4/29/06 FCST: AR/LA

Several models are now in general agreement of a deep upper level low developing Saturday, after the merger of a southwestern US closed low and a progressive northern plains system early in the period, resulting in a warm sector prime for possible significant severe weather over the central and southern MS valley region on Saturday.

12Z 4/26/06 NAM 84hr parameter output, valid 00Z 04/30/06:

Overall setup: Per current NAM, an upper level closed low system is forecasted to develop early in the day from a merger between an existing closed low over NM and a developing trough diving south over the northern plains. The resulting closed system is then forecasted to deepen rapidly during the period with 500mb heights falling to under 5500m by evening centered over northern Oklahoma.

500mb: 70+kt southwesterly jet streak crosses the region during the afternoon, with winds backing as you go farther north into AR.

850mb: 50-60kt southerly jet streak also crosses the area during the afternoon, just ahead of the 500mb jet max. 850mb Tds generally in the 11-13C range.

Surface: 997-998mb surface low centered over northeast OK, with trough extending SSE into the warm sector during the afternoon. Winds generally from the south or SSE over most of the region, backing to southeasterly in northern AR.

Temps: Upper 60s and lower 70s in AR and MS, and mid 70s over central LA. This is likely due to heavy cloud cover over the region due to intense moisture advection, resulting in

Dewpoint Temps: Mid to upper 60s over most of the region, with lower 70s down in LA. Combined with the forecasted temp, this would result in temp/dewp depressions of < 10F over most of the region, resulting in

LCL and LFCs: Both lower to under 400M in parts of the region, especially in areas near the tristate border where LCLs lower to aob 250m.

Instability: MLCAPE 1000J/kg - 1800J/kg from southern AR to central LA, with 500-800J/kg farther north into central AR.

Low level helicity: 0-1km Helicities to 100-250 m^2/s^2 and 0-3km helicities 200-350 m^2/s^2 from west to east across the instability axis.

Assuming the NAM is close on the timing and position and anywhere in the ballpark on the depth of the system, looks to be an afternoon event for discrete supercells, and then line with embedded supercells during the evening into TN/MS/eastern LA. Strength and direction of the nearly juxtaposed 500mb and
850mb jet streaks would suggest the possibility for a few strong tornadoes, even given only marginal to moderate CAPE values.

Other models:
Timing, position, and strength of the system are all likely to change as the day nears, with the location of the warm sector varying also by the time Saturday rolls around. Currently the GFS is a bit more progressive with the system 100-150 miles farther east-northeast Saturday, while the ECMWF more inline with the NAM. The UKMET differs greatly from all three, with the original northern plains upper low and New Mexico closed low remaining separate by 12Z Saturday, with the NM closed low becoming sheared into more of an open wave by Saturday morning but remaining slow to move east. The result is a more westerly and zonal upper level streak over the MS valley, and the absence of a deep surface low, with only a weak low much centered much farther north over Iowa during the day Saturday - basically a non event Saturday per the UKMET.

But 3 out of 4 models depicting a significant system affecting the MS valley Saturday isn't bad. And eastern AR is part of the mix sometime during the day Saturday with all three models, right now anyways.
I'm not gonna go into too much detail this early, given model disparities. I will just say that there is potential for a cold-core setup in se KS, ne OK, sw MO on Saturday. Depending upon how the upper-level low moves out of the southwestern US, there is the potential for a deep surface cyclone over ne OK, with strong wrap-around low-level moisture beneath relatively cold mid-level temperatures near and under a powerful mid-level low over central OK. An occlusion may occur over this area, yielding a potential setup similar to what Guyer and Davies describe for their cold-core setup. One of the main differences at this point is that the sfc low may be E or ENE of the mid-level low, not the more typical southeast position. Regardless, just something to keep an eye on...