F.A.Q.’s

      So here in the weather center we have compiled a list of weather related questions that we have been picking at for years.  This past April, Mark and Drew pushed us all to finally complete them.  So what happened then you might ask?  Well…. nothing, we have a few ideas for them but the Blog is going to get them first.  Eventually we will get these in a list with the answers linked so you ‘ have to scroll through everything.  Possibly in the future there will be video explainer’s as well.   

     Hope you enjoy them, if you have any general weather questions you might want us to add, send any of us an email or post them as a comment below.

-Meteorologist Rob Martin

 

Local Weather Terminology

 

What is a marine push

A marine push occurs when cool air sitting above the Pacific Ocean surges inland, pushing through the gaps in the Coast Range (and along the Columbia River) into western Oregon and Washington.  Common in the summer, a marine push usually results in cooler conditions in the interior valleys west of the Cascades and often times low level clouds.

During the summer (especially during the afternoon), the air right over the land is much warmer than the air right above the Pacific Ocean.  As a result, the air pressure over the warm land is lower than the air pressure over the cool ocean.  This pressure difference causes wind, as the air moves from areas of higher pressure towards areas of lower pressure.  This means that, in the summer, the air moves from the ocean towards the land.  This is also called a “sea breeze” or simply onshore flow.

A marine push typically signifies the end of a heat wave west of the Cascades.  A strong marine push often results in a strong west wind along the Columbia River at the east end of the Columbia Gorge – creating some of the best wind surfing and kite boarding conditions in the world near Hood River.

 

What is onshore flow?

Onshore flow simply means that air is moving from the ocean towards the land.  In the Pacific Northwest, onshore flow is occurring whenever the wind is blowing from the west, northwest, or southwest.  During the winter, spring and fall, strong onshore flow results in much heavier precipitation over the higher elevations of the Coast Range and the Cascades than in the lower valleys.  During the summer, onshore flow is Mother Nature’s way of turning on the air conditioner.  The cooler “sea breeze” of ocean air provides relief from the intense heat.

 

What is offshore flow?

Opposite of onshore flow, offshore flow means that air is flowing from the land towards the ocean.  In the Pacific Northwest, offshore flow is occurring whenever the wind is blowing from the east, northeast or southeast.  During the winter, offshore flow usually means that a bitter east wind is blowing throughout the western Columbia River Gorge and into the Portland area.  In the summer, heat waves are the result of that easterly wind.

 

What is a pineapple express?

Pineapple express is the term used to describe a mild and very wet weather pattern in the Pacific Northwest.  When this weather pattern is in place, a trail of clouds and rain extends from the Northwest all the way back to Hawaii and sometimes back further into the tropical central Pacific.  Most common in the fall and winter, the pineapple express can deliver inches of rain, very high snow levels, high humidity and record high temperatures across Oregon and Washington.  Most of the Pacific Northwest’s historic floods have been a result of the heavy rain and mountain snowmelt caused by the pineapple express weather pattern.

 

What is a thermal trough?

A thermal trough is an area of low pressure caused by intense heating.  It is initially created when the upper air pattern forces a situation where the wind at the lowest levels blows from the east or northeast (offshore flow).  As this air descends the mountains and heads downhill towards the Coast, it gets compressed by the increasing weight of the air above it.  When compressed, air heats up… and when air heats up, the air pressure drops.  The result is a hot area of low pressure…and the beginning of a heat wave for western Oregon and Washington.  After a day or two, the overall upper air pattern will cause eastern Oregon and Washington to get very hot as well… and the thermal trough will shift east of the Cascades.  When this happens, ocean air rushes toward that thermal trough… and western areas get a marine push of cooler ocean air to end the heat wave.

 

What is the jet stream?

The jet stream is a river of air.  Just like a river of water, the jet stream can meander, speed up, slow down, widen and narrow.  It is typically found between 25,000 and 30,000 feet up in the atmosphere, and it is strongest in the winter when the greatest temperature difference occurs between the air at the pole and the air near the equator.  The jet stream interacts with and often steers areas of low pressure (storms) and high pressure (clear weather).

The most common and strongest jet stream is the polar jet stream, which affects the Pacific Northwest almost constantly in the fall, winter and spring months.  When the polar jet dips to our south, it is typically quite chilly.  And when the polar jet remains to our north, it is typically quite mild.  Farther south, there can also be a second jet stream called the subtropical jet.  This jet separates mild air from very warm and humid air near the equator.  When La Nina conditions are present, the subtropical jet often doesn’t exist.  Oppositely, during El Nino episodes, the subtropical jet can be quite active and dominant, directing rain and general storminess into southern California and much of the southern United States.

