|500 millibar map for 7:00 a.m. CDT April 21|
|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.
|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.|
|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.