January 04, 2021

The Amazing Collapsing Cold Front and the Narrow Cold Frontal Rainband

Cold fronts are features in which cold air is displacing warm air.   As shown below, there is relatively uniform warm air on one side of the front and a zone (the frontal zone) in which temperatures fall on the other (see graphic).


The frontal zone is typically a few hundred miles wide, and frontal zones for most Pacific fronts reaching our shores are wide, weak and diffuse.  

Why?  Because our fronts have been over the HUGE and Pacific Ocean, which modifies the low-level temperatures to be similar to that of the ocean surface.  

Take a look at the sea surface temperatures (SSTs) across the Pacific right now (see below).  A broad swath of water around 8-10°C (46-50F) off our coast and stretching all the way to Japan!.


But sometimes we can get strong fronts, when the winds associated a midlatitude cyclone producing converging airstreams from different directions.   For example, strong southerlies (from the south) in front of the cold front bringing up warm subtropical air, while strong northerlies/northwesterlies behind can move in cool air.    The converging airstreams can produce large temperature gradients (change in temperature over distance) that can be impressive.

Such a situation happened on Saturday afternoon.  And I have some unique, amateur collection weather information that documents the hyperfront for you to view!

Here is the forecast sea level pressures, surface winds, and surface air temperatures at 4 PM Saturday from the UW WRF weather prediction model. Low level temperature (about 800 m above the surface) is shown in the colors (yellow is warmer, darker green cooler). 

 Can you see the profound windshift across the front?  And the area of low pressure associated with it (called a trough)?  


The convergence of air near the surface not only concentrated the temperature contrasts it also produced strong upward motion and precipitation.

We can see the strong showers with the front thanks to the wonderful Langley Hill radar near Hoquiam. The picture below shows the radar image a 4 PM and the oval shows the front.  Orange indicates heavier precipitation, followed by yellow and green.  You see the series of precipitation fatures directed southwest to northeast?  That is the front or more specifically the narrow cold frontal rainband with the front.


The front is made of cores and gaps, with the cores the areas of heavier precipitation.  The frontal contrasts are MUCH stronger in the cores than the gaps, something we learned about during the 1970s and 1980s.

And now the fun part of the story.   The UW weather model has been been shown to skillfully forecast these intense fronts, and based on this forecast, a local weather enthusiast and member of Washington Weather Chaser, Michael Snyder, went to the coast with several Kestrel weather sensors, placing them at varied locations.  These inexpensive probes save weather information every 30 seconds.

Here is a plot of a weather sensor observations for a probe he placed that was was crossed by a weather gap region (blue is temperature and orange is dew point).   Rapid cooling of about 2.5 F in minutes, followed by another 3 F  during the next few hours.


But now lets move inland a bit...to Chehalis...to another amateur station (WeatherUnderground
KWACHEHA17 run by Nick Andrew).   A bit later (9PM), but away from the modifying effect of the water this was quite a front, with a steep initial decline and more gradual cooling afterward.  48 to 40F in hours and a very sharp windshift from southeasterly to northwesterly.


Here at the UW, we did even better:  a drop from around 50F to 38F over 4 hours! (see third panel down, time in UTC).


Now because of the moderating effects of the Pacific, our fronts will never achieve the startling strength of the most intense fronts in the central and eastern U.S., but occasionally, when the set up is right, we can experience fronts that you can palpably sense the decreasing temperature over time.

And all our new technology, from coastal radars, portable sensors, to improved models, give us the ability to diagnose and predict the fronts with greater precision and skill.







4 comments:

  1. Hi Cliff,

    Would be curious your views on the expected upcoming pattern change with a strong ridge over Greenland causing a potentially long-lasting ridge over the Western US. If this verifies, this seems very different to the typical La Nina pattern and that the long term models you shared from ECMWF that indicated below normal temps and higher precipitation for the next month.

    Thanks!

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  2. Good stuff, now let's sample an occluded front that makes landfall on the coast!

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  3. Hi Cliff. Any recommendations on brands or features for a home weather station? Stations offered on Amazon or at Costco all seem to have questionable reviews.

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  4. In a future blog could you explain the polar vortex and why a split is noteworthy? How would a split impact the Pacific Northwest? Thanks?

    ReplyDelete

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