Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Thu Aug 10 2023 Valid 12Z Sun Aug 13 2023 - 12Z Thu Aug 17 2023 ...Heat wave to prevail across the South into the Southwest into late next week... ...Hazardous heat possible over the Pacific Northwest next week... ...Overview... Expect the southern tier heat wave, with daily record high temperature records most persistent over Texas and parts of the central Gulf Coast, to continue through at least next week and gradually expand into the Southwest as a strong upper ridge initially centered over Texas and the Lower Mississippi Valley pushes slowly westward and possibly expands northwestward. This ridge should displace an upper low forecast to be near the California coast during the weekend and early next week. A separate Northeast Pacific upper ridge may extend into the Pacific Northwest at times and connect with ridging to the southeast, bringing what may be an extended period of hot temperatures to parts of the Northwest. Guidance shows broad cyclonic mean flow persisting over the northern half or so of the lower 48 east of the Rockies, but with a transition from a series of individual southern Canada/northern U.S. shortwaves toward a larger scale mean trough over and south from Canada by mid-late week. Individual systems, in particular the one forecast to track from the Midwest into the Northeast early next week, may generate episodes of strong and heavy convection. Toward Wednesday-Thursday uncertainty increases regarding strength/location of the northern Pacific/Pacific Northwest ridge and specifics of downstream troughing. ...Guidance Evaluation/Predictability Assessment... Within the leading succession of upper shortwaves and associated surface lows/frontal systems, trends seen in the 18Z and new 00Z GFS runs toward the established ECMWF majority cluster have led to improved agreement versus prior days for the system tracking over southeastern Canada/New England on Sunday and then the unseasonably strong system expected to evolve from the Midwest through the Northeast, supported by a vigorous shortwave/embedded upper low. Among the 00Z runs, the new CMC is the extreme in showing slower timing than other guidance. Into early Day 5 Tuesday there is decent agreement/continuity for a cold front to reach the northern tier at that time, as the character of mean troughing begins to change. After early Tuesday there is increasing divergence among consecutive operational model runs and individual ensemble members for the position and strength of northeastern Pacific/Pacific Northwest upper ridging along with details of corresponding downstream troughing. For example, the 18Z/00Z GFS runs drift the Pacific ridge and downstream trough axis well eastward of the 12Z run, while the 12Z ECMWF also shifted well eastward of its prior 00Z version. Thus far the ensemble means have been fairly stable in keeping the upper ridge more offshore with flatter flow reaching the northern Pacific Northwest and southwestern Canada. After an operational model blend to reflect the most agreeable themes in guidance during the first half of the period, the updated forecast rapidly transitions toward 40-70 percent 18Z GEFS/12Z ECMWF mean input (with the 18Z GFS/12Z ECMWF-CMC making up the rest) to provide reasonable forecast continuity while incorporating a modest nudge of the operational runs in case their recent trends persist. ...Weather/Hazards Highlights... The most prominent focus for active weather during the Sunday-Monday night period reflected in the Days 4-5 Excessive Rainfall Outlooks will extend from the eastern Plains through parts of the Mid-Atlantic and Northeast, as an amplifying upper trough (likely with embedded low) and surface low that would be unseasonably strong for August should produce areas of strong and heavy convection. The Day 4 outlook depicts a Slight Risk area from the east-central Plains into the Mid Mississippi/Lower Ohio Valley, where there is reasonable agreement that instability and anomalous moisture should interact with a warm front lifting northeastward ahead of the primary surface system associated with the strong upper trough. Within the broader Marginal Risk area farther north, there are also some signals for enhanced rainfall near the surface/upper low but instability is less pronounced and there is a fair amount of guidance scatter. Meanwhile no change has been made within the Marginal Risk extending from the central Plains into the southern Rockies. For Day 5, guidance displays typically greater spread for QPF details. One area of relatively better heavy rainfall potential exists over parts central/northern Michigan/far northeast Wisconsin near the surface/upper low center, with guidance suggesting potential for higher localized totals than in the Day 4 time frame. The other area of relatively greater confidence is the central Appalachians where there is the best overlap of meaningful rainfall in the guidance and lower flash flood guidance values. Therefore plan to introduce Slight Risk areas over those two regions. Other locations within the encompassing Marginal Risk may be upgraded at some point (or current Slight Risk areas adjusted) pending better guidance consensus. Latest guidance is suggesting that southern Rockies convection may be heavier during the Day 5 period versus Day 4, but this is a fairly new trend. Therefore preferred to start with a Marginal Risk on Day 5 and await some stabilization in the guidance. The cold front anchored by the Midwest through Northeast system should reach the South by Tuesday-Wednesday, bringing some showers and thunderstorms with it. Some enhanced moisture may lead to locally moderate to heavy showers/storms over Florida early in the period, but for now they do not appear to be widespread enough to merit a risk area in the ERO. Flow ahead of the aforementioned cold front could support some organized rainfall over Florida by midweek. Diurnally favored convection will likely continue into late week over the Four Corners states and possibly some other locations over the West. A cold front dropping into the northern Plains on Tuesday may support showers and thunderstorms through Thursday as it pushes south and east. Forecast temperatures across most of the southern tier show little change, with the oppressive heat likely continuing through at least next Thursday. Greatest emphasis will be over the southern Plains through central Gulf Coast region where highs will likely be 5-10F or so above normal nearly every day. Some locations in Florida should see highs several degrees above normal as well, with a slight decrease possible by next Wednesday-Thursday. Westward expansion of the upper ridge as currently forecast would raise temperatures in the Southwest up to 5-10F above normal by next Tuesday-Thursday (leading to some highs exceeding 110F). Daily record highs will be most likely over the southern Plains, central Gulf Coast, and Florida. High humidity should lead to maximum heat indices at least in the 110-115F range along and near the western-central Gulf Coast, over the Lower Mississippi Valley, and perhaps with slightly less coverage farther east. 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 from Sunday onward. Very warm to hot temperatures may extend beyond the medium range forecast period. Record warm lows should be the most prominent aspect of this heat wave but a few isolated record highs may be possible by Wednesday-Thursday. Areas from the northern Rockies through most of the East will see variable temperatures to system/frontal progression. Of particular note, cool air behind the early week Midwest-Northeast system may bring a day or so of highs 5-10F or so below normal from the northern Rockies/Plains into Midwest. 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