Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM 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 as a weak coastal low pressure system will 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 model guidance for this forecast update looks to be in good agreement with the overall evolution of the upper-level pattern. The period begins during a brief transition in the pattern as various upper-level trough/ridge features evolve from their current placement, but eventually are replaced with a similar blocky pattern that has become common recently with mean troughing in the West, mean ridging in the central U.S., and a developing Rex Block over the East. An initial southern stream short-wave trough will move eastward over top ridging over Texas and eventually settle into the southeastern U.S., expected to deepen into a closed low. Ridging over Texas will then build north and eventually northeastward into southeastern Canada, setting up the eastern Rex Block at the end of the period. Meanwhile, another deep, northeastern Pacific/Gulf of Alaska trough will dig southward over the West, completing the return to a blocky pattern. More subtle differences expectedly arise with the timing and movement of the shortwave energy through the South, within the developing trough over the West, and the degree of ridging that builds northeastward into southeastern Canada. More specifically, the southern stream shortwave is a bit out of phase in the 00Z CMC and UKMET compared to a more similar evolution in the 06Z GFS and 00Z ECMWF. The more recent runs of the deterministic GFS and ECMWF are also both a bit more aggressive with the ridging building into southeastern Canada. However, the 06Z GEFS and 00Z ECens means both also support this this evolution. Despite these differences, a general model blend for the updated WPC forecast is appropriate given the similarity of the evolution in the pattern, with a bit more weight on the 06Z GFS/00Z ECMWF compared to the 00Z CMC/UKMET through the early part of the period. A contribution from the 00Z ECens means is added to replace the forecast time-limited 00Z UKMET for the end of the period, which retained more detail similar to the deterministic models, especially with respect to a stronger ridge building into southeastern Canada and deeper closed low over the southeastern U.S. This forecast blend also keeps general continuity with the prior forecast suggesting good consistency in the guidance and overall forecast, with the main changes being a bit stronger ridge over the central U.S. and a bit deeper cutoff low over the Southeast. ...Weather/Hazards Highlights... Heavy showers and storms are expected to develop Sunday (day 4) from Missouri into the Deep South as the southern stream shortwave passes over a northward moving warm front, ushering in deeper Gulf moisture supported by an overnight low-level jet. A Slight Risk of Excessive Rainfall is in effect for portions of the Mid-South southeast into Mississippi and Alabama where there is a growing signal for areal average rainfall of 1-3", with locally higher amounts. Portions of this region will also likely retain wet antecedent conditions given recent episodes of heavy rainfall late this week and into the weekend. Going into the Day 5 period Monday-Monday night, the guidance suggests 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. The inherited Slight Risk has been expanded northeastward along the southern Appalachians and south to the GA/FL border as the closed upper-low begins to settle in and moist, onshore flow from both the Gulf and southwest Atlantic supports continued heavy areal average rainfall. This heavy rain will likely continue into mid-week as a frontal boundary lifts northward, spreading further northeastward up the coast into the Mid-Atlantic. Temperatures across the region will continue to be below average with the closed-low overhead, influence of clouds/precipitation, and cold-air damming setting up along the Appalachians. Elsewhere across the U.S., the upper trough/low digging into 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. Noticeably cooler temperatures will also be ushered in by Monday as the trough begins to settle in across much of California, Nevada, Washington, Oregon, and Idaho. Afternoon highs should generally be 5 to 15 degrees below average, with the greatest anomalies Monday-Tuesday 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. The excessive heat will continue into next week across much of Texas and into portions of Louisiana, Oklahoma, and New Mexico, with widespread 100s and even some 110s across central and southern portions of Texas, and 90s covering much of the Deep South and the Central Plains. Many daily record high temperatures are possible, and some monthly records may even be challenged. Heat indices as high as 115-120 degrees are possible for parts of southern and southeastern Texas where the greatest combination of high dewpoints and temperatures will likely exist. As mean ridging expands further north, additional much above average temperatures are forecast for portions of the Northern Plains, particularly on Monday-Tuesday, with highs reaching into the mid- to upper 90s. A cold front moving into the region Wednesday-Thursday will bring cooler temperatures and a chance for locally heavy rainfall. Putnam/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 - Heavy rain across portions of the Northern Rockies and the Northern Great Basin, Sun-Mon, Jun 18-Jun 19. - Heavy rain across portions of the Central Plains, the Mississippi Valley, the Northern Plains, and the Southern Plains, Sat, Jun 17. - Heavy rain across portions of the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Southern Appalachians, the Southeast, and the Ohio Valley, Sat-Sun, Jun 17-Jun 18. - Heavy rain across portions of the Southeast, the Southern Appalachians, the Mid-Atlantic, the Central Appalachians, and the Tennessee Valley, Mon-Tue, Jun 19-Jun 20. - Severe weather across portions of the Central Plains and the Southern Plains, Sat, Jun 17. - Severe weather across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, the Ohio Valley, and the Tennessee Valley, Sun, Jun 18. - Flooding possible across portions of the Central Plains. - Flooding occurring or imminent across portions of the Central Plains, the Northern Plains, the Northern Rockies, and the Northern Great Basin. - Flooding likely across portions of the Northern Plains. - Excessive heat across portions of the Lower Mississippi Valley and the Southern Plains, Sat-Wed, Jun 17-Jun 21. - Enhanced wildfire risk across portions of the Southern Rockies, the Southern Plains, and the Southwest, Sat, Jun 17. 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