Convective Addiction

FCST: 04/03/11 – Upper Midwest / Central Plains

Posted on 02. Apr, 2011 by Jesse in Uncategorized

Much of the Midwest is gearing up for what looks to be a severe weather day on Sunday, particularly in the evening into the overnight hours.

An upper-level trough currently centered over British Columbia will gyrate eastward over the next several days, evolving into a more neutral to positively tilted phase as it perambulates towards the Great Lakes region by Tuesday. A mid-level perturbation with associated DPVA will approach the region from the west early Sunday, serving as the forcing mechanism for convective initiation by late Sunday afternoon. An associated “banana low pressure” region ahead of this disturbance will extend from Minnesota WSW into the Oklahoma/Texas panhandle region, with two low pressure centers: a 992 mb pressure area centered over NC IA and a sub-996 pressure area centered over far SE CO.

Across the northern target, a surface WF will extend eastward across parts of IA into NC IL, possibly clearing the I-80 to US 20 corridor by early to mid-afternoon. Ample clearing behind the warm front should allow the warm sector to destabilize, though warm air in the lower levels should foster a strong EML across the entire warm sector through at least late afternoon, inhibiting much of the surface based convection. This is one of the concerns regarding the setup.

500 mb mid-level progs at 18z SUN - note the approaching shortwave

As the system approaches Sunday afternoon, the H85 low-level jet will intensify ahead of the main energy, reaching the Mississippi River valley by 09z Sunday.  An enhanced area of interest, mainly SSW AOA 30 kts with 340 K Theta-e, will move into the N MO – N IL region after 21z Sunday.

NAM 850 mb profile at 00z MON (7 PM SUN)

As this occurs, rather favorable SBCAPE (instability) parameters should exist across large parts of the warm sector.  SBCAPE values AOA 1500 J/KG should exist from NC OK into N IL, per both the NAM and the ECMWF.  These are  seasonably ample instability parameters given the associated shear profiles progged to exist across the region (see below).

NAM SBCAPE - 00z MON (7PM SUN)

ECMWF CAPE - 00z MON (7PM SUN)

These instability parameters will be owed in part of the favorable moisture return (see below) as the Gulf of Mexico will be open for business.  Moisture return will begin in earnest as copious low-level flow commences this evening ahead of the main energy approaching from the NW.

NAM 2m Dewpoint Progs - 12z run SAT

A strong EML (elevated mixed layer) will persist across the warm sector, with 700 mb temps of 10C – 15C across the region ahead of the main forcing.  However, this CINH (convective inhibition) should begin to erode both ahead of the dryline and also SSE of the triple point by 21z as indicated by the lid strength index below.  This is very evident on a forecast skew-t for NE Missouri, per the ECMWF, at 18z (1 PM CDT) Sunday.

NAM 700mb temps holding AOA 10C at 18z SUN (1 PM CDT)

Skew-T - 18z SUN, NE MO per 12z ECMWF

Looking at other convective parameters, ample SRH values AOA 200 m2/s2 will exist across parts of the region, with locally enhanced 0-1km shear INVO of the warm front as well.  LI of -4 to -8 will exist both ahead of the dryline into NC OK and across parts of MO/IA/IL SE of the triple point in the northern sector of the banana low, along with favorable 500-700 mb lapse rates of app. 7 C and surface LCLs of 500 – 750m.  The combination of shear and instability in the lowest 3km should produce some rather magnanimous EHI values of 3-7 (see below) all across the warm sector ahead of the cold front.

NAM LI (lifted index) profile at 00z MON (7 PM CDT)

0-3 km EHI per NAM at 00z MON (7 PM CDT SUN)

Though a lack of forcing and a strongly rooted EML have been concerns with this setup, it’s certainly possible that, though not discernible at this juncture by synoptic-scale models, an embedded perturbation (see below) in the overall pattern ahead of the main shortwave could ignite convection a tad earlier, though in the wake of eroding CINH and ample instability, any storms that do initiate, remain discrete and become rooted in the boundary layer INVO the warm front would have the potential for an enhanced tornado threat.

Note the area of enhanced H7 winds across SE IA/NE MO/W IL

Current thinking is that storms will initiate by 00z proximal to the Mississippi River, perhaps in NC/NE MO or SE IA along and east of a KC – KIRK – KOTM line, moving eastward into Illinois, possibly congealing into a more linear structure between 00z and 03z, with questions remaining as to whether or not anything can root on the warm front.  Storm motions look to be ENE at app. 30 kts.  Further south there is some dubiety about whether or not anything will fire ahead of the dryline before sunset due to a much more stalwart EML, as models suggest CINH increases again after dark.  This suggests that storms firing ahead of the CF may be more elevated in nature, at least initially.

Official Forecast:. Lingering cloud cover looks to be a minimal threat attm.  Otherwise, the aforementioned erosion of the cap will allow for explosive thunderstorm development along the WF and CF stretching a MCS from NW IL to E KS. Severe winds and hail will be prevalent after 02z, reaching the Chicago metro area perhaps toward 04z.

A. Lucio / D. Neal / J. Riz

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