Convective Addiction

FCST: 04/09/11 – Midwest / Mid-Mississippi Valley

Posted on 08. Apr, 2011 by Jesse in Uncategorized

HPC Surface Forecast - 00z SUN (7 PM CDT SAT)

Saturday is looking to be sort of the archetypal “day before the day” severe weather event in the central CONUS.  A powerful upper-level trough currently situated over the Great Basin/Sonora region of the SW CONUS will continue digging eastward as it completes amplification within the longwave regime, taking aim on the Upper Midwest by the week’s end.  The associated H5 mid-level perturbation will transition into a closed mid-level low, as associated height falls spread ENE across the S Plains.  A concomitant 90-100 kt jet core is progged to round the basal side of the mid-level trough, exit stage right on the lee side, and take aim on the Central Plains just in time for Sunday.  By late Saturday afternoon, all of W IA, E NE, far NE KS and NW MO will lie in the exit region of the main jet core; henceforth, this aforementioned area traditionally favored for enhanced PVA, divergence and WAA will remain primed for supercell development by late afternoon.

At the surface, an area of low pressure will sit squarely over the CO Piedmont/Plains border physiographic region, with an attendant WF moving towards the I-90 corridor and into the S Great Lakes Region, while a stalwart dryline entrenches itself from WC NE southward towards the western Red River Valley.  A 30-35 kt H85 LLJ will perambulate poleward, allowing copious amounts of gulf moisture to stream northward behind the warm front, ahead of the maturing trough.  This will be key to fueling Sunday’s SVR WX outbreak (more of this in a forthcoming forecast), as the more deep, magnanimous conveyor belt of moisture, ala ample return flow in the wake of a favorable wind pattern, gyrates towards the Dairy Belt.

Moisture return at 00z SUN (7 PM CDT) per the NAM

ECMWF 850 mb height/temp/humidity at 00z SUN (7 PM CDT SAT)

In the wake of ample surface heating behind the warm front, a large portion of the warm sector will destabilize throughout the day, namely between I-80 and I-70, east of the I-35/I-135 corridor in advance of the pacific front.  Numerical models are progging capacious instability values AOA 3,000 J/KG (SBCAPE) and 2,500 J/KG (MLCAPE).  This will co-exist with sufficient SFC-500 mb bulk shear values AOA 50 kts and 0-1000m SRH values AOA 200 m2**s2 across most of IA, far E NE and WC and NW IL.  The profligate SW flow in the lower-levels will proffer the existence of another EML throughout most of the day, especially across the western portion of the area of greatest risk.  However, the lack of stalwart forcing or an otherwise significant source of lift will leave the seasonably strong instability parameters via ample surface heating to possibly overcome the CINH by late afternoon or early evening.

NAM SBCAPE - 00z MON (7PM SUN)

The SPC has the more substantial threat mainly across IA/MN/WI, where damaging winds and large hail will be the primary threats, although a few tornadoes can not be ruled out.  This will be owed to steep lapse rates as an area of cooler temperatures AOB -13C to -15C overspreads the warm air near the surface and aloft (see SKEW-T).  A secondary “sleeper target” is looking certain to evolve across the tri-states region of IA/MO/IL, especially across WC IL, where a large area of higher CAPE values are being progged in juxtaposition with ample speed and directional shear with 50 kts of bulk shear will be present from the surface up to 500 mb.  While one caveat is the relatively weak, albeit still backed surface winds, the area is favored by the models for the erosion of CINH after 21z.  If a storm can capitalize on the ample instability and maintain itself, all modes of SVR weather are possible across this region as well, despite its not being outlined in the SWODY2.  Nonetheless, absent a significant source of lift, the main show could very well be concentrated across the Upper Mississippi River Valley.

0-1 km SRH per NAM at 00z SUN (7 PM CDT SAT)

Hence, Saturday looks like the classic “day before the day” as a classic twenty-four-carat SVR WX outbreak prepares to unravel across the Upper Midwest on Sunday.  Stay tuned to Convective Addiction for possible live streams from our team members in the field, as well as a comprehensive forecast for Sunday’s major SVR WX outbreak.

Author’s: Jesse Risley / Adam Lucio @ 01z 04/09/11