Day 4-8 Outlook
Day 4-8 Convective Outlook NWS Storm Prediction Center Norman OK 0253 AM CST Sun Dec 06 2020 Valid 091200Z - 141200Z ...DISCUSSION... An upper ridge will spread eastward across the Plains toward the MS River on Day 4/Wed as a shortwave trough digs south and east across the Pacific Northwest and northern Rockies. At the same time, the persistent cut-off upper low off the southern CA/northern Baja coast will develop eastward along the international border. This will bring increasing chances for rain and perhaps a few thunderstorms to portions of the southwestern U.S. Wednesday and Thursday. By Day 5/Thu, the northern Rockies trough and the southwestern upper low will merge into a larger-scale trough, and eject eastward from the Rockies into the Plains by early Day 6/Fri. In response, lee cyclogenesis is forecast over the southern/central Plains. Strong southwesterly deep layer flow will overspread the southern Plains into the mid MS Valley late Thursday, and southerly Gulf return flow will develop across the south-central U.S. As this system shifts eastward through Day 6/Fri and Day 7/Sat, thunderstorm potential will increase across portions of the southern Plains into the lower MS Valley, and perhaps as far north as the mid-MS/lower OH Valleys. However, severe potential is uncertain. The quality of low level moisture return could be limited to a small portion of the Sabine and lower MS Valleys, and may be ill-timed with the passage of the upper trough and surface cold front. Additionally, medium range guidance varies in the intensity of the developing surface cyclone, and in timing of the eastward-advancing cold front from the Plains into the Midwest. As such, predictability is too low to include any severe probabilities.
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