Blocking Ridge, Dry Pacific Northwest, and Southern California Rain

 Not a boring time for meteorologists.  New Orleans Airport is closed with record-breaking snow and cold, while the Pacific Northwest is strangely dry and southern California will get its first decent rain of the season.

The Big Easy

All because of a highly amplified upper-level wave pattern.

The upper level (500-hPa) height map (think of this as pressure around 18,000 ft) today shows a ridge of high pressure aloft over the eastern Pacific while a trough of low pressure is found over the eastern U.S. (red indicates high pressure and blue indicates low pressure).  

Ridges/high pressure are associated with sinking air and dry conditions on their eastern sides.  Like over us.   Troughs are associated with rising air and precipitation.  Between them, cold air moves southward.

By Thursday the eastern Pacific ridge becomes crazy strong.  

Keep your sunglasses handy, as it will be sunny and dry.   What a nice break in the middle of winter for us.   But cold and wet over the eastern US.

By Saturday, something interesting...and important... will occur.   A trough will develop south of the West Coast ridge, resulting in a pattern called a Rex Block.  Very stable pattern.

Such a southern California trough will produce upward motion and precipitation.   Southern California will get wet.  Significantly wet.    

Below are the forecast precipitation totals through next Monday evening.  As much as 1.5 inches in the mountains near LA.  Significant rain over the entire coastal zone.

Such rain will moisten the light fuels and greatly reduce the fire threat until the next Santa Ana.  Very good news.

 

Finally, something else occurred today that I thought was interesting.....a very energetic Chinook wind on the western side of the Rockies around Great Falls, Montana.

Two days ago,  shallow cold air was entrenched over the lower elevations of Montana, with lows of -5 to -20F. (see below)

But with high-pressure building over the east Pacific and lower pressure inland, strong winds from the west descended into eastern Montana.  As the air descended the eastern slopes of the Rockies, warming by compression occurred.   This descending air also displaced the frigid dense air that was in place.

Today's low temperatures were MUCH warmer around Great Falls and neighboring locations near the eastern slopes of the Rockies, with highs getting into the 20s!

Check out the temperature and relative humidity observations at Great Falls during the past day (below).

Yesterday morning at 7 AM, the temperature dropped to around -9F.  By noon it was 12F and by noon today is was around 36F.   All associated with air sinking on the eastern slopes of the Rockies.  

   

The UW WRF model simulated this transition well. The forecast surface temperatures for 4 AM Monday were very cold over most of Montana on Monday (white is very, very cold air).  The winds are shown by the wind arrows.  Weak at low levels.

But by 10 PM PST Monday, the white was gone as westerly winds descended the Rockies.  Blue colors are relatively warm.

Chinook winds are also Foehn winds and are sometimes referred to as "snow easters."