Download the extratropical cyclone

CWS 10A - 1
THE EXTRATROPICAL CYCLONE
1. Print this file and associated images. Also answer the Tuesday "Concept of the Day"
questions in the Daily Summary File.
2. Complete the Investigation by responding to the Chapter Progress Questions (Study
Guide binder) and the introductory portion of Investigation 10A from the Weather
Studies Investigations Manual.
After completing the introductory portion of Investigation 10A in the Investigations Manual,
use the WeatherCycler provided in the Study Guide to answer the following questions.
1. Examine the WeatherCycler with the slide pushed all the way in so the L is displayed in
the large window. It shows that when looking down on a low-pressure system (cyclone),
surface winds blow [(clockwise and outward)(counterclockwise and inward)] about the
center.
2. Precipitation generally occurs in a broad band ahead of the [(warm)(cold)] front and in a
narrow band along the [(warm)(cold)] front.
3. To the south and southeast of the center of a midlatitude cyclone, surface winds are likely
to produce [(warm)(cold)] air advection.
4. To the west and northwest of the center of a midlatitude cyclone, surface winds are likely
to produce [(warm)(cold)] air advection.
5. Dewpoints are likely to be relatively high in the warm-air sector of the cyclone.
Consequently, the most humid air is located generally to the [(north)(south)] of the
cyclone's low-pressure center.
6. Illustrate the passage of a warm front by observing the atmospheric changes in the lower
windows below Point B as the WeatherCycler slide is pulled out. With the passage of a
warm front, the air temperature usually rises and the dewpoint would be expected to
[(fall)(rise)]. With the approaching warm front, atmospheric pressure would [(fall)(rise)]
and following the front, [(fall)(rise or remain steady)].
7. A shift in wind direction usually accompanies the passage of a front. The WeatherCycler
map shows that with passage of the warm front, surface winds shift direction from the
_____________ to the _____________.
8. Ahead of a surface warm front, warm and humid air rides up and over a wedge of cooler
air (a process known as overrunning.) As the ascending warm air expands and cools, its
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relative humidity [(increases)(decreases)], and clouds may form. Most cloudiness
associated with a warm front develops over a broad area, often extending hundreds of
kilometers, [(ahead of)(behind)] the front. From these clouds, light to moderate
precipitation typically falls for 12 to 24 hours or longer.
9. As a cold air mass advances and a warm air mass retreats, the colder, denser air forces the
warmer, lighter air to ascend either along or just ahead of the cold front. Uplift of warm
air triggers cloud development and perhaps showery precipitation. In some instances,
uplift is so vigorous that thunderstorms develop. Typically, as shown by the
WeatherCycler, the band of clouds and precipitation associated with a cold front is
[(narrower)(wider)] than that associated with a warm front.
10. With passage of the cold front, surface winds shift direction from the ______________ to
the _____________. After passage of a cold front, the air temperature usually falls and
the dewpoint would be expected to [(fall)(rise)].
11. With the approach of a cold front, air pressure would [(fall)(rise)] and following the front
passage, it usually [(falls)(rises)], often rapidly.
Over the past weekend a storm system came ashore along the West Coast, crossed the
mountains and reformed to the east of the mountains on the plains. This storm has followed
the classic model of a midlatitude low-pressure system or “wave cyclone”.
12. Image 1 is the surface weather map for 12Z 03 APR 2011. At map time, the lowpressure system was still in a formative stage. Two Ls were shown within a closed,
crescent-shaped 992-mb isobar, located over southeastern South Dakota and eastern
Nebraska. From that center, a [(cold)(warm)(stationary)] front extended generally to the
southeast into Tennessee.
13. From the Ls, a [(cold)(warm)(stationary)] front stretched generally westward to eastern
California. The frontal boundary had several additional low-pressure areas marked along
it, shown by the isobar pattern to be an extension of the central Low.
Additionally, an unusually long dryline, shown by the brown open circles, stretched from
the low center generally southward to western Texas and northern Mexico. Note the
insignificant difference in temperature between Midland (69 °F) ahead of the dryline and
El Paso (70 °F) behind it while there is a thirty-four degree dewpoint difference!
