Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Wed Jun 21 2023 Valid 12Z Sat Jun 24 2023 - 12Z Wed Jun 28 2023 ...Prolonged heat wave continues across Texas and southeastern New Mexico and will expand northeast next week... ...Heavy rain and flooding are possible across the north-central U.S. into Saturday with a low pressure system... ...Overview... The medium range period will begin Saturday with a couple of upper troughs/lows helping to produce rain and thunderstorms, one over the north-central U.S. and another centered near the Lower Great Lakes region. These features are likely to combine into an upper trough that looks to remain atop the East next week, while additional troughing moves from the eastern Pacific into the West. Meanwhile, in between these troughs, an upper high meandering around Texas will cause excessive heat to persist there and even expand in coverage next week eastward into the Lower Mississippi Valley and north into Oklahoma/Kansas at times. ...Guidance Evaluation/Predictability Assessment... Recent model guidance indicates that the upper low over the Southeast in the short term should be lifting north over the weekend, while upper and surface lows track across the northern Plains, and the upper high across the Rio Grande persists. These features all show just some minor differences between models, with nothing too egregious. The first real notable model differences arise early next week with an upper trough/possibly embedded low coming across the eastern Pacific. Models have been waffling some with the depth and timing/track of the trough. The 12Z UKMET seemed to be the first to diverge by showing splitting energy with some tracking into the Pacific Northwest quickly by early Sunday. The 18Z GFS also ended up on the fast side by Monday-Tuesday, whereas the 12Z GFS/ECMWF/CMC all seemed to be in better alignment with the ensemble means with the timing of the trough's push eastward. The incoming 00Z guidance suite generally seems weaker with the depth of the trough and shows less indication for any deep closed low compared to the earlier model cycle, which perhaps allows the trough to move a little faster. So there is still some uncertainty with this part of the pattern, which is often the case when features arise/are reinforced from Pacific and high latitude energies like this. Most guidance agrees well regarding the upper high/ridge pattern over Texas and beyond lasting into next week. There is good agreement in placement of the high, with the exception of the 18Z GFS that pushes the high east sooner and farther than other models. Some isolated 18Z and 06Z runs of the GFS have been showing that in the past few days. Instead favored the more agreeable other 12Z models and ensemble means. This builds ridging across the Four Corners states around Monday and pushing into the Plains Tuesday-Wednesday. Given these considerations, the WPC forecast was based on a blend of the 12/18Z GFS and 12Z ECMWF and CMC early in the period. Gradually reduced (and eliminated the 18Z GFS) the deterministic models in favor of the ensemble means, ending up at about half models/half means by the end of the period. ...Weather/Hazards Highlights... As the upper troughing in the East lifts north and weakens, it will still provide some support for rain and thunderstorms but likely with reduced widespread heavy amounts compared to the short term. Scattered thunderstorms could produce localized heavy rain rates though given the instability in place. There may be a somewhat focused axis of rainfall across the northern Mid-Atlantic region into the Interior Northeast this weekend, so Marginal Risks are in place there for Saturday and Sunday, with areas of low Flash Flood Guidance of particular concern. Farther west, a consolidating low pressure system at the surface and coupled jets (right entrance/left exit region combination) aloft will likely produce heavy rainfall (likely around 2-4" with locally higher amounts) across the north-central U.S. on Saturday. A broad Slight Risk of excessive rainfall is focused on North Dakota and northern/central Minnesota for this potential, with a Marginal Risk for lingering rainfall centered on the Arrowhead of Minnesota on Sunday. Shower and storm chances will likely also extend southward across the Missouri and Mississippi Valleys this weekend ahead of the frontal system. The Mid-South and vicinity looks to see some enhanced organized convection Sunday evening/night with high rain rates, leading to a Marginal Risk issuance there for Sunday. As this upper-level system and surface low pressure/front push eastward, these areas should dry out while additional rounds of rain and storms are likely in the East through midweek. Meanwhile, scattered shower and storm chances return from the Northwest into the northern Rockies/adjacent High Plains by Monday-Tuesday as another upper-level trough approaches from the west. In terms of temperatures, there is really no end in sight for the excessive heat that has plagued particularly south and western Texas/southeastern New Mexico in recent days. In fact, for the weekend and into next week, temperatures over 100F and heat indices much higher will extend farther east into the Dallas-Fort Worth and Houston metros by Saturday and then into the Lower Mississippi Valley as well. Several record temperatures could be set or tied. The Desert Southwest starts to see temperatures well over 100F by the weekend and next week too, though not really anomalous for this typically hottest time of the year there. As mean upper ridging expands farther north, highs in the 90s are likely across much of the Plains and Mississippi Valley, with current forecasts showing temperatures exceeding 100F into Oklahoma/Kansas/Arkansas by next Wednesday. Meanwhile the West should see cooler than normal temperatures particularly for highs with the bouts of upper troughing aloft. The East looks to be within a few degrees of average for early summer. Tate 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