NOAA TM NWS CR-39
F NOAA Technical Memorandum NWS CR-39 F U.S. DEPARTMENT OF COMMERCE NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION National Weather Service
A Synoptic Climatology Of Blizzards On The North-Central Plains Of The United States
Robert E. Black
CENTRAL REGION Kansas City, Mo.
FEBRUARY 1971 NOAA TECHNICAL MEMORANDUM* Central Region Subseries
The Central Region Subseries provides a medium for quick dissemination of material not appropriate or not yet ready for formal publication. Material is primarily of regional interest and for regional people. References to this series should identify them as unpublished reports.
1 Precipitation Probability Forecast Verification Summary Nov. 1965 - Mar. 1966. 2 A Study of Summer Showers Over the Colorado Mountains. 3 Areal Shower Distribution - Mountain Versus Valley Coverage. 4 Heavy Rains in Colorado June 16 and 17, 1965*
5 The Plum Fire. 6 Precipitation Probability Forecast Verification Summary Nov. 1965 - July 1966. 7 Effect of Diurnal Weather Variations on Soybean Harvest Efficiency. 8 Climatic Frequency of Precipitation at Central Region Stations. 9 Heavy Snow or Glazing. 10 Detection of a Weak Front by WSR-57 Radar . 11 Public Probability Forecasts. 12 Heavy Snow Forecasting in the Central United States(An Interim Report).
13 Diurnal Surface Geostrophic Wind Variations Over The Great Plains. 14 Forecasting Probability of Summertime Precipitation at Denver. 15 Improving Precipitation Probability Forecasts Using the Central Region Verification Printout. 16 Small-Scale Circulations Associated With Radiational Cooling. 17 Probabi1ity Verification Results (6-Month and 1 8-Month). 18 On The Use and Misuse Of The Brier Verification Score. 19 Probability Verification Results (24 Months).
20 Radar Depiction of the Topeka Tornado. 21 Wind Waves On The Great Lakes. 22 Seasonal Aspects of Probability Forecasts: 1. Summer. 23 Seasonal Aspects of Probability Forecasts: 2. Fall. 24 The Importance of Areal Coverage In Precipitation Probability Forecasting 25 Meteorological Conditions As Related To Air Pollution, Chicago, Illinois, April 12-13, 1963 - 26 Seasonal Aspects of Probability Forecasts: 3. Winter. 27 Seasonal Aspects of Probability Forecasts: 4. Spring. 28 Minimum Temperature Forecasting During Possible Frost Periods At Agricultural Weather Stations In Western Michigan. (Continued on back inside cover) *Prior to T.K. 37 this series was labeled as the ESSA Technical Memoranda. W 9° s U(fl .06/ m-3i U.S. DEPARTMENT OF COMMERCE NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION NATIONAL WEATHER SERVICE
NOAA Technical Memorandum NWS CR-39
A SYNOPTIC CLIMATOLOGY OF BLIZZARDS ON THE NORTH-CENTRAL PLAINS OF THE UNITED STATES
Robert E. Black (USAF)* , oa
LIBRARY ^ N T O L OCT 2 9 2007 National uceai «c & Atmospheric Administration U.S. Dept, of Commerce
CENTRAL REGION
KANSAS CITY, MISSOURI February 1971 F
* FOREWORD
This paper was subedited in March 1968 to Colorado State University,
Fort Collins, Colorado, in partial fulfillment of the requirements for the Degree of Master of Science, and permission was granted to reproduce it in the Central Region T.M. series. The material herein is produced by direct copy but a few pages of text and a number of figures appearing after the material contained herein were omitted.
TABLE OF CONTENTS
Chapter Page I INTRODUCTION AND BACKGROUND INFORMATION. ... 1
Introduction ...... 1 Purpose and Limitations of the Study ... 2
Area of Study...... 3
Period of Study...... 3
Climatology of Blizzards ...... 5
Origins of Blizzards on the North-Central Plains ...... 8
Cyclones...... 8
Blizzards...... 9
II THEORY OF INTENSE CYCLONE DEVELOPMENT AND DEEPENING...... 10
Cyclogenesis Related to Upper Tropospheric Wave Patterns...... 10
Influence of Mountains on Cyclogenesis ... 11
Contribution of the Jet Stream...... 12
III CHARACTERISTICS OF BLIZZARDS ...... 13
Genesis...... 13
Colorado Lows...... 13
Alberta Lows...... 13 Storm Tracks...... 15 Colorado Lows...... 15
Alberta Lows...... 15 Rate of Movement of Blizzards...... 18 TABLE OF CONTENTS (Cont'd)
Chapter Surface Pressure Distribution
Colorado Lows......
