Extended Forecast Discussion NWS Weather Prediction Center College Park MD 351 PM EDT Tue Jun 13 2023 Valid 12Z Fri Jun 16 2023 - 12Z Tue Jun 20 2023 ***Early season heat wave to persist across much of the south-central U.S. into early next week and beyond*** ...Synoptic Overview... Expect a weak omega block upper pattern to be in place across southern Canada and the northern tier of the U.S. as the forecast period starts at the end of this week. Then from the weekend into next week, a northeastern Pacific/Gulf of Alaska trough with embedded upper low will dig into the western U.S. and western 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 rainfall from the northern-central Plains into Southeast/Mid-Atlantic, with the Southeast 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... From a detail perspective, the most ambiguous part of the forecast involves the shortwave energy that progresses out of the western/central states and eventually settles into the longer term mean trough over the East, along with northern stream eastern trough energy that could interact as well. Recent GFS runs have been leaning on the faster side of the spread with progression of the central U.S. energy, though the 12Z UKMET has come in with a solution different from other guidance as it has a defined shortwave that is even faster than the GFS while also depicting a trailing shortwave more in line with the ECMWF/CMC. By next Monday-Tuesday the models and ensembles have differed over whether this energy, with possible input of northern stream flow, may form an upper low over the East. Latest models have leaned a bit more in the upper low direction versus an open trough while the GEFS/ECMWF means show a little more separation by Tuesday. The updated blend, incorporating a 00Z/06Z operational model composite early and then a combination of models and 06Z GEFS/00Z ECMWF means, reflects this trend but confidence in specifics at any specific valid time remains fairly low. With the system digging into the West, the 06Z GFS was notable in bringing its upper low into the Northwest about 12-24 hours earlier than most other solutions. The new 12Z GFS has slowed down and in fact is a tad slower than the model/mean average. The 12Z UKMET tracks its embedded upper low north of other models and latest GEFS/ECMWF means. The aforementioned blend reflects the best guidance clustering well into day 6 Monday. New 12Z guidance thus far is suggesting that the upper low could be a little slower to eject northeastward by Tuesday, though also with increasing uncertainty over whether some of the upper low's energy could split. ...Weather/Hazards Highlights... There will likely be two main areas for heavy rainfall potential for the Friday to Saturday time period. The first area should be across portions of Mississippi and Alabama where the deterministic guidance indicates a corridor of enhanced QPF, likely as the result of MCS activity with northwest flow aloft. The potential exists for 2-4 inch totals during the Day 4 period Friday-Friday night, but the models continue to disagree on the placement of the main QPF axis, so this precludes anything higher than a Marginal Risk at this time. The second area of potential concern will be just east of the Front Range of the Rockies where additional heavy showers and storms may occur over areas that have received copious rainfall over the past week. Current plan is to maintain a Marginal Risk area for now, while an embedded Slight Risk may be warranted in the near future if shorter-term guidance refines the best region of focus. For the Day 5 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 only minor adjustments for latest guidance, with the expectation that one or more Slight Risk areas could arise if guidance clustering improves. Some of this rainfall may extend farther east/north 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 especially from late week into the 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 northern Rockies from Sunday onward. 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 20 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 normal. Most of the East Coast region should be near or slightly below climatological mid-June levels. Rausch/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 Hazards: - Heavy rain across portions of the Northern Plains and the Northern Rockies Sun-Mon, Jun 18-Jun 19. - Heavy rain across portions of the Central Plains, the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Central Plains, Middle-Upper Mississippi Valley, and the Ohio Valley, Jun 16-Jun 17. - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, and the Southern Appalachians, Sat-Mon, Jun 17-Jun 19. - Flooding possible across portions of the Central Plains. - Flooding occurring or imminent across portions of the Northern Plains, the Northern Rockies, and the Northern Great Basin. - Flooding likely across portions of the Northern Plains, the Northern Rockies, and the Northern Great Basin. - Excessive heat across portions of the Lower Mississippi Valley and the Southern Plains, Fri-Tue, Jun 16-Jun 20. 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