Western Heat Wave

Intense heat is beginning to develop across the Desert Southwest and will spread northward through the Great Basin and Central Valley of California; before it's over, the Pacific Northwest will also be enveloped in the swath of hot air.

The culprit is an atmospheric ridge, a northward bowing of the jet stream along the outer rim of a high pressure dome.  Within the ridge (beneath the dome), sinking air heats up and dries out and cloud formation is impaired; Santa Ana winds may also develop in Southern California.  Triple-digit heat is forecast for much of the region, with upper 90s (F) expected in Oregon and Washington.  Meteorologists warn that the high pressure ridge will remain in place through the upcoming weekend.

As one might expect, the Western heat wave is tied (meteorologically) to the Texas floods.  East of the ridge, the jet stream dips southward and this trough has spawned an upper level low over central Texas.  Counterclockwise winds around the low are combining with clockwise winds around a high pressure dome over the Southeast, pulling copious Gulf moisture across East Texas, Louisiana and the lower Mississippi Valley.  Once this atmospheric pattern shifts eastward, the flooding rains in Texas will cease and the Western heat will abate.

Brazos River Flooding

The Brazos River forms on the Llano Estacado at the junction of its Salt and Double Mountain Forks, northwest of Rule, Texas; it then flows northeastward to Seymour before angling southeast toward the Gulf of Mexico.  En route to the Gulf, the river passes through three major reservoirs: Possum Kingdom Lake, Lake Granbury and Lake Whitney; it also flows through Waco, passes west of College Station and flows west of Houston before emptying into the Gulf of Mexico at Freeport, Texas.

While the Brazos itself is 840 miles long, its watershed stretches back to eastern New Mexico; from its most distant tributary to its mouth, the River's watershed is 1280 miles in length, making it the 11th longest stream in the United States.  Over the past few weeks, heavy rainfall across the Brazos watershed has led to severe flooding, especially below the above mentioned reservoirs.  Not designed for flood control, these lakes must be partly drained as they fill in order to prevent catastrophic dam collapse.

As of this morning, the Brazos crested at 54.6 feet in Richmond, Texas (southwest of Houston), shattering a Century-old record by almost 4.5 feet; unfortunately, more rain is forecast across the watershed in the coming days.  To date, at least 6 individuals have been killed by the flooding and 31 Texas Counties have been declared disaster areas.  See also Texas: Land of Drought & Flood.

Deluge in Houston

The ongoing deluge in Houston, Texas, is the product of a stalled cold front in East Texas and a "blocking high" over the eastern U.S.  The latter dome of high pressure has caused the cold front to become stationary and, at the same time, is funneling a plume of Gulf moisture into the area.  In the Northern Hemisphere, winds flow clockwise around high pressure domes and, in this case, are sweeping across the Gulf of Mexico and into Metro Houston; there, the cold front lifts the moisture-laden air, producing copious precipitation.

In northwest Houston, more than 15 inches of rain have fallen, 5 inches more than the previous record for the entire month of April.  As one might expect, severe flooding is widespread, snarling traffic, damaging structures and threatening lives.  As I write this post, the dome and its moisture plume are slowly shifting eastward and rain is beginning to dissipate in the city.

Indeed, the western edge of the high pressure dome is now entering Louisiana and Gulf moisture is streaming northward along the cold front.  Here in central Missouri, the band of heavy rain and its imbedded thunderstorms are nearing Columbia and should arrive by early evening; two days of rain are forecast before the dome and the front move on to the east.

Prelude to Storms

As the storm system that brought rain and mountain snows to California dips southeastward, a deep atmospheric trough and upper level low are developing across the Western U.S.  Ahead of this system, warm, moist air is moving northward from the Gulf of Mexico, priming the Southern Plains and lower Mississippi Valley for thunderstorms and flooding rains.

Here in Missouri, we expect an afternoon high of 70 degrees F under cloudy skies.  Thunderstorms are forecast to arrive overnight though severe weather will likely remain to our south; indeed, a tornado watch has been posted for much of Texas and States to its north and east.

In addition to the severe weather, copious rainfall is expected in the south-central U.S. over the next few days as the upper level low stalls over Texas, directing a plume of Gulf moisture and imbedded thunderstorms across the region.  Up to a foot of rain is forecast for Louisiana and Arkansas before the system breaks down and moves off to the east.  Flood season is certainly progressing with a vengeance.

A Stormy December Night

It's a stormy evening in central Missouri.  As I sit here watching the coverage of tornadoes east of Dallas, Texas, torrential rain is pelting our roof and thunderstorms are rumbling through the area.  It may be late December but it feels and sounds more like April.

The agents of this stormy weather are an upper level low pushing into west-central Texas, a cold front draped from eastern Texas to southern Illinois and surface lows along that front.  Warm humid air is flowing northwestward from the Gulf of Mexico, providing plenty of moisture and unstable atmospheric conditions across the Southern Plains and mid Mississippi Valley.  Expecting two inches of rain overnight, we are under a flash flood watch in Missouri but should escape the severe weather that is producing damage east of Dallas; a tornado watch covers eastern Texas, southeastern Oklahoma and western Arkansas.

On the backside of the upper level low, a blizzard is developing across northwest Texas, southwest Oklahoma and eastern New Mexico.  At least that segment of the storm is consistent with the season.