Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Wed Aug 09 2023 Valid 12Z Sat Aug 12 2023 - 12Z Wed Aug 16 2023 ...Heat wave to persist over the South into the Southwest through at least the middle of next week... ...Hazardous heat possible over the Pacific Northwest next week... ...Overview... Not much has changed in the guidance across the southern tier, with strong southern Plains through Florida upper ridging likely to build gradually westward toward the Southwest U.S., eventually displacing an upper low forecast to hover near the California coast through the weekend. There is still potential for the northwestern part of this ridge to connect with a separate ridge building over the northeastern Pacific. The expanding ridge will maintain the 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 into the central Gulf Coast region, while supporting a hotter trend over the Southwest by next week. The Pacific Northwest should see very warm to hot temperatures by next week as well. Away from the broad area of upper ridging, broad cyclonic flow will prevail over the northern half or so of the lower 48 east of the Rockies. A succession of embedded shortwaves should support surface waves/frontal systems that may be capable of producing episodes of locally strong/heavy convection. 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... Overall the models and ensemble means have been agreeing on the general theme of shortwaves within the progressive northern tier flow supporting a series of surface lows/frontal systems, but with some typical discrepancies given the scale of the shortwaves. Already by the start of the period early Saturday, recent GFS runs have been differing somewhat from most other guidance for details of flow coming into western Canada, with increasingly noticeable effects downstream with time. The GFS is a bit on the slower side with the leading system tracking east/northeast from the Great Lakes during the weekend, but differences are more significant with the trailing shortwave/surface system. The GFS is in the minority showing a somewhat faster/broader shortwave as of Sunday, and while its shortwave sharpens like consensus by Monday the end result is to lose the well-defined Great Lakes surface system seen in the latest ECMWF/CMC/UKMET runs and the ECens/CMCens means. Latest GEFS means tilt in the GFS direction in slightly more subdued form. From Monday onward, this means a difference between the ECMWF cluster showing yet another rather strong system tracking into New England or southeastern Canada versus the 18Z/00Z GFS depicting a weaker coastal wave. The 12Z GFS did have some aspects closer to the ECMWF cluster though. Another point of contention arises by Tuesday-Wednesday when amplification of incoming Pacific flow and possibly central Canada flow to the north reaches into the northern tier states. The 18Z GFS in particular looked somewhat underdone with its trough amplitude, keeping the associated surface front farther north. Again the 12Z GFS was somewhat closer to the average of other models/means, though it may have been a bit slow with its overall trough axis. Also of note over the West, the 00Z GFS is stronger than other guidance with shortwave energy that lifts into the Great Basin/Intermountain West by early next week. Based on guidance comparisons from the 12Z/18Z cycle, the updated forecast for the first half of the period started with input from the 12Z ECMWF/UKMET/CMC with a small contribution from the 18Z GEFS mean in place of the seemingly less desirable 18Z GFS. Then the blend transitioned toward somewhat more emphasis on the 12Z ECens/CMCens and 18Z GEFS means relative to the ECMWF/CMC runs given typically increasing detail uncertainties. This approach yielded fairly good overall continuity while beginning to trend partially toward the stronger Great Lakes-New England system around the start of next week. ...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. 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