Similar jet streams can be found in both the northern and southern hemispheres, and all move from west to east (with some meandering).   

 

What is the dew point temperature and how does it relate to humidity?

The dew point is the temperature to which air must be cooled in order for the water vapor in the air to condense into liquid water.  In other words, the dew point is the temperature at which dew forms.

The dew point temperature is related to relative humidity.  We consider the relative humidity to be high whenever the dew point temperature is close to the actual air temperature.  A relative humidity of 100% results when the dew point temperature and the actual air temperature are the same.  In this situation, the air is completely saturated, and it will be cloudy or foggy.

Because the relative humidity is also closely related to actual air temperature, the relative humidity is not a very good indicator of how humid it feels.  For example, the relative humidity could be nearly 100% on a cold & foggy winter morning… yet no one will complain about it being uncomfortably humid outside.

To quantify how humid it actually feels, it is better to look at the dew point temperature.  Use this chart to compare the dew point temperature with human perception and humidity:

 

 Dew Point                         Human Perception                                              Relative Humidity (at 90°)               

Less than 49°                       A bit dry for some                                                      Less than 30%

50° to 54°                              Comfortable                                                                   About 35%

55° to 59°                              OK for most, uncomfortable for some                 About 40%

60° to 64°                              Somewhat uncomfortable for most                      About 45%

65° to 69°                              Very humid, quite uncomfortable                        About 50%

70° and higher                      Extremely uncomfortable, oppressive            Higher than 55%

 

 

What is El Nino and how does it affect weather in the Pacific Northwest?

El Nino, or El Nino Southern Oscillation (ENSO), is the name given to an ocean-atmosphere phenomenon in & over the tropical Pacific Ocean.  First discovered in the 1920s, El Nino is characterized by changes in the pressure and wind patterns over the tropical Pacific.  During an El Nino episode, the air pressure (on average) becomes slighter higher over the western tropical Pacific Ocean (north of Australia).  There is no clear scientific consensus as to why this occurs every three to seven years, but this pressure deviation at the beginning of an El Nino episode causes a chain reaction of other atmospheric & ocean responses:

1)      The normal east-to-west winds that blow over the tropical Pacific weaken.

2)      Because of the weaker wind, there isn’t as much “upwelling” in the ocean.  (Upwelling is part of an ocean circulation are results in cooler water from below being transported up to the ocean surface.)

3)      Without strong upwelling, the ocean surface water becomes warmer than normal – especially in the central and eastern Pacific areas (off the coast of South America).

4)      The warmer ocean water results in more clouds & thunderstorms over the central Pacific and different pressure deviations that, in turn, alter the pressure and weather patterns all across the planet. 

When the tropical Pacific Ocean temperature remains warmer than normal (by at least 0.5 degrees Celsius) for at least five months, then the warming episode is officially classified as an El Nino.

The impacts of an El Nino episode are felt globally.  Rain frequently falls in the normally dry western South American deserts.  Areas in northern Australia and Southeast Asia turn drier.  And in the United States, the southern tier of states (California & Arizona east through Texas to Florida) typically experience wetter and cooler than normal winter seasons.

In the Pacific Northwest, El Nino most often results in a warmer than normal winter.  Total winter precipitation in the Northwest isn’t significantly impacted by El Nino episodes, as El Nino events have resulted in a nearly equal number of wetter and drier winters.  However, in the Cascades, the snow season tends to start a little earlier than usual (but not always) when El Nino is occurring.

 

More information about El Nino and La Nina:

http://www.cpc.noaa.gov/products/analysis_monitoring/ensocycle/nawinter.shtml

 

What is La Nina and how does it affect weather in the Pacific Northwest?

La Nina is the opposite phenomenon of El Nino.  During a La Nina episode, the air pressure (on average) becomes slighter lower over the western tropical Pacific Ocean (north of Australia).  There is no clear scientific consensus as to why this occurs every three to seven years, but this pressure deviation at the beginning of a La Nina episode causes a chain reaction of other atmospheric & ocean responses:

1)      The normal east-to-west winds that blow over the tropical Pacific strengthen.

2)      Because of the stronger wind, more “upwelling” in the ocean occurs.  (Upwelling is part of an ocean circulation are results in cooler water from below being transported up to the ocean surface.)

