Month: December 2019

Figure 1: Visible imagery from GOES-16 on 1416 UTC 24 November 2019 (https://www.star.nesdis.noaa.gov/GOES/index.php)

 

A curious cloud formation over the southeastern United States appeared on satellite imagery on the morning of 24 November 2019. Visible imagery taken at 1416 UTC on 24 November 2019 showed a well developed extratropical cyclone over the northeast and further south a line of clouds over eastern Tennessee that suddenly began to spread to the southeast once in Georgia (Fig. 1). An infrared cloud-top imagery loop showed low level clouds that initially (As of 0636 UTC 23 November 2019) stretched in a band from the northeast to southwest, but over time the southern part of the band extended southeast while the northern portion remained stationary and did not propagate eastward at all (Fig. 2). The answer to the cause of this cloud development lies in the height of the clouds, shown on IR imagery to be low in altitude.

Figure 2: Cloud-top IR Imagery from GOES-16 from 0636 to 1421 UTC 24 November 2019 (https://www.star.nesdis.noaa.gov/GOES/index.php)

 

Extending from the northeast to southeast are the Appalachian Mountains, which start in northern Georgia and end in southern Maine (Fig. 3). These mountains are low in height relative to the Rocky Mountains, but in the case of these clouds, they were just high enough. As the band of low-level clouds propagated eastward, the Appalachian Mountains blocked the majority of that movement due to their height. Some of that moist air is simply trapped by the mountains. Any air that is orographically forced upward over the mountains, upon reaching the other side of the mountain, has lost the majority of its moisture on the windward side and is warming, decreasing the relative humidity even further. Thus, there were no clouds on the leeward side. Further south, past the southern tip of the Appalachian Mountains, the clouds can continue on their march southeastward unperturbed and unbothered by the terrain.

Figure 3: Topographic map of the United States (https://jan.ucc.nau.edu/~alew/maps/basemaps.html)

Pre-Thanksgiving travel got a whole lot harder this week in the western region of the US as what is known as a “bomb cyclone” had begun to impact the region on Tuesday, 26 November 2019. A bomb cyclone is caused by bombogenesis, which occurs when an extratropical cyclone rapidly intensifies as a result of the pressure in the center of the storm dropping at least 24 mb in 24 hours. In the case of this most recent cyclone, the storm dropped from 1002 mb to 975 mb in only 12 hours. A mid-latitude storm with a pressure of 975 mb indicates a very strong storm, in this case capable of producing hurricane force winds and winter storm warnings across the west coast. The rapid organization and development of the storm can be seen in Fig. 1.

Figure 1: Airmass RGB satellite imagery from 1730 UTC 26 November 2019 to 0130 UTC 27 November 2019. Source: NOAA

 

The rapid intensification of the storm is due to its location in relation to a strong jet. As can be seen in Fig. 2a, the area of low pressure shifted from the center of the jet to the northern portion of the jet exit region. Strong upper level divergence of air in this portion of the jet leads to rising air at the surface, which in turn lowers the pressure. Between 1200 UTC 26 November 2019 and 0000 UTC 27 November 2019, the pressure at the center of the storm dropped 27 mb as a result of this effect. Over this 12 hour period, the pressure, as seen in Fig. 2b, went from 1 standard deviation below the mean sea level pressure to 5 standard deviations below the mean sea level pressure. Coupled with the other mid-latitude cyclone currently over the central US, travel across the US for the Thanksgiving holiday is not going to be easy this year.

