Very large hail in the Texas Panhandle region

June 12th, 2010 |
POES AVHRR Cloud Top Temperature product

POES AVHRR Cloud Top Temperature product

Severe thunderstorms that developed in the vicinity of a stationary frontal boundary over the northern Texas Panhandle region on 12 June 2010 produced unusually large hailstones (some as large as 5.5 to 6.0 inches in diameter) — additional details of this event can be found at the NWS Amarillo TX website. AWIPS images of the 1-km resolution POES AVHRR Cloud Top Temperature product (above) indicated that CTT values were as cold as -81º C at 19:42 UTC and -80º C at 20:07 UTC. By comparison, the coldest IR brightness temperatures seen on 4-km resolution GOES-13 10.7 µm imagery were -66º C.

The University of Wisconsin Convective Initiation product (below) flagged that particular area of developing convection at 18:40 UTC — about an hour before it produced the first report of 1.0 inch diameter hail at 19:40 UTC (and almost 2 hours before the report of 6.0 inch diameter hail at 20:32 UTC):

GOES-13 10.7 µm IR images (grayscale) + UW Convective Initiation product (colored boxes)

GOES-13 10.7 µm IR images (grayscale) + UW Convective Initiation product (colored boxes)

Long-lived MCV Over Texas

June 3rd, 2010 |

A convective system over Texas spawned a Mesoscale Convective Vortex (MCV) during the day on 2 June, as shown in the 1.5-day loop above (color-enhanced GOES-13 11-micron imagery shown every 12 hours). The convection along the dryline at 0615 UTC on 2 June evolved into the MCV shown at 1815 UTC as a swirl of mid-level cloudiness over central Texas. That swirl supported the development of convection that subsequently evolved into a larger MCV on 3 June.

Atmospheric conditions that support MCVs are fairly well-understood. Wind shear acts to disrupt the warm core circulation of the MCV, thus small values of wind shear are commonly found near MCVs. In addition, MCVs are maintained by latent heat release in convection, suggesting the presence of abundant moisture. Low values of wind shear are noted on 3 June over eastern Texas: Model shear from the RUC (shown here overlain on the 11-micron imagery in a screengrab from AWIPS) has a wide region of weak shear over east Texas. The radiosonde from Fort Worth from 1200 UTC on 3 June also shows low values of shear. Abundant moisture is available to the system. Blended Total Precipitable Water (TPW) imagery (data from GPS over land and AMSU over water — click here and here for more information) from AWIPS shows values nearing two inches over east Texas. A loop of MIMIC TPW suggests moisture is being drawn into east Texas from the adjacent Gulf of Mexico.

Visible imagery from GOES-13 (above) shows convection developing rapidly over central Texas yesterday in the vicinity of the MCV. How well do CIMSS convective products do in predicting that development? Careful inspection of the loop above shows towering Cumulus at 1845 UTC and glaciating towers only 45 minutes later at 1932 UTC. (Click for imagery at 1902 and 1915 UTC). Screengrabs from a webmap server run at SSEC show UW Convective Initiation indicated as ongoing at 1915 UTC for a developing system that is warned as severe 45 minutes later. See the loop of screengrabs below.

Multi-day MCVs are infrequent: usually, an MCV will not persist for more than 12 hours, although famous multi-day examples exist (such as the MCV that spawned the July 1977 Johnstown, PA flood. That MCV could be traced to convective development over the Dakotas three days earlier). Note how the MCV over Texas grows in horizontal size from 2 June to 3 June. At 1200 UTC on 2 June the convective system was confined to a small region of north-central Texas. By 1200 UTC on 3 June, the region of influence has grown greatly, and the convective system covers all of eastern Texas and extends into the Gulf of Mexico. In addition, the strength of the accompanying 500-mb height field perturbation has increased. This upscale development — from smaller scale to larger scale — is one more interesting aspect of this system.

Added: Click here for a 2.5-day animated gif loop of 13-micron imagery data from GOES-13 (48 megabytes of data) produced using McIDAS-V.

This weather event is also discussed at the Hazardous Weather Testbed blog. Link.

