There is a relationship between soil moisture and both precipitation and temperatures that becomes important in the spring and summer months. Forecasters pay close attention to this factor in their forecasts for summer. Soil moisture anomalies of significance tend to persist from spring to summer, except when major events like a landfalling hurricane act to intervene.
Very wet soils favor wetter summers and less extremes of heat (though not necessarily discomfort) in the summer, while very dry soils increase the chances of summer heat and drought.
That is because wet soils and vegetation in the soil lead to more evaporation and transpiration of moisture, which is a cooling process that tends to hold daytime readings down. At night the elevated moisture content reduces the radiational cooling. With more moisture there may be more nighttime fog especially in valley areas and during the day more clouds, lower temperatures and better chances of showers from any lifting mechanisms. In this way wet areas favor more rainfall.
When the ground is very dry on the other hand, there is little evaporation and less plant transpiration especially if water table is greatly lowered. The suns heat is more effective in warming ground temperatures. Higher temperatures and less moisture means lower humidity which desiccate the ground and vegetation further. There is less cloudiness and lifting mechanisms are much less effective in producing rainfall. In this way drought begets drought.
Not surprisingly, there is a high persistence of soil moisture and Palmer Drought Indices from late spring into summer.
Dr. John Christy and others did a study in 2006 of temperatures day and night in the irrigated San Joaquin Valley in California and compared to the nearby non-irrigated Sierra Mountain stations. They noted “A time series of daily maximum and minimum temperatures forstations in the irrigated San Joaquin Valley (Valley) and nearby non-irrigated Sierra Nevada Mountains (Sierra) were generated for 1910-2003.
Results show that 20th century Valley minimum temperatures warmed at a highly significant rate in all seasons, being greatest in summer and fall ( > +0.25 °C /decade). The Valley trend of annual mean temperatures is +0.07 ±0.07 °C /decade.
Sierra summer and fall minimum temperatures appear to cool, but at a less significant rate, while the trend of annual mean Sierra temperatures is an unremarkable -0.02 ±0.10 °C/ decade. A working hypothesis is that the relative positive trends in Valley minus Sierra minima ( > 0.4 °C /decade for summer and fall) are related to the altered surface environment brought about by the growth of irrigated ariculture, essentially changing a high-albedo desert into a darker, moister,vegetated plain.”
There are no lifting mechanisms to speak of in California in summer, so the most the added moisture does is increase nightrime and am fog.
A FEW CASE STUDIES
1974 was coming off a strong La Nina winter like this year. Note the tendencies in May for wet conditions in the Midwest and dryness in Texas (both similar to this year). Little change occurred in this scenario by August, which was in general a cool month for most areas.
Lets look at an extreme example of the opposite, a rapidly growing La Nina in 1988 coming off an El Nino winter of 1987/88 that suppressed the storm track and kept the northern tier dry.. It was a dry spring that expanded into August. The devastating drought summer of 1988 did over $40 billion in damage to agriculture. See the drought expand and deepen. See the very warm August.
Last year, drought was showing itself in the southeast states by May while wet conditions developed in Texas. Note how both persisted through August. Extremely hot temperatures developed over the drought stricken region in late summer while it remained cool in wet Texas.
This year, it has been very wet across the central areas while dryness has persisted in a small area in the southeast and southwest. Top soil moisture is also lacking in the western Plains not shown on the Palmer Index which looks at long term and deep soil moisture.
The following is a Soil Moisture Model run by the Climate Prediction Center that regresses current soil moisture to past soil moisture conditions and correlates with the upcoming summer season. This model shows the most skill this time of year. Note how it keeps it cool and wet across the central areas (June through August) where it is wet now while heat and continued dryness persists or builds from the dry areas in the southeast, western plains and west.
Next week we will look at another year, 1993, a record flood year that so far 2008 is parallelling very well both with regards to rainfall anomalies in the central states and the upper level steering patterns.
Christy, J.R., W.B. Norris, K. Redmond, and K.P. Gallo, 2006: Methodology and Results of Calculating Central California Surface Temperature Trends: Evidence of Human-Induced Climate Change? J. Climate, 19, 548–563.