Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Tue Aug 8 2023 Valid 12Z Fri Aug 11 2023 - 12Z Tue Aug 15 2023 ...Heat wave to persist over the South into the Southwest through early next week... 19Z Update: The 12Z model guidance suite is initially well clustered across the continental U.S. for the end of the week, with some timing differences with a shortwave trough crossing south-central Canada since the GFS is slower and a little stronger. Going into the weekend, more notable differences with the GFS are evident across southern Canada and across the northern Plains/Upper Midwest, but it is relatively in line with the other guidance elsewhere. By early next week, the GFS is a little weaker with the degree of upper level ridging across the northwestern U.S. which could have some impact on the magnitude of the hot temperatures expected. All of the model guidance agrees well on maintaining the southern Plains/Gulf Coast upper ridge, and this results in high confidence of the ongoing heat and oppressive humidity continuing unabated for many of these areas going into the middle of August. In terms of the Day 4 and 5 excessive rainfall outlook, there is still not a clear enough signal for any Slight Risk areas at this time. The fronts/pressures forecast was weighted slightly more towards the ECMWF/CMC along with some previous WPC continuity through Sunday, and then increasing use of the ensemble means to about 40% by next Tuesday. The previous forecast discussion is appended below for reference. /Hamrick ------------------------- ...Overview... Latest guidance is suggesting that the upper ridge initially centered from the far southern Plains through the Florida Peninsula should expand across the southern and western U.S. from late this week into early next week. The western part of this ridge could ultimately connect with a separate ridge forecast to build 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. Temperatures will also gradually increase over the Southwest while very warm to hot temperatures become established over the Pacific Northwest. Meanwhile 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 heavy rainfall, but these features have lower predictability several days out so confidence in important details is not very high. While former Tropical Storm Eugene in the eastern Pacific has weakened to a remnant low, 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. This feature may retrograde early next week as upper ridging builds in from the east. ...Guidance Evaluation/Predictability Assessment... Within the broad cyclonic northern stream flow, most guidance is still signaling a sequence of distinct systems but with ongoing detail differences. As of the start of the period early Friday, the new 00Z UKMET differs from most other guidance by showing less phasing of a short range mid-latitude U.S. wave and the upper low getting close to Maine. Behind this system, there is still a loose clustering for an area of low pressure to track across the northern tier, Great Lakes, and southeastern Canada/New England during Friday-Sunday. There are still notable detail differences while GFS runs have ranged between strong/fast solutions a day or so ago to the new 00Z run that becomes weaker/more disjointed with the surface system compared to most other guidance. Yet another frontal system shows up over the northern Plains by Sunday, but consensus so far suggests it will be weaker than its predecessor (but the new 00Z ECMWF has trended much stronger). By the latter half of the period the models have been differing on how northeastern Pacific/Gulf of Alaska energy may travel around the northern periphery of ridging to the south, with some eventual influence on the Sunday northern Plains system. Sporadic GFS runs, including the 18Z version, have been more amplified than most other solutions with the energy rounding the ridge. The new 00Z GFS compares much better to consensus, as did the 12Z run. CMC runs have at times been a bit on the amplified side with shortwaves reaching the northern Plains and beyond. Farther south, there have been some variations in specifics for the southern tier upper ridge from day to day, affecting details of temperature forecasts at some locations. The most notable trend lately is for stronger ridging to hold on over the Gulf Coast and Florida in addition to the strongest ridging that has been more consistently depicted over the southern Plains/Rockies. Current clustering looks good for the upper low that should be near the California coast Friday-Sunday and then retrograde offshore. Based on the 12Z/18Z guidance available for the latest forecast update, preference incorporated a 12Z/18Z model blend early and than transitioned toward somewhat more 18Z GEFS/12Z ECMWF mean input late. GFS input eventually switched exclusively to the 12Z run due to comparing better to consensus over western North America early next week. ...Weather/Hazards Highlights... There will be two general areas of interest for locally heavy rainfall during the Friday-Saturday night period covered by the Days 4-5 Excessive Rainfall Outlooks, both denoted by a Marginal Risk. One extends from the Southeast into parts of the Mississippi/Tennessee Valleys, and north into the Upper Midwest/central Great Lakes (Day 4) or into the central Appalachians/Great Lakes (Day 5). Northern tier areas will see rainfall focused by surface low pressure and associated frontal system along with supporting upper dynamics, with moderate progression and a mix of antecedent ground conditions continuing to support the Marginal Risk designation. Central and southern parts of this outlook area may have a more favorable combination of instability and moisture but guidance thus far has been very inconsistent among each other and from run to run in depicting where heaviest totals may be. Thus will maintain the Marginal Risk while awaiting any eventual improved focus that could warrant one or more embedded Slight Risk areas. Meanwhile farther west, guidance agrees in principle that flow between the California coast upper low and southern tier ridge should begin to direct shortwave energy and moisture along the Arizona/New Mexico border during Day 4 and expanding northeastward into Day 5, possibly interacting with a front that reaches the central High Plains. Guidance has not yet come into agreement for QPF specifics but the general pattern appears sufficiently favorable to merit a Marginal Risk area that fits that evolution. From daytime Sunday through next Tuesday, continued progression of waves/frontal systems will produce episodes of showers and thunderstorms of varying intensity from portions of the northern/central Plains through the eastern U.S. Expect Florida to see an increase of rainfall this weekend as an area of enhanced moisture moves in from the east. The Four Corners/Southwest and higher terrain of California should see diurnally favored episodes of convection, though with some continued uncertainty over specifics involving moisture and shortwave impulses at any particular location. Expect the heat across the southern tier to continue into 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-Tuesday (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 half of the Gulf Coast, and perhaps at scattered locations 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. Very warm to hot temperatures may extend beyond the medium range forecast period. Some record warm lows may be possible by early next week but forecast highs are not yet reaching daily record values. In contrast, locations from the northern Rockies into the Midwest should see variable temperatures with some areas seeing one or more days of slightly below normal highs. The upper trough/low near California should keep highs over most of the state near or slightly below normal through late week, 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