Monday, January 23, 2012

23 Jan 2012 outbreak: Synoptic evolution

The first moderate risk with tornado potential was forecast last night for portions of AR, MS, TN. A cold front aloft (CFA) associated with a short wave trough at 500 hPa came ripping across OK during the day and became negatively tilted across AR by 00 UTC. Cold advection at 700 hPa was well ahead of the low level front. QG diagnostics at 500 hPa from the 12 hr NAM forecast valid at 00 UTC show the relatively weak cross-front contribution to QG omega confined to AR while further North the along front component was dominant (courtesy of the legacy SUNYPak from UAlbany).

 Soundings were taken at LZK at 21 and 00 UTC which illustrate minor warming (+0.8C at 700 hPa) but but stronger warming (+2.60 at 677 hPa) indicative of the strengthening inversion. Soundings taken at JAN for 00 and 03 UTC showed warming around 600 hPa (+1.3C). Looping the water vapor imagery it appears as though the warming was in advance of the CFA but this effective cap was insufficient to limit convection. All of the available soundings indicate that parcel paths had zero negative area yielding uncapped near surface parcels in the warm sector. With no cap storm coverage was large and relatively uninhibited. With little in the way to focus convection multiple messy lines and clusters formed.

SRH was extreme approaching 500 m2s-2 at JAN at 03 UTC. 1st tor warning south of JAN came out around 0530 UTC. It appears that this line of storms formed along an effective dryline (mostly moisture gradient).

12-14 hr HRRR forecasts valid from 00-02 UTC showed development very similar to observations albeit just about a  or two far east and lacking the secondary more westward line of convection. Given the large 0-1km SRH from observed soundings and the lateness of model convection, it is no surprise why the HRRR failed to show any (and thus not significant) updraft helicity associated with the storms.

This point alone should highlight why it is so tough to forecast severe weather with models that may be only slight late in initiation and slow to develop. 1-2 hours late and 1-2 hours too slow to become significant (in a relative sense) means the models can be as much as 4 hours behind in convective evolution or even later if the environment is evolving and the convection doesn't follow the same evolution as observed.

As for the NAM, lets compare the 36,24,12 hour forecast of the CFA:

The 36 hr (upper left), 24 hr (upper right), 12 hour (lower left) forecast are compared to the analysis (lower right) for the frontal positions (magnitude of the potential temperature gradient) for the 700 hPa (shaded x10-5 K km-1 per 3 hrs), 500 hPa and 850 hPa (6 x10-5 K km per 3 hrs black and blue contours respectively). The difference between 36 and 24 hr is the difference in frontal position at 850 hPa into the cold front aloft. 700 hPa frontal positions were surprisingly stable, albeit with fluctuations in magnitude. So at least in theory this event had some measure of predictability associated specifically with the synoptic precursors, but the dynamical evolution of those precursors had little predictability beyond 36 (or maybe 30?) hours prior to convection initiation.