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ABSTRACT
The latest Old Farmer's Almanac is out with its usual boast of being
80% correct. But are they really that good? From what I have
seen in past years their accuracy is much, much worse than advertised! How
about this year?
This study found that the
national regional forecast for Winter had both the temperature and
precipitation correct only about 13% of the time and Summer forecast was
correct just 6% of the time.
The California forecasts for
temperature were right only 13% of the time and the precipitation was
correct 27% of the time.
DATA
Last November's 2005 edition of
the Old Farmer's Almanac (Yankee Publishing, Dublin NH, 2004) was
examined.
There are seasonal weather graphics for winter and summer (Fig. 1) from the 2005 Old Farmer's Almanac (OFA). The weather forecast section of the is
also divided into 16 regions.
Region 16 encompasses the southern three-fourths of California and is
broken down by month with "forecasts" of how the temperature and
precipitation compare to normal.
Figures 2 is the actual forecast
for Region 16 from the Almanac. The OFA seasonal forecasts for the nation
were evaluated for the Winter (November through March) and Summer (June
through August) using the temperature
and rainfall departures from normal by month for the United States Climate
Divisions (NOAA-CIRES Climate Diagnostics Center
US Climate
Divisions Plotting Page).
| Fig. 1 |
Fig. 2 |
 |
 |
To evaluate the Region 16 forecasts each monthly forecast was compared to the actual monthly temperature and precipitation anomalies by climate
division. Data for climate divisions was chosen
because of the large number of sites that are used in determining the
division average, thus eliminating the bias of a single station.
Temperatures were evaluated for all 12 months while precipitation was only
looked at for
the the California "wet" season of November through April.
Seasonal U.S. Climate Division Temperatures
and Precipitation (Nov. 2004 - Mar. 2005) and (Jun. - Aug. 2005)
| Winter Temperature |
Winter Precipitation |
Summer Temperature |
Summer Precipitation |
 |
 |
 |
 |
The "graded" seasonal forecasts below are a subjective verification of the
Old Farmer's Almanac seasonal forecasts. The basic methodology was
to deduct a grade for each part portion of a regional forecast that was
incorrect and two grades if the sign was also incorrect. If both the
temperature and precipitation forecast were correct and had the
appropriate magnitude then the region would get an "A" grade".
Overall the seasonal forecasts averaged a grade of about "C", meaning at
least the precipitation or temperature was wrong. Only 2 of the 16
(13%) Winter forecasts got both the temperature and precipitation right
and just 1 of the 18 (6%) Summer forecasts verified.
"Graded" Seasonal Forecasts
| Winter |
Summer |
 |
 |
Monthly U.S. Climate Division Precipitation
(Nov. 2004 - Apr. 2005)
|
Nov 2004 |
Dec 2004 |
Jan 2005 |
Feb 2005 |
 |
 |
 |
 |
|
Mar 2005 |
Apr 2005 |
|
|
|
 |
 |
|
|
Monthly U.S. Climate Division Temperatures
(Nov. 2004 - Oct. 2005)
ANALYSIS
The subjective analyses in the table below compares the monthly
forecasts for Region 16 from the 2005 OFA
with the actual observed monthly climate division anomalies of
temperature and precipitation. Because of the scarcity of rainfall in
the California in the summer months,
precipitation for the months May through October was not evaluated.
Furthermore, if either the forecasts or the observed data were geographically
split then separate analyses were done for each geographic area.
The observed data has been color-coded to reflect each of
three categories. Forecasts which were judged correct are
blue. Those
which had the right sign (i.e., above normal observed when above was
forecast or below normal observed when below was forecast) but where the quantity
was incorrect are coded in yellow.
Forecasts that had a large range that was partially corrected are colored
orange. And forecasts which had the wrong sign
are coded red.
SUMMARY
For California, Region 16, a total of
11 precipitation forecasts and 23 temperature forecasts were evaluated.
Of the precipitation cases 27% were correct, 36% were quantitatively wrong
and the remaining 36% had the incorrect sign. The temperatures for
region 16 were only correct 13% of the time, had the wrong sign in 22% of
the cases, were quantitatively wrong 39% of the time and were partially
correct 26% of the time. Overall the
California forecasts were correct only 23% of the time.
Table 1. Analysis Region 16 - California
|
Precipitation |
Temperature |
| |
OFA |
Observed |
OFA |
Observed |
| Nov. 04 |
1" below
normal |
2"-3" below normal
North
Normal South |
4º above normal |
0º-2º below
normal North
2º-4º below normal South |
| Dec. 04 |
1.5" below normal |
1"-3"
above normal |
1º below normal
East
1º above normal West |
0º-1º above
normal North
Normal South |
| Jan. 05 |
2" above normal |
0"-1" below normal North
2"-4" above normal South |
2.5º above normal |
Normal |
| Feb. 05 |
1" below normal |
2"-3" below normal North
0"-1" above normal Central
2"-4" above normal South |
Normal |
1º-3º above
normal North
0º-2º below normal South |
| Mar. 05 |
1" below normal |
0"-1" below normal North
Normal South |
3.5º above normal |
1º-3º above normal North
0º-2º above normal South |
| Apr. 05 |
Normal |
Normal |
3º below normal
East
Normal West |
2º-3º below
normal East
0º-2º below normal
West |
| May 05 |
|
|
2º above normal |
Normal
North
0º-1º above normal South |
| Jun. 05 |
|
|
1º above normal |
2º-4º below
normal |
| Jul. 05 |
|
|
1º below
normal North
2º above normal South |
2º-4º above
normal North
0º-3º above normal South |
| Aug. 05 |
|
|
1º above normal |
0º-2º above
normal North
Normal South |
| Sep. 05 |
|
|
3º above normal |
0º-3º above normal North
0º-2º above
normal South |
| Oct. 05 |
|
|
Normal |
Normal North
0"-2" below coast
South
1"-2" above inland South |
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