14. The wind pattern in the central U.S. around the Low exhibited a generally [(clockwise
and outward)(counterclockwise and inward)] flow as expected. This is also consistent
with the symbols of the advancing front to the west of the Low.
15. Image 2 is the surface weather map for 00Z 04 APR 2011, twelve hours after the Image 1
map. By 00Z, the front extending eastward from the Low was shown as a [(cold)(warm)
(stationary)] front indicating its movement northeastward.
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16. Comparing the 12Z Image 1 map with the 00Z Image 2 map, the cold frontal system
stretching southwestward from the low-pressure center had moved generally toward the
[(northeast)(northwest)(southeast)].
17. The lowest central pressure value on the Image 1 map was 990 mb, labeled twice to the
east and also to the southeast of the two Ls. On the Image 2 map the lowest central
pressure value was labeled to the west of the L in eastern Iowa as 986 mb. Therefore, in
the 12 hours between maps, the central pressure had fallen (decreased) by [(2)(4)(8)] mb
indicating a developing (strengthening) storm system.
18. On Image 2, the wind directions from northern Virginia to Wisconsin were generally
from the [(south)(west)(east)] as would be expected ahead of (preceding) the warm front
of a classic extratropical cyclone.
19. The wind directions from western Minnesota to Nebraska were generally from the
[(southwest)(northwest)(northeast)] as would be expected behind (following) the cold
front of a wave cyclone.
20. In the wave cyclone's warm sector, between the warm and cold fronts, wind directions at
stations from eastern Texas to Kentucky were generally from the [(south)(east)(west)].
Winds behind the dryline were generally strong and from the southwest.
21. Compare the temperature and dewpoint (39 °F and 36 °F, respectively) at Grand Island,
NE, in the cold sector behind the cold front with those at Wichita, KS, in the warm sector
ahead of the cold front. The air was [(warmer and had more water vapor)(cooler and
had less water vapor)] south (ahead) of the cold front.
22. Compare the temperature and dewpoint at Indianapolis, IN, in the warm sector with those
at Detroit, MI, ahead of the warm front. The air was warmer and had more water vapor
[(behind)(ahead of)] the warm front.
23. The air was generally coolest and had least water vapor [(north and east of the warm
front)(in the warm sector between the fronts)(northwest of the cold front)].
24. The area(s) of precipitation associated with this particular midlatitude cyclone, as
represented by the radar echo shadings on the map, was/were located
[(in a broad band ahead of the warm front)(in a narrow band along the cold front)
(in an arc located to the north and northwest sides of the Low center)
(in all of these locations)].
25. Image 3 is the 500-mb upper-air map for 00Z 04 APR 2011, the same time as the Image 2
surface map. Place a bold "L" on the Image 3, 500-mb map where the surface Low
center of Image 2 was shown. The center of the storm system at the surface, as marked
by the L you drew, was located to the east of the axis of an upper air [(ridge)(trough)]
shown on the 500-mb map over the west-central U.S.
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26. Based on the wind directions at 500-mb over the area of the surface storm center, the
system would be expected to move generally toward the [(south-southeast)(eastnortheast)] over the next day with its trailing cold front moving generally southeastward.
27. As a result, persons in Oklahoma, Arkansas and eastern Texas could expect generally
[(cooler)(warmer)] temperatures as the frontal system advanced over the next day or so.
The NWS National Centers for Environmental Prediction (NCEP) also create surface maps
for North America, the continental U.S., Alaska, Hawaii or sections of the country, in either
black and white or color, with or without fronts, and/or satellite or radar data. They are
found from the "NWS Surface Analyses" link on the website under Surface products. Also,
one- and seven-day loops of frontal positions can be created showing the progression of
surface weather features. The National Weather Service Unified Surface Analysis will even
show (and animate) wave cyclones and Highs across most of the Northern Hemisphere. Note
the maps as increasingly intense cyclones form and move across the country with the
progression of spring.
Answer the Current Weather Studies activities on the CWS Answer Form (provided from
the course website on Wednesdays).
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