June 14th, 1998 Clear Spot Behind MCS

A Mesoscale Convective System (MCS) developed over Iowa and Missouri during the night of June 13th and early morning hours of June 14th. At 14z in the image below, note the dissipating system from northeast Iowa through the length of Illinois.

June 14th, 1998 14z Clear Spot Behind MCS

Low pressure was centered over northwest Missouri as seen in the cyan contours.  In dashed green you can see the 3-hour pressure change.  Note the -4.5mb pressure change over southeast Iowa and west central Illinois. Relative to the MCS, you can see higher pressure over Illinois and lower pressure over Iowa. The wind blows from high to low pressure. Hence, an east wind was experienced by people in eastern Iowa that morning.

Now note the clear spot behind the MCS. Air under an MCS generally descends which creates high pressure. We can also call process of descending air subsidence. Subsidence can inhibit cloud development. This is what happened in this satellite picture near the clear spot.

The pressure change was great enough to cause relatively strong wind gusts in our area in that clear slot. Here is a list:

Cedar Rapids
METAR KCID 141252Z 12011KT 4SM BR BKN008 17/17 A2952 RMK AO2 PK WND 11032/1155 RAE1158 PRESRR SLP990 P0000 T01720167=
SPECI KCID 141223Z 11014G22KT 4SM BR BKN008 17/17 A2950 RMK AO2 PK WND 11032/1155 RAE1158 CIG 006V012 PRESRR P0000=
SPECI KCID 141214Z 12017G24KT 4SM BR SCT006 SCT075 17/17 A2950 RMK AO2 PK WND 11032/1155 RAE1158 PRESFR P0000=
METAR KCID 141152Z 12023G35KT 4SM -RA BR FEW006 BKN026 BKN070 17/17 A2949 RMK AO2 PK WND 12035/1143 PRESRR SLP982 P0021 60025 70025 T01720167 10178 20167 58047=
SPECI KCID 141118Z 11018KT 2SM +RA BR BKN022 OVC049 17/17 A2957 RMK AO2 PRESFR P0017=
SPECI KCID 141118Z 11018KT 2SM +RA BR BKN022 OVC049 17/17 A2957 RMK AO2 PRESFR P0017=

Note the remarks (PRESFR - pressure falling rapidly, PRESRR - pressure rising rapidly) indicating the rapid pressure fluctuations.  Cedar Rapids recorded a wind gusts to 35 knots (40 MPH) at 643 AM.

Iowa City
METAR KIOW 141253Z AUTO 12007KT 9SM BKN011 18/18 A2952 RMK AO2 RAE1158 PRESRR SLP991 P0001 T01830183 TSNO=
METAR KIOW 141153Z AUTO 11014G28KT 5SM -RA BR SCT029 18/18 A2949 RMK AO2 PK WND 11028/1151 PRESFR SLP981 P0008 60010 70010 T01780178 10200 20178 58043 TSNO=
SPECI KIOW 141137Z AUTO 13014G22KT 3SM -RA BR BKN029 OVC047 18/18 A2955 RMK AO2 PRESFR P0008 TSNO=
SPECI KIOW 141137Z AUTO 13014G22KT 3SM -RA BR BKN029 OVC047 18/18 A2955 RMK AO2 PRESFR P0008 TSNO=
SPECI KIOW 141131Z AUTO 12015G20KT 2 1/2SM RA BR SCT029 BKN047 OVC075 18/18 A2957 RMK AO2 PRESFR P0008 TSNO=

Again, PRESFR and PRESRR remarks with a peak wind gusts to 28 knots (32 MPH) at 651 AM.

Other sites such as Monticello, Dubuque, Davenport, and Moline recorded wind gusts from 20-35 MPH as the pressure disturbance moved through.

What we have just described is why sometime you can get strong wind gusts BEHIND clusters of thunderstorms.

 

 

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