Extended Forecast Discussion NWS Weather Prediction Center College Park MD 447 PM EDT Mon Jun 19 2023 Valid 12Z Thu Jun 22 2023 - 12Z Mon Jun 26 2023 ...Prolonged heat wave continues across the Texas and southeastern New Mexico throughout this week and even into early next week... ...Heavy rain and flooding potential lingers across the Southeast/southern Appalachians for the latter part of the week... 19Z Update: The 12Z model guidance suite has a very good overall depiction of the large scale pattern across the continental U.S. through late Friday, so a multi-deterministic model blend suffices as a good starting point in the forecast process. More noteworthy model differences emerge by early Sunday with the closed low across Montana and then exiting across the Dakotas, with the UKMET considerably weaker than the GFS/CMC, and the ECMWF is more amplified with it as it reaches the Upper Midwest late Sunday. By next Monday, the GFS is more progressive with the West Coast trough/upper low moving inland, whereas the ECENS/CMC/EC keep it farther back to the northwest. The GFS/GEFS is displaced east with the upper ridge axis over the Southern Plains, whereas the CMC/ECMWF keeps the core of the high over northwestern Mexico, but agrees fairly well across the eastern U.S. In terms of the Excessive Rainfall Outlooks for Days 4 and 5, no major changes were warranted for this forecast update. The previous discussion is appended below for reference. /Hamrick ----------------------- ...Overview... The medium range period will begin Thursday with an amplified and blocky upper pattern across the lower 48, including a deep positively-tilted trough across the northwestern U.S., a warm ridge to remain anchored over Texas/northern Mexico, and a Rex Block in the East as an upper low sits atop the Tennessee Valley while an upper high prevails to the north. Widespread showers and thunderstorms are likely for the Southeast and southern Appalachians given the upper low, which after a wet short range period could cause additional flooding concerns. Meanwhile a low pressure/frontal system set up in the north-central Plains could lead to flash flooding concerns with thunderstorms focusing there. The upper pattern looks to relax a bit as the period progresses, but maintain the general pattern of western troughing/central U.S. ridging/eastern troughing--maintaining continued heat concerns across Texas in particular and into southeastern New Mexico. Rain chances look to spread up the East into the weekend. ...Guidance Evaluation/Predictability Assessment... For the latter part of this week, model guidance continues to show fairly good agreement with the overall blocky synoptic pattern, particularly with the trough in the Northwest and the upper high meandering near the Rio Grande. There has been more waffling with the positioning of the eastern upper low and its eventual drift north and phasing with the northern stream flow by the weekend. In terms of the 12/18Z model cycle available for forecast production, the 12Z CMC seemed to be a northeastern outlier in this low's position even for Day 3/Thursday, and the newer 00Z CMC comes into better alignment with the other models. Though the differences appear somewhat minor, small differences in the position and evolution of the upper low have big implications for the QPF. A model blend favoring the ECMWF/GFS with some inclusion of the UKMET and only a small percentage of the CMC seemed to work well for the early part of the forecast period. Into the weekend and early next week, the eastern upper low should open into a trough and phase with the northern stream, as the initial upper high over southeastern Canada has quickly exited to the east. But behind this feature, another shortwave looks to track from the West across the north-central U.S. that should also push a surface low/frontal system eastward that had been stalled through late week. These troughing features may all combine somehow early next week but still with some uncertainty at this point. Meanwhile models show good agreement that the upper high over western Texas/northern Mexico will continue to hold on, while ridging may poke northward into the Four Corners states and beyond, building ahead of an upper trough and embedded low in the eastern Pacific. Guidance is persistent with showing this low but with variations in its position that are within reason for this lead time. The WPC forecast trended toward including more ensemble means in the blend for the mid-late forecast period amid increasing model spread, to about half of the blend by Day 7/Monday. ...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 Thursday, a Slight Risk of excessive rainfall is once again in place for the southern Appalachians into the Piedmont given the additional rainfall potential over what will be increasingly moist/sensitive soils from rain during the short range period. The WPC Day 1-5 QPF indicates 7-10 inches of rain as an areal average in the southern Appalachians where upslope flow should enhance totals, and there will likely be locally higher amounts. Additionally on Thursday, maintained a Slight Risk across parts of the Florida Panhandle where a stationary front could focus moisture and thus rain/storms. Some additional enhanced rainfall beyond Thursday is possible for the Southeast/southern Appalachians given the slow predicted movement of the upper low together with plenty of antecedent moisture to work with. Most model guidance has lessening rainfall amounts forecast by Friday, with the main exception of the ECMWF runs, but have a Marginal Risk in the Day 5 ERO to cover the potential for additional flooding concerns. By late week into next weekend, rain and thunderstorms are likely to spread farther north up the East Coast into the Mid-Atlantic and Northeast. Upper troughing with embedded upper lows in the West will cause a low pressure and frontal system to stall through late week stretched across the north-central U.S. and provide a focus for thunderstorms. There should be ample instability in this area and thus potential for high rain rates. A Slight Risk in the Day 4/Thursday ERO has been maintained in the central High Plains where the front could stall but shifted a bit southwest into areas more prone to flooding from wetter antecedent conditions and more susceptible soils. There is still uncertainty in where the heaviest rain will set up though, and additional shifts may be needed in future cycles. Then by Day 5/Friday, the upper-level support starts to look favorable across the northern Plains, namely with the potential for a coupled upper jet (right entrance/left exit region combination). A broad Slight Risk is in place there as well. The frontal system looks to start moving faster by the weekend/early next week ahead of an upper low, pushing rain chances eastward. Meanwhile, the excessive heat will continue through this week across much of Texas and into southeastern portions of 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. 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. Meanwhile the West should see cooler than normal temperatures particularly for highs with the bouts of upper troughing aloft, while cooler than average temperatures will also be present in the southeastern U.S. given the cloudiness/rain chances and the upper low. Tate Hazards: - Heavy rain from portions of the central High Plains into much of the northern Plains and parts of the northern Rockies, as well as from along the central to eastern Gulf Coast across much of the Southeast and interior Mid-Atlantic including the eastern slopes of the central and southern Appalachians, Thu-Fri, Jun 22-Jun 23. - Heavy rain across the upper Midwest into portions of the upper Great Lakes and the northern Plains, Sat-Sun, Jun 24-Jun 25. - Flooding possible across portions of the Northern Plains. - Flooding occurring or imminent across portions of the Northern Rockies and the Northern Great Basin. - Excessive heat from southeastern New Mexico through the southern two-thirds of Texas into southwestern Louisiana, Thu-Mon, Jun 22-Jun 26. - Excessive heat across portions of northern Texas/southern Oklahoma into the lower Mississippi Valley, Sat-Mon, Jun 24-Jun 26. 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