Northern Prairie Wildlife Research Center
Precipitation is light and nearly always in the form of snow during the winter in North Dakota. Monthly water equivalents of melted snowfall for the winter months average mostly from 0.3 to 0.6 inches, but January amounts range upwards to 0.9 inch in the northeast (Figures 26, 27, 28). In all months, precipitation is 0.1 to 0.2 inches heavier in the east than in the west and it is also generally heavier in the north than in the south. Precipitation is usually light in North Dakota during winter because nearly all of the moisture-laden low pressure systems that result in large amounts of snow follow tracks well to the south of the state. Low pressure systems forming on the eastern slopes of the Rocky Mountains in Canada or originating in the northern Pacific Ocean frequently move southeastward across the state during the winter. These low pressure systems are better characterized by the cold air invasions which follow their passage across the state rather than by the moisture that falls.
The effect of topography on the amount of precipitation that falls in any section of the state is complex and difficult to assess. As discussed earlier, the major contribution of topography to the precipitation process is providing a mechanism to lift the air mass and cause precipitation. Thus, one would expect to find the higher amounts of precipitation on slopes facing the prevailing winds and lower amounts on lee slopes.
The Turtle Mountains well illustrate this effect of altered precipitation patterns. More precipitation falls on the northwestern slope and on top of the Turtle Mountains than the surrounding terrain because they forced the prevailing northwesterly air flow upward. On the other hand, less precipitation is received in the area southeast of the mountains because the prevailing flow of air from the northwest moving down the southeast slopes stifles the precipitation process.
More precipitation also falls in western Divide, Williams, McKenzie and northwestern Dunn counties because of higher elevations in those areas. The higher precipitation amounts in the northeast are due to the Pembina Mountains where elevations increase by more than 600 feet in less than eight miles. In this case, the higher amounts are brought about by the orographic uplift of air from the east. In January and February, more precipitation is also measured in the eastern portion of the Missouri Slope in the southwestern portion of the state as easterly winds are lifted.
The first substantial rains of spring sometimes fall in late March, but usually in early April. Monthly precipitation amounts increase as spring wears on because the storms which traveled well south of the state in winter now follow more northerly tracks (Figures 29, 30, 31).
During the spring months, the southeast corner of the state receives more precipitation than the rest of the state because it is closer to the main storm tracks and the Gulf of Mexico moisture source. Therefore, precipitation is lightest in the northwest where amounts in April and May are more than an inch lighter than in the southeast. The southwest and northeast areas of the state receive about the same amount of precipitation in April and May.
Topographical influences on precipitation amounts in the spring are especially evident in the area immediately south of the Turtle Mountains where amounts are nearly always lighter than surrounding areas. The persistence of this deficiency pattern during the spring months is thought to be due not only to the shielding effect of the Turtle Mountains, but also due to a similar shielding effect of the Pembina Mountains from rain-bearing easterly winds and the comparatively high terrain to the southeast. Precipitation amounts higher than surrounding areas are also found in the eastern portion of the Missouri Slope, where the gradually rising terrain uplifts winds from the east.
Rainfall hits its high peak in June (Figure 33). Amounts range from just over three inches in the extreme northeast and northwest to more than four inches at several locations in the southern half of the state. In July and August, average rainfall amounts have decreased from June levels, but monthly averages as high as about three inches are still found over the Red River Valley (Figures 34, 35). However, in the extreme west rainfall decreases rapidly in both July and August, so that by August the Red River Valley has about twice as much rainfall as western border counties.
During the summer months, the importance of topography in affecting precipitation patterns is variable. Air mass thunderstorms often occur during June. Since air mass thunderstorms generally move from the southwest to the northeast, heavier precipitation is frequently found on the southwestern slopes or near the tops of rises because of the lifting and cooling of the air as it moves upslope. However, in July and particularly in August, most of the precipitation is caused by cold fronts lifting the air and causing thunderstorms which generally cross the state from a northwest to southeast direction. Because of this, the lines in the August maps are much more regular with fewer pockets of high and low rainfall attesting to the lessening importance of topography in inducing precipitation.
Precipitation amounts decrease rapidly throughout the fall months, and by November rainfall ranges from only about a half-inch in the west to nearly an inch in parts of the Red River Valley (Figures 35, 36, 37).
The rapid decrease in rainfall in combination with mild fall temperatures usually produces several weeks of beautiful Indian Summer. The chances of receiving measurable precipitation are lower in late October and early November than at any other time during the year.
