New Data Available on the October 14th Microburst Storm

It turns out that there was additional data from the Lawrence Airport on the damaging ‘wet microburst’ event of October 14th.   After a lightning caused power outage at the airport caused the automated weather observing equipment at the Lawrence Airport to stop transmitting data at about 4:14 PM, the equipment kept on observing internally for about a half hour on backup battery power. The last good observation was stored internally at 4:42 PM.  This means that the ‘wet microburst’ event was, in fact, captured from the vantage point of the Lawrence Airport!

Thanks to the staff at NWS in Topeka for providing me with these un-transmitted aviation weather observations that were stored internally during this widespread damaging wind event across northeast Lawrence.

Long story short (more or less), the automated equipment at the Lawrence Airport recorded a first peak gust of 32 knots (37 MPH) from 190 degrees at 4:23 PM.  A second peak gust of 62 knots (72 MPH) from 160 degrees occurred 4 minutes later at 4:27 PM.  According to a note from the NWS, the maximum sustained wind (that is, the greatest average wind speed over a two-minute period) was 39 MPH from 190 degrees recorded at 4:28 PM, one minute after the maximum peak gust.  However, the two peak gusts are of greatest interest to us in analyzing the event, as they indicate the beginning time and magnitude (4:23 PM – 37 MPH) and peak wind time and magnitude (4:27 PM – 72 MPH) of the event at the airport.

The southerly direction of the maximum winds reflects that the airport was not in the direct path of the microburst, but rather, more toward the north side of it.  The winds closer to the center of the microburst would have been out of the southwest, or from around 240 degrees, spreading outward in other directions around the edges.  This means that the winds were likely stronger closer to the core, or central path of the microburst, than what was recorded at the airport.  This is because the core winds of the microburst are added to the prevailing wind direction as well as the speed and direction of travel of the parent storm, which were both from southwest to northeast.

In addition, more precipitation was recorded.  From 4:22 PM to 4:26 PM, concurrent with the two peak wind readings, 0.27 inches was recorded.  This may not sound like much but over 4 minutes this is a rainfall rate of 4.05 inches per hour.  How accurate the measurements are with these rain rates combined with this kind of wind are anyone’s guess, but this kind extreme rate of rainfall concurrent with the strongest winds adds further credence to the notion of a ‘wet microburst’ type of event.

For the storm, 0.99 inches was recorded up to 4:42 PM, which radar imagery indicates was the vast majority of it.  For the day, a total of 1.03 inches was recorded at the airport when some light early morning showers are taken into account.  Additional light to moderate rain showers of another quarter of an inch fell later in the evening during the period the automated equipment was totally out.  So a reasonable estimate of the total rainfall for the day up at the airport is 1.28 inches, 1.03 inches of which was actually recorded.

That is the crux of it.  For a more detailed analysis of this unusual event, we must interpret in a blow by blow manner all the un-transmitted observations now available in conjunction with the earlier transmitted observations.  So, for the other diehards out there, lets get started:

KLWC 142052Z AUTO 35005KT 9SM -TSRA FEW055 FEW075 OVC100 24/22 A2976 RMK AO2 LTG DSNT ALQDS RAB31 TSB48 SLP073 P0001 60002

The above observation is the regularly scheduled hourly that was transmitted normally at 3:52 PM.   There is thunder at the airport, which began at 3:48 PM, and there is light rain.  Winds are light out of the north (350/05KT).


KLWC 142104Z AUTO 30014KT 1 3/4SM +TSRA BR FEW036 BKN075 OVC095 23/23 A2978 RMK AO2 VIS 3/4V5 LTG DSNT ALQDS P0011

At 4:04 PM, a special is generated for visibility criteria (down to 1 3/4 mile) in heavy rain and thunder.  Winds are now WNW and have increased (300/14 KT).  0.11 inches has fallen in the the 12 minutes since 3:52 PM (a rainfall rate of 0.55 inches per hour).


KLWC 142113Z AUTO 28012G24KT 1/2SM +TSRA FG BKN020 BKN055 OVC095 22/21 A2979 RMK AO2 WSHFT 2056 LTG DSNT NE AND SW AND W P0049

The above observation skips two special observations ahead to the last observation actually transmitted. Now the visibility is 1/2 mile in heavy rain and thunder.  Winds have picked up out of the west (280/12G24 KT) or gusting to 28 MPH.  This is pretty standard for local thunderstorms.  Rain is very heavy with 0.49 inches since the hourly (a rate of 2.53 inches per hour over the previous 9 minutes and 1.40 inches per hour over the 21 minutes since the hourly).

20171014_211516_bref1_gray.png

Radar image at 4:15 PM.  The southward extending appendage over Lawrence (dark red blob) just west of the ‘+’ for the location of ‘LWC’) is the apparent incipient microburst.

20171014_212032_bref1_gray.png

4:20 PM Radar image.  The microburst is a subtle, yet potent feature and is right over Iowa Street at this time.

KLWC 142122Z AUTO 24012G16KT 1SM TSRA BR SCT018 BKN042 OVC065 22/21 A2980 RMK A02 WSHFT 2056 LTG DSNT NE AND SW AND W P0057

Above we see the first stored but un-transmitted observation.  Winds are southwest now but not real strong (240/12g16 KT).  The special is for visibility criteria, now up to a mile due to a bit of a lull in the rainfall, just ahead of the big event.

20171014_212550_bref1_gray.png

4:26 PM:  Here we see the microburst moving eastward mostly just south of the airport (+) just west of ‘LWC’.   This kind of verifies the idea that the airport caught the north portion of the microburst and was not in the core area of it, which was a bit more to the south across north Lawrence.

