National Weather Service Forecast Office
Synoptic scale mid-latitude cold fronts, as analyzed from surface data, which move into Wyoming from the north is a common occurrence especially from Autumn through Spring. The resultant upslope low level wind flow following the frontal passage produces most of the snow events over the lower elevations of the northern and eastern parts of the state. Some of these anticyclonic snow events can produce snow accumulations, which may affect travel, depending on amount of available moisture and position of an upper level jet or jet streak (Dunn 1988). Even though these are synoptic-scale fronts, there are similarities to small scale surges in Wyoming (Davis 1995).
For this study a significant cold surge is one with a large and sudden temperature and wind change, precipitation and westward extent to the west of the Continental Divide. A minor cold surge is one that generally has a smaller and less sudden temperature and wind change, very little or no precipitation and a westward extent along or east of the Continental Divide.
The western and southern extent of low level cold air associated with invasions of Canadian Polar (CP) and Arctic (A) air masses are difficult to forecast in Wyoming. Wyoming is frequently on the western edge of the cold air, and therefore forecasts of temperature and wind can be difficult, requiring accurate timing of the cold front passage. The purpose of this study is to examine the observational characteristics of aforementioned cold fronts in Wyoming from September 1995 through May 1996.
DATA AND METHODOLOGY
From September 1995 through May 1996, documentation of Wyoming fronts included surface and upper air analysis as well as area radiosonde observations, area wind profile data and area 88D VAD wind profile data. Other than discussion of depth of the cold air revealed, the radiosonde data, wind profiler data and the 88D VAD wind profile data will not be shown. These data included evolution of the following features:
Data from Daily Weather Maps (NOAA 1990-1995) for the period of study (September 1995 through May 1996) were used to identify every Cold Front (CF) occurrence in Wyoming from August to June. This data was examined mainly for climatological purposes. Note that the period of study is different from the 5-year climatology used to define CFs during that period of study.
CFs moving through the state of CP or A air differs from maritime Pacific CF's moving through the state (Blair 1965). Maritime Pacific CF's are usually north-south oriented and usher in a cool air mass of an eastern Pacific Ocean origin. The CP or A fronts originate in northwest Canada, has much colder temperatures, move from north to south, and are generally east-west oriented.
An average of 30 CF's per season affected Wyoming during the five-year period of 1990-1995. The largest number was 37 from the early fall of 1993 through the late spring of 1994. The data suggests that CFs tended to cluster at times, spaced only a few days apart when the synoptic scale regime was favorable for CFs to move into Wyoming. Figure 1 shows the dates of the CP or A fronts into Wyoming from the early fall of 1995 through the late spring of 1996. When the synoptic-scale regime "aloft" was unfavorable for CFs to move into Wyoming (southwest wind flow), there can be periods of a week to ten days without a CF moving into the state.
The average number of CFs per month in the past five seasons varies from one in August to five in January. From Figure 1, there are typically fewer CP or A fronts affecting Wyoming in the early fall and late spring, and more in the remainder of the cold season.
The CFs and the associated cold air masses have a variety of effects on Wyoming. This can range from simply a wind shift and minor cooling of ten degrees or less to a more drastic temperature and wind change with associated significant banded precipitation. All CF's are critical to aviation and other interests, and have some common features.
All CP or A CFs introduced a cooler air mass into the area and was associated with a change in low level winds to a more northerly direction. However, some CFs were more intense, with a dramatic low level wind change to north and a drastic lowering of the temperature along with increased precipitation. Some of these fronts move through the entire state, while others will enter only portions of the Wyoming eastern plains. The CFs also move through the state at speeds ranging from approximately ten knots to over fifty knots.
Figure 1. Dates on which cold surge initially moved into Wyoming.
The terrain plays an important role in the movement of the CFs southward over Wyoming. Many of mountain ranges tend to block or halt the progression of the surge of cold air at the low levels and alter the progression of the fronts. The main blocking features include the Big Horn mountains, Absaroka Range, Wind River Range and the Laramie Range (Figure 2).
The cold air must exhibit a depth great enough to move across those ranges which varies from about 12000 feet MSL for the Wind River Range to about 7500 feet MSL for the Black Hills. Most of Wyoming's lower elevations range from four to seven thousand feet MSL, with the largest area of lower terrain in the eastern plains.
Figure 2. Terrain Map of Wyoming (shading represents elevations in 500 foot intervals starting with 4500 feet or less in white).
