Just 11 days after deadly Cyclone Nargis made its devastating landfall in Myanmar (also known as Burma), Meteosat-7 IR imagery from the CIMSS Tropical Cyclones site (above) showed a new tropical disturbance that was moving northwestward across the Irawaddy Delta region of Myanmar on 13 May 2008. The Joint Typhoon Warning Center issued the following Tropical Cyclone Formation Alert at 20:30 UTC:

WTIO21 PGTW 132030
MSGID/GENADMIN/NAVPACMETOCCEN PEARL HARBOR HI/JTWC//
SUBJ/TROPICAL CYCLONE FORMATION ALERT//
RMKS/
1. FORMATION OF A SIGNIFICANT TROPICAL CYCLONE IS POSSIBLE WITHIN 150 NM EITHER SIDE OF A LINE FROM 16.5N 96.1E TO 19.4N 92.1E WITHIN THE NEXT 12 TO 24 HOURS. AVAILABLE DATA DOES NOT JUSTIFY ISSUANCE OF NUMBERED TROPICAL CYCLONE WARNINGS AT THIS TIME. WINDS IN THE AREA ARE ESTIMATED TO BE 25 TO 30 KNOTS. METSAT IMAGERY AT 131800Z INDICATES THAT A CIRCULATION CENTER IS LOCATED NEAR 16.7N 95.7E. THE SYSTEM IS MOVING WEST-NORTHWESTWARD AT 06 KNOTS.
2. REMARKS: RECENT ANIMATED INFRARED SATELLITE IMAGERY AND A 121210Z SSMI MICROWAVE IMAGE INDICATE PRONOUNCED LOW-LEVEL CYCLONIC TURNING OF CONVECTION AT THE WESTERN END OF THE MONSOON TROUGH. HOWEVER, MORE RECENTLY, CONVECTION HAS WANED SOMEWHAT IN RESPONSE TO LAND INTERACTION. THE CIRCULATION CENTER IS CURRENTLY TRANSITING
GENERALLY NORTHWESTWARD ACROSS THE YANGON DELTA REGION OF MYANMAR. OBSERVATIONS FROM YANGON AS OF 131300Z, SUPPORT A 25 TO 30 KNOT CIRCULATION WITH SEA LEVEL PRESSURES NEAR 1000 MB (3 MB PRESSURE FALLS OVER THE PAST 24 HOURS) AND SUSTAINED WINDS AT 10 KNOTS GUSTING TO 20 KNOTS OUT OF THE NORTHEAST. A PARTIAL 130301Z ASCAT
IMAGE ALSO INDICATES STRONG WESTERLIES TO THE SOUTH OF THE CENTER WITH SUSTAINED EASTERLIES TO THE NORTH, FURTHER PROOF OF CYCLONIC TURNING. THE CENTER CURRENTLY LIES UNDER LOW VERTICAL WIND SHEAR JUST SOUTH OF THE SUBTROPICAL RIDGE AXIS WITH FAVORABLE SOUTHWESTERLY DIFFLUENCE ALOFT. THOUGH THE DISTURBANCE IS CURRENTLY OVER LAND, MINIMAL DEGRADATION OF THE LOW LEVEL IS EXPECTED DUE TO THE LOW LYING TOPOGRAPHY AND FAIRLY QUICK TRANSIT
OVER THE LOW-LYING COASTAL REGION OF SOUTHERN MYANMAR. EMERGENCE INTO THE BAY OF BENGAL IS EXPECTED WITHIN THE NEXT 12 TO 18 HOURS. MAXIMUM SUSTAINED SURFACE WINDS ARE ESTIMATED AT 25 TO 30 KNOTS. MINIMUM SEA LEVEL PRESSURE IS ESTIMATED TO BE NEAR 1000 MB. THE POTENTIAL FOR THE DEVELOPMENT OF A SIGNIFICANT TROPICAL CYCLONE WITHIN THE NEXT 24 HOURS IS GOOD WITH THE ONLY LIMITATION BEING TEMPORARY LAND INTERACTION.

CIMSS satellite-derived atmospheric motion vectors and products (below) indicated that the upper-tropospheric winds, divergence, and deep layer shear were all relatively weak  over the region at that time.

