A Cooling North Wind Across Northern California

Alexander Tardy, Meteorologist
National Weather Service Sacramento CA
American Meteorological Society - Sacramento CA

Case Study October, 2000

Introduction

Northern California usually experiences a type of Foehn (warming) wind several times during a typical year. The warming effect is the result of compressed air which has been forced adiabatically to flow down the southern Cascades and Coastal Mountain ranges. This type of wind is most common during the season transitional months, autumn and spring. The weather pattern conducive to a north wind is primarily created by higher surface pressures over the Pacific Northwest and Great Basin, combined with lower pressures associated with an inverted pressure trough along the central coast of California. The wind not only is responsible for warm temperatures, but it also significantly dries out the boundary layer of the atmosphere. For most people this means a refreshing, dry and clean airmass, but to those with fire weather interests, this type of weather usually brings a substantial increase to the fire danger.

For the operational meteorologist a north wind gradient adds difficulty to the forecasting of high temperatures, and with respect to minimum temperatures, whether these winds will remain strong enough at night to prevent decoupling of the boundary layer. It is usually more clear how the north winds will affect the Delta and Clear Lake area. Typically in these regions it is common to see maximum temperatures rise 10 to 15 degrees Fahrenheit (F) in one day with the change of wind direction alone. The effects are most noticeable in the Delta since a shift to a northerly wind component will shut off the cooling effects of Pacific marine air. A north wind adiabatically mixes out the boundary layer and particularly can add some artificial compressional warming. During stronger events, when the marine layer is reduced to a few hundred feet on the Oakland upper air sounding (KOAK), temperatures can reach above 105F in parts of the Carquinez Strait and Delta. This case study will not focus on how warm temperatures can get, but rather analyze an event where a deep layer of cold air advection behind a short wave more than offset the normal warming effects of a downslope flow, and instead northern California experienced cooling from a north wind.

Synopsis

On September 26^th 1999 a typical north wind pattern became established over northern California (Fig. 1). This pattern is the result of strong surface pressure rises that occur over the Pacific Northwest and eventually the Great Basin. These pressure rises are directly related to synoptic scale subsidence, or sinking air, that occurs in the wake of short wave troughs. The magnitude of the north to south gradient usually depends on the strength and proximity of individual short waves, and the associated area of higher pressure that develops or approaches northern California. In this case a strong short wave and cold front moved across the Pacific Northwest and gusty north winds developed (Fig. 2).

Model forecasts and observations

This study used the 0000 UTC run of the Eta model on 27 September 1999. This guidance would have been available to the forecaster. As is normally the case in the autumn months, the surface cold front made little penetration across northern California. However, at the 850-mb level Figure 3 shows that a tight thermal gradient was present over far northern California. This figure depicts the Eta forecast at 18-hr or 1800 UTC 27 September. The Eta 850-mb geopotential height field showed that winds at this level were nearly perpendicular to the isotherms, which would indicate cold air advection. This theory was further supported by the Eta forecast at 1800 UTC of cold air advection in a layer between 850-mb and 700-mb (Fig. 4). Figure 4 shows that 700-mb winds were also advecting colder air into northern California which would indicate there was substantial depth to the cold air mass. During the warm season it is common for afternoon high temperatures to be a result of dry adiabatically mixing air to the surface from around 850-mb to as high as 700-mb. Noticing changes to the air mass as these levels is crucial to forecasting 2-m (surface) temperatures. It is demonstrated here that with the use of the Advanced Weather Interactive Processing System (AWIPS), model graphics can be created to display a derived quantity such as advection (in this case for temperature). The advantage to this is that it can be visually observed as a graphic and quantified, rather than estimated by the forecaster.

Results and Forecasting Applications

Table 1 shows that the forecaster identified the northerly winds correctly on September 26^th and forecast very warm temperatures in the Delta. However, the cooling effects were underestimated, and observed high temperatures in the northern Sacramento Valley were several degrees cooler than forecast. The forecast for September 27^th recognized this trend, however, the cooling was still underestimated. It is apparent in Figure 5 that an additional short wave, of near equal strength, moved across the Pacific Northwest, and reinforced the cold air advection into northern California. Note that temperatures on the 27^th were observed to be cooler than the 26^th.