Tornado outbreak in Oklahoma and Kansas

May 10th, 2010 |
GOES-13 0.63 µm visible images

GOES-13 0.63 µm visible images

On yet another relatively rare SPC “High Risk” Convective Outlook day, a major outbreak of damaging tornadoes occurred across parts of Oklahoma and Kansas on 10 May 2010 (see: SPC storm reports | NWS Norman OK | NWS Witchita KS). McIDAS images of the GOES-13 0.63 µm visible channel data (above) showed the development of numerous long-track supercell thunderstorms across the southern Plains region — much of this development was along and ahead of an advancing dryline, where GOES-13 Sounder Derived Product Imagery (DPI) showed Lifted Index (LI) values in Oklahoma as low as -14º C 21 UTC (along with Convective Available Potential Energy (CAPE) values as high as 6000 J kg-1 at 19 UTC).

Also note the large hazy plume of blowing dust that was moving northeastward from the Texas panhandle across western and central Oklahoma, as well as a smaller area of blowing dust that moved southward out of southwestern Kansas and into the Oklahoma panhandle region later in the day — this was an indicator of the strong lower tropospheric wind fields that were present over much of the region on that day.

The corresponding set of GOES-13 10.7 µm IR images (below) revealed several “enhanced-v” storm top signatures and a number of pronounced cold overshooting tops — cloud top IR brightness temperatures became progressively colder into the late afternoon and early evening hours, with a number of cloud tops in the -70º C to -80º C range (black to white enhancement).

GOES-13 10.7 µm IR images

GOES-13 10.7 µm IR images

A 1-km resolution MODIS water vapor image (below) showed a very complex array of middle tropospheric wave structures, an indication that the very strong winds were interacting with the terrain of the region.

We will begin by focusing our attention on the severe storm that can be seen developing over northern Oklahoma, near the triple point intersection of the cold front, the dry line, and the warm front.

MODIS 6.7 µm water vapor image + surface fronts

MODIS 6.7 µm water vapor image + surface fronts

The GOES-13 satellite was placed into Super Rapid Scan Operations (SRSO) to support the VORTEX 2 field experiment — so satellite imagery was available at 1-minute intervals for short periods during the afternoon and early evening hours. GOES-13 SRSO visible images (below; also available as a QuickTime animation) showed some very interesting storm evolution and structure associated with a few of the earlier thunderstorms that developed across the Oklahoma/Kansas border region.

Of particular interest was the appearance of inflow feeder clouds that were seen to develop and become ingested into the southwest quadrant of one of the stronger storms during the 20:15 – 20:45 time period — and not long after these inflow feeder clouds were seen on the visible satellite imagery, this storm intensified and produced hail of 4.25 inches in diameter and a large tornado.

GOES-13 0.63 µm visible images (SRSO at 1-minute intervals)

GOES-13 0.63 µm visible images (SRSO at 1-minute intervals)

A comparison of the GOES-11, GOES-15, and GOES-13 visible images that were available during the 20:30 – 20:45 UTC period when these inflow feeder clouds were being ingested into the storm show the value of more frequent imaging, which allows the evolution of such features to be more clearly visualized and understood (below). The images are shown in the native projections of each GOES satellite.

GOES-11, GOES-15, and GOES-13 visible images during the 20:30-20:45 UTC period

GOES-11, GOES-15, and GOES-13 visible images during the 20:30-20:45 UTC period

A 1-km resolution MODIS 11.0 µm IR image at 19:49 UTC (below) revealed the presence of a well-defined cold/warm cloud top temperature couplet associated with a developing “enhanced-v” signature — both cloud top signatures can be indicators a potential for that storm to produce severe weather (a 20-30 minute lead time is typical for such satellite storm top signatures). In fact, this storm did later produce hail of 4.25 inches in diameter at around 20:10 UTC, and then a strong tornado around 20:33 UTC. Note that neither the cold/warm couplet nor the developing enhanced-v signature was apparent on the 4-km resolution GOES-13 10.7 µm IR imagery at that time.

MODIS 11.0 µm IR image + SPC storm reports

MODIS 11.0 µm IR image + SPC storm reports

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POES AVHRR 12.0 µm IR image

POES AVHRR 12.0 µm IR image

Farther to the south, additional large areas of severe convection developed a couple of hours later over southwestern Oklahoma, which then moved eastward and produced large hail and tornadoes in the Oklahoma City and Norman areas. An AWIPS image of the AVHRR 12.0 µm IR channel data (above) showed the dramatic increase in areal coverage of cold cloud tops in southern and central Oklahoma at 22:32 UTC. A close-up view of that AVHRR IR image with an overlay of SPC storm reports (below) revealed a cluster of very cold cloud top brightness temperatures (as cold as -84º C, purple color enhancement) that was likely associated with the reports of tornadoes and hail of 4.00 and 3.75 inch in diameter in the Norman and Tinker Air Force Base areas.