3)      Because of the increased upwelling, the ocean surface water becomes colder than normal – especially in the central and eastern Pacific areas (off the coast of South America).

4)      The colder ocean water results in fewer clouds & thunderstorms over the central Pacific and different pressure deviations that, in turn, alter the pressure and weather patterns all across the planet.

When the tropical Pacific Ocean temperature remains colder than normal (by at least 0.5 degrees Celsius) for at least five months, then the cooling episode is officially classified as a La Nina.

The impacts of a La Nina episode are felt globally.  Areas in northern Australia and Southeast Asia become much wetter than normal.  And in the United States, the southern tier of states (California & Arizona east through Texas to Florida) typically experience very dry & warm conditions during the winter.

In the Pacific Northwest, La Nina most often results in a wetter than normal winter.  Cascade snowfall totals can be significantly above normal during strong La Nina episodes.  In the lower elevations, temperatures during La Nina winters are usually near normal or cooler than normal.

More information about El Nino and La Nina:

http://www.cpc.noaa.gov/products/analysis_monitoring/ensocycle/nawinter.shtml

 

 

What is the freezing level and is it the same as the snow level?

The freezing level is the elevation where the temperature is 32 degrees.  It is the boundary that divides above-freezing air from below-freezing air.

Occasionally during the winter months, there can be more than one freezing level.  When cold, below-freezing air is trapped in the lowest valleys, the first freezing level will mark the boundary between the cold, below-freezing air and a middle layer of milder air.  Higher up, a second freezing level, called the free-air freezing level, is the level where the air drops below-freezing again.

The snow level is usually defined as the lowest elevation at which you’ll find snow sticking to surfaces.  The snow level is often about 1,000 feet below the freezing level.  Depending on the weather pattern at the time, the temperature at the snow level is usually between 33 and 36 degrees.

What is hail?

Hail is formed in heavy showers where the air is violently moving up and down above and below the freezing level.  Hail begins its life as a liquid raindrop that encounters a strong updraft of wind inside its home cloud.  If the raindrop is lifted above the freezing level, then the raindrop will freeze into an ice pellet.  With each up-and-down cycle, additional water collects on and freezes onto the growing ball of ice.  Eventually, the ball of ice falls to the ground as hail.

 

What is sleet?

Sleet is another name for ice pellets.  Formed differently than hail, sleet only occurs when the temperature at ground-level is at or below freezing. 

Sleet begins its life as snow.  As the snow falls into warmer, above-freezing air, the snow melts into liquid rain.  If there is a layer of colder, below-freezing air trapped near the ground, then the raindrops may freeze into ice pellets before they reach the ground.  If this happens, then it is sleet.  However, if the layer of cold, below-freezing air near the ground is very shallow, then the liquid raindrops may not have time to freeze into ice pellets before they reach the ground.  In this situation, the liquid raindrops hit the below-freezing ground and quickly freeze into ice.  This is freezing rain

 

What is freezing rain?

Freezing rain is simply regular, liquid rain that freezes into a glaze of ice once it hits the ground.  Freezing rain only occurs if the ground temperature is at or below freezing. 

Freezing raindrops begins there life as a snowflakes.  As the snow falls into warmer, above-freezing air, the snow melts into liquid rain.  If there is a layer of colder, below-freezing air trapped near the ground, then the raindrops may freeze into ice pellets before they reach the ground.  If this happens, then it is sleet.  However, if the layer of colder, below-freezing air near the ground is very shallow, then the liquid raindrops may not have time to freeze into ice pellets before they reach the ground.  In this situation, the liquid raindrops hit the below-freezing ground and quickly freeze into ice.  This is freezing rain… and an ice storm is the result. 

 

What are showers and how are they different than regular rain?

When the forecast calls for “rain”, it means that a period of steady, continuous rain is expected.  Showers can best be described as “off and on rain”.  Large-scale weather systems often bring rain, while smaller, cellular-type clouds bring showers… sometimes with breaks of sun between them.  The intensity of rainfall can range from very light to very heavy in showers, whereas the intensity of steady rain is typically light to moderate.

 

Climate

 

Which month is the driest?

Typically, the driest month in the Pacific Northwest is July.  In Portland, the average July precipitation total is 0.63 inches.

Which month is the wettest?

Typically, the wettest month in the Pacific Northwest is December.  In Portland, the average December precipitation total is 6.13 inches.

Which month is the coldest?

