Wednesday, April 8, 2009

April 9 forecast

The 00Z models show a slight southward shift from previous runs, which is a real downer given the poor chase territory over much of northeast OK. I think the potential for a pseudo cold core play looks better in some regards on the 00Z NAM, as previous runs had 35-55 kt 500mb flow (southeast of the mid-level low) overspreading the surface low and adjacent warm front. The 25-35 kt 500mb flow forecast off the 00Z run looks better for keeping storm motions slower (lessening the risk of them either weakening or quickly becoming elevated), and is in the range consistent with a lot of other cold core tornado events (particularly those tied directly to the cool side of a baroclinic boundary). I'm a little leary of sticking close to the surface low tomorrow though, given the progressive nature of the low and strong low-level cold advection forecast in its wake. I'm also not sure how "tornado friendly" the cool side of the effective warm front will be, given the rather late arrival of the substantial low-level moisture (if it in fact arrives as forecast) and 700-500mb temperatures that are not especially impressive for an early April cold core event. My initial hunch is to play the northern end of the warm sector, east of the low a bit (say, the far eastern KS-OK border), and hope something goes up on the northern sections of the dryline and gets its act together in far northeast OK or far southeast KS before outrunning the narrow instability axis. It is certainly possible that a better cold core play might evolve farther west, tied to the surface low/warm front intersection, and surface obs and visible satellite later on tomorrow will help with figuring that out. If we can't get upper 50s dews up toward the KS-OK border before dark, the sfc low target (w/ the coldest temps aloft and strong PVA) may be the only viable option.

The just-in 00Z NAM-WRF-NMM 4km WRF from Matt Pyle has no trouble initiating an arc of storms across northeast OK between 5 and 7 P.M. local time, which is interesting.

This would be a much tougher forecast (from a chase strategy standpiont) if it was occurring smack dab in the middle of KS. As it is... I refuse to mess with "Ozark" country (especially around, if not after, sunset), where parameters are more typically "stacked up" ... i.e., stronger deep layer shear, stronger moisture/CAPE, and thus bigger EHI/STP values than points farther northwest. Thus, the cold core or pseudo cold core (i.e. open warm sector ESE of the sfc low) looks like the most practical target at this time.

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