The CFs that move into Wyoming and their associated weather vary due to a number of factors. For the autumn 1995 through spring 1996, the common elements associated with CFs moving into Wyoming are cooler temperatures and a low level wind shift. A summary of the observed characteristics of the evolution of significant and minor events is provided in Table 2. A case study of a significant and minor event is provided in the next section, illustrating some of the following points.
Once a cold airmass (CP or A) has developed in its source region over northwest Canada (or even Alaska), it begins to move south or southeast after a shortwave trough passage from the west. Often a shortwave trough moving across western Canada initiates pressure rises at the surface after its passage, signaling southward movement of the cold air as implied by Sangster (1980). Much of the time, at least during significant cold surges, a stronger shortwave trough remains upstream as the air mass begins to move southward.
Often a surface low develops over the northern Rockies, the nearby high plains or southern Canada and is associated with the initial shortwave trough. As the low moves east or southeast, the cold air begins to move south after initial shortwave trough passage.
Every CF recorded from fall 1995 through spring 1996 exhibited these characteristics.
Intensity of the cold air surges in Wyoming depends on differences in the common elements above as well as depth of the cold air mass. Composite maps of some of the parameters for significant and minor cold surges are shown in Figures 3 through 8.
For significant CFs into Wyoming the synoptic pattern and the parameters were very similar with each event (Figures 3 through 5). Based on observed upper air and surface data, most events have an initial, and weak shortwave trough passage across Canada over the air mass. Shortly after that initial feature's passage, the more significant shortwave trough is generally moving from western Canada or the Pacific northwest to the south or southeast. Twenty-four hours before the arrival of the CF in Wyoming, this shortwave trough is usually in western Canada or the Pacific northwest. A strong 300 mb jet is usually located just to the west of the shortwave trough allowing it to move, or dig further south. Twenty-four hours before the CF entering Wyoming the surface front was located over southern or central Alberta and Saskatchewan, with a surface low over eastern Saskatchewan. Wyoming soundings and those to the north of the state revealed that the depth of the cold air needs to be at least 9000 feet MSL to produce the weather (clouds and precipitation) associated with a significant CF.
Figure 3. Composite Map of synoptic features twelve hours before cold front enters Wyoming. Arrow represents 300 mb jet stream, dashed lines represent 500-mb shortwave trough positions and frontal symbol represents cold front location. This is for significant events.
Figure 4. Composite Map of synoptic features at time when front enters Wyoming in significant events. Arrow represents 300 mb jet stream, dashed lines represent 500-mb shortwave trough position and frontal symbol represents cold front location.
Figure 5. Composite Map of synoptic features twelve hours after cold front enters Wyoming in significant events. Arrow represents 300 mb jet stream, dashed line represents 500-mb shortwave trough position and frontal symbol represents cold front location.
The approach of the main shortwave trough to Wyoming results in a surface pressure rise/fall couplet enhancing the southward movement of the CF. Couplets exhibit pressure rises between 2mb and 6mb behind the front and 2mb - 5mb falls to the southeast and south of the associated surface low. The pressure tendency equation (Palmen and Newton 1969) illustrates surface pressure change is a function of the vertically integrated density (temperature) advection. Thus the stronger the lower tropospheric cold air advection, the greater the surface pressure rises.
Twelve hours before the CF enters the state, the feature is typically located along the Montana and Canadian border. Although the shortwave trough can be located anywhere from Alberta to Nevada. Strong cold air advection becomes apparent at 500-mb and 700-mb. When the front enters the state, large surface pressure rises continue behind the front over Montana while the shortwave trough is located from Saskatchewan to Idaho. When the CF is at its western most extent, the shortwave trough is over the northern high plains, with significant cold air advection at 500-mb and 700-mb over Wyoming. The largest surface pressure rises have shifted further east into the Dakotas and Nebraska at that time.
With the minor CFs, the positions of the important features are quite different (Figures 6 through 8). The shortwave trough 24 hours before its arrival in Wyoming is further north. Then, the position is over northwest or western Canada. The feature then moves more east across southern Canada, with much less amplification than with a significant CF. The 300 mb jet is also further north and develops across southern Canada or over the extreme north part of the country. At the surface, the low is over North Dakota or southern Saskatchewan and moves rapidly east or southeast into the Great Lakes area shortly after the final westward extent of the CF is reached. The depth of the cold air is usually from 5 to 7 thousand MSL feet deep. Although good pressure rises occur in the cold air as the shortwave trough approaches, the rises quickly move from western Montana to the Dakotas and then points east.
