Saturday, March 21, 2009

March 23 forecast update

Just a dismal forecast update for Monday's severe weather potential. It's really too bad we don't have another day of return flow ahead of this system, or rather, that trajectories don't improve sooner than forecast. It looks like the boundary layer air mass over the southern two-thirds of the Gulf of Mexico has modified further, with moisture deepening considerably. The Brownsville TX 00Z RAOB looked far better than I would have expected, with a 1.5 km deep moist layer and a 100mb ML dewpoint of 63°F! In fact, dense MVFR stratus has already materialized over portions of the Rio Grande valley and up as far north as the TX Hill Country. But none of this seems to matter, as our trajectories just aren't forecast to improve in time to get low-level moisture of anywhere near that quality into the central Plains prior to darkfall Monday. I'll do one last forecast tomorrow night when the new data is in... with the system being onshore the models should have a very strong lock on it by then. The 00Z GFS tonight still seems to be having issues with keeping the warm sector socked in with clouds and cooler temps. Anyway, my hunch is that this could be the type of system where dewpoints on the north end of the warm sector aren't terribly weaker than well to the south... e.g. broad mid 50s°F at the surface. If so, though again the impressively cold 7H-5H air will probably be lagging the warm sector a bit, there could still be some enhancement to the low-level destabilization process up north. Any cold core potential looks like a "warm sector" variety up there, as a synoptic warm front will not be present (rather just an inverted trough extending N of the surface low). That target is out of my reach regardless, but it will still be interesting to watch how that area may evolve... especially w.r.t. low-level destabilization and ultimate convective mode. The NAM is actually showing strong crossover of the 0-6 km bulk shear vectors with the bent back/occluded front up there, though they back with time. The models are really still all over the place on many mesoscale facets of this system though, and hopefully tomorrow night things will look clearer.

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