Extended Forecast Discussion NWS Weather Prediction Center College Park MD 320 AM 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... ...Multiple days of heavy rain and potential flooding are possible in the southern Appalachians/Southeast... ...Overview... An amplified and blocky pattern will be present over the lower 48 through the latter part of the week, including (1) a deep positively-tilted trough across the northwestern U.S., (2) a warm ridge to remain anchored over Texas/northern Mexico, with some extension of the ridge traversing the Great Lakes region, and (3) 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. ...Guidance Evaluation/Predictability Assessment... Model guidance continues to show general agreement with the synoptic scale pattern through late this workweek. The trough over the Northwest and the Rio Grande ridge are particularly agreeable, while there remain some model differences with the positioning of the Southeast upper low. CMC runs from both the 12Z and newer 00Z show a farther north position of the low compared to other models, impacting its QPF that is farther north as well (with more rain into the Mid-Atlantic). Then the 12/18Z GFS runs became weaker with the feature by around Friday, which may be too fast to weaken given the blocky pattern. Generally favored the ECMWF in the forecast blend as a good proxy middle ground of position (similar to the ensemble means) and maintaining strength for longer. The QPF amounts for the week ended up staying similar to the previous forecast. By the weekend, the overall pattern may relax/deamplify a bit. The future of the Southeast upper low remains somewhat uncertain as to where it tracks and how/when it may try to phase with the northern stream. The WPC forecast has the energy weakening but maintains a trough across the Southeast, which seemed to be a reasonable compromise. In the West, there is some general signal for an upper low to lift into the northern Plains/perhaps south-central Canada that could finally press the front/low pressure system in the northern half of the Plains eastward. The details for this remain uncertain at this point. But at least farther west, the 00Z model guidance appears to be converging better in the position of an upper low diving south through the Pacific (farther west/offshore than some previous models). For the latter part of the forecast period, maintained a fair proportion of the ECMWF and small portions of other deterministic models while increasing the ensemble mean proportion to about half given the increasing spread. ...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. 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