Increasing supercell chances for the bluegrass

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Updating the forecast from this afternoon to increase the supercell coverage from mid afternoon to evening from isolated to scattered. These supercells most likely from Lexington south and west will contain very large hail, damaging winds and tornadoes in most cases. Anyone in the path of the supercell should take the warning very seriously as tornadoes with these will be strong and long lived and track over several counties. I still expect the supercells around sunset to form a squall line which moves east with widespread damaging winds with a more isolated threat for tornadoes and hail regionwide.

Can this set-up be compared to some legendary textbook outbreaks?

My answer is no but close. The warm sector in the April 1974 case was more pronounced with warm and moist air being a month later. However in terms of severe events this should be the top event of the year and perhaps of the past few years.

Why is that?  ( Model images are invalid for this post after 00z Saturday Mar 3rd.)

1. Powerful jet at the 250 mb pressure level. 110-130 KT with our region in the favored exit region ( ahead of the jet streak-strongest winds at the 250 mb level)

../GemPakTier/MagGemPakImages/nam/20120302/00/nam_namer_024_250_wnd_ht.gif

Decent 500 mb shortwave and vort max to our northwest.

../GemPakTier/MagGemPakImages/nam/20120302/00/nam_namer_021_500_vort_ht.gif

our region is just southwest of the triple point (warm front, cold front low pressure center meeting spot) in the prime heating hours of the day.

../GemPakTier/MagGemPakImages/nam/20120302/00/nam_namer_018_850_temp_mslp_precip.gif

A key indicator for me in terms of a tornado threat is the risk for updrafts into storms known as helicity.

Values around 150-250 m/s^2 are favorable for a tornado threat. In this case the entire is that then some. Wednesday values were from 400-600 m/s^2 but that was with less instability.

NAM HLCY forecast valid 21 UTC Fri 02 Mar 2012

twister data is the source. I’ll double check to see if the site is added in the links section.

instability, a good measure of that is CAPE. Values from 1000-2000 J/KG indicate moderate instability sufficient for severe weather.

NAM CAPE forecast valid 21 UTC Fri 02 Mar 2012

Another indicator is the lapse rate temp difference with height. In the case of tomorrow it will steep increasing the instability and promoting the large hail chance.

In terms of chasing there will be a lot of interest in the region but keep in mind storm motions will be very fast and these storms will be dangerous as well. Please do any chasing very safely.

I’m not going to discuss the Monday morning snow again till Sunday, severe weather is of most concern, please refer to last update.

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One Response to “Increasing supercell chances for the bluegrass”

  1. mitch Says:

    The SPC did not go with a high risk in the new day 1 outlook though it’s a close to high as possible.

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