North of the Dome

Over the past week, a subtropical dome of high pressure has been developing across eastern Mexico and the northern Gulf Coast.  Pushing into the Southern Plains and Lower Mississippi Valley, it is both shunting Pacific storm systems to the north and fueling the Southwest Monsoon.

Along the outer edge of this atmospheric dome, moisture flows across Mexico (from east to west) and then up through the Four Corner States; this flow is augmented by low pressure over the Desert Southwest, which draws in moisture from the Sea of Cortez.  Just north of the dome, the moisture combines with inflow from the Pacific and moves eastward along a stationary front; pulses of low pressure move along the front, igniting thunderstorms and torrential rain.  "Training" across the same path (from eastern Kansas to southern Ohio), the storms will drop up to 5 inches of rain in some areas; northern Missouri is forecast to receive the brunt of the deluge.

Here in Columbia, light drizzle developed at 6:30 AM; steady rain has since moved in and will likely continue (off and on) for the next two days.  While the rains will provide welcome relief from our ongoing drought, heavy downpours on sun-baked soil will rapidly drain into creeks and rivers, setting the stage for flash floods.

Flooding in Appalachia

Yesterday afternoon and evening, a line of strong thunderstorms stretched across the Appalachians, from the Ohio River Valley to south-central Virginia.  Aligned along a stationary front and steered by the jet stream, the storms "trained" over the same areas, unleashing torrential rain (up to 9 inches in some locations).

Falling on the dissected terrain of the Appalachian Plateau and the Ridge and Valley topography west of the Blue Ridge, the copious precipitation drained rapidly from the steep ridges to the primary river valleys; mudslides and valley floods were the result, most severe in southeastern West Virginia and adjacent sections of Virginia.  Unfortunately, most Appalachian towns lie within the valleys and widespread damage has been reported.

Flash flooding is especially dangerous in mountainous terrain since it develops rapidly and escape routes are often limited.  Those living in or visiting such regions are thus advised to heed weather forecasts and take note of potential routes to higher ground; multiple routes should be considered since some roads may be closed due to high water or mudslides.  Beautiful as it may be in sunny weather, mountainous terrain poses unique risks when thunderstorms approach, even when heavy rains fall far upstream.

Cool Down in Denver

After two weeks of hot, humid weather in Missouri, I returned to Colorado today and was greeted by a cold front.  As I approached Metro Denver, a thick band of clouds was moving south toward the city and the Front Range peaks were obscured by an upslope haze.

By the time I reached our Littleton farm, scattered showers and thunderstorms had formed above the urban corridor and the air temperature, thanks to the upslope flow, had dropped to 70 degrees F, a welcome change from the sunny, hot conditions on the Great Plains.

As I write this post, it is 65 degrees at the farm and skies are clearing behind the cold front.  To our south, however, a swath of thunderstorms stretches along the Palmer Divide and into the foothills north of Pike's Peak.  One of the storms, located near Wilkerson Pass, west of Colorado Springs, has become severe and radar indicates that it has spawned a tornado.  The storms will likely weaken as the sun sets and our cool reprieve will be brief; if the current forecast is accurate, we can expect high temperatures near 100 degrees F in the Front Range cities tomorrow afternoon.

Within the Dome

A massive atmospheric ridge of high pressure extends across much of the U.S., stretching from Southern California to the Mississippi Valley.  Within this dome, air is sinking, causing it to heat up and dry out; as a consequence, cloud formation is minimal, augmenting the intensity of the high June sun.

High temperatures will range from the mid 90s (F) to 120 degrees within the dome; the most extreme heat will develop in the low deserts of the Southwest while triple digit heat indices (combining temperature and dew point) will be widespread across the Great Plains and lower Mississippi Valley.  Here in central Missouri, we expect a high of 99 degrees F under sunny skies and anticipate afternoon highs in the mid to upper 90s for the next week.

Thunderstorms will erupt along the northern and eastern edges of the dome, where the hot, humid air interacts with a cooler and drier air mass; this clash zone currently stretches from the Great Lakes to the Ohio River Valley and Mid Atlantic Region.  On the back (northwestern) edge of the dome, a less intense band of precipitation curves from Northern California to Montana.  As the atmospheric ridge inches eastward, the storms will move in concert and relief from the intense heat won't arrive for a week or more.  By July, such high pressure domes begin to settle over the Southern Plains, triggering the Southwest Monsoon.

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.

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.

A Formal Sendoff

Preparing to leave for Missouri this morning, I heard the distinctive call of an olive-sided flycatcher in the "backyard" of our Littleton, Colorado, farm.  Sure enough, the large-headed silhouette of that summer mountain resident occupied the end of a dead limb; he would intermittently dart out to snare a flying insect before returning to the same perch.  Though formally known as the olive-sided flycatcher, I have long thought he should be called the "tuxedo flycatcher" since his greenish-brown flanks part to reveal a vertical white band on his chest and abdomen.

Passing through the urban corridor on his way to the mountains, this insectivore will spend the summer near forest clearings or along the alpine timberline where he and his mate will aggressively defend their nest (usually placed in a conifer) from all intruders.  Olive-sided flycatchers breed from Alaska to eastern Canada and southward through the Western Mountains and higher stretches of the Appalachians; come September, they will head for wintering grounds in Central and South America.

As for ourselves, we left the farm by early afternoon and enjoyed sunny, mild weather as far east as central Kansas.  There, however, we encountered severe thunderstorms and stopped for the night in Hays, where tornado sirens sent us to the first floor hallway; fortunately, we escaped the brunt of the storms, receiving torrential rain and intense lightening.  Indeed, as I write this post, a spectacular light display continues to our east, promising more stormy weather when we reach Missouri.

Birding in Bad Weather

Since we are leaving on our California road trip tomorrow, I decided to get in one more visit to South Platte Park.  While recent visits to the preserve occurred on warm, sunny days, this morning promised a different experience; low clouds, chilly air and a steady northeast breeze greeted me at the parking lot.

Despite these conditions (and, indeed, because of them), birding was excellent at the floodplain refuge.  Violent-green swallows were especially abundant, strafing the lakes and the woodlands; other sightings of note included 55 yellow-rumped warblers, 17 yellow warblers, 6 yellow-breasted chats, 4 western tanagers, a few chipping sparrows and a lone great horned owl that stared at me from a lakeside tree.  In all, my two-mile walk turned up 33 species, better than my counts on the warm, sunny days.

A gray overcast produces lighting that mimics dawn and dusk, when many species tend to be most active.  In addition, the chilly air invigorates birds and mammals alike, stoking the drive to move and to feed.  As a consequence, birding is often most productive on days that are less appealing to birders; today's visit to South Platte Park reinforced that fact.