Various observations on both days showed northerly winds sustained between 15 and 25-kt from Sacramento northward. The wind was strong enough to prompt the issuance of a wind advisory for the northern Sacramento Valley on the 26^th and 27^th . The cooling affects further south were not evident as temperatures soared to highs between 95 and 100F over much of the Delta region. Meteorologically this makes sense since these areas are much farther from the source of cold air. However, on the 27^th despite a very warm air mass under a persistent 588-dm upper level ridge at the 500-mb level (Fig. 5), the cold air advection appeared to have slightly lowered temperatures in the Delta as well.

In this case it is evident that if low level north winds are forecast it does not necessarily preclude a significant warming. In fact, temperatures on September 25^th, were actually several degrees warmer in the northern Sacramento Valley with upper 90s observed at Red Bluff and Redding. On this day, a north wind at speeds of 10 to 15-kt prevailed at both sites. The surface wind was stronger on the 26^th and 27^th with Redding and Red Bluff both recording north wind gusts to 30-kt. One could argue that such strong wind and subsidence, as that observed on the 26^th and 27^th , would result in mixing air from a greater depth in the atmosphere thus lowering the observed 2-m temperature. However, it was observed on May 22, 1999, that under an upper level ridge an increase in mixing from strong north winds and atmospheric subsidence does not necessarily lower the temperature. On this day, the Sacramento Valley (Sacramento to Redding) experienced a sustained north wind at speeds of 15 to 25-kt, while temperatures reached between 95 and 98F.

This case suggests that cold advection between 850 and 700-mb, often the source of the air that will eventually be brought dry adiabatically to the surface, can significantly lower surface maximum temperatures. Not shown in this case, but often a good rule of thumb for Valley locations, a forecaster can predict maximum temperatures during sunny conditions by using a Skew-T log P diagram. Following the dry adiabat from the model forecast 850-mb temperature to 1000-mb (near surface level) would yield the forecast maximum temperature. In extreme cases the air may originate from as high as 700-mb. In this case, observed 0000 UTC 850-mb temperatures mixed to the surface adiabatically on the 26^th and 27^th were very close to the actual high temperatures over northern California (not shown). The Foehn wind warming effect was evident in Delta and Clear Lake area, but probably only accounted for an additional few degrees of artificial warming. More likely, an offshore flow, such as a north wind, would have its largest impact by drying the airmass and reducing the marine layer, and sometimes accelerating the morning temperature increase.

When the pressure gradient is sufficient, a north wind may prevent the boundary layer from decoupling, resulting in very mild observed minimum temperatures. These situations appear to be even more difficult to forecast. There is no evidence, however, that observed mild minimum temperatures would result in warmer maximum temperatures during the summer months. Instead these situations are more a reflection of an overall warm airmass, and afternoon high temperatures can only be as warm as the atmosphere supports (approximately 850-mb temperatures), with the exception of some artificial warming from downslope effects such as a Foehn (north) wind. In this case, recognizing the cold air advection, rather than the north wind was more crucial to accurately forecasting temperatures under this maybe not so typical weather pattern.

Conclusion

Since no weather event is ever the same, a forecaster must gain experience in recognizing patterns that are somewhat similar in characteristics. This pattern recognition is crucial to out performing the high resolution numerical guidance that has been developed (such as MOS and Eta 2-m temperatures), and separates a good forecast from an excellent one. It is also as important to fully utilize all the appropriate data from the numerical weather prediction models by reviewing all available guidance for different parameters and atmospheric levels.

A weather pattern in northern California that is sometimes over looked is the cold air advection north wind event. Sometimes a north wind is identified to be associated with additional warming to surface temperatures. However, this paper showed clearly that other atmospheric factors must be considered before a forecaster can accurately forecast temperatures. It is important to recognize whether or not the lower levels of the troposphere are experiencing cold air advection from a northerly air flow. If this cold air advection is strong enough it may offset adiabatic mixing or any compressional warming, and in some cases may result in lower maximum temperatures than otherwise anticipated by the forecaster. Unfortunately, at this time, it is difficult to quantify how much cold air advection would be needed. However, once this type of pattern is identified a forecaster can at least make the necessary adjustments and trend the forecast in the right direction.

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