Hot temperatures in the central Plains

August 3rd, 2008 |

MODIS Land Surface Temperature product (Animated GIF)

03 August 2008 marked the 22nd  consecutive day of daily high temperatures of 90º F or higher at Denver, Colorado (the old record was 18 consecutive days, set back in 1874 and 1901). AWIPS images of the MODIS Land Surface Temperature (LST) product (above) revealed daytime LST values as high as 140º F in southeastern Colorado and 134º F in southwestern Nebraska — while the surface “skin temperatures” were quite warm, the actual air temperatures (measured within shaded instrument shelters located about 5  feet above the surface) were only the in upper 90s to low 100s F.

It is interesting to note that the LST values were significantly lower across much of southcentral and southeastern Nebraska (in the upper 80s to low 90s F, green to yellow colors), even though the air temperatures were similar to those seen in eastern Colorado at that time (in the upper 90s to low 100s F).  These lower LST values were due to a much higher density of vegetation in eastern Nebraska, as shown by a comparison with the MODIS Normalized Difference Vegetation Index (NDVI) product (below) — NDVI values were greater than 0.7 in Nebraska, compared to 0.1 to 0.3 across much of eastern Colorado. In addition, very dry conditions had prevailed across eastern Colorado — Denver had only received 3.28 inches of precipitation so far in the year (the normal is 10.25 inches for the 01 January  – 31 July period) — in contrast with Hastings in eastern Nebraska, which had received 6.77 inches of precipitation so far in the year (the normal precipitation is 3.81 inches for the 01 January – 31 July period). The combination of drier soils and sparse vegetation  helped contribute to such high MODIS LST values.

MODIS LST + NDVI + visible image (Animated GIF)

MODIS true color imagery from the SSEC MODIS Today site (below) confirmed the presence of a much higher density of vegetation (denoted by darker green colors) across eastern Nebraska, compared to that found across eastern Colorado.

MODIS true color image

Convective Heat Burst in South Dakota

August 3rd, 2008 |
AWIPS images of GOES-12 10.7 µm IR channel (Animated GIF)

AWIPS images of GOES-12 10.7 µm IR channel (Animated GIF)

AWIPS images of the GOES-12 10.7 µm IR channel (above) showed a cluster of strong thunderstorms moving eastward across parts of Nebraska and South Dakota on 03 August 2008 — at one point these storms exhibited an “enhanced-v” signature on the IR imagery around 02:32 UTC over north-central Nebraska. As the cluster of storms began to dissipate and collapse, a convective heat burst was observed at Sioux Falls in southeastern South Dakota around 09:15-09:45 UTC (04:15-04:45 am local time). The surface temperature abruptly rose from 74º F to 101º F, with wind gusts of 50-60 mph.

GOES-12 + GOES-13 IR images (Animated GIF)

GOES-12 and GOES-13 10.7 µm IR images (Animated GIF)

A comparison of GOES-12 and GOES-13 10.7 µm IR channel images (above) showed a closer view of the dissipating convection (GOES-13 had just been taken out of on-orbit storage for a period of operational testing). Note the rapidly warming cloud top temperatures as the convection approached Sioux Falls — the coldest cloud top IR brightness temperatures warmed from -68/-69º C at 05:02 UTC to -42/-43º C at 09:45 UTC (below).

GOES-12 and GOES-13 IR brightness temperature plot

GOES-12 and GOES-13 IR brightness temperature plot