Alberta Lows ...... 500-mb Circulation ......
Colorado Lows......
Cyclogenesis ......
Alberta Lows ...... 1
CHAPTER I
INTRODUCTION AND BACKGROUND INFORMATION
Introduction
Blizzards are the most destructive winter storms en countered on the North-Central Plains. They often cause much hardship and economic loss and affect the safety of persons and animals in their path. A storm of blizzard proportions is potentially dangerous to all living animals due to low visibility, strong winds and cold temperatures. Each year several persons perish from exposure during bliz zard conditions. Animals wander ahead of the storm and be come lost or frozen. Often entire herds of livestock or wild animals become smothered by accumulations of fine, packed snow which blows over their shelter. Over 163,000 head of livestock were lost in one blizzard in the Dakotas and Nebraska in 1966 (U.S. Dept. Comm., 1966). Some areas of South Dakota lost 100 percent of their pheasant crop during the same storm^. Another blizzard in late March,
1966 accounted for 34 human deaths in two days. The origin of the term "blizzard" is somewhat obscure. Among the first to use it was a mariner, Henry Ellis, who
wintered on Hudson Bay in 1746. His account of a storm
^Private communication with the South Daxota Depart ment of Fish, Game and Parks. 2 mentioned strong northwest winds, intense cold and the air filled with fine, hard particles of snow (Greely, 1888).
The blizzard, which is considered primarily a wind,
has different names in different geographical areas. In
Siberia it is called a buran or gurga, and in France it is called a blizard or boulbie (Hus^hke, 1959) .
Blizzard conditions were chosen to limit the data
sample. These conditions are defined by the U.S. Weather
Bureau as: wind speed greater than 28 knots, temperature less than or equal to 20F and visibility not greater than
500 ft; a severe blizzard has winds in excess of 39 knots,
temperatures not greater than 10F .and visibility less than
or equal to 100 ft. Visibility restriction may be due to
blowing or drifting snow and/or snow in one of its forms
(HusJjke, op. cit.). Purpose and-Limitations of the Stud*. This study was undertaken to investigate the synoptic climatology3 of
blizzards on the North-Central Plains of the United States
in an attempt to provide further insight into the character
istics of blizzard or near-blizzard situations. Tnere is
little information in the current literature concerning
2"Othe. r meteorological services define blizzards by different criteria. ^„nnnf-ic climatology is defined as the study and i f innate in terms of synoptic weather informa analysis of climate m remit, r funshke. tion, principally in the form of synoptic charts (llushke, op. cit.). 3 characteristics of severe winter storms. Many articles of a general nature have been written about individual storms, but little has been written about the gross synoptic characteristics. Storr (1953) wrote a forecast bulletin for the Department of Transport describing blizzards on the western Canadian prairies. A number of studies on the sub ject of severe cyclogenesis have been published but have only generalized application. Certainly not enough is known about blizzards and further work is needed before
more accurate forecasts will be a reality. This study investigates only the gross characteristics of severe winter storms meeting blizzard criteria. It is
intended only as a basic study which may be tailored for
use at local forecasting levels. Area of Study. The area chosen for the study is the
north-central portion of the United States encompassing all
or a part of 12 states (Figure 1). The region was chosen
for several reasons: first, it limits the data sample;
second, it avoids consideration of effects of the Great
Lakes and mountain ranges; third, the terrain is genera-xy flat or gently rolling plains, so continuous tracking of low pressure systems is relatively easy. Elevations in the
area vary from about 400 ft MSI, in southeastern Illinois
and Missouri to 6200 ft MSL in southeastern Wyoming. Period of Study. The data sample includes the five months of November through March for the years 1957 through
1967. This was done to limit the total number of storms F 5 investigated. Quality of data from this period is fairly consistent.
Climatology of Blizzards
According to Haurwitz and Austin (1944) most blizzards
occur in Koeppen’s type D climates (Koeppen, 1931). D cli
mates are characterized by winters with snow cover and def
inite seasonal temperature cycles. They are continental in
nature and, as such, are free from the stabilizing influence
of large bodies of water. There are two major subtypes of
the D climates—the Df, or moist winter, and the Dw, or dry
winter. Blizzards are more frequent in Df climates. Only
the Northern Hemisphere has land masses large enough to
provide continentality for D climates. In the United States
D climates are found east of 100W longitude and north of
38N latitude. In the United States, blizzards occur most frequently
in the northern Great Plains and upper Mississippi Valley.
