Fly early morning to avoid convection?

Well, if you believe what most pilots will tell you, sure...fly early in the morning and you'll always avoid any nasty convection. Right? Well, not in the Plains and Midwest (including "tornado alley") where much of the convection that meets SIGMET criteria tends to occur in the overnight and early morning hours. For the afternoon hours, it's a good bet west of the High Plains and throughout the Southeast U.S. that most convection is in the afternoon as you can see below from this convective SIGMET climatology study that looked at issuances over a several year period. The period shown in this image is from 20Z through 01Z. Warm colors (red, orange and yellow) indicate a high number of convective SIGMETs issued in the afternoon. Cooler colors (greens and blues) represent a low number of convective SIGMET issuances during this time frame.

But for the overnight hours into the early morning hours, it's a different picture. The convection that meets SIGMET criteria in parts of the Plains and Midwest are more likely to occur in the overnight and early morning hours. Shown below is for 11Z to 16Z. This is predominantly due to what are referred to as Mesoscale Convective Systems which are nocturnal beasts that develop in the evening and persist through the night and often dissipate leaving behind convective debris between 9 and 11 am.

Actually, we can see this for later tonight and early tomorrow morning as a mesoscale convective system (MCS) is expected to blossom at sunset and persist into the overnight and early morning hours in the central Plains creating the potential for a slight risk of severe weather. The forecast below is from the Localized Aviation MOS Program (LAMP) convective potential found in the EZWxBrief static imagery (for those EZWxBrief members) shown as a loop from 00Z through 11Z (depending on your browser settings, you may need to click on the image to see the loop).

This is all occurring in somewhat benign-looking zonal (west-east) flow at 500 mb shown below in a loop from 00Z to 12Z. This is a very common environment for these mesoscale convective systems to develop within. Notice also in this loop that the weather system creates it's own upper level disturbance as shown by the absolute vorticity field (colors) and slight kink in the height lines shown in the red circle. It will also likely create its own mesoscale high pressure (cold pool) immediately behind this MCS that will be analyzed on the surface analysis chart after the system develops.

This will develop along a weak stationary front analyzed on the WPC surface prog chart shown below valid at 12Z.


It's always good to look at other sources to include the simulated reflectivity to get a better understanding of the big picture how the weather system will be organized. Shown below is the forecast from the WRF-ARW model for 00Z to 12Z demonstrating a well organized weather system that blossoms in western Kansas during the early evening and moves through the central Plains in the overnight and early morning hours. So pilots waking up for an early morning flight out of southern Missouri and northwestern Arkansas might be faced with a dangerous convective threat.

This is also echoed in the High Resolution Ensemble Forecast (HREF) probability of echo tops above 30,000 feet MSL from 01Z to 12Z.

Most pilots are weatherwise, but some are otherwise™


Dr. Scott Dennstaedt

Weather Systems Engineer

Founder, EZWxBrief

CFI & former NWS meteorologist





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