Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sun Jun 18 2023 Valid 12Z Wed Jun 21 2023 - 12Z Sun Jun 25 2023 ...Prolonged heat wave continues across the southern Plains throughout this week into the weekend... ...Multiple days of potential flooding rains across the Southeast into the southern Appalachians... ...Overview... An amplified and blocky pattern will remain in place through this week across the mainland U.S. This pattern will feature (1) a deep positively-tilted trough across the northwestern U.S. that is forecast to be less amplified with time, (2) a warm ridge to remain anchored over Texas/northern Mexico, with some extension of the ridge traversing the Great Lakes region, (3) a broad trough lifting out of New England with a cool surface High feeding Atlantic moisture toward the East Coast, and (4) an upper low that will likely bring heavy rain and potential for flooding into the Southeast/southern Appalachians. Meanwhile, the potential for heavy rain exists from the central High Plains to the northern Plains as a front is forecast to become quasi-stationary in the vicinity midweek. In addition, tropical moisture could stream northward into Florida by the end of the week. ...Guidance Evaluation/Predictability Assessment... Despite the prevailing blocky synoptic pattern as noted above, model guidance continues to show relatively good agreement with each other through next weekend. The trough over the Northwest and the ridge over Texas remain particularly agreeable. Meanwhile, models continue to indicate subtle amplifications of the upper low that is forecast to move into and then meander over the Southeast by the middle of this week. This feature, which already contains a well-defined circulation aloft over Missouri today, will play a critical role in producing multiple days of heavy rain and potentially flooding impacts across parts of the Southeast this week. Since yesterday, the ECMWF and the EC mean have become the most bullish guidance in terms of the QPF amounts along the eastern slopes of the Appalachians as a stronger and deeper influx of Atlantic moisture is forecast ahead of a stronger and broader upper low over the Southeast. The GEFS is not too dissimilar to the EC QPF pattern while the CMC/CMC mean favor a faster northward progression of the rain by the end of the week with higher amounts staying closer to the coast. The ECMWF appears to have initialized better for the upper low this morning. This may lend us a higher confidence to its forecast performance for this heavy rain event. The WPC medium-range forecasts begin with a composite blend of 40% from the 06Z GFS/GEFS, 40% from the 00Z EC/EC mean, and 20% from the 00Z CMC/CMC mean, and then lean toward the EC mean solution for Days 6 and 7. ...Weather/Hazards Highlights... The upper low meandering over the Southeast/Tennessee Valley or so will provide support for multiple rounds of rain and thunderstorms for the southeastern quadrant of the country through much of this week. Into the medium range period starting Wednesday, a broad Slight Risk is in place from the Florida Big Bend into Georgia and the Carolinas as tropical moisture from the Caribbean and from the Gulf focuses near and overruns the frontal system in the region. Parts of this region could be rather sensitive to additional rainfall by Wednesday after a wet pattern in the short range period. It still appears that the biggest threat of the heavy rain in the Southeast will be through midweek, but with some continuation of rain chances through the latter part of the week as well. By Day 5/Thursday, a broad Marginal Risk is in place except for an embedded Slight for the southern Appalachians, where upslope flow could cause rain totals potentially nearing 10 inches over the next 5 days or so. Future Moderate Risks are certainly possible for the southern Appalachians in future forecast cycles, but may take until the short range once it becomes clearer if/how/where the early week rainfall causes flooding. By late week into next weekend, rain and thunderstorms could spread farther north across the East Coast, but with uncertainty in placement and amounts dependent on the upper low track. Upper troughing with embedded upper lows in the Northwest will cause an initially eastward-moving cold front to then stall around mid-late week, providing another focus for thunderstorms in the northern/central Plains. There should be ample instability in this area and thus high rain rates especially with evening/overnight convection on Wednesday night when the low-level jet is enhanced. A Slight Risk has been maintained in the Wednesday-Wednesday night ERO with this activity near the front, within the broader Marginal covering the northern/central Plains Wednesday and Thursday. Considered a Slight Risk for Thursday as well, but at this point it appears support for heavy rain may wane slightly as the upper pattern pivots toward more ridging by then, but a Slight may eventually be needed, especially if the QPF looks to focus over Colorado given the wet antecedent conditions there. Cooler than normal temperatures are forecast for the West with the upper trough overhead, but with a slow moderating trend into next weekend. Meanwhile, the excessive heat will continue through this week across much of Texas and into portions of Louisiana and New Mexico, with widespread 100s and even some 110s across central and southern portions of Texas, and 90s with heat indices well over 100F elsewhere. 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 upper ridging expands farther north, highs in the 90s are likely across much of the Plains and Mississippi Valley, while 80s and low 90s in the Upper Midwest/Great Lakes region will be 10-15F above normal. Kong/Tate 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