Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 PM EDT Thu Jun 13 2024 Valid 12Z Sun Jun 16 2024 - 12Z Thu Jun 20 2024 ***Early season heatwave from the Midwest to the East Coast, and heavy rainfall becoming more likely for the Western Gulf Coast*** 19Z Update: The existing forecast from overnight did not require many changes for the new day shift forecast, and the 12Z model guidance suite is in decent overall synoptic scale agreement on most aspects of the medium range forecast. Recent runs of the ECMWF have been hotter across much of the Eastern U.S. for the middle to end of next week compared to the more reasonable CMC and GFS solutions, and well above the ECENS in many cases across the Northeast. The 13Z NBM served as a good starting point for temperatures, and this was blended with some of the 6Z GEFS mean which appeared to have the best overall depiction of the expected heat wave next week. The NBM appeared a bit too widespread with higher QPF totals across the central/western Gulf Coast region, so this was blended with some of the deterministic guidance to portray a more realistic looking QPF forecast. Little in the way of changes were necessary to the Day 4 and 5 excessive rainfall outlooks. The previous forecast discussion is appended below for reference. /Hamrick ------------------------------- ...General Overview and Guidance/Predictability Assessment... The latest models and ensembles generally remain in good overall agreement into next week with respect to the overall pattern, featuring a strongly building upper ridge across the eastern half of the U.S., and a developing amplified upper trough over the West. Guidance indicates a southern stream shortwave over the Southwest will lift northeast, and around the western periphery of the building ridge with additional embedded waves crossing the northern tier as the main upper trough begins to take hold over the West. An upper-level disturbance now over Florida should slowly work westward over the Gulf of Mexico this weekend into next week. This will bring improving conditions for Florida, while increasing rainfall chances for the central and western Gulf Coast and vicinity going into Sunday and early-mid next week. The WPC medium range product suite was primarily derived from a composite blend of best clustered guidance of the 18 UTC GFS/GEFS mean and 18 UTC ECMWF/ECMWF ensemble mean, the 01 UTC National Blend of Models through medium range time scales all next week in a pattern with near average predictability overall. ...Weather/Hazards Highlights... A main axis of tropical moisture is forecast to shift westward across the Gulf of Mexico and affect the central to western Gulf Coast region next week. The latest Day 4/Sunday and Day 5/Monday ERO Marginal Risk areas remain valid for the west-central Gulf Coast given the moist pattern. Uncertainty remains, particularly with the potential amount of rainfall and with westward extent into the western Gulf into later next week. Elsewhere, convective precipitation event is likely across an unstable Northern Plains and the Upper Midwest/western Great Lakes as southern stream troughing ejects northeastward from the Southwest as well as lagging northern stream upper trough approach. The Marginal Risk areas remain in effect for the updated Days 4/5 Sunday/Monday excessive rainfall outlooks. There remains a more consistent signal that the subsequent upstream approach of an amplified upper trough out from the West/Northwest will act to also support the Marginal Risk of heavy rains over the Northern Rockies/High Plains Day5/Monday. This focus is expected to slowly work across the north-central U.S. Tuesday into next Thursday as aided by renewed cyclo/frontogenesis. As for anomalous temperatures, an amplified upper ridge slated to steadily/broadly shift from the Southwest through the south- central U.S. to the East over the coming week will produce widespread anomalous heat and humidity, with the potential for some daily record highs to be established. Temperatures will already be rather hot over portions of the South before the even broader region of heat expands into the Midwest/Ohio Valley then Mid- Atlantic/Northeast next week as the upper-level ridging builds in. Highs are forecast well into the 90s, and with lows in the upper 60s to upper 70s providing little overnight heat relief. This heat wave will likely have some staying power beyond Thursday. Schichtel/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 forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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 https://www.wpc.ncep.noaa.gov/#page=ovw