The first significant snowfall of the season usually occurs during the middle or latter part of November. However, measurable amounts of snow (0.1 inch or more) may fall in September about once in every 10 years in the west and north, but only rarely in the southeast.
Topographic effects on precipitation are minor during the fall, although by November the area of lighter precipitation southeast of the Turtle Mountains is again evident.
North Dakota's precipitation is usually adequate for the commercial crops grown, although occasionally they do not come at exactly the optimum time. Approximately 50 to 60 percent of the annual precipitation falls in the four-month period from April through July, while nearly 75 percent occurs in the six-month period from April through September. Distribution of the average daily precipitation is also favorable for crop production. Rainfall in spring is light, allowing fields wet with melting snow to dry which facilitates rapid land preparation. Average daily rainfall increases until late June before declining gradually until December. The declining rainfall along with abundant sunshine is very favorable for maturing and harvesting crops.
For both the April through July and the April through September periods, precipitation is about 50 percent greater in the southeast than in the northwest (Figures 38,39). On both maps, as on the annual map, an area of higher rainfall than in surrounding areas is located in the southwest, and an area of lower rainfall is found southeast of the Turtle Mountains.
A difference of one or two inches of rainfall during the growing season may not seem very great, but it can be significant agriculturally. This can be illustrated easily by considering that on an average day a grain crop during heading and filling will use about 0.25 inch of water. Thus, a one-inch greater rainfall will supply water for four additional days and two inches of additional rainfall will supply water for about eight days. For a barley crop that can be grown in about 70 days, this is a decided advantage which will translate into extra bushels of grain for each additional inch of water.
Precipitation for the nongrowing season ranges from three inches in parts of the north and west to about five inches in parts of the east (Figure 40). In all sections of the state this is approximately 25 percent of the annual precipitation. Most of the nongrowing season precipitation falls as snow, although some rain is still likely in October. The higher precipitation totals in the eastern portion result from that area being nearer the average track of major snow-producing winter storms coming out of the Colorado-Oklahoma-Kansas low pressure spawning grounds.
Annual precipitation ranges from less than 13 inches in the northwest to more than 20 inches in parts of the Red River Valley and southeast (Figure 41). The lines of equal precipitation, although subject to some meandering, are oriented north-south, so that as a generalization one can say that precipitation increases about an inch for every 50 miles of eastward movement.
There are two areas in the state in which the general increase of precipitation in an easterly direction does not apply. One area is located in the southwest where the annual precipitation of more than 16 inches is higher than the surrounding area. This area of higher precipitation is largely a result of topographic uplift. The other area is in the north central where the annual precipitation of less than 16 inches is lower than surrounding areas. This area is caused primarily by air moving downhill from all but a southerly direction, which works against the precipitation process.
Annual snowfall in North Dakota ranges from less than 26 inches in parts of Mountrail and McLean counties in the west central to about 38 inches in a belt extending diagonally across the state from the northeast corner to the southwest (Figure 42). The snowfall maps are based on the period from 1931-60, while snow depth maps are based on the years 1948-49 to 1968-69.
Table 4. Average annual number of snowfalls of stated amounts and the percent chance that a snowstorm will produce snowfalls of given amount at nine representative stations in North Dakota.
|Mean annual no. of snowfalls with||Percent of all measurable snowfalls (0.1 inch or more) resulting in stated amounts in 24-hour periods.|
|0.1 in. or more||More than 1.1 in.||0.1-1.0||1.1-2.0||2.1-3.0||3.1-4.0||4.1-5.0||5.1-6.0||6.1-7.0||7.1-8.0||8.1-9.0||9.1-10.0||More than 10.0|
|West Central||Watford City||1936-66||19.4||9.2||52.5||27.2||11.3||4.0||2.2||0.7||1.2||0.2||0.3||0.5||*|
North Dakota is favorably located with respect to annual snowfall despite its northerly location. North Dakota's annual snowfall is considerably less than any state to the east of it which borders Canada, and all states west of North Dakota have large areas which receive considerably more snow. For example, most of the New England states, nearly all of New York, northern Pennsylvania, and about three-fourths of Michigan receive well above 100 inches. Even such southern states as Arizona, New Mexico and California have considerable areas at higher elevations which receive much more than 38 inches of snow annually.
Average monthly snowfall amounts for any location in the state during the winter period of December through March average only five to eight inches, depending upon month and location. Average monthly snowfalls for November and April group in the range of two to four inches. Smaller average amounts fall in October and May.
