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

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Other Related Site: La Niña Early Winter to Mid Winter Singularity

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

 

Composite 500 mb Heights

 

Departure of Mean 500 mb Heights from Average

Impact of 1999 On Long Term La Niña Composite

 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

 

Composite Surface Pressure Field Anomalies

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

 

Surface Wind Field

 

Surface Wind Field Anomalies

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

 

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.