MODIS true color images (30 September, 01 November, 10 November)

A sequence of three MODIS true color images from the SSEC MODIS Today site (above) showed an increasing level of turbidity of the water along the Gulf Coast — the 3 images are from 30 September, 01 November, and 10 November 2009. This increase in turbidity can be directly attributed to the runoff of sediment-rich water due to heavy precipitation across much of the Gulf Coast states from late October into early November, as shown in the 14-day observed precipitation map (below). Special thanks to Steve Davis and Jeff Craven at the National Weather Service forecast office at Milwaukee/Sullivan for creating/capturing these images and bringing this case to our attention!

14-day observed precipitation

AWIPS images of the MODIS Sea Surface Temperature (SST) product (below) showed that the Gulf of Mexico immediately offshore was significantly colder due to this discharge of sediment-rich water from rivers draining from the Gulf Coast states — SST values were in the low to mid 60s F (darker green colors) right along the coast, compared to the mid 70s to near 80º F (darker red colors) farther offshore.

MODIS Sea Surface Temperature product (November 10 - 12)

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A morphed microwave imagery loop (from the MIMIC website at CIMSS) shows 48 hours of structural changes as observed from microwave imagery. Note the strong eyewall-like structures at the beginning of the loop, with persistent strong convection noted west of the storm center. As the storm moves over colder water, and as southwesterly shear increases, the structure deteriorates and strong convection becomes concentrated in regions some distance east and north of the storm center.

The color-enhanced loop, above, of 11-micron brightness temperature also shows Hurricane Ida (dowgraded to a Tropical Storm at 9 AM EDT on Monday) moving northward from the Straits of Yucatan into the central Gulf of Mexico. As it moves, the appearance of the storm deteriorates markedly. At the start of the loop, a relatively warm region in the center of the central dense overcast (CDO) that might be the eye of the storm migrates to the southwest edge of the CDO and then vanishes. This change shows the effects of strong southwesterly winds that are moving the tops of the thunderstorm away from the center of the storm.

Maps of shear (wind vector differences between the upper and lower troposphere) show a more hostile environment for the storm between 1200 UTC Sunday — when Ida was a strengthening category 1 hurricane in the Straits of Yucatan — and 1200 UTC Monday, when Ida was a weakening category 1 hurricane over the central Gulf of Mexico. Maps of mid-level shear, that is changes in wind between the lower and middle troposphere, for 1200 UTC Sunday and Monday tell a similar tale.

Ida’s path is forecast over progressively cooler sea-surface temperatures. That in combination with the hostile shear environment suggest that re-strengthening to hurricane status is unlikely. Strong high pressure off the East Coast of the United States, however, as shown in an analysis here suggest a large pressure gradient that will support strong winds over the entire southeast part of the United States as Ida approaches the central Gulf Coast.

Visible imagery, above (from GOES-14), show the results of shear on the cloud patterns. Deep convection is offset to the northeast of the circulation center (shown as the yellow dot in the imagery, from the 1500 UTC National Hurricane Center discussion). Latent heat in the storms cannot affect the southern/western semicircles of the storm if the storms are all displaced by shear to the north and east, as in this rocking loop. A similar loop from GOES-12 is shown here.

Precipitable water estimates from microwave imagery, above, show deep tropical moisture over the Gulf of Mexico. As this tropical moisture moves over the cooler air at the surface over the southeast part of the United States, it will cool and water will condense out. Heavy rains are predicted in the next two days as that happens.

For more information on Ida, please visit the CIMSS Tropical Weather Website or the National Hurricane Center website.

Added: Late afternoon infrared satellite imagery from GOES 14 and visible imagery from GOES 14 show convection nearly wrapping around the center of the storm, just south of the mouth of the Mississippi River. However, persistent shear appears to be over-riding that tendency.

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Tropical Storm Ida has formed in the southwestern Caribbean Sea. Strong convection near the center is evident in the visible image, above, as well as suggestion of a banded feature northeast of the center. The southwestern Caribbean Sea is a region where tropical cyclogenesis can still occur at this time of year because of the combination of two things: (1) still-warm ocean waters (see an analysis of sea surface temperatures from CIMSS here; and (2) small values of vertical wind shear, as shown here. Microwave estimates of Precipitable water (using MIMIC) show that Ida formed in a region of enhanced precipitable water.

Ida is forecast to take a path over Nicaragua and Honduras (the path is depicted in the image of SSTs above (and here) before re-emerging over the still-warm waters of the far western Caribbean.

Update, 5 November. Visible imagery from the morning of 5 November, above, show upgraded Hurricane Ida on the coast of Nicaragua. The visible image shows banded structures over the Caribbean that speak to the increased organization to the system. However, its motion over the rugged terrain of Central America should cause weakening. Torrential rains over the mountains of Nicaragua and Honduras will accompany the storm.

Loops (Visible and Infrared) taken from the CIMSS tropical webpage show the evolution of Ida after landfall. Ida quickly lost intensity to depression status late on the 6th. The storm is forecast to be over the northwest Caribbean, where waters are very warm, over the weekend, so re-intensification is possible.

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The long nights of November allow ample cooling in clear airmasses, and fog is a frequent occurrence over rivers that are still relatively warm compared to the surrounding land. In these near-water locations, cooling to the dewpoint, and resultant saturation, allows fog to form in and along River valleys as shown in the above visible image from GOES-12. The Ohio River and its many tributaries in Pennsylvania, West Virginia and Kentucky are plainly visible.

Detection of fog at night occurs by comparing observed brightness temperatures at about 11 microns and about 3.9 microns. Small water droplets in fog are not effective emitters of radiation at 3.7 or 3.9 microns — that is, they do not emit like blackbodies — but the water droplets are effective emitters of radiation at 11 microns. Thus, the temperature inferred by the satellite (that assumes all bodies are emitting like blackbodies) is cooler for the 3.9-channel than for the 11-micron channel. A difference between the two temperatures, then, can be used to highlight fog.

The MODIS Fog product image, above, from AWIPS, shows the channel difference at 0736 UTC on 2 November, and it suggests ongoing fog in river valleys from New York State southwestward to Kentucky. The nearly simultaneous GOES image (from 0730 UTC) is below. The degraded resolution of the GOES infrared sensor over the Ohio Valley (about 5-km pixels, vs. 1-km pixels for MODIS) means that developing fog, by its nature at very small horizontal scales, is not initially detected. The finer-resolution detector on the MODIS instrument provides earlier warning of developing fog.

Fog in the river valleys is more obvious in the GOES Fog Product image from 1030 UTC, three hours later. However, the pixel footprint of GOES IR sensor means a less defined horizontal mapping of the true fog locations. The higher resolution MODIS instrument gives earlier detection and better horizontal delineation of fog events.

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