Deep snow is not commonly found in North Dakota. Surprisingly, the average maximum snow depth measured at any time during the winter season ranges from only nine inches in the southwest to 15 inches in the northeast (Figure 43). Maximum snow depths are lower in the southwest because of the higher average temperatures which allow more melting and settling of the snow cover. Maximum snow depths vary greatly. During some winters very little snow accumulates, while in other winters, such as 1968-69, snow depth in early March ranged from 10 inches in the extreme west to more than 40 inches in the extreme southeast. This, however, was the deepest snow pack to cover the state since 1897.
To the delight of snowmobilers and other winter enthusiasts, the number of days with snow cover of one inch or more ranges from approximately 80 days in the southwest to more than 120 days in the northeast (Figure 44). That thick snow packs are not normal in North Dakota, the average maximum depth ranging from only nine to 15 inches, is again reflected in the maps of days with snow cover of six inches or more (Figure 45). Such a snow cover can be expected on only 20 days in the southwest to about 60 days in the northeast. A snow pack of 18 inches or more averages less than one day a year over large areas of the south and west half and only on about 10 days in the northeast corner (Figure 46).
The date on which there is a 50 percent chance of having a snow depth of at least one inch (that is, in half of the years the snow depth will be less than an inch and in half of the years the snow depth will be one inch or more on that date) in fall or early winter ranges from about November 20 in the north central to late December in the extreme southwest (Figure 47). In the spring, a snow depth of one inch or more can be expected to remain as late as March 15 in the extreme southwest and south central to early April in the deeper snow area in the northeast (Figure 48).
Most people are surprised to find that snowfalls totaling 0.1 inch or more occur on only 17 to 26 days during the winter. Snowfalls of 0.1 inch or more happen most frequently in a broad diagonal band extending from the northeast and north central areas southwestward into the southwest corner. This band corresponds roughly with the area of highest annual snowfall in Figure 42. Snowfalls of 0.1 inch or more occur less frequently on either side of the band. Northwest of this diagonal band snowfalls of at least 0.1 inch are measured on about 20 days, while southeast of the band snowfalls of 0.1 inch or more are measured on only 17 days.
Chances for precipitation during any particular hour of the day depends upon the season and the time of the day. This is clearly evident in Figure 49 where the chances for precipitation for each hour of the day and by seasons are plotted for Bismarck and Fargo. In winter, the chances for precipitation at any particular hour are about twice as great as for the same hours in summer and fall. Chances for precipitation at any hour of the day during spring is less than winter, but greater than fall or summer.
Diurnal variations in chances for precipitation show a rather consistent pattern for Bismarck. In all seasons, chances for precipitation are highest in the early morning hours between 6:00 and 8:00 a.m. The chances then decline until about midday, thereafter remaining more or less constant until night when the odds for precipitation once again begin to climb toward early morning high levels. Only in summer is there a significant change in this pattern. During summer, the chances for rainfall increase late in the afternoon, which reflects the commencement of thunderstorm activity due to daytime heating.
Daily variations in precipitation are small for Fargo in spring and fall. In winter, odds for precipitation increase after 5:00 a.m. until about 8:00 a.m. and thereafter remain at levels well above those at night until darkness at which time the odds drop sharply. In summer, there is a pronounced maximum in the chances for precipitation around 6:00 a.m. due to thunderstorm activity associated with the low-level jet stream, a fast northward-moving river of air which is best developed at night and whose strongest winds are usually only 2,000-4,000 feet above the earth surface.
The percentage of hours for each month in which snow, rain or drizzle, and freezing rain or drizzle occur is shown in Figure 50 for Grand Forks. The trend lines are representative for other locations in the state, although the percentage frequency of the hours in a month in which precipitation occurs varies somewhat.
Snow falls during 23 percent of the hours in Grand Forks in January, the highest frequency of precipitation for any month. Somewhat surprisingiy, the greatest average monthly snowfall at Grand Forks occurs in December when 6.6 inches of snow falls during 18 percent of the hours. At Grand Forks, the frequency of snow shows a slight increase in March over February, but at other stations for which data is available, March has a slightly lower frequency of hours during which snow falls than does February.
The hourly frequency distribution of rain or drizzle for the year shows peaks in May and September. These peaks are associated with the seasonal migration of low pressure storm tracks. In summer, few low pressure systems travel south of the state so that most of the rain falls in a short period of time in showers or thundershowers associated with cold fronts moving through the state, or because of air mass instability. During the rest of the year, many more low pressure systems follow tracks which keep them south of the state. Consequently, most or all of the precipitation that falls is steadier and lighter.