They can, however, occur southward over the plains to Texas
and Oklahoma and westward to the Pacific Coast ranges. They also occur eastward over the Great Lakes to the New
England states. Blizzards in the United States can occur from the first'of October to the end of April but most frequently occur from early November to the end of March—
the time period considered here. A total of 53 blizzards occurred in the study area
during the ten years considered (Table 1). Of these, 25 6 were the Colorado (Trinidad) Low type, 20 were the Alberta Low type and eight were classified as "Other.” These eight
Other cases were blizzards caused by rapidly moving high pressure systems, rapidly moving cold fronts, or lows other
than the two mentioned above. Colorado Lows, named for their most frequent origin,
are one of the major year-around weather producers in the
United States. They form in one of the most favorable
areas of cyclo^genesis in the U.S. and then migrate onto
the plains. These lows occur an average of 39 times
annually from November through March (Sands, 1966). Of
these 39 lows, usually two to three storms are blizzards.
Table 1. Frequency of occurrence of blizzards by year*.
Total Frequency Year** Colo. Low Alba. Low Other of Occurrence
1957- 1958 1958- 1959 1959- 1960 1960- 1961 1961- 1962 1962- 1963 1963- 1964 1964- 1965 1965- 1966 1966- 1967 Totals *Data from U.S. Dept. Comm, a, b, 1966, and private communi cation with state climatologists. *A year is defined as a blizzard season, from 1 November to 31 March. 7
Alberta Lows, also.named for their favored geographi cal origin, occur most frequently of all lows over western North America. Average frequency of occurrence for this
system during the blizzard months is 42 (Sands, op. cit.) .
These systems cause storms on both sides of the Canadian-
U.S. border but most frequently in the prairie provinces of
Canada. Approximately two blizzards a year are caused by
Alberta Lows. March had the greatest number of blizzards (14) during
the 10 year sample; 10 of the 14 were the Colorado type (Table 2). The greatest number of Alberta Low blizzards
occurred during December, January and February, the months
when the Canadian storm track is farthest south. In
November, as in March, there were more blizzards from
Colorado Lows than from Alberta Lows. The storm traces
are in more northerly positions during these months.
Table 2. Frequency of occurrence of blizzards by month*.
Total Frequency Month Colo. Low Alba. Low Other of Occurrence
November 5 2 0 7 December 4 5 1 10 January 2 6 3 11 February 4 4 3 11 March 10 2 2 14
*Data from U.S. Dept. Comm, a, b, 1966, and private communi cation with state climatologists. 8
North Dakota and South Dakota had the greatest number of blizzards—the result of their position with respect to the paths of both storm systems. Except for South Dakota, states not along the U.S.-Canadian border, did not have blizzards caused by Alberta Lows (Table 3).
Table 3. Frequency of occurrence of blizzards by state*.
Total Frequency State Colo. Low Alba. Low Other of Occurrence 1 Colorado 0 1 2 0 1 3 Illinois 12 Iowa 10 0 2 4 0 2 6 Kansas 14 Minnesota 8 3 3 2 Missouri 2 0 0 11 Montana 1 8 2 14 0 3 17 Nebraska 24 N. Dakota 8 13 3 12 6 5 23 S. Dakota 8 Wisconsin 6 0 2 2 Wyoming 2 0 0
Data from U.S. Dept. Comm, a, b, 1966, and private commun ication with state climatologists.
