Severe Storm Outlook Narrative (AC)
Issued by NWS
Issued by NWS
775 ACUS02 KWNS 201727 SWODY2 SPC AC 201725 Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1225 PM CDT Fri Sep 20 2024 Valid 211200Z - 221200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF EASTERN NM AND WEST TX... ...SUMMARY... Scattered severe storms are possible Saturday afternoon to early evening across parts of the southern High Plains. Isolated strong to severe thunderstorms are also possible from the mid Missouri Valley to the Upper Midwest, and also from western Pennsylvania into northern/central Virginia. ...Southern High Plains vicinity... A southern stream mid/upper-level low and attendant trough will progress east across the Four Corners and southern Rockies on Saturday. Strong southwesterly flow aloft associated with the ejecting trough will become oriented over eastern NM into the OK/TX Panhandles by 00z/Sunday. While diurnal heating will result in some boundary layer mixing, south/southeasterly low-level flow will result in persistent theta-e advection, and dewpoints from the upper 50s to low 60s F are expected. Steepening midlevel lapse rates will contribute to modest destabilization, with 1000-1500 J/kg MLCAPE forecast across parts of eastern NM into the TX Panhandle/South Plains vicinity. Vertically veering wind profiles amid strengthening midlevel flow will support supercells, though a tendency toward increasing storm coverage and redevelopment may result in some clustering with time. Large hail, sporadic strong/severe gusts, and a tornado or two are all possible. Additional strong storms are possible to the north and west across northern NM into south-central CO beneath the core of mid/upper low. Instability will be weaker across this area, but isolated strong gusts and hail will still be possible with the strongest storms. ...Western PA into northern/central VA... A low-amplitude mid/upper-level shortwave trough is forecast to move across the Northeast on Saturday. Northwesterly deep-layer flow will overspread the Upper OH Valley toward the Mid-Atlantic coast. A corridor of low to mid 60s F dewpoints and pockets of stronger heating will support weak to locally moderate destabilization. Modest vertical shear (around 25-30 kt effective shear) will aid in at least transient organized updrafts. Elongated/straight hodographs and relatively cool temperatures aloft suggest marginally severe hail will be possible with the stronger cells. Locally strong gusts also will be possible where stronger heating and steepening of low-level lapse rates occurs. ...Central Plains to Mid-MO Valley and Upper Midwest... The forecast for this region remains quite uncertain for Saturday. Most guidance depicts ongoing elevated convection across parts of IA/MO Saturday morning. The evolution of this activity will impact how the airmass destabilizes/recovers late in the afternoon across the region. Nevertheless, thunderstorm development may increase during the afternoon/evening over the Upper Midwest into the mid MO Valley, as a northern stream upper trough traverses the Canadian Prairies and the international border vicinity, and an attendant cold front moves through the region. In areas that are not overly affected by early-day convection, diurnal heating and relatively steep midlevel lapse rates atop 60s F dewpoints will support moderate destabilization, while deep-layer shear becomes at least marginally supportive of organized storms. Some guidance suggests a cluster of elevated convection may develop across Iowa during the morning and intensify as it moves east-northeastward. Otherwise, isolated storms may develop near the front during the afternoon, with increasing storm coverage into the evening. Relatively elongated hodographs will support some hail potential with the strongest storms, while localized damaging winds will also be possible, especially if any stronger clusters do evolve with time. Farther south, the magnitude of destabilization into central KS remains uncertain and could be rather limited, though increasing midlevel flow would support some organized convection if robust storms can develop. A strong storm or two also cannot be ruled out with morning convection across MO, and again near any outflow boundaries during the afternoon/evening. However, confidence remains quite low regarding storm evolution in this area. ..Dean.. 09/20/2024 $$