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An outbreak of severe weather with very large hail and few strong tornadoes seems likely across eastern OK into MO and IL. A deep trough over northern Plains surrounded by strong polar jet on its southern side translates towards Great Lakes in the next 24hrs. At surface, a cold front extending from SFC low in south Canada SW-wards across upper Mississippi valley towards KS/OK and TX Panhandle is racing SE-wards across the Plains. Impressive 50-60kts SW-erly LLJ jet ahead of this front supports healthy moisture advection NEwards from central Plains into MO and W IL.

A well defined MCV was on going over E OK moving NE-wards into S MO, where at least one outflow boundary blown N-wards is evident on VIS satellite and radar images. This boundary could serve a focus for afternoon’s storm initiation along it, when airmass will destabilize. Very steep ML lapse rates will spread over the warm sector and in combination of rich BL moisture, surface dewpoints expected to be in upper 60s to low 70s from E OK into MO, extreme instability with MLCAPE values in excess of 3000 J/kg will result. Visible clearing skies behind the warm front over central MO allow strong surface heating during the day and weakening cap is expected later in the afternoon with an approaching forcing. Storms will likely form ahead of NE-wards moving residuals of MCV while additional initiation of storms will take places along numerous SFC boundaries/convergences from past storm systems on going over these areas. Expect rapid development of severe storms in this very unstable enviroment with strong shear. Stronger shear will be in place over MO/IL closer to the polar jet, while only weak to moderate shear will be in place more SW-wards over OK. Therefore, strong tornadoes are possible over MO/IL given the high 0-3km SR helicity values in range 400-600 m^2/s^2 and locally enhanced LL shear. More isolated severe storms are expected over OK where a few tornadoes are possible, but mainly threat for very large to giant hail exists.

Strong SW LLJ jet and quite linear hodographs suggest storms to rather quickly take linear mode, so threat for stronger tornadoes can be expected in the early stages of any supercell that forms in this favorable environment. Given the significant eroding of cap over larger area in the afternoon, widespread coverage of storms can be expected and for this reason, SPC has issued MDT risk with 10% area for tornadoes and 45% hatched area for very large hail from E OK towards W IL.

PUBLIC SEVERE WEATHER OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1136 AM CDT WED MAY 13 2009

…SEVERE THUNDERSTORMS EXPECTED FROM THE CENTRAL/SOUTHERN PLAINS TO THE MID MISSISSIPPI VALLEY LATER TODAY AND TONIGHT…

THE NWS STORM PREDICTION CENTER IN NORMAN OK IS FORECASTING THE DEVELOPMENT OF TORNADOES…LARGE HAIL AND DAMAGING WINDS FROM THE CENTRAL/SOUTHERN PLAINS TO THE MID MISSISSIPPI VALLEY LATER TODAY AND TONIGHT.

THE AREAS MOST LIKELY TO EXPERIENCE THIS ACTIVITY INCLUDE

NORTHWEST ARKANSAS
ILLINOIS
SOUTHEAST KANSAS
MISSOURI
OKLAHOMA

As mentioned above, storms will be more isolated across E-CNTRL OK given the stronger cap and less forcing in place. Weak to moderate shear does support supercells while SR helicity remains low, but enhanced values along SFC boundaries can support tornadoes as well, along the very large to giant hail given the extreme instability with MLCAPE over 4000 J/kg in place. A bit higher chances and veering wind profiles exist in southern OK, but this area still looks well capped and might not see storms initiation before front surges in during the evening.

Later towards the evening, as cold front continues to race SE-wards across the MDT area, storms will cluster into linear MCS or two with likely embedded strong squall-line. The system will continue as a large mesoscale complex overnight moving NE. Therefore threat for large hail and damaging winds seems likely where bowing segments might occur.

As for the place to be today, its quite a hard one. There could be three best places today… one over central MO just north of expected MCV location in the afternoon as OFB can serve as a trigger for initiation, storms there will quickly go tornadic given the strong shear and moderate instability. Secondary target is the one I like the most and could be a NNE-ern OK in triangle Enid-Coffeyville-Tulsa, as shear is still moderate and extreme instability is there, cap is not too strong and a tail-end charlie supercell would be the game. Latest RUC does have breaking precips there as cap erodes towards 22z. Third area could be SW OK ahead of E-wards mixing dryline from W TX, some backed LL flow in weak shear and extreme instability is in place, but also the strongest cap. If storms manage to initiate there, they will rapidly become severe with threat for giant hail and also a weak tornado or two.