Origins of Blizzards on the North-Central_Plajjis
Cyclones. Low pressure systems on the North-Central
Plains usually result from eastward migration of cyclones from the lee side of the Rocky Mountains. These lows are of various origins along the Rockies from southern Colorado
to west-central Alberta. Cyclones are generally accompanied by a trough aloft and tend to follow the flow at 500 mb. Lows which form in 9 the southern portion of the region generally move east- northeasterly, while those from Alberta move easterly or east-southeasterly. Blizzards. Blizzards on the Central Plains are generally caused by deep or deepening cyclones. Since only a small portion of the lows crossing the area in the winter develop into blizzards, a system of large intensity
is necessary in order to produce the strong winds. Cyclones
developing to blizzard intensity originate in three geo graphic areas—southern Alberta, east-central Wyoming and
southern Colorado. Since no Wyoming Low blizzards occurred
during the ten year period, they will not be discussed
further. 10
CHAPTER II
THEORY OF INTENSE CYCLONE DEVELOPMENT AND DEEPENING
Cyclogenesis Related to Upper Tropospheric Wave Patterns
Many factors contribute to cause intense cyclo genesis and deepening which is necessary to produce bliz
zards. One of the best known theories describing the pro
cess is Sutcliffe's development equation as presentee by
pettersen (1956). If one considers a two level model of
the atmosphere with divergence aloft and convergence in the
lower levels, there must be a level where the divergence vanishes. This is called the level of nondivergence (LND)
Under conditions of conservation of angular momentum, pro duction of surface vorticity occurs. One can consider the
vorticity equation in the form
at <« + f)o = ' Do (c + f)o (1)
where C is relative vorticity, f is the Coriolis parameter,
D is divergence and the subscript "o" refers to the surface
level. Considering the thermal wind, WT, ana the vorticity
of the thermal wind, *T, equation (1) can be rewritten as
(2) 3 U + f)0 = - \Y- VU + f) - at ^t 31
where y is the horizontal wind at the level of nondiver gence. Nonsubscripted parameters refer to the level of
nondivergence. The local rate of change of absolute vor
ticity results from the advection of absolute vorticity at 11 the LND as well as the local rate of change of the vorticity of the thermal wind. Cyclonic vorticity advection usually occurs in that portion of the long wave between the trough line and the adjacent downstream ridge. The second term on the right in equation (2) is the local rate of change of the relative vorticity of the thermal wind. It is propor tional to the thickness advection from the surface to the
LND and is an expression of the mean warming of this layer.
The rate of change of CT is a function of the increase m the temperature of the layer due to processes such as ad vection of warmer air, local heating and latent heat release
from condensation. With southerly flow at the surface, warm air is advected into the layer from the surface to the LND.
Pettersen (op. cit.) states that both a large positive vor
ticity tendency and positive vorticity production are
found in the region of maximum warm air advection.
Influence of Mountains on Cyclogenesis
Intense cyclogenesis on the lee side of a mountain range is the result of vertical stretching of an air
column. This is associated with downward motion at the
surface, convergence in the middle and lower levels and the approach of a positive vorticity maximum associated
with a long wave trough. Assuming adiabatic motion, the
potential vorticity equation can be written as
(5 + £L = constant (3) e 8p 12
v/here 6 is potential temperature and p is pressure. Since JL® becomes smaller with convergence, (S + f) must become 3p larger. The center of surface development will occur in the
region where the effects of vorticity advection at the middle and upper levels and warm air advection and vertical
stretching in the lower layers all combine to give the
greatest positive vorticity tendency. Thus, the lee of a
mountain range becomes a very favorable area for intense cyclogenesis when an upper trough approaches from the west.
The extent of cyclogenesis is the primary factor in
determining whether blizzard conditions will occur. Intense cyclogenesis is necessary to obtain the strong surface winds
as well as to cause the required cold air penetration from
the north.
Contribution of the Jet Stream
Cyclones which produce blizzards are associated with
jet stream maxima. The high speed center of the jet
stream passing across a trough line is apparently associ ated with the intense cyclogenesis necessary for a blizzard
Furthermore, the passage of the high speed center must occur when the trough is located in a position favorable
for cyclogenesis in the Central Plains region. Because
this combination of events occurs infrequently, severe
blizzards are a relatively rare phenomenon. 13
CHAPTER III
CHARACTERISTICS OF BLIZZARDS
Genesis
Colorado Lows. Colorado Lows develop in statistically
favorable areas of Southwestern United States over the
Great Basin, the Colorado Plateau, and to the lee of the
Colorado and New Mexico Rockies (Figure 2) . The three cyclones that formed in the Great Basin of Nevada and Utah
were a result of cyclogenesis associated with a deep trough
off the west coast. Those lows whose origin locations were
in southern Arizona and New Mexico developed from either a wave on a Pacific cold front or as an extension of the semi
permanent lew over north-central Mexico. The remainder of
the cyclone origins, in Colorado and Texas, resulted from
cyclogenesis in the lee of the Rockies. Alberta Lows. Alberta Lows result either from migra
tion of strong lows formed in the Gulf of Alaska and along the western Canadian coast or as a result of cyclogenesis
to the lee of the mountains. Four.of the origins shown in
Figure 2 were the result of strong low pressure formation off the west coast of British Columbia, and three resulted
from cyclogenesis to the lee of the mountains. Three cyclones originated near northern British Columbia as a
result of development of a wave along the Arctic Front. Four more blizzard lows developed from cyclogenesis to the
lee of the Rockies. These developments resulted from lows Figure 2. Origins of lows causing blizzards on the Nortn Central Plains. £ Alberta Lows. Q Colorado Lows. 15 filling off the west coast of Washington or British
Columbia and redeveloping in an area from Calgary (YC) to
Great Falls (GTF).
