Extended Forecast Discussion NWS Weather Prediction Center College Park MD 327 PM EDT Thu Aug 10 2023 Valid 12Z Sun Aug 13 2023 - 12Z Thu Aug 17 2023 ...Severe weather and flash flooding likely across parts of the central to eastern U.S. early this week... ...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. Meanwhile, 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 convection with heavy rainfall. Toward Wednesday-Thursday uncertainty increases regarding strength/location of the northern Pacific/Pacific Northwest ridge and specifics of downstream troughing, affecting the temperature forecasts at that time. ...Guidance Evaluation/Predictability Assessment... The first area of concern with model diagnostics is with the vigorous shortwave/embedded upper low and its unseasonably strong surface low reflection tracking through the Midwest to Northeast. Timing differences remain with this feature, with GFS runs including the newer 12Z run farther east/faster than other guidance while many GEFS ensemble members and the mean do the same. Since this can be a fairly typical model bias of the GFS suite, preferred the non-NCEP guidance for the most part, favoring especially the ECMWF and UKMET as the CMC was perhaps too slow. The incoming 12Z cycle other than the GFS did converge even better on a placement with the EC and UK slowing down a bit with the CMC speeding up a bit. These differences do have significant implications on the timing and placement of QPF. A multi-model blend worked well for continued ridging across the south-central to southeast U.S. as well as a weak upper low offshore near California. Additionally, agreement is increasing for ridging to poke into the Pacific Northwest through around Tuesday, leading to rising temperatures there. Models are still variable though with how long this ridgy pattern sticks around, with the 00/06Z models more aggressive with it maintaining it and pushing northern Canada energy east of that compared to the 12Z guidance. With this uncertainty, transitioned to using and increasing the amount of GEFS and EC ensemble means to just over half of the blend by the end of the period, after the deterministic model start favoring the ECMWF. ...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 and Severe Weather 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 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. The Slight Risk is quite broad given model disagreements in the exact timing and track of the fronts and the possibility of multiple axes of enhanced rainfall. A Marginal Risk extends northward into the Upper Midwest closer to the low track on Sunday/Day 4, with good forcing in place. Model guidance shows that instability could get tucked into the low as it moves east into the Upper Great Lakes region Day 5/Monday, and thus maintained a Slight Risk in the ERO by then for higher rain rate potential. Areas near the eastern Ohio Valley and central Appalachians could also see heavy rainfall totals that could overlap with lower Flash Flood Guidance values to cause some flooding issues, so the Slight Risk was maintained there and expanded a bit westward. Meanwhile rain amounts trended down in this forecast cycle farther east in the Northeast on Monday. As usual, outlooks could change pending future forecasts. The good support aloft could also contribute to severe weather over eastern parts of the central Plains into the Mid-Mississippi Valley and Ohio Valley Sunday-Monday. 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. But flow ahead of the aforementioned cold front could support some organized rainfall over Florida by midweek. Diurnally favored convection will likely continue across the Four Corners states and possibly focus generally in the southern Rockies through much of next week. Marginal Risks are in place in the EROs Sunday and Monday, with a possible peak in coverage/magnitudes on Monday. 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. The 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 early to mid-next week, with highs reaching 10F to locally 20F above normal over some locations from Sunday onward. Widespread record warm lows will be a prominent aspect of this heat wave but some record highs are possible as well. Areas from the northern Rockies through most of the East will see variable temperatures due to system/frontal progression. Of particular note, cool air behind the early week Midwest-Northeast system may bring a day or so of highs 5-15F or so below normal from the northern Rockies/Plains into Midwest. Tate/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