Gardner, KS

A Cool Down - wet - Then Warm?

Brian E Erwin

No question, the storms that moved through Kansas City packed a punch on Tuesday evening. Hail and straight-line winds were the main threat. Localized flooding was also a concern. Here was the setup:

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Surface-Based CapeWeather Bell

The greatest instability was to the north and west.

A severe thunderstorm watch was issued for the Kansas City Region.

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Severe Thunder-Storm WatchNWS

The other big threat with Tuesday's storms was localized flooding.

"Tuesday's storms did produce some wind gusts that knocked over tree limbs. Early in the evening, there was some thunderstorm development which threw the atmosphere into a chaotic mess. The storms that came through the metro were a wave from Central KS - and that was the complex that got us late in the evening," Fox 4 Meterologist Joe Lauria.

Lauria went on to explain - the whole thing eventually turned into this fascinating wave. On radar, it looked like "an exaggerated comma." This formation is called a book-end vorticity and is characteristic of a Mesoscale Convective System. They actually have happened many times in the past but this one was a vivid one as it came closer to KC.

Weather Lesson of the week:

BOW ECHOES and BOOKEND VORTICES

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BOW ECHOES and BOOKEND VORTICESNational Weather Service

The above figure is a prototype of the evolution (stages A through E) of a bow echo (from Fujita 1978). The black contour lines are meant to depict radar reflectivity. Dr. T. Theodore Fujita, a professor at the University of Chicago, coined the term "bow echo" in the late 1970s. The terminology was based on how bands of rain showers or thunderstorms "bow out" when strong outflow winds associated with the storms reach the surface and spread out like pancake batter. The bowed rain band is near the leading edge of the damaging winds, and frequently marks the location of where the rear-inflow jet contacts the ground. The storm system on which Fujita based the "bow echo" terminology produced a strong derecho over northern Wisconsin and adjacent states on July 4, 1977.

Derechos typically are associated with a long-lived bow echo or a series of bow echoes. These bow echoes may vary in size but usually go through an evolution that displays at least some of the aspects of the prototype shown above. The time span involved in the schematic varies with the size of the bow and with the background thermodynamic and kinematic (wind/mass field) environment. Small bows tend to evolve more quickly than larger structures, and those that form in very thermodynamically unstable and/or strongly sheared environments usually evolve more rapidly than those forming in more settled regimes. A smaller-scale bow, for example, one whose length extends through perhaps three or four average-sized counties, might evolve from stages A through E in about an hour. In contrast, for a larger-scale bow (such as those associated with many of the more significant, longer-lasting Derechos described in Noteworthy Events), the time span involved is on the order of several hours. That is a different topic to be discussed at a different time.

Tuesday's situation was a different case. During the development of a bow, counter-rotating storms (or areas of storms) commonly appear at both ends of the larger-scale bowing segment, straddling the rear-inflow jet. These storms are known as bookend vortices. In the above schematic, such features are likely to be best developed during stages "C" and "D" at the locations marked by the red dots. The presence of bookend vortices can enhance the rear-inflow jet and thereby initiate or accelerate the bowing process. If a bow echo persists for some time, i.e., for more than two or three hours, the influence of Coriolis force becomes significant. This causes the poleward (cyclonic) member of the two vortices to become dominant and, over time, the overall convective system to become increasingly comma-shaped. To emphasize that the poleward member of a bookend pair most often is dominant, the red dots on the left side of the dotted downburst ("DB") path in the schematic have been drawn larger than those on the right, with the hatched depiction of the southern vortex in stage "E" signifying its demise.

Source: https://www.spc.noaa.gov/misc/AbtDerechos/bowechoprot.htm

A composite radar view of a comma-shaped derecho-producing convective system with a bookend vortex is shown below.

You can see how an innocuous area of thunderstorms in northern KS came together and on the western end, a larger storm became the dominant one…

It was "weak enough," not to become a Derecho. It packed a punch and brought plenty of rain with it for sure.

The next significant change is a strong May cold front that will be dumping chilly air into the Plains later Tonight (Friday night) With the flow aloft and above this front coming from the WSW and cooler air flowing in that is a recipe for the rain to develop into the cooler air and that will set us up for a chilly Saturday, especially IF we can’t get a lot of sunshine happening until later in the day.

This severe weather season in KC, while not completely uneventful has once again - not produced a Tornado Watch. May 28, 2019, will make it three full years without the entire metro area being under a Tornado Watch. Johnson County and Wyandotte county have not been under a Tornado Watch since that day three years ago. It's an incredible record.

The risks are bouncing around us as well. Even when we have had a more significant risk, an ingredient or two are just not there to produce major severe weather in our area. One key factor is the storms have weakened before they get to KC.

I still think there is a chance for at least one significant severe weather outbreak in the KC Region. One key factor is La Nina is going to continue to influence our weather pattern into summer. A big factor is the Jet stream may struggle to move north, and that means storms will move across the eastern part of the country with rounds of severe weather.

So, what is the weather story for the weekend -

After the cold front moves through, it is going to cool down quite a bit for Saturday. As the front first moves through, it will be somewhat seasonable behind the front with the May Sunshine. However, overnight, the clouds will increase and the winds above the surface will be from the south and stronger. This is overrunning and it normally leads to rain.

Saturday, will not fill like Spring - but more like fall. We will struggle to get out of the middle 50s. The actual high may occur around midnight. It may not be the coldest high for 5/22 but it may be pretty close. Here are the coldest highs for that date.

Record low HighFox 4 Weather Blog

For Sunday the focus is on the Chilly morning, the record is 41°

Record Low for May 22ndFox 4 Weather Blog

Something else of note according to Fox 4,

Last week when we were in the lower 90s and tying or challenging record highs? I believe there was one day in there that the record was indeed from 1963! The record low of 41 occurend on May 22nd 1963. An amazing coincidence.

So what is the forecast for the next few days?

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GFS American Model Forecast Highs and LowsWeather Bell

I think this model is way too warm for Saturday by the way. I think we will be about 10 degrees cooler than is forecasted.

Although Sunday will be sunny, it will be cool.

Our next chance of rain comes on Monday. Here is the pattern of the jet stream:

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Jet Stream Pattern (lower jet) 19,000 feet upWeather Bell

It looks to be an active week into the last week of May.

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Long Range OutlookNWS Pleasant Hill

Our next chance for severe weather will be Memorial Day Weekend.

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Severe Weather ThreatsWeather Bell

This will likely change!

Be looking for the summer outlook soon!

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Gardner, KS
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