Storm Tracks
Colorado Lows. Individual Colorado Low blizzards tend to follow the mean storm track (Klein, 1957) across the Central United States (Figure 3). Less severe bliz
zards migrate toward central Kansas, then along the mean track into Michigan. More severe storms migrate in a more northerly direction—from Colorado, across Nebraska, into
Minnesota. These migration tracks are primarily due to the
deep trough associated with development of these severe
storms. Cyclones forming in southern New Mexico and Arizona
tend to migrate around the southern edge of the Rockies. Those whose genesis occurs over the Great Basin or Colorado
Plateau migrate through the lower elevations across southern
Wyoming. Lows which form in the lee of the Rockies migrate
almost directly eastward during their early stages. Alberta Lows. In nearly all cases the track of an Alberta Low is east-southeasterly from its origin. These lows generally follow a path from central British Columbia
to central Minnesota. As the low center reaches the Great
Lakes, it recurves to the northeast. This path is con siderably farther south than the mean storm track for the
months of November through March (Figure 4). Since tms study considers only blizzards in the United States, the \ O M fS 16 17
F 18 data are biased toward a storm track more southerly than the mean track. Alberta Lows having their origins off the west coast of British Columbia migrate through the lower elevations near Hazelton, British Columbia (HZ), along the mean storm
track. Those with origins near Fort Nelson (YE) move
southeast for a great distance before turning east. Lows
which result from lee-side cyclogenesis move east-southeast
until they near the eastern Dakotas then turn east or
northeast. Alberta Lows generally move more rapidly than Colorado
Lows. Once away from their origin, there is almost no
tendency for them to stagnate or meander. More rapid move
ment plus less tendency to stagnate makes blizzard con
ditions associated with Alberta Lows generally less severe
and of shorter duration than Colorado Low blizzards.
Rate of Movement of Blizzards. During the early
stages of a Colorado Low the cyclone moves quite rapidly
across the southern portion of the study area. As the
cyclone approaches the eastern portion of the region, it
is, or is becoming, an old storm and slows from 30-45 knots
to an average speed of 10-20 knots. Alberta Low blizzards move at an average speed of 35
knots with a range of 15 to 80 knots. Colorado Low bliz
zards move at an average speed of 31 knots with a range or
10 to 74 knots. Both blizzard systems move more rapidly
than the estimated 26 knots of general low pressure systems
(Bvers, 1959) . 19
Surface Pressure Distribution
Cyclones which develop into blizzards exhibit certain surface characteristics which may aid in evaluating or pre dicting their behavior. The central pressure of a blizzard in the study area averages about 13 mb lower than that of a typical non blizzard cyclone with physical dimensions nearly the same.
Colorado Lows which produce blizzards had an average lowest central pressure near 983 mb; the Alberta type cyclones which produce blizzards averaged near 988 mb. The differ ence in average central pressure between Colorado and
Alberta Lows accounts in part, for the greater ferocity of
the Colorado Low blizzard. Colorado Lows. Cyclogenesis occurs much more rapidly
for cyclones which produce blizzards than for nonblizzard
producing lows. This is an important predictor in early
deepening. Rapid deepening of Colorado Lows generally
occurs over the Rockies near their origin and can on.en be
detected within the first 24 hours after cyclogenesis.