Figure 6. Composite Map of synoptic features twelve hours before cold front enters Wyoming in minor events. Arrow represents 300 mb jet stream, dashed line represents 500-mb shortwave trough position and frontal symbol represents cold front location.
Figure 7. Composite Map of synoptic features near time when cold front enters Wyoming, during minor events. Arrow represents 300 mb jet stream, dashed line represents 500-mb shortwave trough position and frontal symbol represents cold front location.
Figures 8. Composite Map of synoptic features twelve hours after a cold front enters Wyoming during a minor event. Arrow represents 300 mb jet stream, dashed line represents 500-mb shortwave trough position and frontal symbol represents cold front location
CASE STUDY OF A SIGNIFICANT EVENT (NOVEMBER 25-26, 1995)
This case is presented to illustrate some of the results described earlier in the study. This significant CF produced one to three inches of snow in most of the lower elevations of the state. Some areas received larger amounts, notably Lander with ten inches of snow and Story with seven inches. Ahead of the front were very mild temperatures and strong west winds. After frontal passage, temperatures dropped twenty degrees in a short time, with brisk north winds. Light snow fell in most areas within an hour of frontal passage.
Depth of the cold air increased from 7000 feet MSL at Great Falls and Glasgow at 1200 UTC 26 Nov to 10000 feet MSL by 1200 UTC 27 Nov. The cold air depth was between 8000 and 9000 feet MSL at both Riverton and Rapid City by 1200 UTC 27 Nov.
Figure 9 shows the surface low and frontal movement at six hour intervals from 1800 UTC 25 Nov to 1200 UTC 27 Nov. This frontal movement, along with 2 to 5 mb surface pressure rises behind the front, is similar to other significant cold surge events from fall 1995 to spring 1996. The cold air takes about twelve hours to move from the Montana/Alberta/Saskatchewan border to extreme northern Wyoming. The front then takes another twelve hours to move into southeast Wyoming and reach its final westward extent.
Figure 9. Surface frontal positions (for time shown) for significant event of Nov. 25-26, 1995.
The front began to move south after a weak shortwave trough moved across central Canada on Nov 25. At the same time, a shortwave trough developed over southwest Canada and moved southeast. At 1200 UTC 26 Nov, the feature was over eastern Washington and eastern Oregon and moved over western Wyoming by 0000 UTC 27 Nov. One hundred to 150 meter height falls were noted at 500 mb, ahead of that feature. Behind the shortwave trough, temperatures were from -20°C to -28°C that were 7 to 10 degrees colder than ahead of the feature. The jet at 300 mb was on the backside of the feature, over western Wyoming at 0000 UTC 27 Nov and oriented north-south. Figure 10 shows the movement of the shortwave trough at 500-mb.
Figure 10. Shortwave trough positions at 500-mb (times shown) for the significant event of November 25-26, 1995.
At 700-mb, very mild westerly wind flow was over Wyoming at 000 UTC 26 Nov. With shortwave trough passage, temperatures were -2°C to -11°C over the state, by 0000 UTC 27 Nov, with convergent wind flow over Wyoming.
The front remained over southeast to northwest Wyoming through 1500 UTC 29 Nov (frontal position not shown). Area soundings through that time indicated that the air mass became more shallow as height rises and strong warm air advection were present in the northwest flow "aloft". Pressures were falling at the surface from early evening on Nov 28 through Nov 29.
The NGM run of 1200 UTC Nov 25 was reviewed during this significant CF and, as with other significant cold surges, was too slow. In addition, the winds were forecast over the state to be more northwest, instead of the observed north to northeast. The poor handling of the cold fronts by the Nested Grid Model (NGM) in this part of the country has been examined by Norman Phillips and Dennis Deaven (1986).
CASE STUDY OF A MINOR EVENT (JANUARY 14-15, 1996)
This case is also presented to illustrate some results described earlier in the study. A minor surge of cold air moved into eastern Wyoming on the morning of January 14, 1996. This event produced colder temperatures along with widespread fog and areas of light snow or flurries in northeast and east central Wyoming.
