Mesoscale Convective Vortices (MCVs) will occasionally emerge from under the eroding cirrus canopy of a Mesoscale Convective System (MCS). Typically, an MCS will dissipate shortly after sunrise, but in atmospheres that include plentiful moisture and little vertical wind shear, the MCV that very frequently develops in an MCS can persist and serve to force subsequent convective development. (Previous MCVs have been documented in the CIMSS Satellite Blog here, here and here).

Strong convection that developed over western Texas early in the morning on Monday 11 August grew into a MCS that quickly eroded near sunrise. However, a swirl of mid-level clouds in the IR image loop above, and in the visible loop here, clearly show the persistence of an MCV. Note in the visible loop how subsequent convection develops very near the propagating MCV, first just to the northwest of the vortex (with the convection spreading south) and then to the north and east.

An MCV is driven mostly by the release of latent heat in convective clouds. Such heating will alter the stability and thus force the development of a vortex. The persistence of the vortex is a balance between the effects of ongoing convection releasing latent heat (maintaining the vortex) and the effects of strong vertical wind shear that serves to weaken the vortex. On this day, a rich moisture source is evident, as noted in the plot here of surface (yellow, in degrees Fahrenheit) and 850-mb dewpoints (white, in degrees Celsius). In addition, GFS model output of 850-500mb shear at 1200 UTC and 1800 UTC show minimal values in Texas where the MCV persisted.

Because forcing associated with the MCV can aid subsequent convective development, noting their presence — an easy task in satellite animation — is vital.

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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 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.

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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 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

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Parts of interior Alaska experienced heavy rainfall and significant river flooding during the last week of July 2008; AWIPS  composite images of the GOES-11 and GOES-12 water vapor channels (above) showed a rather unusual moisture plume — one having a long northwesterly fetch — stretching from the Russian arctic coast across the Chukchi Sea and into the interior of Alaska on 30-31 July. Rainfall amounts during the 28-31 July period were in the 2-6 inch range; in the Fairbanks area, 2.17 inches fell at North Pole (their second-highest daily rainfall amount on record), and this rainfall helped Eielson Air Force Base reach 7.30 inches for the month of July (their wettest July on record).

A comparison of the 06:00 UTC GOES water vapor image and the 05:20 UTC POES AMSU Total Precipitable Water (TPW) image (below) revealed  that TPW values were as high as 38 mm or 1.49 inches (green colors) within the moisture plume over the Chukchi Sea.

The heavy rainfall caused flooding along parts of the Chena River and the Tanana River in the Fairbanks area — the flood crest of the Tanana being the highest since August 1967 — and many residential areas had to be evacuated. The photo composite shown below (taken just after attending the Alaska Environmental Satellite Workshop) is of the Chena River at the “Ice Bridge” at  Pike’s Landing in Fairbanks on the afternoon of 31 July, when portions of the river were near flood stage (for additional photos and video, see the Fairbanks Daily News-Miner and the Alaska Superstation websites). In addition, the Alaska Railroad was forced to suspend passenger service north of Denali National Park because of rising waters in the Nenana area, with train passengers  being bused between the park and Fairbanks.

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