Tuesday, March 8, 2016

Cry Me A (Atmospheric) River

The general weather pattern that has set up over the U.S. the last few days has brought mild weather to much of the central and eastern part of the country. More ominously, it has established two narrow regions of concentrated atmospheric moisture that will bring a myriad of problems to California and the central U.S. These atmospheric rivers are not uncommon, and they are responsible for much of the transport of atmospheric moisture outside of the tropics. These atmospheric river setups are often responsible for heavy rain and flooding when they impact the U.S. You have probably heard about the Pineapple Express, which is one of these atmospheric rivers.

The strong southern jet stream across the Pacific Ocean, a manifestation of the warm Pacific waters associated with El NiƱo, is funneling moisture from eastern Asia to the west coast of the U.S. Precipitable water is a measure of the moisture vapor in the atmosphere that, if condensed, would accumulate as rain. In the image below you can see the stream of moisture extending across the Pacific, and another plume of moisture extending from the southern tip of Mexico north through the Midwest.

Precipitable water for 6:00 p.m. CST Tuesday, March 8, 2016.

The Pacific atmospheric river is feeding into a strong trough of of the west coast, combining to bring rain to California and heavy snow the Sierras. A cutoff low developed in the trough late yesterday, with strong southerly winds on the east side of the trough.

500 millibar map for 6:00 a.m. CST Tuesday, March 8, 2016. Shaded areas indicate wind speeds.

As this system moves slowly east the moisture over the Pacific will wrap around and enhance the flow of moisture into the central U.S from the Gulf of Mexico.  The atmospheric river in the Pacific, as well as the one developing from Mexico to Texas into the central U.S. (dubbed the Maya Express) are clearly evident on the precipitable water maps for today (above) and the forecast map for Thursday morning.

Precipitable water forecast map for 6:00 a.m. CST Thursday, March 10, 2016

The precipitation forecasts for the next several days are impressive. Over the next three days the heaviest precipitation is expected to occur in Texas and east along the Gulf coast with perhaps amounts up to 12 inches, and flooding is likely in many areas if this amount of rain does in fact occur. this is in addition to 2 to 6 inches of rain in Texas that occurred in the 24 hour period ending Tuesday morning.  Heavy precipitation is also expected in northern California. Much of California has received from one to two inches of rain in the past few days. While California needs the rain, this much rain will be causing flooding problems.

Quantitative precipitation forecast for the 72 hour period ending 6:00 p.m. CST Friday, March 11, 2016

Looking ahead the next seven days, more rain is expected beyond the three-day period as additional storms reach the coast. These will also add to the snowpack in the Sierras and Cascades, adding to the water storage for the warm season.

Quantitative precipitation forecast for the 7 day period ending 7:00 p.m. CDT Tuesday, March 15, 2016.
You can read more about atmospheric rivers in this Forbes article by Dr. Marshall Shepherd of the University of Georgia and host of WXGeeks on the Weather Channel.

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