CENTRAL REGION APPLIED RESEARCH PAPER 17-07


A Downslope Fog Event Across Northwest Kansas During the Warm Season

Eric J. Martello
Nexrad Weather Service Office
Goodland, Kansas


INTRODUCTION

During the early morning hours on August 12, 1996, an unexpected fog event developed over the Goodland, Kansas County Warning Area (CWA). Most of these events are in correlation to orographic lifting of air known as upsloping, sufficient deep low level moisture, and radiational cooling (Barker 1994). Orographic lifting of moisture in northwest Kansas occurs with winds containing an easterly component. Terrain over the region increases in elevation from east to west (Table 1). However, this case proves that orographic lift is not always needed for fog formation. The purpose of this study is to discuss the interrelationship of wind, moisture, and terrain on fog formation from 00 UTC to 15 UTC 12 August 1996.

Barker/USGS Elevation Stats

LocationElevationDistance and Direction
from Goodland
Elevation Gradient in ft/mi*
Goodland (GLD) 3640 feet
St. Francis 3362 feet 25 miles North 11.1
Atwood 2862 feet 40 miles Northeast 19.5
Colby 3180 feet 30 miles East 15.3
Russel Springs 2960 feet 36 miles Southeast 18.9
Sharon Springs 3450 feet 28 miles South 6.8
Burlington, CO 4160 feet 27 miles West -19.3
*Negative values indicate descending terrain to GLD.
Positive values indicate ascending terrain to GLD.

TIME FRAME 1 (00 UTC-06 UTC) 8/12/96

At 00 UTC (Figure 1), a surface high pressure center (inferred from the mean sea-level pressure field) was located over western Iowa with a ridge extending southwest into northeast Colorado. Northeast winds averaging 10 knots resulted across the Goodland CWA. Dew points of 60°F and higher were noted east of a Dodge City, Kansas to North Platte, Nebraska line. Moisture advection and orographic lift were taking place at this time over the CWA. The clear skies over Goodland and mostly cloudy skies at Hill City, Kansas to the east-northeast reflected this process clearly. Soundings at 00 UTC (not shown) from Denver, Colorado and Dodge City, Kansas indicated a northeast flow of 10 to 20 knots. The northeast flow prevailed up to 700-mb (approximately 9,000 ft AGL) at Denver and up to 800-mb (approximately 6500 ft AGL) at Dodge City. The sounding from North Platte, Nebraska indicated southeast winds of around 10 kts up to 850-mb (approximately 5300 ft AGL). Precipitable water ranged from 0.54 inches at Denver to 0.94 inches at Dodge City. The 800-mb to 825-mb layer was the most nearly saturated at all the upper-air sites. Moreover, 700-mb temperatures ranging from 16°C at North Platte to 13°C at Dodge City indicated that a nocturnal inversion would form once radiational cooling began after sunset.

Figure 1. Analysis of MSL Pressure in 1 mb increments at 00 UTC 12 August 1996. Plotting is conventional.

By 03 UTC (Figure 2), a new surface high pressure center had formed over south central Nebraska near Kearney. This caused southeast winds over northwest Kansas. Clear skies prevailed from 00 UTC-03 UTC at Goodland. Strong radiational cooling combined with moisture advection and orographic lift to drop the Goodland CWA average dew point depression to 3°F. At this time, the forecasting staff assumed that fog formation would soon occur because the surface layer continued to moisten and cool while warm temperatures remained aloft.

Figure 2. Same as Figure 1 at 03 UTC.

Three hours later at 06 UTC (Figure 3), the surface high over southern Nebraska split into two centers, with the main center of concern located near Hill City. Just to the southeast of the CWA, surface dew points of 60°F or more and southeast winds of 10 knots were causing moisture advection. The depth of the moist layer under the subsidence inversion was approximately 1500 ft. To remove all of the moisture would take some sort of air mass displacement by a frontal boundary or surface trough. No such process occurred this night.

Figure 3. Same as Figure 1 except in 2 mb increments at 06 UTC.

