WMOD00499

• Title: An Objective Forecasting Technique for High Plains Thunderstorm Development
• Date: 10/16/1978
• Weather Modification - Doc Type: Report

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This leads to a strong gradient of both potential
temperature and moisture between western Kansas
and the Rockies during the afternoon hours. The
strongest portions of the gradient will depend
upon the terrain. However, the deeply-mixed layer
tends to progress eastward during the afternoon
and the most intense changes will often appear in
eastern Colorado, Figure 2 is an example of a
developed gradient. The isentropic gradient will
be reinforced on many occasions by cooler air
masses drifting southeastward from Wyoming and
Montana onto the central High Plains.

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Figure 2. Potential temperature (solid
and every degree) and equivalent potential temp-
eratu~e (dashed and every 5 degrees) cross section
between Limon, Colorado (LIe) and Goodland, Kansas
(GLD) .

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GLO

The distinction between the mixed air
of eastern Colorado and the less-mixed surface
layer over Kansas and Nebraska is also noticeable
in the surface streamline analysis. The eastern
air is generally a region of organized, large-
.scale flow with little directional convergence or
divergence. The mixed-air region, on the other
hand, will usually consist of less organized flow
appearing as a series of eddies. The zone between
the two air masses consists of a band of confluent
flow (Fig. 3). The strength and relative position
of this pattern will depend upon the synoptic con-
~itions such as the position of the dominant upper-
.ir features.

Carlson and Ludlum (1968) found that
on occasions preceding troughs in the eastern High
Plains of Oklahoma, Kansas and Nebraska, a stream
or plume of potentially warm, dry air from the

Mexican Plateau appeared to restrain small-scale
convection in the moist trade-wind stream flowing
inland from the Gulf of Mexico, The western edge
of the Gulf flow was usually marked by a"dry line"
which appeared as a confluence line west of which
was a very dry current. Carlson and Ludlum sug-
gested that near the dry line, the relatively cool
trade wind air flows out from under the plume of
Mexican air aloft and undercuts the dry air fur-
ther west, so that in a narrow zone, the restraint
of the lid disappears and large cumulus form
locally in a stratification providing a 'Ilery
large potential bouyancy,

Analysis of tte Goodland HIP][,EX data
indicatE!s that the low-level air from eastern
Kansas clOd Oklahoma will mix wi th in the conflu-
ence zone bringing some moisture westward into the
flow p;et.ern of the deeply-mixed western air. This
entrained air now contains a higher potential
tE!mperat:ure than the unmixed air to the E!ast and
will remain with the more disorganized western
flow. If the movement is cross-isentropi.c, from
warmer to cooler air, mechanical lift will provide
conditions for possible cloud development.

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Modahl (1974) found that apparently, f
011 the most intensive 1974 National Hail Research!
Experiment storm days in northeastern Colorado,
~clonic meso-scale streamline features and asso-
ciated confluence bands or lines \{ere usually
present. These areas had an excellent correlation
with storm location. The dominant feature appear-
ed to be a cyclonic swirl, suggestive of a vortex-
sink circulation.

-\

-

JIOUNTAIII- IlIFLUENCED,
IlUP MIXED LAYER

AIR-MASS INFl:.uENC:ED.
SJIALLOW MIXEI) LAYER

-

Figure 3. Schematic illustration' of the
deeply-mixed and Shallow-mixed air masses through
the central High Plains.

3.

FORECAST TECHNIQUE

Potential temperature and wind-vector
plots ~re obtained from the Environmental Data
Network (Politte et aI" 1977). Detailed surface
isentropic and streamline-isotach analyses are
conducted daily for 1800 GMT. A grid field con-
sisting of, 20 elements, each having an area of
22, !:iOO km", is centered near Goodla.nd and placed
over the analysis, The average iser,tropic grad-
ient and cross-isentropic wind compeonent jln
meters per second is computed foreilch elE!ment.,
The average vertical isentropic gradient Is - '
calculated from the 1800 GMT G<.'l">dland sounding.

180