Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Tue Jun 13 2023 Valid 12Z Fri Jun 16 2023 - 12Z Tue Jun 20 2023 ***Early Season heatwave to persist across much of the south-central U.S. into early next week and beyond*** ...Synoptic Overview... A weak omega block pattern across southern Canada and the northern tier of the U.S. is expected to be in place for the end of this week, with a broad upper low situated over southern Quebec/New England, a ridge across western Ontario/Upper Midwest, and a trough over Saskatchewan and into Montana. This breaks down some going into the weekend and more of a quasi-zonal flow pattern develops across the south-central states. A much stronger trough begins to build southward across the Pacific Northwest and into the northern Rockies and Intermountain West for the beginning of next week, and the East Coast trough also builds back in with ridging from Texas to the Great Lakes. Meanwhile, the core of the upper ridge over Texas and the western Gulf of Mexico will continue with the ongoing heatwave unabated through the forecast period, and even beyond next Tuesday based on recent Climate Prediction Center outlooks. ...Guidance Evaluation/Predictability Assessment... The 00z model guidance suite continues to indicate good overall synoptic scale agreement through the majority of the forecast period for the upper level pattern, but there is some variability in the mesoscale details. Some of the greatest model differences continue with some weak shortwave energy crossing from the Upper Midwest to the Ohio Valley this weekend, adding to uncertainties in the rainfall forecast across these regions as well. The UKMET differed the most with a stronger low crossing the Carolinas by the weekend. There are also timing uncertainties of waves crossing the Northeast ahead of the aforementioned feature, and the placement of a deeper upper low over southwest Canada or the Pacific Northwest for early next week. The CMC is farther south with the placement of the upper low across the West by Tuesday when compared to the model consensus. Most of the other differences are smaller in scale and may take until the short range period to fully resolve. A general model blend of the deterministic models worked well for Friday and then dropping the UKMET by Saturday, but the second half transitioned towards about 40% of the ensemble means, with some contributions from the deterministic ECMWF and GFS for added system definition. This approach fits well with previous shift WPC continuity. ...Weather/Hazards Highlights... There will likely be two main areas for heavy rainfall potential for the Friday to Saturday time period. The first area is expected 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 are not yet locked in 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 are forecast over areas that have received copious rainfall over the past week, and this may eventually result in a Slight Risk as the event gets closer in time, however a Marginal Risk area is planned for now. For the Day 5 period Saturday-Saturday night, a broader signal for scattered heavy showers and storms exists from Minnesota to the Deep South near a frontal boundary and shortwave aloft, and a Marginal Risk is warranted for now. Additional showers and storms are likely across the Southeast and portions of the East Coast region going into early next week as well, and the amplifying trough across the northern Rockies and Pacific Northwest is also expected to result in scattered to numerous showers late in the forecast period. The excessive heat continues going through the remainder of the week and into the weekend 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. 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 building upper trough across 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 5 to 15 degrees below average, with the greatest anomalies over the northern Great Basin. Snow showers are even within the realm of possibility for the highest mountain peaks. 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