TIME FRAME 2 (06 UTC-12 UTC) 8/12/96

By 08 UTC (Figure 4), the surface winds began to change dramatically. The high pressure center that had been near Hill City had drifted southwest over Garden City, Kansas. Surface winds dropped at Dodge City as a result and fog formed. Eastern areas of Goodland's CWA also began to develop fog, which was to be expected considering the low-level moisture and light winds that had been evident there earlier in the evening. Conversely, Goodland's winds veered to 210 degrees and had a sustained speed at 10 knots by 08 UTC. A downslope regime was taking hold. Temperatures continued to fall, yet more slowly due to adiabatic warming. Saturation was temporarily delayed as the warming combated the radiational cooling under clear skies. The dew point depression at Goodland by this time had fallen to 1°F. However, fog formation seemed relatively unlikely because downslope flow usually dries the surface layer.

Figure 4. Same as Figure 3 at 08 UTC.

At this point, all mention of fog was removed from the Goodland TAF because higher dew points had retreated into extreme southwest Kansas and northwest Oklahoma. Also, the winds had become 10 knots from 210 degrees. Winds from this direction have a considerable downslope component because of the effects of the Palmer Divide, an east-west ridge south of Goodland. However, visibilities at Goodland dropped to ½ statute mile from 10 UTC-13 UTC.

TIME FRAME 3 (12 UTC-15 UTC) 8/12/96

At 12 UTC (Figure 5), surface analyses showed that a surface trough remained over northwest Kansas just east of Goodland. Similar to what was analyzed at 00 UTC, a strong high pressure center extended from north-central Kansas into western Iowa. Goodland remained in an air mass with a 1°F dew point depression. A moisture ridge was moving north along the trough axis across southwest Kansas. Soundings at North Platte and Denver indicated a very shallow saturated surface layer with southwest winds of up to 20 knots. The Red Willow, Nebraska profiler and the Goodland doppler radar VAD wind profile also indicated the strong southwest flow.

Figure 5. Same as Figure 3 at 12 UTC.

Finally and most importantly, the initial analysis from the 12 UTC Rapid Update Cycle (RUC) model indicated high relative humidities at 850-mb (Figure 6) over extreme southeast Colorado and southwest Kansas into the Texas and Oklahoma panhandles. Corresponding analyses also indicated south-southwest winds of up to 15 knots at 850-mb.

Figure 6. Initialization of the Rapid Update Cycle (RUC) 850-mb percent RH at 12 UTC 12 August 1996.

Three hours later at 15 UTC (Figure 7), Goodland's surface wind remained at 210 degrees, yet had diminished to approximately 5 knots. The dew point depression by this time at Goodland had risen to 7°F, indicating drying of the surface layer with the onset of diabatic heating. The surface trough was now directly over Goodland. Areas to the east and south of the surface trough continued to experience fog and a low stratus deck. Also, winds in these areas were light and variable as a center of high pressure encompassed the eastern two-thirds of the state.

Figure 7. Same as Figure 3 at 15 UTC.

SUMMARY AND CONCLUSION

Wind, moisture, and terrain all played essential roles in the occurrence of fog over Goodland. The components that made up the fog event from 00 UTC to 15 UTC were discernable in retrospect, but subtle. It can be deduced from the 00 UTC analysis that an upslope flow from the northeast helped to increase the depth of the saturated layer. However, with the formation, dissipation, and relocation of several surface high pressure centers, winds at 10 knots at 210 degrees resulted at Goodland by 06 UTC. Furthermore, the 12 UTC RUC 850-mb moisture analysis showed relative humidities over eighty percent along the southwest Kansas-southeast Colorado border. In addition, stratus and fog formed in the vicinity of LaJunta, Colorado. The fog and moisture advected north and northeast along a surface trough axis, reaching Goodland by 10 UTC. Any adiabatic warming incurred by the advection was countered by the depth of the moist layer to the southwest. Thus, fog advection occurred with a southwest wind. This scenario suggests methods for more accurately predicting forecasting fog formation when increasing moisture pools occur in the same direction as the prevailing wind.

ACKNOWLEDGMENTS

John Kwiatkowski (SOO) for his assistance in gathering soundings, AFOS surface plots, and RUC data which helped greatly in the analysis of this fog event.

REFERENCES

Barker, L., 1994: Forecasting Considerations Pertaining to Upslope Fog Formation at Renner Field, Goodland Kansas. National Weather Service, Central Region Applied Research Paper 12-10.

 


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