Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sat Jul 09 2022 Valid 12Z Tue Jul 12 2022 - 12Z Sat Jul 16 2022 ...Heat wave to last through early next week across the southern Plains and Mid-South as a heavy rainfall threat emerges for the central Gulf Coast states... ...Guidance Evaluation/Predictability Assessment... An uncertain aspect in the medium-range period remains near the central Gulf Coast where an upper feature from the Gulf of Mexico and possible surface low may interact with a stalled front and could result in heavy rainfall in the vicinity. The ECMWF/ECMWF ensemble mean and especially now the UKMET focus the heaviest rains farther east toward MS/AL coasts and western Florida Panhandle than the GFS and GEFS mean. The CMC also indicates such interaction, but with less QPF. Even a composite blend acts to emphasize a threat that seems plausible given surface pattern/tropical moisture potential along with the slow moving and amplified nature of the upper flow that would tend to keep a deep moisture and heavy rain threat near the central Gulf Coast region next week. Another recent model difference of note concerned consistency with later period amplitude of upper trough energy near the Pacific Northwest, but forecast spread has decreased, with most guidance now showing moderate or better amplification and near neutral tilt. The WPC medium range product suite was mainly derived from a composite of the 18 UTC GFS and 12 UTC ECMWF/UKMET/Canadian along with 01 UTC National Blend of Models Tuesday into Thursday. Gradually increasing forecast spread into days 6/7 (Friday/next Saturday) prompted a preference transition toward the 12 UTC ECMWF ensemble mean aloft, whose most amplified ensemble mean solution seemed to best fit the pattern. WPC product continuity is good in a pattern with above normal predictability and 00 UTC guidance generally remains in line. ...Overview and Weather/Hazards Highlights... A mean upper trough position will develop off the Pacific Northwest by early next week as lead Midwest upper troughing digs into the East where associated frontal passages will focus rain and thunderstorms. However, deeper moisture is expected to trail from the eastern Mid-Atlantic to the Southeast along a wavy stalled lead front forecast to interact with an upper low from the sub-tropics to support heavy rain potential near the central Gulf Coast. This system could focus slow-moving/repeat cells with very heavy downpours leading to a local runoff risk. The northern Plains to Upper Midwest should also see chances of rain and strong thunderstorms as ejecting impulses round the ridge. Meanwhile, a persistent monsoonal moisture flow into the Southwest offers chances for some locally heavy downpours. Very hot weather is expected to persist across the southern Plains/Lower Mississippi Valley underneath lingering upper ridging early next week, before easing slightly with frontal approach. Early week high temperatures near or exceeding 100F are expected, which is between 5 and 15 degrees above normal. Combined with the humidity, daily maximum heat indices upwards to 105-110F+ are likely. The repeating days of high heat/humidity and lack of appreciable nighttime relief (lows in the upper 70s to low 80s for some areas) will increase the potential for impacts from this heatwave to vulnerable populations. Several daily record high temperatures are possible, particularly for warm lows, but a few highs may be close to records in Texas. As the upper ridge drifts westward toward Four Corners, much of the West. By late week, heat is forecast to develop and overspread the northern Plains as an upper ridge exits the northern Rockies ahead of an upper trough advancing into southwestern Canada. Downstream, the East can expect slightly cooler than normal temperatures early in the week behind the cold front, before warming to near/just above normal. Schichtel 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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