Hello, and welcome to the National Weather
Service Seattle, Washington office presentation
on the Puget Sound Convergence Zone.
The Convergence Zone is a local phenomenon
that allows the Puget Sound area to experience
prolonged periods of rainfall, thunderstorms,
and even snow!
To outline the contents of this lesson: first
we’ll give a definition, [*] we’ll talk
about the ingredients required, [*] how it
works, [*] why it causes precipitation and
[*] what kind of precipitation.
[*] Then we’ll take a closer look at an
actual Puget Sound Convergence Zone event.
[*] Lastly, we’ll give you some safety tips
[*] and conclude our lesson with a summary.
Let’s start with a general definition.
[*] A convergence zone occurs when air converges
over Puget Sound.
Convergence means ‘collide’, but we’ll
talk about this more in the upcoming slides.
[*] This convergence causes rain, thunderstorms,
and even snow, as you will see!
[*] You can see a convergence zone in the
radar image to the right.
The red arrow is pointing out an area of precipitation
over Seattle detected by a weather radar.
A radar image can tell you where it is raining
in an area.
Colors such as green, yellow, and red show
increasing intensities of rainfall or precipitation!
In order for a convergence zone event to occur,
a few “ingredients” are required.
[*] Winds need a westerly component in order
to encounter [*] a barrier with plenty of
[*] moisture present.
Meteorologists like to define winds by the
direction that they are coming FROM.
This is because we like to know where we’re
getting our weather.
[*] These winds are coming from the west,
towards the east, so they are called ‘westerly’
winds.
[*] Our barrier, in this case, is the Olympic
mountains on the west coast of Washington.
[*] Finally, we need moisture in order for
clouds and precipitation to form.
[*] When the air cannot easily travel over
the mountains, it must find another way to
continue on its path towards the east.
[*] Usually, the easiest way for the air to
do this is to go around the mountains on both
sides, [*] as you can see in this image.
When the air meets on the other side of the
mountains, [*] it is called “convergence”
because the air is colliding.
[*] This convergence causes precipitation.
Precipitation is a term for forms of water
that falls from clouds, such as rain or snow.
[*] When air converges it is forced to rise,
which results in clouds and precipitation
(as long as moisture is available).
This short animation will show you that, where
there is convergence, there can be precipitation!
[* ten times]
[*] This short cartoon will show you how convergence
causes clouds and precipitation in some areas,
and clearing in other areas.
[* 8 times] After the air has traveled around
both sides of the Olympics, [* 9 times] it
curves towards the Seattle area (often to
the north of Seattle).
[*] Once the air collides, [* 7 times] it
begins to rise.
When moist air rises, it eventually reaches
its dewpoint, [* 3 times] which means that
water can begin to condense and clouds can
form.
When cloud droplets get big enough, [* 4 times]
they fall out as rain or snow.
[* 6 times] The air is then pushed out the
sides of the clouds and descends to the north
and south of the convergence zone area.
Since the air has lost some of the moisture,
and it is warming as it descends, [* 5 times]
clear sky areas can be seen (as we will see
in a satellite image later).
The convergence zone can even result in snow
in the puget sound area if freezing levels
are low enough.
Also, an unstable airmass in the area can
allow for destructive thunderstorms complete
with hail and lightning!
Now we have pictures of satellite, radar,
and model output from the national weather
service for a convergence zone event on the
afternoon of July 28th, 2001.
[*] The first image is a visible satellite
image along with local observation station
reports.
[*] The next image is a radar image along
with mean sea level pressure.
[*] Notice the clear areas to the north and
south of this convergence zone event.
The radar shows the highest reflectivities
in the convergence area.
Here are some safety suggestions to keep in
mind during a convergence zone event.
[*] Be aware of the risk for flooding during
heavy rain events, particularly ponding on
roadways, and watch out for thunderstorms
with lightning and hail. 
If cold enough, snow is also possible with
a Puget Sound convergence zone.
[*] Remember to always keep your weather radio
nearby and listen for updates from your local
National Weather Service office.
[*] And, as always, be careful.
In conclusion, [*] recall that a Convergence
Zone is caused by air colliding (or converging)
on the lee side of the Olympic Mountains (over
Puget Sound!).
[*] This convergence causes the air to rise,
forming clouds and rain.
[*] This can lead to thunderstorms and even
snow in the Puget Sound area!
Make sure to check us out on your favorite
social media site or get your latest forecast
at www.weather.gov/seattle.
