The Gulf of Mexico and Gulf Coast states have seen a plethora of thunderstorms since about mid-April. One of these episodes was a multi-day event that spanned April 16 – April 20. Multiple rounds of thunderstorms with very heavy rainfall (especially in and around Houston) and some significant severe weather (more hail and wind than tornadoes) traversed the region. The below animation shows these events and I included an example Mesoscale Precipitation Discussion (MPD) from WPC as a reference to the heavy rain.
GOES-13 Infrared imagery with GOES-R Lightning Detection (Density) spanning five days (04/16-4/20).
The Weather Prediction Center (WPC) Mesoscale Precipitation Discussion (MPD) for southeast Texas on 04/18/15.
More recently, on the evening of 04/22/15, a couple strong supercell thunderstorms moved off the Texas coast near the Corpus Christi area and traversed the offshore waters passing relatively close to a couple oil platforms. One of the more amazing attributes to the storms was the strong reflectivities of near 75 dbz! Although these were measured at high altitudes (based on the beam height), it’s interesting to note that large hail may have been occurring well offshore in the Tropical Analysis and Forecast Branch (TAFB) offshore zones. I have included a few animations showing the evolution of these supercells.
Animation of the GOES-R Convective Initiation product with GLD-360 lightning strokes overlaid valid 4/22-4/23.
National WSR-88D Mosaic base reflectivity with the Overshooting Top Magnitude (OTM) product overlaid highlighting the most intense updrafts associated with the supercells. The OTM product provides the difference between the overshoot and the surrounding cirrus clouds.
National WSR-88D Mosaic base reflectivity overlaid with the GOES-R Lightning Detection (Density) product in 5-minute increments. Note the extreme amount of lightning occurring with the dominant supercells in the Gulf of Mexico.
Here is a post from the National Weather Service Office in Houston-Galveston which nicely summarizes the threat posed to mariners from offshore supercells.
All mariners out there…this is why you should ALWAYS heed a Special Marine Warning. Buoy 42019 measured a 76 kt / 87 mph wind gust at 729 PM associated with this Gulf of Mexico thunderstorm!
Conventional satellite imagery depicted an upper low digging slowly southeast into the Central Rockies. Meanwhile, an active subtropical jet was focused farther south underneath it lifting northeast across northern Mexico and downstream across the wind across the Gulf Coast states.
GOES Sounder Air Mass RGB animation showing the evolution of heavy thunderstorms over the Gulf Coast centered near Houston (0600 UTC 04/16/15 – 0600 UTC 04/17/15).
While there was broad warm air advection, instability and a very moist airmass focused across the western Gulf Coast region, there were no identifiable features on the larger scale in conventional satellite imagery that suggested a forcing mechanism for organized convection to initiate. However, upon closer inspection of morning GOES-RGB airmass imagery, a subtle and small scale, but identifiable feature appeared that reflected a shortwave impulse. This impulse was tracked crossing the Rio Grande river at 12Z/16, and in conjunction with deep moisture and diurnal instability proved to be the catalyst for increasingly organized convection after 18Z/16 that continued past 00Z/17 along the Upper Texas coast and coastal areas of southern Louisiana. The result was 4 to 8 inches of rain, and flash flooding in the suburbs of Houston, TX.
GOES Sounder Air Mass RGB valid at 1200 UTC on 04/16/15. The red circle denotes the subtle shortwave that initiates downstream convection.
GOES-13 Water Vapor image valid at 1215 UTC on 04/16/15. The red circle denotes the same shortwave identified in the Air Mass RGB image.
GOES Sounder Air Mass RGB image valid at 1800 UTC on 04/16/15.
GOES-13 Water Vapor image valid at 1815 UTC on 04/16/15.
GOES Sounder Air Mass RGB valid at 0000 UTC on 04/17/15.
GOES-13 Water Vapor image valid at 0015 UTC on 04/17/15.
The NSSL 24-hour QPE product valid at 1200 UTC on 04/17/15.
Squinting into blowing dust rivals trying to see through a lake-effect snow squall. However, unlike snow, this particular phenomenon is nearly invisible to radar and satellite alike. So the question is, how do we see and forecast for something so obvious to the naked eye and yet so invisible to our traditional instrumentation? This is a question that AWC forecasters at the FA desks face on various occasion and is what makes blowing dust SIGMETs so difficult to issue. One such occasion occurred this past Wednesday, April 8th. A strong upper level system was positioned over the Southwest U.S. and the associated upper level jet had surface winds in the afternoon gusting to 30+ mph, particularly across New Mexico (Figure 1).
Figure 1. 20140408 1800 UTC GOES-W visible imagery and surface obs. Winds were 20-30 mph, with 30 mph+ gusts.
The forecaster working the FA desk was expecting these winds to result in blowing dust and at the beginning of shift requested to use the VIIRS/MODIS dust enhancement (provided by CIRA) to try and better identify any areas of dust that would arise. With approval for a temporary transition of the product to operations, the tool was able to be utilized in this case. By 1800 UTC winds had begun to pick up and radar reflectivity from Albuquerque showed some weak echoes indicating perhaps some dust being lofted. However, surface observations for the next couple of hours remained clear.
At 1940 UTC the first useable MODIS pass arrived, shown below in Figure 2. While it showed some weaker indications of dust in the Four Corners area, the majority of New Mexico remained clear.
Figure 2. 20150408 1940 UTC MODIS dust enhancement over the Southwest. Note the pale yellows in the Four Corners area
Additionally, the forecaster took a look at GOES sounder dust enhancement imagery available on the web. Though a coarser resolution, the weak indications of dust in the Four Corners were noticeable in this imagery as well (Figure 3).
Figure 3. 20150408 1802 UTC GOES sounder dust imagery. Note the yellows in the Four Corners area.
Though there may have been some lofted dust, the few weak radar echoes and overall clear surface observations indicated that it was not widespread or thick enough to warrant the need for a SIGMET. The FA forecaster was collaborating with ZAB (Albuquerque CWSU) during the afternoon hours and passed on the link (http://rammb.cira.colostate.edu/ramsdis/online/goes-r_proving_ground.asp) to both versions of the dust enhancement imagery. As the satellite-derived imagery seemed to be supporting their original assessment and lacked any strong dust signals, a final decision was made not to issue the SIGMET.
Although this ended up as a null case, it still shows the value of the satellite-derived imagery as a decision support tool at the AWC. As previously mentioned, dust is very difficult to pick up on any traditional imagery. However, the VIIRS/MODIS dust enhancement is able to bring this invisible hazard into view and provides additional and valuable situational awareness for blowing dust SIGMETs.