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Above:
Mark Pickett (left) and Ryan Craig (right) work to rescue Bruce Salley
,
who was trapped in his car by flood waters in a supermarket parking lot
in Rockford,
Illinois, on Monday, June 18, 2018
. An evening
thunderstorm brought heavy rains across the Rock River Valley of
northern Illinois,
causing vehicles to get stuck in flood waters and
stranding motorists. Image credit: Arturo Fernandez/Rockford Register
Star via AP. |
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Flash flood watches were strewn
from northern Montana to the Texas Gulf Coast on Tuesday, the result of
unusually rich moisture stranded across the United States beneath
lackadaisical upper-level winds. It’s a classic summer setup for
generating the kind of slow-moving showers and thunderstorms that can
lead to huge rainfall totals and serious flash flooding. Unlike river
flooding, flash floods tend to be localized in both time and space,
which means people at risk need to be prepared to act quickly.
The
most pressing concern on Tuesday for the largest number of people is
the risk of flash flooding across southeast Texas, including the Houston
area. The NOAA/NWS Weather Prediction Center has flagged most of the
Texas coastal region as being in a moderate risk of excessive rainfall on Tuesday (this is separate from the Storm Prediction Center’s risk areas for severe weather).
The
main trigger for the rains across coastal Texas is a nearly stationary
disturbance with tropical origins that traveled from the western
Caribbean into the western Gulf late last week, then stalled out near
the Texas coast. Rich moisture continues to circulate from the Caribbean
and Gulf around the low into Texas. The radiosonde launch at
Brownsville, Texas, on Tuesday morning showed 2.49” of precipitable
water (the amount of water vapor in the air above a given spot), which
is close to a record value for June.
Fortunately, the rains from this system will be nowhere near as widespread or as heavy as the 30-60” totals observed in catastrophic Hurricane Harvey,
and the impacts are thus expected to be much more localized. However,
there is still plenty of uncertainty in exactly where the pockets of
heaviest rain will set up. In situations like this, the heaviest cells
often lodge atop small-scale (mesoscale) boundaries that may not be in
place until a few hours beforehand. The prediction challenge was plain
to see in a forecast discussion issued by the NWS office in Houston on Monday afternoon:
“Models are still having a very very very very very difficult time
indicating where [the heaviest rainbands] could possibly happen, with
different locations and amounts evident on nearly every run that comes
out.”
Multi-day amounts over southeast Texas will end up well above 10” in some spots. Already, weather.com reports
that Alice, Texas, racked up 9” of rain in 24 hours as of 10 am Tuesday
morning. At least 11" fell between Monday and Tuesday in a rain gauge
that overflowed about seven miles southwest of Premont (between Corpus
Christi and Laredo), according to the NWS office in Corpus Christi.
Update: Major
flash flooding was under way Wednesday morning in south Texas,
including cities along the lower Rio Grande Valley, where a flash flood emergency was in effect from McAllen to Harlingen. Close to 13" of rain had fallen in the Corpus Christi area.
Major flood woes in Upper Michigan
On Sunday
and Monday, mid- and upper-level moisture from ex-Hurricane Bud
combined with lower-level moisture from the disturbance now over Texas
to yield an exceptionally water-rich air mass along a stationary front
in the Upper Midwest. Dozens of sinkholes opened up across the region,
according to weather.com.
The highest-impact flooding was along a belt running from northern
Minnesota into the Upper Peninsula of Michigan. Two U.P. counties,
Houghton and Menominee, were declared disaster areas by Governor Rick
Snyder on Monday. At least one flood-related death was reported
, a 75-year-old man found near his truck in Wisconsin on Sunday.
On
Monday night, it was eastern Colorado’s turn for storms that dumped
heavy rain and very large hail from the northern Denver metro area onto
the High Plains. A
3-inch-diameter hailstone fell in Superior,
between Boulder and Denver, and reports of 2-2.5” hail (golf ball to
tennis ball size) were widespread in the northern urban corridor.
“Yesterday's Denver-area hail storms certainly have the feel of a big cost event,”
tweeted Bryan Wood, a meteorologist and storm damage analyst at Assurant, on Tuesday morning.
Bob Henson
WU meteorologist Bob Henson, co-editor of
Category 6, is the author of "Meteorology Today" and "The Thinking
Person's Guide to Climate Change." Before joining WU, he was a longtime
writer and editor at the University Corporation for Atmospheric Research
in Boulder, CO.
Tomorrow, July 10, 2018, there will be a low of 76F (24C) and a high of 83F (28C)
