Extended Forecast Discussion NWS Weather Prediction Center College Park MD 252 AM EDT Wed Jun 14 2023 Valid 12Z Sat Jun 17 2023 - 12Z Wed Jun 21 2023 ***Early season heat wave to persist across much of the south-central U.S. into early next week and beyond*** ...Synoptic Overview... For the beginning of the forecast period this weekend, a well defined northeastern Pacific/Gulf of Alaska trough with embedded upper low will build southeastward across the western U.S. and southwestern Canada, while central U.S. into Canada ridging eventually rebuilds after initial western U.S. shortwave energy progresses through the Plains and settles into the long-term eastern U.S. mean trough. The core of the upper ridge over Texas and the western Gulf of Mexico will persist through the forecast period, maintaining a south-central U.S. heat wave that should extend beyond next Tuesday based on Climate Prediction Center outlooks. Meanwhile, the pattern evolution should favor an area of enhanced rainfall from the northern-central Plains into Southeast/Mid-Atlantic, with portions of the Deep South likely having the highest totals overall. The northwestern U.S./northern Rockies should see precipitation increase early next week as the western system moves in, and the initial upper trough/low over the Northeast will also support periods of rainfall there. ...Guidance Evaluation/Predictability Assessment... The 00Z model guidance suite at the beginning of the forecast period Saturday is in good synoptic scale agreement for most areas. The GFS is a little faster in ejecting the lead shortwave trough across the western High Plains and the CMC is more amplified with the trough over the East Coast region, but are within the margin of error and a blend of the ECMWF/GFS/CMC works well as a starting point in the forecast process for the weekend. The past couple of UKMET runs had a stronger low near the Southeast U.S. coast that did not align well with the model consensus. Going into next Monday, the CMC is a bit weaker with the shortwave trough over the Ohio Valley compared to the more amplified ECMWF/GFS solutions, but the model agreement remains good for the major trough building in across the northwestern states. Looking ahead to the middle of the week, there is strong consensus for a developing omega block pattern with an upper ridge axis extending well into central Canada and a trough and potential upper low situated near the Appalachians, with the CMC remaining on the weaker side of the guidance. There are more in the way of QPF differences regarding placements of heavy rainfall swaths that will be heavily dependent on smaller scale shortwave and MCS tracks, so the 1Z NBM was the main solution of choice with some GFS/ECMWF included. The use of the ensemble means for mass fields increased to about 40-50% by next Wednesday. ...Weather/Hazards Highlights... For the Day 4 period Saturday-Saturday night, there is a broad signal for scattered heavy showers and storms from parts of the north-central through south-central Plains into the Deep South near a frontal boundary and shortwave aloft. The outlook again maintains a Marginal Risk area with the eastern portion of the old Day 5 outlook trimmed back on the northeast side to account for a slight southwestern trend in the guidance, and parts of this region may eventually qualify for a Slight Risk as future models runs come into better agreement on the main QPF axis. Going into the Day 5 period Sunday, the guidance suggest the core of the heaviest rainfall will likely settle across interior portions of the Deep South with the potential for 2-4 inch rainfall totals on a localized basis, but models vary in the placement of the heaviest QPF maxima. Although a broad Marginal Risk area is currently planned for the new Day 5, parts of Alabama and western Georgia are borderline for a Slight Risk area since multiple rounds of heavy rainfall are expected in the short range forecast period for these areas. Elsewhere across the U.S., lighter rainfall will likely extend farther northeast into the Ohio Valley/Mid-Atlantic/Southeast from late weekend into early next week. The Northeast should see periods of rainfall with possible late-day thunderstorms this weekend with the upper trough/low over the region. Meanwhile, the vigorous upper trough/low reaching the West will likely increase the coverage and intensity of precipitation over the Pacific Northwest and especially the northern Rockies from Sunday into early next week. Some locally heavy rain may fall over areas in the northern Rockies that could be sensitive due to well above normal rainfall recently. The depth and track of the upper system could also support some snow at the highest elevations. The excessive heat will continue through the remainder of this week and into next week across much of Texas and into much of Louisiana and Oklahoma, with widespread 100s across central and southern portions of Texas, and 90s covering much of the Deep South and the Central Plains (mainly Monday-Tuesday for the latter). Some daily record high temperatures and heat indices on the order of 105 to 115 degrees are likely during the hottest parts of the day for many of these areas, and some isolated instances of heat indices up to 120 degrees are possible for parts of Deep South Texas where the greatest combination of high dewpoints and the heat will likely exist. There is strong model support for this event to have additional staying power into much of next week as well. On the other end of the spectrum, the upper trough digging into the northwestern U.S. will herald the arrival of noticeably cooler temperatures by Sunday and especially into early next week across much of California, Nevada, Washington, Oregon, and Idaho. Afternoon highs should generally be 5 to 15 degrees below average, with the greatest anomalies over the northern Great Basin. A few places could challenge daily records for cool highs. Elsewhere, the central Rockies will be on the cool side Friday-Saturday before moderating, while parts of the Upper Midwest/northern Plains will be rather warm with highs 5 to 15 degrees above average. Most of the East Coast region should be near or slightly below climatological mid-June levels. Hamrick 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