Thursday, April 21, 2016

Large Impacts from Slow-Moving System

The low pressure system on the weather maps today, both on the surface and aloft, has been taking its sweet time moving through the U.S. The upper level system moved into the Pacific Northwest a week ago (April 14), and this morning was located over Iowa.

500 millibar map for 7:00 a.m. CDT April 21
 Over the last week this system wreaked havoc on the central Rockies with heavy snow, and in southern Texas with record heavy rain. The slow progression of the low was due to what is called and omega block in the upper atmosphere. That effectively parked the upper low over the Great Basin and Rockies for several days. As is typical in these situations this was a good news, bad news situation. The good news was sunny, warm spring weather over the eastern half of the country. The bad news was a heavy spring snowstorm in the Rockies, and record heavy rainfall and flooding in southern Texas, particularly in the Houston area.

500 millibar map for 7:00 a.m. CDT April 17.

With the upper low parked over the Rockies, southerly winds on the east side of the low funneled copious amounts of moisture  northward through Texas and into the central Rockies and western Plains. Very cold air aloft, ample moisture, strong upward motion, and the fact that the system was barely moving resulted in an extended period of snow from Wyoming south through Colorado into northern New Mexico last weekend. By late Sunday more than 4 feet of snow - heavy, wet snow - had fallen in some locations on the east side of the front range above 9000 feet. Two to three feet was common in the Front Range foothills.

72 hour snowfall ending the morning of April 18.
4-day CoCoRaHS snow totals for locations in Colorado

Denver (Stapleton Coop site) picked up 8.4 inches of snow from the storm, but amounts varied from 6 to 12 inches across the metro area. This latest storm boosted Denver's snow for the season to 71.4 inches (11.4 inches for the storm and a season total of 69.3 inches at Denver International). That was enough to bump the Accumulated Winter Season Severity Index (AWSSI) for Denver back into the severe category for this year.

Far bigger problems were in store for Texas. A cold front trailing south from the surface low in Kansas stalled out and provided a focus for the development of heavy showers and thunderstorms. Southerly winds fed moisture laden air with dewpoints in the low 70s into southern Texas. This air collided with the colder, drier air north and west of the front and was forced upward, helping sustain heavy thunderstorms from Dallas south to Houston.

Surface map for 4:00 a.m. CDT April 18 for the southwest U.S.

The rain was heaviest in the Houston area, and was enhanced by an outflow boundary from thunderstorms that helped further sustain the rainfall. Thunderstorms regenerated and trained repeatedly over the same area. Houston's Intercontinental Airport set a one-day rainfall record of 9.92 inches on April 18, breaking the old record by almost two inches (8.16 inches in 1976). However, 10 inches was far from the highest amounts recorded. Those occurred in the northwest quadrant of the metro area. Measured rainfall amounts were in excess of 15 inches for the 24-hour period, and radar estimates were as high as 20 inches.

Quantitative Precipitation Estimate for southern Texas for the 24-hour period ending at 7:00 CDT April 18.

48-hour precipitation for the period ending 7:00 a.m. CDT April 19.
Source: Harris County Flood Warning Service

A number of CoCoRaHS observers recorded 12 inches or more on the morning of April 18. Much of this fell in a 12 to 13 hour period.

Southern Texas CoCoRaHS observations on April 18.
Flooding was rapid and widespread. Despite warnings to stay off the streets and not drive into flooded roads, eight people lost their lives after being trapped in floodwaters. Unfortunately the rain didn't quit completely, and another two to three inches fell in the Houston area through this morning.

Total precipitation for the 72-hour period ending at 8:00 a.m. April 20

Flooding is going to remain a problem in Houston for a number of days. Two dams in the area are considered "extremely high risk" by officials and are being closely monitored. The reservoirs behind them are at about 80 percent capacity.

Houston flooding on April 18.
Credit: Reed Timmer via Twitter

The rain wasn't limited to the Houston area. Five to eight inches of rain fell in the Dallas-Ft.Worth area resulting in flooding in northern Texas.

The pesky upper low responsible for this week of stormy weather will finally weaken and move out into the Atlantic late Saturday. Then, we'll turn our attention to the next system in the Pacific Northwest which may mean more snow for the Rockies and an unsettled week in the Plains and Midwest.

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