Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Thu Jun 22 2023 Valid 12Z Sun Jun 25 2023 - 12Z Thu Jun 29 2023 ...Prolonged heat wave continues across Texas and southeastern New Mexico and will expand northeast next week... ...Overview... The overall pattern next week will be fairly slow-moving, as a couple of shortwaves/lows should combine to produce upper troughing across the East and allow for additional rain and thunderstorms, while another upper trough moves into the West. In between, an upper high centered over Texas will continue to bring persistent excessive heat to the southern Plains and into the Lower Mississippi Valley. ...Guidance Evaluation/Predictability Assessment... Across the East, early in the week/forecast period, most guidance showed good agreement with the overall pattern including a compact upper low tracking across the Midwest/Great Lakes while additional troughing gets absorbed and the combined energy leads to an upper trough over the eastern U.S. by around Tuesday. The 12Z UKMET seemed to be an outlier with the details of the Midwest low, tracking across the region more slowly and keeping a closed low farther south of other guidance into Tuesday. The newer 00Z UKMET looks to be somewhat more in line with other guidance. By Wednesday there continue to be some differences with the position/timing of the trough gradually moving east, with GFS runs on the faster side and the ECMWF on the slower side. A blend of the 12/18Z guidance along with the ensemble means seemed to give a good middle ground position. Farther west, good confidence remains for the hot upper high to persist across Texas. There has been some spread by the latter half of the period on the centroid of the high's exact position, as 12/18Z GFS and CMC runs track east while the ECMWF is held back west. The ensemble means were more agreeable and show placement in the middle of this spread. Meanwhile ridging extending to the north of the high will be affected by the trough coming into the West. Models show fairly good consistency in a positively tilted trough with its axis near the West Coast around Sunday-Monday. The trough should push east while weakening as the week progresses, with increasing spread in timing and strength along with some signal for energy to split and race east across the north-central U.S., with low confidence. Thus a blend of the 18Z GFS/12Z ECMWF/12Z CMC at first and increasing amounts of the 18Z GEFS and 12Z EC ensemble means with time worked well with these features and across the CONUS for this forecast. ...Weather/Hazards Highlights... At the start of the period Sunday, a low pressure system aloft and at the surface could cause some rain to linger across the Upper Midwest, with locally heavy amounts possible given the good dynamical support. Farther south, shower and storm chances will likely also extend southward across the Mississippi Valley along and ahead of the frontal system, and areas of the Mid-South may see some heavy rain rates in potential mesoscale convective systems near a dissipating warm front. Meanwhile, there may again be a somewhat focused axis of rainfall across the northern Mid-Atlantic region into the Interior Northeast on Sunday, with some flooding concerns over areas with low Flash Flood Guidance. All these areas have Marginal Risks of excessive rainfall in place for Sunday-Sunday night. As the systems consolidate with time, convection looks to overspread much of the East on Monday, with severe weather a possibility too. A large Marginal Risk has been introduced for much of the East as there may be locally heavy rain with these scattered to widespread storms in an unstable environment. Embedded Slight Risk(s) are possible as well in future cycles if model guidance agrees on areas of focus for heavy rain, especially if those areas have been greatly impacted by the recent and short range heavy rains. By Tuesday and Wednesday, the front should push slowly eastward with drying behind the front, while additional rounds of rain and thunderstorms continue for the Eastern Seaboard ahead of it. Meanwhile the next system into the West will also allow scattered showers and storm chances to return across parts of the Northwest and into the northern Rockies and Plains next Monday-Wednesday. In terms of temperatures, there is really no end in sight for the excessive heat that has plagued particularly Texas/southeastern New Mexico in recent days. In fact, into next week, temperatures over 100F and heat indices much higher will continue expanding east into the Lower Mississippi Valley. Several record temperatures could be set or tied, as temperatures nearing or exceeding 110 could again occur in western and southern Texas. The Desert Southwest starts to see temperatures well over 100F 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 the southern two-thirds of the Plains and Mississippi Valley, with current forecasts showing temperatures exceeding 100F into Oklahoma/Kansas/Arkansas by Wednesday-Thursday. Meanwhile the West should see slightly 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, with a trend toward below average by around 5-10 degrees for the Great Lakes region. 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