Aug 12, 2005
Wind and Dust Event of August 12,
2005
This weather event was unusual in
many ways. The overall synoptic pattern that led to this event was more typical
of January rather than mid-August. A ridge of high pressure had built over the
Gulf of Alaska and amplified northward into Alaska. This brought record warm
temperatures of upper 70s and lower 80s (normals are in the 60s) to the 49th
state. In response, the jet stream over western Canada dug to the south, bringing
a cool air mass into the northern Rockies and Northwest U.S. The hot temperatures
of early August began to slowly moderate on the 10th as the Canadian jet gradually
sagged into the area. The initial surge of a cooler low-level air mass arrived
into the Inland Northwest on Thursday evening, August 11th as winds shifted
to the northeast.
What transpired on the afternoon
and evening of August 12th intially appears to have originated as a outflow
gust front from thunderstorms over northeast Washington. The loop below shows
the gust front dropping southwestward as the thunderstorms continue to move
off to the southeast.
While this is not an unusual occurance,
what was atypical was the longevity and persistent strength of this boundary.
By 03Z (800 pm PDT), the boundary had progressed all the way to Walla Walla
to the south, and Ellensburg to the west, more than 4 hours after it's inception.
Additionally, the wind gusts experienced at Hanford and Pasco were equal to
the gusts as it moved through Spokane.
Typically a thunderstorm-generated
gust front will slowly decay over time. This is due to the fact that the initial
surge of cold air which originated from the thunderstorm is limited in supply.
As the gust front spreads out, this supply of cold outflow air becomes increasingly
more shallow. Additionally, the outflow air will also undergo a mixing process
with the environment as well as frictional affects.
The boundary on August 12th did have
a couple of things working in its favor. The pre-existing flow was already from
the northeast. So the boundary moving to the south didn't have a head wind to
"fight". Also, the topography from Spokane to the Tri Cities is a
gradual downslope, changing in elevation from about 2500' to around 400' above
sea level. Thus, gravity would have been assisting the boundary as well.
The problem with assigning the thunderstorms
as the cause of this boundary, is that the numerical models also predicted a
cold front well before any thunderstorms has developed. The loop below shows
the 500mb heights and vorticity during this event. Note the strong short wave
that moves over the area during the afternoon and evening hours.
At the 850mb level, the model predicted
a front to back into the area from the northeast. Note the strong temperature
gradient with this front. Also note the near-perfect placement of this front
in the model forecast compared to the actual boundary on the radar. The end
of this loop is 03Z (800 pm PDT), which is the same time as the radar mosaic
above.
As the 500mb trough moved over the
area, the cooling temperatures aloft helped to destabilize the atmosphere. The
dynamic lift from this 500mb short wave also aided in the thunderstorm development.
Below is a loop of the hourly CAPE and CIN as analyzed by the LAPS model. CAPE
values of near 2000 J/kg developed during the afternoon. But note how the atmosphere
rapidly stabilizes over northeast Washington as the boundary moves south. More
importantly, note how this stabilization occurs north and east (in southern
BC and western Montana) of where the boundary was first spotted on radar at
about 2145Z.
When the boundary moved through the
Spokane area, the VAD Wind Profile from the radar observed the deepening and
strengthing winds.
Again, note the pre-existing northeast
flow before the passage of the boundary (around 2235Z). After the boundary passage,
northeast flow up to about 7000 ft MSL was present under a prevailing northwest
flow aloft. As the boundary continued to progress southward, it's depth was
suprisingly consistent, remaining at about 8000 ft MSL over an hour later.
The Eta model actually did a good
job of predicting this, again, before any convection had developed. It clearly
shows a windshift just before 00Z (500 pm PDT) with the depth of the northeasterlies
up to about 8000 ft. The white trace line is model omega, and shows a marked
change from upward (negative) to downward (positive) motion as the front moved
through.
Since the Eta model cannot predict
outflow boundaries from convection, and the scale of the 850mb wind and thermal
field is much larger than any convective complex, the data appear to indicate
that this boundary was more of a synoptic scale front rather than from thunderstorm
outflow.
The MSAS analysis of pressure (green
lines) and 3-hour pressure change (brown lines) below, clearly shows a "bubble"
of pressure rises that pinch off over western Montana and move into northeast
Washington. This pressure rise center developed well behind any convection But
more important are the observations at Cranbrook (CYXC) and Creston (CWJR),
BC, just north of the Idaho Panhandle. At the start of the loop, Cranbrook has
just experienced a wind shift to the northeast. Two hours later, Creston has
a similar wind shift. Using the time of these two wind shifts and extrapolating
ahead in time, note how well the cursor (purple circle with a dot inside) lines
up with the wind shifts in eastern Washington at Spokane and Hanford as the
front moves south.
Lastly, look at the loop below. This
time, the extrapolation line extends backwards in time. The "boundary"
that appears south of the thunderstorms at the end of the loop was extrapolated
backwards. Note how convection is enhanced ahead of this line, but then dies
behind it. Thus, the question becomes, did the boundary exist up in BC, but
just wasn't observable on radar until it got close enough to Spokane? Or did
the thunderstorms actually produce outflows that became the front.
Taking another look at the initial
radar loop, the boundary does appear to be thunderstorm outflow in origin. But
a closer look shows that this boundary is much too continuous to have been generated
all at once by thunderstorms in different stages of development and decay.
Inspection of some of the radar images
does show a discontinuous nature to the boundary, including some instances where
it appears that semi-circles of outflow intersect the pre-existing boundary.
Additionally, there is another boundary that appears in southwest Shoshone county
and moves into Benewah county that does not appear to have originated from any
thunderstorms. But our pre-conceived ideas of smooth continuous synoptic fronts
is likely unjustified in complex terrain. The problem is that we aren't able
to actually "see" the surface front.
In conclusion, it appears from observed
and modeled data that the boundary on Aug 12th, 2005 was primarily a synoptic
scale front which moved into the the area from southeast BC and northwest Montana.
Convection developed ahead of this front due to surface-based instability and
upper-level dynamics. As the front progressed southwestward under the convection,
it stabilized the atmosphere. Fairly steep lapse rates in the mid-levels coupled
with the dynamics aloft were able to sustain some of the convection in elevated
form in the northwesterly flow aloft. The front proceeded southwestward in a
somewhat discontinuous manner, likely due to the topography. Some modifcation
and enhancement of the front due to thunderstorm outflow cannot be ruled out.
But as the front moved over the smoother terrain of the Columbia Basin, it became
better organized and more continuous in nature.
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