The storm system reaches its lowest central pressure
in the northeastern portion of the area--near the Great Lakes. Stagnating or meandering cyclones reach their lowest
pressures in the central portion of the area. Figure 5a-e is a series of typical surface cnarts at
12 hour intervals showing the development of a low into a 20
b Pc = 997mb Figure 5. Typical surface pressure distribution associate . with Colorado Lows which produce blizzards. i»nz zard conditions started at A+24. Charts are at 12 hour intervals. Pc is the central pressure of pri mary cyclone. AP = 4 mb. 21
Figure 5. (Continued). 22
Figure 5. (Continued). 23 blizzard. Figure 5a (A+00 hrs) ^ shows a low with its origin location in eastern Arizona-western Mexico. Twelve hours later (A+12) the low has started to deepen and move northeasterly. Twelve hours after the cyclone has started to move (A+24), it is sufficiently intense to produce the strong winds necessary for blizzards. When blizzard con ditions are observed, they occur to the north and northwest of the low center. The winds are normally from northeast through northwest. At A+36 most of the western portion of
the area of study is experiencing blizzard or near blizzard
conditions. At A+48 (Figure 5e) only the northern portion
of the area is affected by the deep cyclone. Alberta Lows■ The area of lowest central pressure reached by Alberta Lows is not well defined. There is a
tendency for the lowest central pressure to occur in the
area between 100W and the Great Lakes, but it has also
been observed to occur with lee-side cyclogenesis near
Great Falls (GTF) or Medicine Hat (XH). Figure 6a-d is a series of typical surface charts at
12 hour intervals depicting the development of an Aloerta Low into a blizzard. As the surface system of Figure 6
(A+00) approaches the area of study, it begins to deepen. Twelve hours later (A+12) strong north winds occur west and
^^.fOO hrs refers to initial map time. Subsequent map times are indicated by A plus a two digit number indicating the number of hours elapsed since the initial map. 24
mary cyclone. AP = 4 mb. 25
Figure 6. (Continued). 26 northwest of the cyclone center behind the fronts. At. A+24
the strong winds and blizzard conditions have spread over much of the western portion of the study area. Blizzard
conditions also exist north of the low center. By A+36
(Figure 6d) only the northeast section of the area is
affected by winds strong enough to cause blizzard
conditions.
500-mb Circulation
Colorado Lows. There are wide variations in the
actual shape and rate of movement of the circulation pat
tern at 500 mb which account for variations in location of
cyclogenesis as well as in individual storm traces. Circulation at 500 mb associated with the Colorado Low, however, is quite distinctive. Figure 7a-e is a typi
cal series of 500-mb charts at 24 hour intervals showing
this circulation. Approximately 36 hours prior to cyclo
genesis in the Great Basin or Colorado Plateau, a trough
will appear off the western coast of the United States. Also, there will be a ridge over the Central Plains and
another trough either over the eastern seaboard or the
western Atlantic Ocean (Figure 7a). During the ensuing 48
hours, the trough will move inland to a position over the
eastern Great Basin or Colorado Plateau (Figure 7b-c).
About this time a closed low will develop in the trough.
A cold tongue of air will accompany the trough through ius
migration eastward, with the axis of the cold tongue 27
hours. — are height contours (Ah=60m), -- are isotherms (AT=5C) . 28
Figure 7. (Continued) . Figure 7. (Continued). 30 coinciding closely with the trough line. Cyclogenesis will usually occur at this time (A+48). Within 24 hours after cyclogenesis (A+72), the closed low or trough aloft will move into eastern or central Colorado or Nebraska. The trough line at this time will be about 200 to 300 n.mi. to the west or southwest of the surface low, depending upon the rate of movement of the surface system (Figure 7d). Thirty- six hours after cyclogenesis the 500 mb low position essen tially coincides with the surface low position indicating
that the storm is no longer young (Bjerknes and Holmboe,
1944). Later (A+96) the closed low will move farther from
the cold air source but will continue to remain nearly above
the surface low position (Figure 7e). Sixty to 72 hours
after cyclogenesis the system will no longer affect the
study area unless stagnation occurs. Cyclogenesis. After cyclogenesis commences, the
tongue of cold air or the cold dome will lag behina the
movement of the upper trough or closed low. Approximately
six to twelve hours prior to cyclogenesis, cold domes rorm followed by a general shrinkage in area (Figure 8). Shrink
age of the closed isotherm indicates subsidence of the cole"
air aloft. Sinking of the cold dome is a source of energy needed for cyclogenesis. According to Riehl and Teweles
(1953) strong cyclogenesis frequently follows the appearance of elongated and nearly isolated cold domes aloft over the
northwestern United States. 31
dome
cold
the
of
position.
subsidence low
showing surface
the . isotherm
-25C
represents
mb, □
500
1960.
the
of
December
4
-
3 Shrinkina
8.
