Convective Addiction

FCST: 03/20 – 03/21 – Winter Event

Posted on 19. Mar, 2010 by Jesse in Forecasts

Today’s 12Z model runs have started to come into better agreement regarding the upcoming storm system that should move out of the Southern Plains and into the Ohio River valley by early Monday.  Models seem to be centering on a more southerly track from what was being indicated much earlier in the week, which isn’t at all unexpected.  The 12Z NAM and GFS, as well as the most recently available UKMET, Canadian GEM, GFSx/MRF and the JMA model solutions are fairly congruous in the relative track of the storm system taking a more southerly route than previously anticipated, with the storm system centered over far western Kentucky by ooZ Monday, which subsequently shifts the heaviest axis of precipitation amounts south of the US 36 corridor.

Adequate moisture will be transported northward as a cold air mass sags southward in advance of the impending storm system.  Unresolved questions remain regarding the southward extent of the cooler air mass, as well as the exact location of 0C surface temperatures, along with the potential for warm air aloft to intrude upon the augmentation of frozen precipitation as the system makes it way to the NNE.

It’s is likely that actual accumulation amounts will be less than what both the GFS and the NAM are indicating, especially given the fact that boundary layer temperatures have been mild and tepid over the past seven to ten days, especially in the most favored locations for deformation band snowfalls.  However, given uncertainties about the extent of cold air advection, as well as the likelihood for some evaporative cooling, some locales may indeed surpass accumulations of 4 – 6 inches, especially on grassy surfaces and elevated areas.   Per the SREF, at 00Z Sunday the 850 mb zero degree isotherm is located along an axis from KPTS to KUIN, with 850 mb temperatures sitting at -2 to -6 C across the region north of this line; however, 700 mb tems site at -4 to -6 C across the same region as the 500 mb low migrates across the lower Mississippi valley.  Best divergence at 300 mb, along with primordial 700 mb upward vertical motion and strongest 700 mb theta-e advection occurs late Saturday and early Sunday over a broad region from the western Ozarks into central and EC MO as well.

JLR