Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM 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... Latest guidance maintains the general theme of good agreement for the large scale pattern evolution through the period, and at least into day 6 Tuesday guidance has improved its clustering for the vigorous system reaching the Northwest. However there is continued spread for shortwave energy progressing from the Rockies/Plains into the East and some detail differences remain for lingering Northeast mean troughing behind an upper low that departs after the weekend. In general, UKMET runs have generally compared most poorly to other guidance for various aspects of the eastern U.S./western Atlantic forecast. As of the 12Z cycle the model is now getting closer to consensus for the eastern U.S. aloft but is now using leading shortwave energy to develop a tropical-looking western Atlantic surface low. By the end of its run it strays southeastward with its axis of the Northeast upper trough. Meanwhile GFS runs continue to extend its Plains/Midwest shortwave trough farther southeast than consensus (including the GEFS mean) from Sunday onward. This results in the associated surface pattern over the East being more suppressed than most other solutions for a period of time. Guidance has maintained a signal for an embedded upper low that would most likely track approximately from the Middle Mississippi Valley into Ohio Valley/Appalachians during Monday-Wednesday. There are typical low-predictability detail differences for how energy within northeastern U.S. troughing will look by late in the period. 00Z/06Z guidance comparisons favored starting with the 00Z ECMWF, a split of the 06Z GFS/GEFS mean (to downplay less desirable GFS specifics in the East), and 00Z CMC for about the first half of the period and then trending to a nearly even weight of models and GEFS/ECMWF means by day 7 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 Excessive Rainfall Outlook maintains a Marginal Risk area but with some ongoing adjustments on its periphery to reflect latest guidance trends, including a somewhat drier trend over Alabama/Georgia and an increased potential for a band of enhanced rainfall extending back into the central High Plains. It is likely that some locations within this overall area may eventually qualify for a Slight Risk as future model runs come into better agreement on the main QPF axis. Meanwhile this afternoon's update plans to add a Day 4 Marginal Risk area over portions of the Northeast, as guidance is suggesting potential for locally heavy maxima under the region's upper low. Most of this area should have fairly wet ground conditions by that time, whether from recent rainfall (especially in Maine) or what is expected in the short term. Going into the Day 5 period Sunday, the guidance suggests the core of the heaviest rainfall will likely settle across interior portions of the Deep South and possibly extending a bit to the northwest 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 maintained in the Day 5 outlook, just a little more improvement in guidance clustering will be needed to merit an embedded Slight Risk. Another aspect to monitor will be how much overlap there will be between Day 5 activity and what occurs in the days leading up to that point. The Day 5 outlook also maintains a Marginal Risk over parts of the northern Rockies with the strong system and leading front approaching the region. Elsewhere across the U.S., the Midwest into South rainfall will likely extend farther northeast into the Ohio Valley/Mid-Atlantic/Southeast from late weekend into the early-middle part of next week, with some locally moderate to heavy totals possible. Locally enhanced rainfall with late-day showers/storms over the Northeast with the upper low overhead should begin to trend lighter and more scattered on Sunday. 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 Day 5 Excessive Rainfall outlook maintains a Marginal Risk over parts of the northern Rockies with this activity and additional rainfall is likely into Tuesday. The depth and track of the upper system could also support some snow at the highest elevations. Then the northern Plains may see a band of rainfall by midweek as the leading front pushes into the region. 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 onward 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 into 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. 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 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