c55.209:73/8

HIGHLIGHTS: A succession of storms continued to
batter California with
with rain, gusty wind, and
mountain snow. The north central part of the
State was hardest hit, and floods and erosion
affected the area. After midweek the flow of
moisture shifted northward and gave some respite
to California. Heavy snow fell at high elevations
of the central and northern Rockies. Lighter snow
fell from the northern Great Plains to New England.
Southerly flow of air into the southern Plains
brought record-warm temperatures to Texas and then
all across the South. The moisture laden air
brought showers and thunderstorms to the East from
the Tower Mississippi Valley to Georgia and north-
ward to Maryland. Early week rain and then snow
covered the Northeast.

SUNDAY...Moist air continued to push eastward
across the Western States bringing more rain,
mountain snow, and gusty winds. Rain was moderate
to heavy from the upper third of California to
Southern Washington. Snow fell from the northern
lains to southern New England and a band of
Freezing rain reached from southern Iowa to
ennsylvania. Rain showers were
showers were scattered from
Mississippi and Alabama to the lower Ohio Valley.

A FRONTAL SYSTEM IN THE PACIFIC OCEAN APPEARS TO BE HEADING TOWARD THE PACIFIC NORTHWEST OF

THE UNITED STATES INSTEAD OF CENTRAL CALIFORNIA AS IN PRECEDING DAYS. A FRONTAL SYSTEM, MEAN-

DERING FROM IDAHO THROUGH OKLAHOMA AND GEORGIA, SEPARATES COLD POLAR AND ARCTIC AIR FROM MUCH

WARMER TROPICAL AIR TO THE SOUTH. THE WARM AIR IS CAUSING SNOW TO MELT IN MUCH OF THE WEST.

The Drought Severity (Long-Term, Palmer) Index
Lyle M. Denny and Thomas R. Heddinghaus

conditions

The Drought Severity or Palmer Index is and index of meteorological
drought (or moisture excess ) and indicates prolonged abnormal
effecting water sensitive economics. The index usually ranges from about
-6 to +6 with negative values denoting dry spells and positive values, wet
spells weather (categories of values are given under the accompanying map).
The equations for the index were derived from monthly average data and based
on the concept of a balance between moisture supply and demand (Palmer,
1965). The equations have been modified to compute the index on a weekly
basis for publication in the Bulletin. Input data consists of weekly
temperature averages and precipitation totals for 350 climate divisions in
the United States and Puerto Rico.

The index is a sum of the current moisture anomaly and a portion of the
previous index to include the effect of the duration of the drought or wet
spell. The moisture anomaly is the product of a climate weighting factor
and the moisture departure. The weighting factor allows the index to have a
reasonably comparable significance for different locations and time of year.
An index value for a division in Florida would have the same local
implication as a similar value in a more arid division in western Kansas.
The moisture departure is the difference of water supply and demand.
Supply is precipitation and stored soil moisture, and demand is the
potential evapotranspiration, the amount needed to recharge the soil, and
runoff needed to keep the rivers, lakes, and reservoirs at a normal level.

The index is measured from the start of a "wet" or "dry" spell and is
sometimes ambigious until a weather spell is established. A week of normal
or better rainfall is welcome in an area that has experienced a long
drought, but may be only a brief respite and not the end of the drought.
Once the weather spell is established (by computing a 100 percent
"probability" that an opposite spell has ended), the final value is
assigned. To make the program have a real-time significance, a. value is
assigned based on a greater than 50 percent "probability" that the opposite
weather spell has ended. This is not entirely satisfactory, but it does
allow the index to have a value when there is a doubt that it should be
positive or negative.

One aspect that should be noted, is that the demand part of the
computations includes three parameters: potential evapotranspiration,
recharge of soil moisture, and runoff--any one of which may produce negative
values. If only enough rain fell to satisfy the expected evapotranspiration
but not enough to supply the recharge and runoff, then a negative index
would result. If such an odd situation continued, agriculture would
progress at a normal pace but a worsening drought would be indicated.
Shallow wells and springs would dry and the levels of rivers, lakes, and
reservoirs would fall. Serious economic stress to the livestock trade,
industries, and cities would eventually result. Then if rainfall fell below
the minimum needed for agriculture, crops would suffer drastic and rapid
decline because there would be no reserve water in the soil. Such a
situation, to some extent, occurred during the Northeast drought in the mid-
1960's when New York City almost ran out of water.

A detailed explanation and examination of the index is given by Alley
(1984). Both Alley and Karl (1983) address the sensitivity of the index and
list some limitations.