Convective Addiction

FCST: 05/25/11 – Middle Mississippi/Ohio River Valleys

Posted on 25. May, 2011 by Jesse in Forecasts

Another significant severe weather day appears in store for parts of the Midwest and Corn Belt regions, especially over parts of the Middle Mississippi and Ohio River valleys. As yesterday’s system treks eastward, the strong mid and upper-level dynamics will once again coalesce with a very unstable warm sector to potentially yield an outbreak of severe weather across the aforementioned region. All modes of severe weather are possible, though there are some additional caveats for today’s setup.

Currently a 996 mb low pressure system is centered over EC Kansas, with a stationary frontal boundary extending eastward roughly along the US 36 corridor towards the Mississippi River, and then ENE towards KPNT – KRZL – KFWA – KFDY corridor in the western Great Lakes region. Ongoing convection continues along and north of I-70 over parts of E MO, Central IL, IN and W OH; OFBs are plenteous in areas E/S of the ongoing convection. A secondary area of low pressure is situated near Streator, IL (see surface map overview below).

12z/25 Surface Analysis from the HPC

The mid/upper-level low will gyrate eastward today, although the upper-level jet stream dynamics are weather than they were yesterday when the main trough moved out of the four corners region and took aim on the Central and Southern Plains, producing a rather substantial outbreak of SVR weather from Texas northward into S Kansas.

RUC 250 mb profile at 18z/25

However, the mid-level profile shows that a strong mid-level perturbation will take aim on the main risk area today, witha 75 kt mid-level jet core approaching the region from the SW by late afternoon (see 21z/15 RUC  mid-level profile below).  As this best forcing for ascent approaches the region, CINH should not be in place to the same extent it was yesterday, so favorable wind shear profiles will support the formation and maintenance of supercells by early this afternoon.

500 mb absolute vorticity - 18z/25

In initiation should occur ahead of the surface low/cold front between 18z and 21z, likely over parts of E MO and NE AR, moving ENE with time.  One possible limiting factor across northern portions of the warm sector is the ongoing convection and remnant cloud debris that is present inhibiting destabilization closer to the triple point and the warm front.  Whether or not the noted area of secondary breaks in clouds (see below) will be able to entrench itself over parts of NE MO and WC/Central IL remains to be seen.  If that does occur, a pocket of directional shear will coexist with some instability, and that area would be prime for tornadic activity as well.  Nonetheless, rapid destabilization does seem to be occurring over other parts of the warm sector well SE of the surface low and ahead of the cold front (primary surface forcing mechanism).  Deep moisture is streaming northward towards I-70, and SBCAPE (instability) values in excess of 2500 J/KG should be realized further south this afternoon.

RUC 18z/25 SBCAPE forecast

The 12z RUC progs show that continuous moisture influx should not be a problem, with ample surface dews in the upper 60s and lower 70s working into the region ahead of the cold front/surface low.  Additionally, LI values should be very magnanimous with values AOB -5 by early afternoon.

RUC 21z/25 surface dewpoint/LI values

The extensive atmospheric instability and moisture profiles are reflected in the rather impressive forecast Theta-E profiles for later this afternoon, which points to the great concern for this setup as that cool, drier SW air aloft intersects a very unstable lower troposphere with ample backed surface winds.

RUC 21z/25 Theta-E profiles

While low-level jet profiles may be somewhat more marginal early this afternoon, they should really improve between 21z and 00z, helping to enhance hodographs closer to the Ohio River Valley, which could spell trouble for parts of S IL/S IN and N KY.

RUC 850 mb LLJ profile at 21z/25

Initiation is likely to occur sometime between 18z and 21z.  Due to a somewhat weaker cap and different dynamics than Tuesday, it’s possible that when initiation does occur, the environment could be hindered by too much ongoing convection as a opposed to the more favored mode of a few discrete supercells.  OFBs could also be a major player today, as always, along with a few surprise cells that root in the warm front boundary and take advantage of any helicity and instability that would reside there if ample clearing occurs.  Finally, there’s always the possibility that better ongoing convection across the southern part of the risk area could intercede some of the better moisture and cause a more paltry show further north or that convection could evolve into a more linear mode much more quickly.

Interested parties will once again want to stay tuned to the latest forecasts, watches and warnings in association with this potentially potent severe weather outbreak later this afternoon.

CA – JLR