At 1800 UTC 13 Jan, the front was located over southern Saskatchewan and southern Alberta with the surface low over extreme southeast Saskatchewan (Figure 11). The front moved slowly south and by 0600 UTC Jan 14 extended from north central South Dakota to central Montana to southwest Alberta. The front continued to move slowly south and ended up from the southwest Nebraska panhandle to the east foothills of the Big Horn mountains at 1500 UTC 14 Jan. The front remained in that area until it moved east between 0600 UTC and 1200 UTC 15 Jan.
Figure 11. Surface frontal positions (for times shown) for minor event of Jan. 13-14, 1996.
Surface pressure rises behind the cold front were from 4-6 mb starting at 0600 UTC 14 Jan over eastern Montana. After that time, the largest surface pressure rises then was mainly to the east and northeast of Wyoming.
"Aloft", the polar front jet stream at 300 mb was over central British Columbia, central Alberta and southern Saskatchewan at 1200 UTC 13 Jan. This feature developed east-southeast, with the jet from southern Alberta to southern Manitoba at 1200 UTC 14 Jan. A shortwave trough was entering western British Columbia at 1200 UTC 13 Jan, with 50 to 70 meter 500-mb height falls ahead of the feature over western Canada. By 0000 UTC 14 Jan, the trough was located from northern Alberta to southwest British Columbia, with 40 to 60 meter 500-mb height falls ahead of feature over southern and western Canada. At 1200 UTC 14 Jan, there was west-northwest channeled flow over southern Canada. Cold air advection at 500-mb and 700-mb was over western Canada at 0000 UTC 14 Jan, and spread eastward by 1200 UTC 14 Jan. Figure 12 shows the position and movement of the shortwave trough at 500-mb.
Figure 12. Shortwave trough positions at 500-mb (times shown) for minor event of Jan. 13-14, 1996.
The depth of the cold air was 5000 to 7000 feet MSL per raobs at Glasgow, MT (GGW), Great Falls, MT (TFX) and Rapid City, SD (UNR) at 1200 UTC Jan 14. This same depth was noted earlier upstream, even in southern Canada as early as Jan 12. This cold air depth was less than that of the significant case study and all of the significant events observed from early autumn 1995 through late spring 1996.
RETREAT OF THE COLD AIR MASS
Data from September 1995 through May 1996 indicated that the cold air masses usually only remained in Wyoming a relatively short time (anywhere from 1 to 4 days). Nearly all of the retreating events in the fall 1995 and spring 1996 were associated with surface pressure falls. The cold air eroded from west to east as warm air advection at 500-mb and 700-mb occurred and the resulting surface pressure falls began over the state. The 300 mb jet was located east of Wyoming. Height rises at 500-mb also occurred west of Wyoming before the cold air erosion.
The height rises corresponded to a strengthening of the westerly wind component throughout the depth of the atmosphere. This appeared to aid in the flushing of the cold air (Lee et al. 1989). The cold air mass eroded from the top, so the deeper significant cold surges usually remained entrenched longer than minor surges, usually by a length of one to three days.
OTHER ASPECTS OF COLD FRONTS
Some potentially serious aspects of weather associated with CP or A CFs are major snowfall and strong winds that can cause near blizzard conditions and very low wind chill temperatures. However, not all of the significant cold fronts produce major snowfall or strong winds.
Dunn (1988), illustrated how the interaction of a jet streak (usually westerly) moving over low level cold air can enhance snowfall amounts associated with CF passages. The ascent in this case is usually caused by ageostrophic jet streak circulations, primarily in the left exit and right entrance regions. In addition, convergence at 700 mb aids in the development of the significant snow. Both features are present during portions of most significant frontal episodes.
In some CF cases, strong winds of over 50 knots developed behind the cold front and lasted for several hours. Interestingly, both minor and significant events can produce these conditions. Both Weiland (1984) and Kapela (1995) showed that the isallobaric component can greatly enhance the wind speeds. This component is often due to the location of strong 300 mb wind flow parallel to the cold surge leading to synoptic-scale sinking motion. It has been noted during the fall of 1995 through spring of 1996 that CFs with pressure rises of 7 mb or greater upstream of Wyoming produce these strong winds in the state. Due to the strong subsidence, very little or no precipitation is observed. However, if fresh snow is on the ground, the strong winds can cause considerable blowing and drifting snow.
CP or A CFs is a common occurrence in Wyoming in the autumn, winter and spring months. These events are difficult to forecast. Further the rapid change to colder and windy conditions associated with major snowfall can be life threatening.
