What’s the Weather Like Outside?

• Well, if a Low is coming toward you, it’s about to turn foul

Passage of Cyclone—Fig. 9-13

Weather With a Cyclone--See Fig. 9-13

• You notice clouds are common wherever the air is being forced to rise: ahead of both the cold front and warm front, and wrapped around the Low center

Weather With a Cyclone--Fig. 9-22 (b)

Weather With a Cyclone--See Fig. 9-22 (b)

• When a cyclone is mature (occluded front has already formed), the cloud structure often resembles a comma on satellite imagery

Weather With a Cyclone--Fig. 9-12

Weather With a Cyclone--See Fig. 9-12

• Not surprisingly, the precip is found in the same areas as the cloudiness

Weather With a Cyclone--Fig. 9-13

Zonal Flow

• True-False Question

Zonal Flow

• True-False Answer
• When the flow is zonal, the wind is nearly ____________, and mid-latitude cyclones ________ to develop

Cyclogenesis

• When a wave forms aloft, instead of zonal, the flow is __________--so there are _______ and _______
• This upper-level wave, so important to instigating cyclogenesis, can be generated by surface factors such as ____________, land-sea _______ contrasts, or ocean current influences

Airflow Aloft

• Recall that Low (High) pressure systems cause surface _____________ (_____________) as air flows to the Low (from the High)
• How could the pressure in the Low go down if air is coming into it? Adding air should make the pressure rise

Airflow Aloft

• Something has to take out the incoming air to maintain the low pressure
• ____________ aloft is needed
• Since ____________ is air spreading out, ____________ above the Low takes air out of the column

Airflow Aloft

• If ____________ aloft exceeds surface ____________, the Low will ________ (central pressure will lower)
• If it is less than surface _____________, the Low will ________ (central pressure will rise)

Airflow Aloft

• ____________ aloft occurs downstream (ahead) of the _______
• Therefore, surface Lows often form just downstream of an upper-level trough, centered below the jet stream

Airflow Aloft--See Fig. 9-15

• Convergence aloft occurs as air approaches the trough, so surface Highs are favored upstream of (behind) the upper-level trough

Airflow Aloft—Fig. 9-15

The remaining notes will not be covered in lecture, but you should read them and the relevant pages of the book. A few questions on Test #4 may come from this material. Look for hints as to the most relevant material.

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Patterns of Movement

• Cyclonic disturbances that affect the U.S. often originate in the Pacific Ocean
• As they traverse the Rockies, they sometimes dissipate
• However, they can and do reform lee of the Rockies

Cyclogenesis Areas--Fig. 9-16

Typical Cyclone Paths--Fig. 9-17

Patterns of Movement

• Some redevelop in Canada and move southeast to the Great Lakes before turning northeast
• In winter, these are often referred to as Alberta Clippers because they form in the province of Alberta and move rather quickly--bringing cold air and some snow to the Upper Midwest

Patterns of Movement

• The most common area for Midwestern Lows to form is E Colorado
• These Lows could be reforming from Pacific disturbances, or they could be brand new formations due to unstable Gulf of Mexico air advecting in

Patterns of Movement

• The Colorado Lows generally move SE at first, before taking an easterly path for a bit
• Then they could continue eastward, or they could swing to the NE, or even N
• In winter, when they track east to SE Kansas, and then turn NE through Missouri, that’s when Iowa can get big snowfalls

Patterns of Movement

• Other cyclones can form farther south by New Mexico and Texas
• These usually move eastward and pass to the south of the Midwest
• Cyclones can form east of the Appalachians and turn north along the Atlantic seaboard and cause a "Nor'easter"

Cyclone Migration

• The 500 mb level is often called the "steering level" for mid-latitude cyclones
• The cyclone path tends to be directed by the 500 mb wind directions
• The speed of the cyclone is generally about half as fast as the 500 mb wind speeds

Cyclone Migration--Fig. 9-18

Anticyclonic Weather

• Winter anticyclones often move southeast from Canada to bring cold air (usually a cP air mass) to the United States after a cyclone has passed
• If the anticyclone came from the Arctic, it could be a cA air mass and bring a cold air outbreak to the country

Anticyclonic Weather—Fig. 9-19

Blocking Highs

• Notice in the last picture, the cyclone and fronts to the west were being blocked by the High to the east
• When Highs are strong and cover a large area, they often will not move for weeks and are called Blocking Highs

Blocking Highs

• Once in place, the stagnant Highs block the eastward migration of cyclones, forcing them to move poleward or equatorward instead
• Thus, areas under the High get dry conditions, while other areas continually get Lows and receive a lot of precip

Blocking Highs

• Also, since Highs inhibit vertical motion, and cause light horizontal winds, air pollution episodes often occur in large cities affected by blocking Highs
• The pollutants released cannot rise up and disperse and instead build up near the surface causing humans to breathe them in