Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Thu Jun 15 2023 Valid 12Z Sun Jun 18 2023 - 12Z Thu Jun 22 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 Sunday, a well defined northeastern Pacific/Gulf of Alaska trough with embedded upper low will build southeastward across the western U.S. and southwestern Canada, and a weak coastal low pressure system will likely be exiting New England. The core of the upper ridge over southern Texas and northeastern Mexico will persist through the forecast period, maintaining a south-central U.S. heat wave that should extend through next week. Meanwhile, the pattern evolution should favor an area of enhanced rainfall from the Midwest states into the Southeast U.S., 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 is generally in above average agreement on most aspects of the synoptic scale pattern, even going into next Wednesday and Thursday owing to the amplified flow across much of the forecast domain. 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, but the latest run has the low well away from the coast and it looks reasonable elsewhere, so it was part of the deterministic blend through Monday night. Similar to yesterday, the GFS is a little faster with the shortwave across the Midwest on Sunday, but it aligns closer to the other guidance by Tuesday. 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. The use of the ensemble means for mass fields increased to about 35% by next Thursday. ...Weather/Hazards Highlights... For the Day 4 period Sunday-Sunday night, there is a broad signal for scattered heavy showers and storms from Missouri into the Deep South near a frontal boundary and shortwave aloft. The outlook now includes a planned Slight Risk area across portions of Mississippi and Alabama, where heavy rainfall through tomorrow will result in more saturated ground conditions leading up to this event. The eastern portion of the old Day 5 Marginal Risk has been trimmed back on the northeast side to account for a slight southwestern trend in the guidance. Going into the Day 5 period Monday-Monday night, the guidance suggest the core of the heaviest rainfall may exhibit more of a bi-modal distribution across the southeastern U.S., with one maxima over northern Georgia and the southern Appalachians, and another maxima near the GA/FL border. A Slight Risk for Day 5 is now planned for the northern maxima where a slightly better concentration of model guidance currently exists. Elsewhere across the U.S., lighter rainfall will likely extend farther northeast into the Ohio Valley/Mid-Atlantic/Northeast U.S. going into early next week. 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 Tuesday. Some locally heavy rain may fall over areas in the northern Rockies that could be sensitive due to well above normal rainfall over the past few weeks. The depth and track of the upper system could also support some snow at the highest elevations, mainly for the Cascades and western Montana. The excessive heat will continue into next week across much of Texas and into portions 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. 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 central/southern Texas, and some instances of heat indices over 115 degrees are possible for parts of Deep South Texas where the greatest combination of high dewpoints and the heat will likely exist. 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 where highs could be up to 20 degrees below average. A few places could challenge daily records for cool highs. Most of the East Coast region should be near or slightly below climatological mid-June levels, and pleasant by early summer standards. 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