F.iqu^e 32
Alberta Lows. Circulation at 500 mb associated with an Alberta Low does not have the readily distinctive features that are characteristic of the Colorado Low. Pre blizzard flow is normally one of two patterns. The flow may be nearly zonal from British Columbia to the Central
Plains with a shallow trough over the Plains which steers the storms into the Plains region. A second typical pat tern might be a trough off the west coast which migrates inland.
Nearly zonal flow over British Columbia and Washington prior to cyclonic development will frequently be perturbed by short waves. These perturbations may aid either in development of the surface cyclone or in deepening of the shallow trough over the Central Plains. Movement of cy clones is usually more rapid in cases where upper flow is nearly zonal. This results in blizzards of shorter duration than those of the Colorado system.
In the early stages there is greater tilt between the surface and 500-mb features in the Alberta blizzard system than appears later. As the upper air low approaches the eastern Dakotas, Minnesota or Manitoba, stacking becomes more vertical. This occurs approximately 48 to 60 hours
after low formation. Figure 9a-d, a series of 500-mb charts at 24 hour
intervals, is typical of the circulation associated with
Alberta Lows which produce blizzards. Figures 9a (A+00)
and 9b (A+24) show the strong, nearly zonal flow over 33
Figure 9. Typical 500-mb circulation associated with Alberta Lows which produce blizzards. Blizzard conditions started at A+48. Chart interval is 24 hours. — are height contours (Ah=60m), -- are isotherms (AT=5C). 34
Figure 9. (Continued). 35 southwestern Canada into the shallow trough over the Great
Plains. Figure 9c (A+48J shows a short wave over Montana which will cause surface cyclogenesis to occur to the lee of the Canadian Rockies. Twenty-four hours after cyclo genesis (A+72) blizzard conditions will occur over most of the western portion of the area of study. The closed low at 500 mb will be displaced approximately 100 to 150 n.mi. northwest of the surface low. 36
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Office of Forecast Development, 1962: Climological snowfall patterns and synoptic climatology of precipitation of winter storms in the Central U.S., Part I, Heavy Snow Project. Tech. Note. No. 10. Pettersen, S., 1956: Weather Analysis and Forecasting. New York, McGraw-Hill Book Co.,428 p. See Chapter 16.
Reiter, E.R., 1963: Jet Stream Meteorology. Chicago, Univ. of Chicago Press, 515 p. Riehl, H. et al., 1952: Forecasting in the Middle Latitudes. Meteor. Monogr. Boston, Amer. Meteor. Soc., 80 p.
and S. Teweles, 1953: A further study of the relationship between the jet stream and cyclone formation. Tellus, 5, 66-79.
, 1962: Jet Streams of the Atmosphere. Colorado State University, Atmosph. Sci. Tech. Rpt. No. 32. Sands, R.D., 1966: A Feature-of-circulation Approach to Synoptic Climatology Applied to the Western United States. University of Denver, Publications in Geography No. 66-2, 332 p. Storr, D., 1953: An investigation into means of forecasting blizzards on the Western Prairies. Cir-2278 TEC-345 Dept, of Transport, Met. Div. U.S. Department of Commerce, a: Climatological Data for 12 states, November-March, 1957-1967. U.S. Department of Commerce, b: Storm Data,- 1959-1967.
U.S. Department of Commerce, 1966: Some Outstanding Blizzards. Pub. LS 6107. Washington, D.C., U.S. Government Printing Office. 38
U.S. Department of Commerce, 1967: Heavy snow forecasting in the Central United States. Central Region Tech. Memo. No. 12.
U.S. Weather Bureau, 1954: A collection of reports on the storm of.November 6-7, 1953. t
29 An Aid For Tornado Warnings. 30 An Aid in Forecasting Significant Lake Snows. 31 A Forecast Aid for Boulder Winds. 32 An Objective Method For Estimating The Probability of Severe Thunderstorms. 33 Kentucky Air-Soil Temperature Climatology. 34 Effective Use of Non-Structural Methods in Water Management. 35 A Note On the Categorical Verification of Probability Forecasts .
36 A Comparison of Observed and Calculated Urban Mixing Depths . 37 Forecasting Maximum and Minimum Surface Temperatures at Topeka, Kansas, Using Guidance from the PE Numerical Prediction Model (FOUS).
38 Snow Forecasting for Southeastern Wisconsin