Greg Perkins [KC9GHZ]

SUMMARY

Thunderstorms are likely all day Tuesday, with a slight risk that some of these thunderstorms may be severe.  The main threats will be damaging winds and large hail.  It currently appears that the greatest threat will occur in the mid- to late afternoon hours.  Spotter activation may be necessary.

DETAILED DISCUSSION

The Storm Prediction Center places us in a Slight Risk area on Tuesday:

Currently, a progressive shortwave pattern aloft characterizes the northern 2/3 of the continental US.  A mid/upper low is now evident across eastern Ontario with a trough extending southward over Ohio.  This feature will lift northeastward over Quebec and weaken as an upstream trough moves quickly east-southeastward.  The latter trough, now evident in water vapor imagery and RAOB soundings over British Columbia and the interior Pacific Northwest, should cross the northern Plains and form a closed 500MB low in the vicinity of Wisconsin or the Upper Peninsula of Michigan by 7:00 AM CDT.

At the surface, a warm frontal zone is becoming better defined at the moment from southwestern Nebraska east-southeastward across northwestern Missouri into southern Illinois.  Cyclogenesis will occur along this front over the central Plains today.  The resulting low is forecast to move east-northeastward and deepen between eastern Iowa and lower Michigan, reaching Lake Huron as an occluded low nearly vertically stacked with a mid-level cyclone by the end of tomorrow.  As this occurs, the warm front should lift northeastward across lower Michigan into southern Ontario and across western New York.  A cold front should move southeastward across western Illinois, Missouri, and Oklahoma early tomorrow, then by late tomorrow night the front should reach the central and southern Appalachians, northern Alabama, northern Louisiana, and south central Texas.

A narrow plume of return flow low level moisture, evident this morning in the 925 and 850 MB analyses and RAOBs, is expected to widen and spread northeastward ahead of the surface cold front, supporting severe potential across the Great Lakes to the mid-Mississippi Valley.

You can see this setup illustrated on this series of 4-panel plots from today’s 12Z run of the NAM model:

The severe threat should continue near and ahead of the surface low throughout the morning and afternoon.  Weak convective inhibition should be quickly eroded by the combination of heating and convergence.  Therefore, the activity is expected to build southwest-westward along and ahead of the cold front through mid-afternoon.  Large hail is likely with the initial morning activity near the Great Lakes and into Iowa, followed by increasing potential for damaging wind gusts as the inflow layer becomes surface based through the late morning and early afternoon.  Nearly unidirectional deep-layer wind profiles, parallel to the primary belt of low level forcing, suggest a primary threat of damaging wind gusts.  However, any discrete isolated convection or small bow echoes may pose a slight tornado risk as well.

Analysis of the forecast soundings for O’Hare from today’s 12Z run of the NAM suggest that the peak of instability will be reached at around 5:00 PM CDT.  Here is the BUFKIT sounding for 5:00 PM:

Convective inhibition at that point has been eroded to 0, with CAPE values approaching the 900 mark.  The Lifted Index is at -3, with the equilibrium level at around 30,500 feet and the wet-bulb zero at around 7,800 feet.

The hodograph from this same sounding, indicating nearly unidirectional winds and a straight shear profile, indicates a threat of severe damaging wind gusts, shown below:

This entry was posted on Monday, April 2nd, 2007 at 3:47 pm and is filed under Severe Weather, Local Weather. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.