Typically, the coldest month in the Pacific Northwest is January.  In Portland, the average January high temperature is 46 degrees and the average January low temperature is 34 degrees.

Which month is the warmest?

Typically, the warmest month in the Pacific Northwest is August.  In Portland, the average August high temperature is 80 degrees and the average August low temperature is 57 degrees.

What time of day is it usually the coldest?

Typically, the coldest time of the day is in the early morning just after sunrise.

To understand why this is the case, one must first understand how the earth heats and cools.  During every hour of the day and night, the earth is radiating energy outward towards space.  This is a cooling process for the earth.  But during the daytime hours, areas exposed to the sun are receiving its intense energy.  These areas experience warming, as the energy received from the sun exceeds the energy lost to space.

At night, there is no energy received from the sun.  Yet, the earth continues to emit energy into space.  So the air cools during the night, and it continues cooling until the sun’s energy becomes strong enough to counteract the earth’s cooling – which is usually right after sunrise.  After that, sun’s heating overpowers the earth’s cooling – and the temperature rises.

Weather factors such as wind, clouds, cold fronts and warm fronts can also have a significant effect on the temperature… which is why the coldest time of the day isn’t always in the early morning.

 

What time of day is it usually the warmest?

Typically, the warmest time of the day ranges from mid-afternoon to early-evening.

To understand why this is the case, one must first understand how the earth heats and cools.  During every hour of the day and night, the earth is radiating energy outward towards space.  This is a cooling process for the earth.  But during the daytime hours, areas exposed to the sun are receiving its intense energy.  These areas experience warming, as the energy received from the sun exceeds the energy lost to space.  This warming continues until the sun’s energy is no longer strong enough to counteract the earth’s cooling – which typically occurs between 4:00 and 6:00 p.m.  After that time, earth’s cooling overpowers the sun’s heating – and the temperature drops.

Weather factors such as wind, clouds, cold fronts and warm fronts can also have a significant effect on the temperature… which is why the warmest time of the day isn’t always in the late afternoon or early evening.

 

What are the most common wind directions in the winter (in the Portland area)?

The most common wind directions during the winter months are east winds (during dry spells and when storms are approaching) and south winds (during stormy periods).

What is the most common wind direction in the summer (in the Portland area)?

The most common wind direction during the summer months is a northwest wind.

Where can I find detailed local climate information?

The most detailed local climate information is maintained by our local National Weather Service (NWS) offices.  You can access specific climate information from the following websites:

Portland NWS climate page (serving northwest Oregon and southwest Washington):  http://www.weather.gov/climate/index.php?wfo=pqr.

Pendleton NWS climate page (serving central and northeast Oregon and southeast Washington):

http://www.weather.gov/climate/index.php?wfo=pdt.

Boise NWS climate page (serving eastern Oregon and Idaho): http://www.weather.gov/climate/index.php?wfo=boi.

Medford NWS climate page (serving southern Oregon and northern California):

http://www.weather.gov/climate/index.php?wfo=mfr.

Seattle NWS climate page (serving western Washington):

http://www.weather.gov/climate/index.php?wfo=sew.

 

Local Weather Extremes

What is the hottest it has ever been?

In Portland, the all-time record high temperature is 107 degrees… occurring on three different occasions:  July 30, 1965, August 8, 1981, and August 10, 1981.

In Oregon, the all-time record high temperature is 119 degrees… occurring in Pendleton on August 10, 1898.

In Washington, the all-time record high temperature is 118 degrees… occurring at Ice Harbor Dam on August 5, 1961.

 

What is the coldest it has ever been?

In Portland, the all-time record low temperature is -3 degrees… occurring on February 2, 1950.

In Oregon, the all-time record low temperature is -54 degrees… occurring in Seneca on February 10, 1933.

In Washington, the all-time record high temperature is -48 degrees… occurring on December 30, 1968 at two locations:  Mazama and Winthrop.

5 Responses to F.A.Q.’s

  1. ken says:

    I have noticed that the east winds have started later for the most part each day. I also have been watching the moon and it seem that when the moon gets close to the same spot in the western sky the winds really seem to pick up at that point. Does the moon have the same effect on the high and low pressures that cause the east wind as it does on the tides.

  2. Ken Durham says:

    Re: What is the warmest it ever has been in Washington? -48 degrees?

  3. Very interesting and informative. Thank you!

    Earthquake predicted for Sept. 10th! http://youtu.be/6M7hO8cwfy

    -Della Richmond

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