Extended Forecast Discussion NWS Weather Prediction Center College Park MD 411 PM EDT Wed Jul 05 2023 Valid 12Z Sat Jul 08 2023 - 12Z Wed Jul 12 2023 ...Heat expected to rebuild from the Southwest to the northern Gulf Coast this weekend into next week... ...Heavy rain/runoff concerns likely to focus over portions of the south-central Plains into Tennessee/Lower Ohio Valleys and then possibly into parts of the East Saturday through at least Sunday night... ...Overview... The weather pattern over the medium range period will feature mean troughing over the eastern half of the country and a strengthening ridge over the southwestern states. Several frontal systems are forecast to push across the Central and Eastern U.S. as a deep upper level low descends into northern Ontario. This pattern will likely lead to an active period of shower and thunderstorm activity for multiple areas east of the Rockies and above average temperatures from the Southwest to the northern Gulf Coast. ...Guidance Evaluation/Predictability Assessment... The models and ensembles continue to show good agreement for the overall pattern evolution, but uncertainty remains in the smaller scale details. A general blend of deterministic models was used for the weekend forecast, then ensembles were introduced to the blend for early next week's forecast to smooth out smaller scale differences. The CMC starts to diverge from the other model solutions early next week with the position of the deep upper level low, pulling it considerably further south over the Great Lakes while the other models keep it near the southern Hudson Bay. With the CMC being a clear outlier, it was not included in the model blend for the later part of the period. Models are in good agreement that an upper level shortwave will round the base of the trough and swing over the Ohio Valley towards the Northeast Monday through Wednesday next week. The GFS and the ECMWF align fairly well with the strength and position of the shortwave, so a blend of the GFS/ECMWF/GEFS/ECENS was used to capture it in the forecast. Weak troughing will also persist over the coastal Pacific Northwest, with some uncertainty in the strength and position. For this forecast, the general model blend resulted in a middle ground solution. ...Weather/Hazards Highlights... 18Z Update: Models remain in good agreement in heavy rain potential from the Central/Southern Plains to the Mid/Lower Mississippi and Tennessee Valleys over the weekend. Only minor changes were made to the Day 4 and 5 EROs to account for latest model guidance. There is a signal for heavy rain focusing over parts of the northern Mid-Atlantic region on Day 4, but there was not enough confidence to introduce a new Slight Risk area at this time. This area should be monitored for a possible upgrade in future forecasts as a wave of low pressure moves north ahead of an approaching frontal boundary. The broad cyclonic flow over the eastern half to two-thirds of the country with embedded shortwaves will promote an active period for showers and thunderstorms. During the Days 4-5 period covering Saturday through Sunday night, the most prominent focus for episodes of strong and heavy convection should extend over parts of the south-central Plains into the Tennessee/Ohio Valleys with a combination of one or more surface waves/fronts and upper shortwaves, along with favorable moisture/instability. Both days of Excessive Rainfall Outlooks depict a Slight Risk area over parts of this overall region. Model/ensemble probability guidance clusters fairly well for the central/eastern part of the Day 4 Slight Risk and most of the Day 5 Slight Risk, while on Day 4 the 00Z GFS/UKMET/CMC offer a developing cluster hinting at a band of heavy rainfall potential along a moisture/instability axis extending back through the south-central Plains. Meanwhile on Day 5, the latest GFS/ECMWF runs and their corresponding ensembles offer the best clustering for heavy rainfall potential from the northern Mid-Atlantic into parts of the Northeast with a shortwave/surface system that may provide a fairly pronounced moisture focus. The CMC/UKMET differ with their shortwave/QPF details so confidence in the specifics is a bit lower than over some areas farther west. One area that may be worth monitoring for a Marginal Risk on Day 5 may be over northern Minnesota into the Upper Great Lakes, as guidance suggests development of convection from a combination of a weakening stalled front near the area and then a stronger front dropping south from Canada. Potential east-west orientation of enhanced rainfall could be an issue, but guidance differs on the specifics and most of this area has had below average rainfall over the past couple weeks with minimal totals expected in the short range. Thus will wait for improved clustering in location/amounts before depicting an area. After early Monday, expect the potentially heavy rainfall over the Northeast to continue progressing through the region while south-central U.S. convection may drop toward the eastern half of the Gulf Coast. The front dropping into the northern tier by Monday should push southward with time and may serve as a focus for showers/storms through midweek. Most of the West should remain dry through the period. The strengthening southern Rockies/High Plains ridge, which may drift a bit west with time, will likely expand coverage of hazardous heat across the southern tier during the forecast period. Expect the greatest temperature anomalies of plus 10-13F for highs to be near the upper high center (western Texas and southeastern New Mexico) with broadening coverage of highs at least a few degrees above normal elsewhere, including into the Southwest by the start of next week. Likewise, expect gradually increasing coverage of maximum heat index values reaching 110F or greater over southern/eastern Texas into the Lower Mississippi Valley, while lower humidity will lead to maximum heat index values over the Southwest near or slightly below the expected air temperatures reaching up to 110-115F from late weekend onward. Low temperatures in the upper 70s to low 80s will not provide much relief overnight and could exacerbate heat stress as well as yield some daily records for warm minimum temperatures. Meanwhile parts of the northern-central Plains should see highs up to 10-15F below normal on Saturday followed by a trend toward normal readings for the first half of next week. The Interior Northwest/far northern Rockies should see highs 5-15F above normal Saturday-Monday and then a moderating trend. Much of the eastern half of the lower 48 should see near to slightly below normal readings. Dolan/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 - Heavy rain across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, the Southern Plains, the Ohio Valley, and the Tennessee Valley, Sat-Sun, Jul 8-Jul 9. - Heavy rain across portions of the Central Plains, the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Southern Plains, and the Ohio Valley, Sat, Jul 8. - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, and the Tennessee Valley, Mon, Jul 10. - Heavy rain across portions of the Mid-Atlantic and the Northeast, Sun-Mon, Jul 9-Jul 10. - Flooding possible across portions of the Northern Plains. - Flooding occurring or imminent across portions of the Northern Plains. - Flooding likely across portions of the Northern Plains. - Hazardous heat across portions of the Lower Mississippi Valley, California, the Southern Plains, and the Southwest, Sat-Wed, Jul 8-Jul 12. 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