La Niña May-June Composites
from the Period 1951-1999
Foreshadowing of Great Plains
Spring Patterns 2000
John P. Monteverdi*
Department of Geosciences
San Francisco State University
*Spring 2000 Visiting
Scientist NSSL Norman and Visiting Scientist NWSFO Monterey
1. Nature of Update
The purpose of this site is to present an update
of the work done last year in presenting composite May-June patterns
for La Niña years as a possible foreshadowing of what was
to occur in 1999. I think most will agree that the "foreshadowed
patterns" provided a reasonable characterization of what
actually happened.
NCEP/NCAR reanalysis data sets have been expanded
and now include data back to 1948. Thus, it was possible to not
only include the data for 1999 but also that for the La Niña
years of 1956 and 1951. This makes the compositing more robust,
but, interestingly, does not change the overall patterns from
those developed last year at
this time (which did not include the years 1951, 1956 and, obviously,
1999).
2. Methodology
The NCEP/NCAR reanalysis package allows one
to specify ranges of months and years and to do simple statistics
on the data available for the selected time periods. In this case,
I chose the May-June period over which to composite.
Means and long term May/June means and anomalies
(1948-1996) for 500 mb height, 700 mb omega, 250 mb wind speeds,
surface pressure, surface wind vectors and surface relative humidity
were obtained. In addition, in order to assess the CHANGE in the
composite by adding the 1999 data, I obtained the difference in
the composites between the composited 500 mb heights and anomalies
with and without the year 1999 included.
The opinions stated here are my own. They have
not been subjected to peer review and represent preliminary views
on the nature of the impact of La Niña on circulation patterns.
3. Charts
A. May-June La Niña Composite 500 mb
Patterns
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Composite 500 mb Heights
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Departure of Mean 500 mb Heights from Average
|
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Impact of 1999 On Long Term La Niña Composite
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In review, the May/June 500 mb pattern during La Niñas
(above left) shows a mean trough over the West and mean ridge
over the Plains. The anomalies (above right) show that the La
Niña pattern has higher than normal heights over the southern
Plains, normal heights over the central Plains and lower than
normal heights over the northern Plains.
Note that the impact on the long term composite of the 1999
data (left) was to lower the composite 500 mb heights in the
western portion of the Great Plains from the Texas Panhandle
northward.
|
B. May-June La Niña Composite 250 mb
Wind/700 mb Omega Field Anomalies
 |
 |
|
250 mb Anomalous Flow
Looks like a merger of a subtropical branch of the jet and
polar jet over New Mexico. And it also looks like the region
from the northern TX panhandle through KS, northeastern New Mexico,
eastern Colorado, western Nebraska would be in an area of diffluence
and divergence in the right rear of polar jet isotach pattern
and left front of subtropical branch isotach pattern. CAPE robbing
suggested for most of SW TX.
|
700 mb Anomalous Omega
Consistent with the 250 mb isotach pattern, the 700 mb anomalies
show a bullseye of omega more negative than average (stronger
upward motion in lower mid troposphere) in region mentioned in
box at left.
|
C. May-June La Niña Composite Surface
Pressure Pattern
|
Composite Surface Pressure Field
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Composite Surface Pressure Field Anomalies
|
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The composite sealevel pressure field features a leeside trough
that extends the length of the high Plains. At first glance,
this suggests pretty decent advection of moist air from the Gulf
through the length of the western Plains.
Given the high 500 mb heights over TX in the composite, though,
capping is suggested as a major problem for the southern Plains.
|
The composite sealevel pressure anomaly has lower than average
pressure over the inter mountain region, consistent with the
more frequent cyclogenesis one might expect there given the 500
mb patterns above.
The chart suggests an anomalous strong southerly component
to the surface wind field in the western Plains, with a southeasterly
component in northwestern Kansas and western Nebraska.
|
D. May-June La Niña Composite Surface
Wind Fields
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Surface Wind Field
|
Surface Wind Field Anomalies
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Sorry, could not find a way to get length of wind arrows (not
vectors) longer.
The pattern shows influx of air from the Gulf of Mexico with
an axis through the Rio Grande River Valley into the western
High Plains and northward into Kansas.
The pattern suggests that the dryline would be well west,
in the composite view.
|
The composite wind field anomalies shows a stronger
than average southeasterly flow in the Panhandles and western
Kansas. The mean shear profile suggested by the composite 500
mb and surface wind fields looks favorable for severe thunderstorms
(possible supercells, if these were actual charts) from TX Panhandle
into the western portions of KS, Nebraska and eastern Colorado.
The 500 mb pattern suggests that capping will be a problem in
the south, though. |
E. May-June La Niña Composite Surface
Relative Humidity
|
Surface RH Composite
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Composite Surface RH Anomalies
|
|
Dewpoint temperature is no longer an option in the "plot
variables" list, and specific humidity data are not available
after 1996. So, I plotted composite surface relative humidity.
I think this is somewhat consistent with what one would infer
from the advections suggested by the surface wind field above.
It does look like there would be a dryline in the west, but I
don't really know if that directly follows from the RH field.
|
The composite RH anomalies are equally difficult to interpret,
but I have included them for consistency.
Anyone who would care to help me with interpretation of this,
please email me.
|
F. Upshot
The composites suggest that this upcoming May-June
period will feature synoptic scale features of the same sort and
in the same general position as last year. The 500 mb height patterns
suggest that the southern Plains may be capped out, but such capping
in the mean may suggest that isolated events that occur might
be more explosive (due to the higher CAPE in combination with
the decent shear suggested by the 500 mb pattern).
Western Kansas, eastern Colorado, western Nebraska
are beckoning. No wonder VORTEX-3 will be in Goodland.