POES AVHRR 12.0 µm IR image + SPC storm reports + surface METAR data

POES AVHRR 12.0 µm IR image + SPC storm reports + surface METAR data

AWIPS images of the CIMSS experimental “Convective Initiation”, “Cloud Top Cooling Rate”, and “Overshooting Top” products are shown below; these products appeared to have some skill in providing some lead time to the development of this severe convection that later affected Oklahoma City and Norman. These CIMSS products are being provided to the Storm Prediction Center as part of the Hazardous Weather Testbed evaluation (a component of the GOES-R Proving Ground activities).

GOES IR + Convective Initiation + Cloud Top Cooling + Overshooting Top products

GOES IR + Convective Initiation + Cloud Top Cooling + Overshooting Top products

UPDATE: Added an N-AWIPS loop of Visible imagery with lightning and Convective Initiation, along with an N-AWIPS loop of Visible Imagery with Cloud-top cooling and warning polygons.

Convection Returns to the southern Plains

April 23rd, 2010 |

Spring convection and associated severe weather returned to the southern Plains on April 22nd. Did predictors of convection do a good job in locating the severe cells?

CIMSS has recently started to produce synthetic satellite imagery from the Weather Research and Forecasting (WRF) model run at the National Severe Storms Laboratory (NSSL). Output from daily runs at 00 UTC is produced for 9 infrared bands that correspond to those of the Advanced Baseline Imager (ABI) that will fly on GOES-R. The hourly loop of the 11.2-micrometer channel, above, for the period between 1800 UTC 22 April and 00 UTC 23 April, shows convection initially forming along the dryline in the Texas Panhandle between 1900 and 2000 UTC before progessing northeastward into Oklahoma and Kansas. Synthetic imagery of the middle of 3 ABI water vapor channels (6.95 microns), show a similar story. Model predictions give clues on where to look for convective development. How did real-time predictors of convective development perform?

The UW Convective Initiation algorithm combines observations of 10.8-micron cooling (from GOES-13) with cloud phase changes. When cooling occurs as cloud phase is changing (suggesting growing cumulus towers that are glaciating), GOES-13 pixels are flagged as showing convective initiation. Depending on the cloud phase — all water, mixed water and ice, or all ice, the initiation is flagged in the screengrabs from N-AWIPS above as pre-CI cloud growth (blue), CI likely (green), or CI occurring (yellow). Once glaciation has occurred, CI detection turns off. A previous blog entry on this method is here.

UWCI does flag individual cells that subsequently develop, ignoring adjacent towering cumulus. Thus, it can draw forecaster attention to the updrafts that, for whatever reason, are the most vigorous. For example, the image at 1701 UTC show convective initiation indicated in one spot along the dryline in west Texas. By 1745 UTC, convection has developed. Shortly after 1800 UTC, UWCI identifies individual cells along a line from the extreme western portion of the Oklahoma panhandle northward into east central Colorado. These cells subsequently spawn severe weather. UWCI also flags nascent convective development for cells that eventually develop into an arc of convection over central Kansas at 2301 UTC. Note also that UWCI flags specific convective towers within a large cumulus field over the southern Panhandle. (Consider the three images at 2131 UTC and 2145 UTC and at 2231 UTC; convection initiation flagged at the earlier two images develops most vigorously as shown in the final image). This can focus forecaster attention to the clouds that are growing most rapidly.

The two images above show where convective initiation was diagnosed to be ongoing at some time on 22 April, as well as a preliminary Storm Report from the Storm Prediction Center. Note the good general overlap of UWCI points over the High Plains and storm reports. That more Storm reports exist than UWCI points reflects the UWCI philosophy of keeping the false alarm rate low, perhaps at the expense of detection.

There are several UWCI hits over the northeast on 22 April as well. There, cold air at upper levels promoted self-destructing sunshine and shower and thunderstorm development. Clear skies early in the day (1431 UTC) gave way to cumuliform development. The strongest updrafts likely yield the strongest cloud-top-cooling signal (as shown in this loop) and evolve into the most vigorous shower or thundershower. Even though severe weather was not reported with these cells, lightning was produced, starting around 1900 UTC as shown here. Cloud-top cooling can give a forecaster a head’s up that a particular cell might become vigorous enough to electrify.

(Note: this post has been corrected to remove images from before 1645 UTC on 22 April that may have included mis-navigated regions of convective initiation).