KLWC 142126Z AUTO 24012G16KT 1/2SM RA  FG BKN016 BKN042 OVC065 22/20 A2976 RMK A02 PK WND 19032/2123  WSHFT 2056 TSE24 PRESFR P0084

In the above 4:26 PM observation, we see the first indication of the microburst.  The rain is heavier again with the visibility down to a half mile.  While the average wind over the preceding 10 minutes is still 240/12G16 KT there is a peak wind remark, showing 190 at 32 KT at 4:23 PM, or south at 32 knots at 4:23 PM.   This could be considered the start of the microburst event at the airport.  Rainfall is now up to 0.84 inches since the hourly.  This represents a rate of 4.05 inches per hour in the 4 minutes since the last special and 2.1 inches per hour in the 24 minutes since the hourly.   This is wet microburst stuff.

20171014_213309_bref1_gray.png

4:33 PM:  Same time as the un-transmitted observation below.  The airport already appears just out of the microburst zone on radar.

KLWC 142133Z AUTO 24017G45KT 1/4SM RA FG BKN016 BKN044 OVC080 23/21 A2977 RMK A02 PK WND 16062/2127  WSHFT 2056 TSE24 P0092

Here we have recorded the maximum wind of the event in the peak wind remark.  The observation shows a peak wind of 160 degrees at 62 knots (72 MPH) at 4:27 PM.  The south wind from the microburst has probably been rising steadily over the previous 4 minutes since the 32 knot peak wind of the previous observation at 4:23 PM.  Visibility is down to 1/4 mile, indicating the very heavy rain and whatever debris might be blowing in the wind.  Rainfall is now 0.92 since the hourly.  This is a rate of 1.91 inches per hour over the previous 11 minutes and 1.34 inches per hour in the 41 minutes since the hourly.  So the rain rates, while very high, are lower than the last observation.


KLWC 142136Z AUTO 24019G45KT 3/4SM -RA BR BKN019 BKN044 OVC085 22/21 A2978 RMK A02 PK WND 16062/2127  WSHFT 2056 VIS M1/4V4 TSE24 P0093

Three minutes later another special is generated for visibility criteria.  The visibility shows at  3/4 mile while the rainfall shows as light, verified by only another 0.01 inches in the last 3 minutes.  This I cannot readily explain.  One possible reason is the rain bucket is not registering all the rain.  Another possible explanation is there is actually a strong lull in the rain right behind the microburst, which is equally plausible.  There is a also variable visibility remark, showing the prevailing visibility ranging from less than 1/4 mile to 4 miles.  Visibility in an observation goes back goes back over the previous 10 minutes, so the less than 1/4 mile visibility is probably during the height of the microburst wind and rain event between about 4:23 and 4:28 PM.

20171014_213840_bref1_gray.png

4:38 PM – Two minutes after the previous un-transmitted observation above.

KLWC 142142Z AUTO 22020G32KT 2SM RA BR SCT029 BKN044 OVC085 22/21 A2977 RMK A02 PK WND 16062/2127  WSHFT 2056 VIS M1/4V4 TSE24 P0097

The above is the last good observation before the battery went dead.  Visibility is up to an average 2 miles in rain with the same variable visibility remark as the previous  observation.  Winds are pretty strong, averaging out of the southwest (220 at 20 gusting 32 knots), which is the ambient or prevailing direction of the winds.  Rainfall is up to 0.97 inches.  This is a rate of 1.20 inches per hour over the previous 20 minutes and 1.16 inches per hour in the 50 minutes since the hourly.

20171014_214417_bref1_gray.png

4:44 PM – Two minutes after the last un-transmitted observation above.

20171014_214940_bref1_gray.png

4:49 PM.  The microburst seems to have created a void behind it which is now moving across Lawrence.

Another interesting observation of the event came from one of our CoCoRaHS rainfall observers located just west of Trail Road and Lawrence Avenue in north-central Lawrence:

“Thunderstorms late Saturday afternoon; severe with very strong winds (microburst?) for a few minutes beginning at 4:18 PM. Much tree damage in my neighborhood, but appeared to be less just to the west near Kasold Drive and south of 6th Street on Lawrence Avenue. Suspect measured total precipitation is less than accurate as rain was “falling” sideways during the strong winds.”

Storm reports of considerable damage for north Lawrence were reported to be at about 4:20 PM (generally north of 9th Street and east of Iowa Street).

The microburst is first recorded at the airport at 4:23 PM with a south wind gusting to 37 mph, peaking 4 minutes later at 4:27 PM at 72 MPH.  The south wind direction at the airport, however, indicates a glancing blow.  Nevertheless, the strong winds are concurrent with a measured rainfall rate of 4.05 inches per hour in the 4 minutes between specials generated at 4:22 and 4:26 PM (0.27 inches in 4 minutes).

The above paints a fairly clear picture of a wet microburst event that started near Lawrence Avenue and Trail Road at 4:18 PM (near Dillon’s Grocery at 3000 W 6th St), moved into northeast Lawrence at 4:20 PM (mainly north of 9th Street and east of Iowa Street) and then registered at the Lawrence Airport at 4:23 PM with a 37 MPH peak gust, then maxing out at 4:27 PM  with a another peak gust of 72 MPH.  A brief but very intense deluge accompanied the strong winds.

Due to the apparent glancing blow at the airport (indicated by the southerly wind direction) an even stronger event was likely unfolding within the wide zone of extensive damage across north Lawrence.

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