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AWIPS images of the MODIS visible channel along with the Normalized Difference Vegetation Index (NDVI) and Land Surface Temperature (LST) products (above) depicted the areal extent of the Mississippi Alluvial Valley (MAV) on 12 May 2008. This physiographic feature represents the historical flood plain of the lower Mississippi River, which stretches from the confluence of the Ohio River to the Gulf of Mexico. Much of this land has been converted to agricultural use during the past century — because of the marked differences in soil composition and vegetation density between the MAV and the surrounding forest-covered areas, the MAV shows up as a lighter shade of gray on the visible image, with significantly lower NDVI values around 0.2 to 0.3. In addition, significantly warmer LST values were seen in the MAV, which were 20-30 degrees F warmer (darker red colors) compared to the surrounding forested areas (similar warm LST values were also noted over cities and other heavily urbanized locations).

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The severe thunderstorm that was producing a significant long-track tornado (approximately 75 miles long, and around 1 mile wide at times) across parts of northeastern Oklahoma and southwestern Missouri on 10 May 2008 exhibited a classic “enhanced-v” signature on both GOES-12 and NOAA-16 IR imagery. The coldest cloud top brightness temperature values in the region of the enhanced-v were -79 C on the 1-km resolution NOAA-16 AVHRR data, compared to -67 C on the 4-km resolution GOES-12 IR data. In addition, the visible imagery revealed a well-defined anvil plume spreading northeastward from the overshooting top region. Large hail (up to 2.75 inches in diameter) and wind gusts to 65 mph were reported around or just after the time of these images at Joplin, Missouri (station identifier KJLN). The long-track tornado produced EF-4 damage, and was responsible for at least a dozen fatalities in the Picher, Oklahoma and the Seneca/Racine, Missouri areas (SPC storm reports | Tulsa OK NWS summary | Springfield MO NWS summary).

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On the morning of 08 May 2008, we received the following email from David Zaff, Science and Operations Officer at the National Weather Service Forecast Office at Buffalo NY:

The NWS BUF office was marveling over the fine line starting from below the thumb of MI through Toronto and along the northern edge of Lake Ontario, but we had a hard time describing why it was there. A cold front had moved through overnight with widespread rain and this boundary appeared as the main synoptic event passed. It doesn’t appear to be tied to land/lake interactions or terrain. It’s clearly drier on one side and more moist on the other with a cap in place. There are also some neat gravity waves that pass into lake Ontario as the flow passes over the boundary when looping the vis earlier in the day.

Great question Dave, and a very interesting satellite imagery example! After some initial pondering, the satellite geeks at CIMSS came to the consensus that the sharp quasi-stationary cloud boundary seen on GOES-12 visible channel images (above, oriented WSW to ENE across lower Michigan,  southern Ontario, and northern Lake Ontario) was possibly due to an atmospheric hydraulic jump. A hydraulic jump can occur when fluid flow at a higher velocity discharges into a zone of lower velocity — a thinner layer of faster, laminar flow abruptly transitions to a deeper layer of slower, more turbulent flow.

But why would a hydraulic jump cause the sharp clearing line seen in the field of cumulus clouds? The answer might lie in the vertical structure of the atmosphere over that region; rawinsonde data plotted for Gaylord, Michigan, Detroit, Michigan and Buffalo, New York (below) revealed that a very strong subsidence inversion existed over lower Michigan that morning. The base of the temperature inversion (which was most dramatic at Detroit, yellow sounding plot) was around 900 hPa, with very dry air in place above that level — if turbulent flow associated with the hydraulic jump acted to rapidly entrain some of that very dry air to lower altitudes, such a marked cloud clearing could result.

GFS40 model fields (below) indicated that there was a weak post-frontal trough at 850 hPa, with a band of convergence just ahead of the trough axis that seemed to correspond to the sharp cloud boundary on the GOES-12 visible imagery. Such a band of convergence is consistent with the idea of “higher-velocity flow discharging into a zone of lower velocities” — the slowing of the flow would effectively create convergence.

A vertical cross section using initial condition (0-hour forecast) fields from the GFS40 model (below) showed the very low relative humidity values that existed above the low-level temperature inversion (over the northern 2/3 of the cross section area). Both the model ageostrophic vertical circulation and the omega fields suggested that there was a shallow region of boundary layer downward motion over  southern Ontario, approximately where the hydraulic jump and cloud clearing line were located around 12 UTC.

In addition to the hydraulic-jump-induced sharp cloud clearing line seen on GOES-12 visible imagery, there was also evidence of a downstream undular bore; parallel cloud bands were apparent on 250-meter resolution MODIS true color imagery (from the SSEC MODIS Today site) centered over western Lake Ontario (below), as well as farther to the west over Lower Michigan.

Another question that remains is: what role (if any) did topography play in the formation and/or maintenance of such a hydraulic jump? An AWIPS topography image (below) shows that there is some slightly higher terrain over  southern Ontario, but the general orientation of the topography does not match that of the sharp clearing line seen on satellite imagery.

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