Agricultural Forecasting Aids for the Northern Sacramento Valley which consists of Tehama, Glenn, Butte, and Colusa Counties

Agricultural Forecasting Aids for the Northern Sacramento Valley which consists of Tehama, Glenn, Butte, and Colusa Counties

Hygrometric Formulas

       Chico                                     Hamilton City
  (Spring Formula)                              (Winter Formula)

MIN=D - H-35 + V + V1                        MIN=D - H-35 + Vd + Vh
        ----                                         ----
          3                                            4

D               V                            D        Vd    H       Vh
---------------------                        ---------------------------
10-12          +7                            14-15   +12    0-39     0
13-16          +6                            16      +11    40-44   -1
17-19          +5                            17-18   +10    45-48   -2
20-22          +4                            19      +9     49-53   -3
23-25          +3                            20-21   +8     54-57   -2
26-28          +2                            22      +7     58-59   -1
29-31          +1                            23-24   +6     60-75    0
32-35           0                            25      +5     75-80   +1
36-39          -1                            26-27   +4     81-82   +2
40             -2                            28      +3     83-84   +3
41 and over    -3                            29-30   +2     85-86   +4
                                             31      +1     87-88   +5
                                             32-33    0     89-90   +6
                                             34-37   -1     91-92   +7
                                             38 +    -2     93-94   +8
                                                            95-97   +9
                                                            98-100  +10

H          V1 
0-50%      -2 
Over 50%    0 

MIN - Forecasted minimum temperature for the following morning

D - 4PM Dewpoint taken at the observation site

H - 4PM Humidity

V, V1, Vd, Vh - These are just variables

Sprinkler Frost Control Start

Dew Point                   Turn on temperature

26                            34
24-25                         35
22-23                         36
20-21                         37
17-19                         38
15-16                         39

Forecast Helps

The name of the game is Micro/Meso-Meteorology. You'll see nights when orland will stay at 45 degrees with the help of the north wind, and chico drops to 25 by losing it. Usually your pressure gradients and changes will indicate this.

The winter hygrometric formula is OK for southerly flow and ground fog, or with high humidities even in spring, but is usually 1-2 degrees too warm for key station forecast.

RDD mos dewpoints are a good first guess for minimum temperatures in the AG district.

Especially during north wind situations, use spring formula when dewpoints are low, even during winter season.

If you have the opportunity to put ion alert stations, you'll have a great asset. Spread them across the valley. This is a windy district and winds vary greatly from one side of the valley to the other as well as between north and south. The east side is usually the last place to get north winds and the first place to lose it. It is therefore the coldest except for the drainages around oroville. Watch your gradients and pressure tendencies. MFR-RDD, RDD-SAC, and RNO-SAC are the most helpful. When RNO approaches MFR, north winds are undependable for help. As long as RNO>SAC, oroville will be warmest except for #38. Orland gets more wind than Hamilton City.

Watch overhead and upstream dewpoints. For increasing dewpoints watch south winds, and RDD-SAC gradients, also watch pressure tendencies and SUU winds...these should help you forecast arrival of fog which will often come up to about Durham or South Chico and stop so that the south end of the district stays protected, but cool in the days, and the north end has excellent radiation. With strong westerly gradient, small MFR, SAC, and negative RDD-SAC, watch for marine layer intruding into the district.

Large rises in APP groups to north and west indicate increasing north winds for help towards warmer mins. Conversely, falls indicate decreasing winds and colder mins. Falling tendencies north and west with rises in Nevada indicate dying winds and colder west of Chico, but warmer in Oroville. Note that with fropa coming, the amount of falls are very good for where heaviest rain might be headed.

Know what your critical temperatures are for the current critical fruits and nuts. You'll look like a foul harping on 29 degrees when the plant's critical temperature is 25 at that time of the year. The crop calendar should help you there. Also the files are full of crop info. Farm advisors can also help.

More Rules of Thumb

Old Timer's Say

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