Extended Forecast Discussion NWS Weather Prediction Center College Park MD 327 PM EDT Wed Aug 09 2023 Valid 12Z Sat Aug 12 2023 - 12Z Wed Aug 16 2023 ...Heat wave to prevail across the South into the Southwest through at least the middle of next week... ...Hazardous heat possible over the Pacific Northwest next week... ...Overview... The heatwave across the southern tier will endure through at least next week as ridging builds westward. This expansion will displace an upper level low anticipated offshore the coast of California through the weekend. The potential remains for the for the northwestern part of this ridge to connect with a separate ridge building over the northeastern Pacific. This heat will lead to daily high temperature records, particularly for Texas and parts of the central Gulf Coast. A series of embedded shortwaves within a broad cyclonic flow will will track east of the Rockies across the northern tier. Multiple frontal systems will usher locally strong/heavy convection with them as they progress eastward. These features have varying degrees of predictability, with greater confidence in the general pattern of progressive surface waves/fronts than some of the finer but important details. ...Guidance Evaluation/Predictability Assessment... The latest runs of the model and ensemble guidance continue to have a decent handle on the large scale pattern by keeping ridging across the South while the northern flow is progressive with a series of surface lows/frontal systems. Typical model differences persist given the scale of the shortwaves, but are within reason. The GFS once again depicted displaced QPF from the cluster with the initial system over the Northeast and continued to be an outlier with the next round moving across the Upper Midwest/Midwest/Great Lakes region. The ECWMF, CMC and UKMET where best clustered and maintained features through the week, especially with another system that is depicted for the Northeast; whereas, the GFS suggested a weak coastal wave. The GEFS mean was not as reflective as its parent the GFS and leaned toward the ECWMF/CMC/UKMET although a bit muted. Followed a similar forecast approach for this issuance by utilizing 00Z ECWMF/CMC/UKMET with a some inclusion of the GEFS mean through Day 5 of the extended period. For the latter portion of the extended period the UKMET was dropped and replaced with the EC and CMC ensemble means. This approach helped maintain forecast continuity. ...Weather/Hazards Highlights... During the Saturday-Sunday night period covered by the Days 4-5 Excessive Rainfall Outlooks, both days should see some potential for locally heavy convection over parts of the southern Rockies/south-central High Plains with some shortwave energy/moisture lifting northward between the upper low over the California coast and ridge to the east, while a couple fronts may drop into the central High Plains to provide added focus. A Marginal Risk area accounts for this potential. Areas from the Southeast into parts of the Mississippi/Tennessee Valley and northeastward into the and near the eastern Great Lakes are also in a Marginal Risk area for Day 4, with a couple frontal systems interacting with above normal moisture and instability. One or more embedded Slight Risk areas could eventually arise if shorter term guidance eventually starts to refine where more pronounced focus may exist. There is also a diffuse signal for locally heavy convection over parts of the north-central Plains/Midwest on Day 4 but clustering does not appear sufficient to warrant a risk area at this time. On Day 5, leaning toward the majority ECMWF cluster for upper level and surface evolution over the Midwest would support a fairly broad area of locally heavy convective potential that could link back to the south-central High Plains. Thus plan to depict a Marginal Risk area over the Midwest and connecting with the aforementioned southern Rockies/High Plains area. Florida may see some increased rainfall during Days 4-5 as moisture anomalies increase but compared to a couple days ago the guidance has become more diverse regarding the timing and magnitude of moisture anomalies, so will not depict a risk area there at this time. Moisture and shortwave energy should produce some diurnally enhanced showers and thunderstorms over other portions of the West during the period, with confidence tempered by guidance differences in specifics of location/strength of the upper level energy and moisture details. Meanwhile the system which the guidance majority depicts over the Great Lakes by Monday could produce another episode of convection that could be strong/heavy over some areas of the eastern U.S. Continue to monitor forecasts as guidance eventually comes into better agreement on system details. Yet another cold should drop into the northern Plains by Tuesday-Wednesday, with some associated rainfall possible. The heat across the southern tier will likely continue through at least the middle of next week with the greatest emphasis 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, while westward expansion of the upper ridge as currently forecast would also raise temperatures in the Southwest up to 5-10F above normal by next Monday-Wednesday (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 will be possible by early-mid week but forecast highs are not yet reaching daily record values. Areas from the northern Rockies through northern-central parts of the East will see variable temperatures up to 5-10F on either side of normal, corresponding to system/frontal progression. The upper trough/low near California should keep highs over the Southwest slightly below normal into the weekend, followed by a warmer trend as the feature drifts westward. Campbell/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