible record highs tending to decrease gradually thereafter. High humidity should lead to maximum heat indices at least in the 110-115F range along and near the western half of the Gulf Coast. The Desert Southwest should see a brief lull in the 110F+ highs through late week but temperatures may reach into the lower half of the 110s once again by Sunday-Monday as the upper ridge strengthens over the West. The Florida Peninsula could also see a few record highs. Record warm lows will be possible across a broad portion of the southern tier each night. Meanwhile strengthening upper ridging should bring higher temperatures to the Pacific Northwest during the weekend and early next week, with highs reaching 10F or more above normal over some locations. Very warm to hot temperatures may extend beyond the medium range forecast period. In contrast, the forecast pattern will tend to favor near to moderately below normal high temperatures from the northern Rockies into the Midwest/Ohio Valley. The upper trough/low developing near California should keep highs over most of the state near or slightly below normal at least through late week. Putnam/Rausch 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, 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