Freezing rain or drizzle is rain or drizzle which falls in liquid form and freezes on impact to exposed objects such as telephone or electric lines, tree branches, or on the ground. It is formed by warm, moist air being forced over colder air below. A necessary condition for freezing rain or drizzle is that the water droplets be super-cooled before impact; that is, the water droplets have been cooled beyond their normal freezing point of 32° F. The smaller and purer the water droplets, the more likely is supercooling. Super-cooled water droplets have been observed in clouds at temperatures as low as about -40° F. Super-cooling of water droplets accounts for the observation that freezing precipitation occasionally occurs when shelter air temperatures are as low as the "teens."
Sleet is often confused with freezing rain. Sleet is defined as generally transparent, globular, solid grains of ice which have formed from the freezing of raindrops, or the refreezing of melted snowflakes which fall through a below-freezing layer of air near the earth's surface.
Freezing rain or drizzle occurs during 1.7 percent of the hours in December and 1.2 percent of the hours in February, but in other months freezing precipitation is observed during only a few tenths of one percent of the hours. Under North Dakota conditions, freezing rain or drizzle is normally a transient condition which occurs for only a very short period of time when a cold air mass is replaced by a warmer air mass in the cold season. Severe icing storms causing heavy damage over a wide area are not common, occurring on average about once every five years somewhere in North Dakota.
During an average year, a measurable amount of precipitation (0.01 inch or more) can be expected on 67 days in the extreme northwest at Crosby. But in the southwest at Dickinson, a measurable amount can be expected on 90 days (Table 5). In most places around the state, 0.01 inch or more will be recorded on 68 to 79 days. Precipitation will total a half-inch or more during a 24-hour period on only about seven days in the northwest and north central, and up to 10 days in the east central and southeast.
Table 5. Average annual number of precipitation events of stated amounts and the percent chance that precipitation will produce a given amount of precipitation at nine representative stations in North Dakota.
|Ave. annual no.events
amount of precipitation
|Percent of all measurable precipitation events (0.01 inch or more) resulting in stated amounts in 24-hour periods.|
|0.01 in. or more||More than 0.50 in.||0.01-0.10||0.11-0.25||0.26-0.50||0.51-0.75||0.76-1.00||1.01-1.50||1.51-2.00||2.01-3.00||3.01-4.00||More than 4.00|
|West Central||Watford City||1936-66||68.2||8.1||45.5||27.8||14.9||6.4||2.5||2.1||0.56||0.23||0.09||*|
Data of the percentage frequency of particular precipitation amounts are also given in Table 5. Most precipitation events are light, as 46 to 62 percent of all 24-hour amounts of precipitation amount to only 0.10 inch or less. Rainfall amounts of more than four inches in a 24-hour period are unusual. In fact, five of the nine stations in the table have never recorded a four-inch rainfall in a 24-hour period.
Sometimes it is necessary for planning and design purposes that some estimate be made of the probability of certain precipitation amounts occurring over some period of time. For instance, engineers designing dams, size of ditches or culverts, or even hard surfacing of parking lots must balance the likelihood of extreme precipitation occurrence against the cost of building certain safeguards into the structure. Estimates of extreme precipitation occurrence are given in Table 6 which lists the precipitation amounts for one-hour, one-day and 10-day periods in which precipitation events are likely to be equalled or exceeded once on the average for return periods of 2, 5, 10, 25, 50 and 100 years. Thus for Bismarck, precipitation during a one-hour period is likely to equal or exceed 1.1 inches on the average once every two years or 2.7 inches once every 100 years. Similarly, during a 10-day period at Bismarck, precipitation will equal or exceed 3.5 inches on an average of every two years or 8.3 inches every 100 years.
Table 6. Return period values in inches of precipitation for selected duration at several locations in North Dakota.
|Location||Duration||Return Period In Years|
Most of the precipitation that falls during the summer occurs during periods of thunderstorm activity. Records of thunderstorm activity have been summarized for four locations in North Dakota (Table 7). The average number of thunderstorms annually ranges from 22 to 34, with the fewest number of thunderstorms occurring in the northwest. July is the peak month for thunderstorm activity when eight to 10 storms occur, although thunderstorms occur nearly as often in June and August as in July. Thunderstorms occur about once a month in April and September and only rarely in March and November.
Table 7. Average number of thunderstorms by month and for year at selected locations.
|1Number of years of record.|
* Less than 1/2 day.