 

Figure 2a (left): Mean Sea Level Pressure (MSLP) in black, jet stream shaded in blue/pink. Source: http://www.atmos.albany.edu/student/abentley

Figure 2b (right): Mean Sea Level Pressure (MSLP) in black, standard MSLP anomaly shaded. Source: http://www.atmos.albany.edu/student/abentley

 

Sources:
http://www.atmos.albany.edu/student/abentley
https://www.wpc.ncep.noaa.gov/archives/web_pages/sfc/sfc_archive.php
https://www.washingtonpost.com/weather/2019/11/26/bomb-cyclone-could-break-records-it-slams-into-west-coast-bringing-mph-winds-blizzard-conditions/
https://www.weather.gov/

Flash flood warnings were issued by the National Weather Service across portions of Arizona, Utah, and California due to the rainfall associated with a low pressure system moving across the region. This can be seen in Figure 1. The system formed originally as a tropical invest region off the west coast of Mexico, then moved northeastward over the Gulf of California. Over the Gulf of California, the system was able to continue to grow as the warm, moist air above the water contributed to convection. This can be seen in Figure 2. This figure depicts the moisture transport associated with the system. As shown, the moisture is being advected from over the Gulf of California northward to the Mexico and Arizona border.

Figure 1. National Weather Service issued map of warnings and advisories. Issued 19 November 2019 at 2006 UTC. Depicts majority of Arizona under a flash flood warning. Over 13 million people were under a flash flood warning at the time this was taken.

 

Figure 2. SPC Mesoanalysis Page produced moisture transport plot. Taken 2000 UTC 19 November 2019. Depicts the transport of precipitable water (fill) northward. Moisture transport vectors are plotted. https://www.spc.noaa.gov/exper/mesoanalysis/new/viewsector.php?sector=12#

 

Figure 3. Visible Satellite Image taken 1956 UTC 19 November 2019. Depicts Southwestern United States, including the system over Arizona bringing rainfall to the region. http://www.aos.wisc.edu/weather/wx_obs/GOES17_sw.html

 

The system is expected to propagate northeastward, bringing rain to more of the southcentral United States, and even snowfall at higher elevations. The current cloud structure can be seen in Figure 3. The system is currently located over the border of Mexico and Arizona. The flash flood warning will remain in place through Wednesday evening as the system moves northward, bringing continuous rainfall to the rest of Arizona. In regions that are as dry as the southwest, it does not take a lot of rainfall to induce flooding. Though there is some potential for hail formation and strong winds, the main threat will be the flooding.

Amongst preparations of turkey, cranberry sauce, and stuffing for Thanksgiving, West Coast residents in California and Oregon are also preparing for a potentially record-breaking winter cyclone. A powerful low pressure system, which is rapidly intensifying, is expected to make landfall near the border of Oregon and California during the afternoon of 26 November. With anticipated wind gusts up to 100 mph, this low-pressure system is expected to cause wind speeds comparable to a low-end Category 1 hurricane. Additionally, this system is predicted to cause up to 4 feet of snow in the Sierra Nevada and wave heights up to 37 feet along the coast.

Currently, the system’s central pressure values are predicted to drop 40 mb in 24 hours, which effortlessly surpasses the “bomb cyclone” requirements of a 24 mb decrease in pressure over 24 hours. In Fig. 1, a GIF of water vapor satellite imagery shows the variations of brightness temperature to the east and west of the low pressure system. To gain further insight into this pattern, it is helpful to use the Air Mass RGB satellite product. As shown in Fig. 2, dry upper level air with a cold air mass to the south, as well as high, thick clouds, contribute to the spiraling system. Fig. 3 shows a GFS model run of MSLP and 10m wind speed knots, which depicts the counter-clockwise wind barb orientations, which reflects the low pressure environment. Furthermore, if the central pressure decrease of 40 mb proves to be true, this could cause the region’s lowest pressure readings on record. The previous record was set in 2010, with a central pressure reading of 978 mb.

Figure 1: To view a GIF of water vapor satellite imagery of the low pressure system just off the western coast of the United States, please go to https://twitter.com/NWSBayArea/status/1199414866475270144 )

 

Figure 2: This Air Mass RGB satellite imagery shows the interactions of the dry upper level air (red), cold air mass (blue), and high, thick clouds (white) which spiral counter-clockwise at the bomb cyclone off the coast of California and Oregon.

 

Figure 3: This map from Tropical Tidbits shows a GFS model run of MSLP and 10m wind speeds. Here, the central pressure of the low system is predicted to be 971 mb.