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.

Mammatus Clouds

As a line of thunderstorms approached Columbia this evening, our daughter called our attention to mammatus clouds above the city.  Indeed, a broad area of dipping lobes was observed overhead, the product of sinking air beneath the advancing cummulonimbus clouds.

Often associated with severe thunderstorms, mammatus clouds (named for their breast-like shape) have long been rumored to predict the development of tornadoes.  However, though their development indicates a potent updraft within the thunderstorm, mammatus clouds are actually most commonly associated with weakening storms.  Nevertheless, they do indicate the presence of wind shear, ice crystals and potent temperature gradients and weather-savvy pilots know to avoid these cloud formations.

For those of us on the ground, mammatus clouds offer a beautiful sight, especially when illuminated by a rising or setting sun; my thanks to Ally for the heads-up!  Fortunately (or unfortunately) we should have more opportunities to observe these clouds this weekend as a potent storm system moves in from the west.

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.

Tale of Two Domes

This morning, an atmospheric dome of high pressure, centered over Minnesota, was bringing cold, Canadian air to most of the central and eastern U.S.; another high pressure dome sat over the Desert Southwest.  Beneath the center of atmospheric domes, air sinks while, along their rims, air flows in a clockwise direction; the eastern dome lies within a broad atmospheric trough (a dip in the jet stream which allows cold air to plunge southward) while the southwestern dome is the heart of an atmospheric ridge (a northward curve in the jet which allows warm air to flow northward).

Northwesterly winds created by the cold, eastern dome were producing lake-effect snows in the Great Lakes region and sweeping frigid air as far south as the Mid-Atlantic States.  The western edge of this dome had backed up against the Colorado Front Range, creating dense morning fog as cold air settled above relatively warm, moist ground.

As the Southwest dome expands eastward, warm conditions are returning to the Front Range and will spread across the Southern Plains, eventually pushing into the Heartland.  In concert, the eastern dome will also drift eastward, bringing the coldest air of the winter to New England, the Great Lakes States and the Mid Atlantic region.

A Night of Tornadoes

Awakened by tornado warnings on our cell phones last night (just before 3AM), we turned on the TV and watched as a twister raked Siesta Key and moved northeastward through Metro Sarasota; three others followed in its wake but were less intense than the first.

The severe weather developed ahead of a potent cold front and were fueled, in part, by a strong subtropical jet stream, an atmospheric feature associated with El Nino.  Indeed, while most Florida tornadoes occur during the annual hurricane season (June to November), the strongest tend to develop during the winter months, especially in El Nino years.

Here on Longboat Key, we received heavy rain last night and continue to experience strong northwesterly winds behind the cold front this morning.  Fortunately, we escaped the brunt of the storms and were treated to a spectacular light show across Sarasota Bay as they moved onshore.  Unfortunately, at least two individuals were killed by the tornadoes.

A Winter Flood

The numerous December thunderstorms that lashed the American Midwest have produced extensive flooding throughout the Mississippi River watershed.  While the crest of the flood is reported to have passed to the south of St. Louis, the Missouri and its tributaries are still running high and flooding along the Illinois River is not expected to peak for another few days.  According to hydrologists, the crest of the Mississippi flood will not reach New Orleans until January 19.

Crossing the Mississippi just north of St. Louis this morning, we observed widespread flooding, especially along the Illinois side of the river.  Farther east, almost all of the Illinois streams that pass beneath I-70 were bank full and the Wabash River (a major tributary of the Ohio and thus a secondary tributary of the Mississippi) is still spilling across its floodplain.

Midwestern floods are most common in late winter or early spring, when snowmelt and heavy rains run across frozen ground and when dormant trees are not actively transporting moisture from the soil to the atmosphere.  This winter flood, spawned by an El Nino weather pattern and likely aggravated by our warming climate, may prove to be especially disastrous if a normal or heavy spring flood season follows in its wake.  Saturated soil from the current flood will not absorb the early spring rains, runoff will be excessive and the Mississippi levee system will be stressed; as we have observed in the past, our ability to control nature's fury has its limitations.

A Sleet Storm

Driving back from the Kansas City Airport this morning, I encountered a sleet storm along Interstate 70, having developed on the backside of this week's massive winter storm.  Stretching for more than 50 miles along the highway and driven by a strong north wind, the swath of heavy sleet decreased visibility and produced a slippery surface on the roadway.

Unlike hail, which forms in the cold upper layers of thunderstorms and eventually falls to earth when gravity overcomes the force of the updraft, sleet forms as rain or partially melted snowflakes fall through a cold layer of air near the ground, refreezing into small ice pellets that coat lawns, fields and roads.  Freezing rain, generally more common and destructive than sleet, refers to liquid rain drops that freeze when they contact cold objects or surfaces (tree limbs, fence posts, cars, roads, etc.).

Needless to say, driving was treacherous in the midst of the sleet storm.  Unfortunately, the greatest danger arose from drivers that did not heed the conditions, racing by in the passing lane; as usual, SUV owners and truck drivers were the primary offenders and many of them paid the price, their cars, jeeps and sixteen-wheelers soon littering the median and grassy margins of the highway.

December Tornadoes

Over the past week, an atmospheric ridge developed across the eastern U.S., allowing warm, humid air to flow northward from the Gulf of Mexico.  Yesterday, a potent storm system, centered over Iowa, swept its cold front into this unstable air mass, igniting severe thunderstorms, unleashing torrential rain and spawning tornadoes.

While tornadoes are not rare in the Gulf Coast States during the winter months, yesterday's outbreak, which included 24 twisters, was unusual for December.  As of this morning, at least seven deaths have been attributed to the storms, which pummeled communities from Arkansas into Mississippi and Tennessee and northward to the Ohio Valley.

As the cold front continues to push eastward this morning, the risk of severe weather persists through eastern Alabama, northern Florida, Georgia and the Carolinas.  This week's atmospheric imbalance in the U.S., characterized by a strong jet stream that divided cold, dry air in the West from warm, humid air in the East, was bound to break down and, when it did, tragic consequences ensued.

Addendum:  As of December 25, the storm system death toll has reached 14.  In addition, meteorologists suspect that a significant percentage of the twister sightings were related to the same long-lasting tornado, that may have been on the ground over a distance of 145 miles (from Clarksdale, Mississippi, into west-central Tennessee).

Pacific Northwest Deluge

Several storm systems have been riding the Pineapple Express over the past few days, dropping copious rain and high elevation snow across the Pacific Northwest.  The last storm is the series should arrive tomorrow, aggravating the mudslides and flooding that have plagued the region.

On the bright side, the moisture flow will shift south as the jet stream begins to dip across the Western U.S. and is expected to drop at least two feet of snow on the Sierra Nevada, welcome news for drought-stressed California.  The developing atmospheric trough will also allow the Pacific Northwest to dry out and bring plenty of snow to the mountains (and ski areas) of Utah and Colorado.

Of course, potent storm systems have negative consequences as well.  As warm, humid air is swept northward ahead of the trough, producing record highs from the Gulf Coast to the Upper Midwest, the atmosphere will be primed for severe thunderstorms.  These storms, some of which may spawn tornadoes, will ignite along the cold front and are expected to boil up from Texas to the Great Lakes over the coming weekend.  Behind the front, winter will return to the Heartland.