Extended Forecast Discussion NWS Weather Prediction Center College Park MD 304 AM 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 elongated upper ridge axis to extend from the Rio Grande Valley eastward through the Florida Peninsula. By the weekend and early next week the western part of this ridge should become stronger/more dominant and build 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... The latest models and ensembles maintain a lot of the general ideas seen over the past couple cycles but with continued detail spread and in some cases trends. Consensus still shows a leading southern Canada into Great Lakes/New England upper trough late this week helping to push a cold front into those regions while a separate shortwave to the south promotes some degree of surface waviness from the Ohio Valley/Middle Mississippi Valley to the East Coast. The small scale nature of this shortwave and differences in strength/timing have been producing considerable spread for specifics of the surface reflection by Friday. The new 00Z UKMET is a notable strong extreme with its wave. Behind these features, guidance is gradually getting better defined with what may be a fairly vigorous system reaching the Great Lakes by Saturday. Recent GFS runs have strayed a bit on the faster side of the spread due to a combination of faster timing/more southeastward amplitude of the upper shortwave. The new 00Z run has come back some from its 18Z run, now matching the GEFS mean. An intermediate solution seems reasonable given typical predictability/error ranges a few days out in time. The trailing cold front seems to be trending somewhat farther southward over the central U.S., perhaps in response to model consensus better resolving the strength of the upper trough. This system should continue into eastern Canada after Saturday. The next system in the series may reach the northern tier by Sunday-Monday. As was the case for the preceding system 6-7 days out, the signal for this last system is fairly diffuse in the models/means overall but with enough overlap to depict a defined, albeit weak, system. By next Monday the 00Z GFS/CMC develop issues not seen in the 12Z-18Z model/mean cycle, with the GFS bringing a fairly strong shortwave through the Northeast Pacific/western North America mean ridge and the CMC showing more amplitude/sharpness of a northern Plains shortwave. An operational model blend transitioning toward a model/ensemble mean mix reflected continuity for the large scale forecast while incorporating some trends where suggested by the majority of guidance. ...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 Mid-Atlantic within an overall signal for some locally heavy rainfall. This recommends no change to the Marginal Risk area. 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, and possibly over the northern tier again 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 may provide a focus for heavier showers and thunderstorms over that region, 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. 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. 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. 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. 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