However, similarities in synoptic patterns and timing between the significant and minor events do exist. Cold air is brought southward after a shortwave trough passage. In significant events, a stronger shortwave trough remains to the west or northwest of Wyoming. The westward extent of the cold air is maximized when that stronger shortwave trough moves to the east of Wyoming. A significant CF typically has a shortwave trough further south and west than a minor CF. Most shortwave troughs in a minor event move southeast across southern Canada into the northern plains and the Great Lakes. The 300 mb jet, likewise, is further west and south (and has a more north-south orientation) during a significant event than a minor CF.
Cold air advection throughout the atmosphere takes place behind the shortwave trough with a north or northwest wind flow. Well after the trough passage, warm air advection begins and the winds become more westerly. The cold air then begins to erode out of the state.
The cold front in a significant event usually takes twelve hours to move from just north of the Montana-Canada border to northern Wyoming. The greater the surface pressure rises in the cold air, the faster the cold front will move. An important factor in determining westward extent of the new air mass is the depth of the cold air. For significant events, the cold air needs to be 9000 feet MSL or greater, while for minor surges the depth is usually less than 8000 feet MSL or less. Upstream upper air observations in Canada can give a good indication of the depth of the cold air even several days before the arrival of the cold air into Wyoming.
Further, if there is enough available moisture and the cold air is 8000 to 9000 feet MSL deep, significant snow can fall if a westerly jet stream is over the area in the presence of mid level convergence. In addition, if surface pressure rises are 7mb or greater in the cold surge, very strong winds but little precipitation will occur behind the front.
Blocking Topographical Features in Wyoming (Refer to Figure 2)
|1||Big Horn Mountains|
|4||Owl Creek Mountains|
|5||Wind River Range|
|Significant Events||Minor Events|
|300 mb jet located over or to west of state.||300 mb jet north of state.|
|500 mb main shortwave trough moving from southwest
Canada to central/northern Rockies
|500 mb shortwave trough moving
from west Canada to south central
Canada or Northern Plains.
|Significant height rises and falls associated with main
|Some height rises and falls
associated with shortwave trough.
|Significant cold air advection behind shortwave trough
at 500 and 700-mb
|Some cold air advection behind short-
wave trough at 500-mb and 700-mb
|Surface low moving from southwest Canada to Northern/-
|Surface low moving from south central
Canada to Great Lakes
|Cold front moving south from south central/southwest
|Cold front moving southeast into
|Significant pressure rises behind front||Good pressure rises behind front|
|Front stalling along the Continental Divide or west of
|Cold front stalling in eastern Wyoming|
|Depth of cold air to 9000 feet MSL or more behind front||Depth of cold air to less than 8000 feet
MSL behind front
|Significant Events||Minor Events|
|Strong west to southwest winds ahead of front and strong
north to northeast behind front
|Weak westerly winds ahead of front.
Brisk northerly winds behind front
|Significant Events||Minor Events|
|Mild ahead of front and much colder behind front. Very
tight temperature gradient
|Mild ahead of front and cooler behind
|Significant Events||Minor Events|
|Light snow which can be moderate or heavy at times.
Clouds for at least 6 to 12 hours behind front. Snow
remain in state for a number of days.
|Flurries or no snow. Only brief clouds
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Davis, C.A., 1995: Observations and Modeling of a Mesoscale Cold Surge During WISPIT. Mon. Wea. Rev., 123, 1762-1780.
Dept. of Commerce, 1995: Daily Weather Maps. NOAA, National Weather Service, Climate Prediction Center, Washington, D.C.
Dunn, L.B., 1988: Vertical Motion Evaluation of a Colorado Snowstorm from a Synoptician's Perspective, Wea. Forecasting, 3, 261-271.
Kapela, A.F., P.W. Leftwich, and R. Van Ess, 1995: Forecasting the Impacts of Strong Wintertime Post Cold Front Winds in the Northern Plains, Wea. Forecasting, 10, 229-244.
Lee, T.J., R.A. Pielke, R.C. Kessler, and J. Weaver, 1989: Influence of Cold Pools Downstream of Mountain Barriers on Downslope Winds and Flushing, Mon. Wea. Rev., 117, 2041-2058.
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Sangster, W.E.,1980: A new concept in diagnosing large-scale vertical motion. Central Region Technical Attachment 80-20, 6pp.
Weiland, M.S., 1984: The Blizzard of February 4-5 1984 over the Eastern Dakotas and Western Minnesota. CR Tech Memo CR-73, 10pp.