Thursday, September 11, 2014

Divergent forecasts

The view from my office window today.
The weather station on top of my office building attests to the cold and drizzly conditions.
Overnight, it might even get cold enough to snow in some parts of northeast Colorado. The NWS forecaster wrote:
Area forecast discussion
National Weather Service Denver/Boulder Colorado
754 PM MDT Thursday Sep 11 2014

Update...
issued at 754 PM MDT Thursday Sep 11 2014

Cold moist airmass will remain over northeast Colorado tonight and into Friday afternoon. Northeast winds will continue to usher in colder air. Temperatures look to stay in the 33 to 37 degree range late tonight along the Front Range...as the latest hrrr and rap indicate. However...areas above 6000 feet will likely reach and drop freezing.
I wouldn't be surprised if the snow line falls between my apartment and my office.  (First snow in early September!)  That's the main reason I moved into an apartment complex across the street from the NCAR shuttle stop.  I wasn't keen to drive to an office at 6400 feet elevation in the winter.

Meanwhile, Bad Dad says it is quite warm in the familial home.  The LA Times graphic for the NWS Sunday forecast shows that it will only get toastier. 96F at the beach?
That's not really what the NWS said for coastal Los Angeles. This part is true:
Southwest California area forecast discussion
National Weather Service Los Angeles/Oxnard California
805 PM PDT Thursday Sep 11 2014

Synopsis...
High pressure will bring a warming trend to southwestern California through this weekend and into early next week. Triple digit heat can be expected for the valleys...foothills and Antelope Valley through Tuesday...with Sunday and Monday being the warmest days. A slight cooling trend should then take place by the middle of next week.
But, a subsequent paragraph shows a chance of a cooling marine layer near the coast.  Whether it reaches inland in the South Bay is still not clear.
Latest acars sounding near lax indicated a very strong and relatively shallow marine layer inversion around 700 feet deep. At 700 feet the temperature was 70 degrees...at the top of the inversion around 1800 feet...the temperature was 84 degrees. A very strong inversion indeed. Fog product satellite imagery was showing widespread low clouds pushing down the outer waters pushing in to the central coast. The inversion is shallow as well to the north...so expect patchy dense fog to develop along the immediate coast. If fog does spread inland a bit farther...a dense fog advisory might be needed later tonight. Will let the next shift look at this closer. For areas S of Point Conception...confidence is pretty high that low clouds will stay away from the coast...but would not be surprised if some patchy dense fog developed near the la/vtu County coast just before sunrise.
acars refers to Aircraft Communications Addressing and Reporting System
(ACARS), which became briefly famous in the aftermath of the MH370 crash. Until then, I wasn't even aware that ACARS existed for any other reason than collecting weather data.

Every time a plane equipped with ACARS lands or takes off from LAX, we collect a vertical sampling of ambient weather conditions.  From those reports, the NWS forecaster saw a very strong temperature inversion, which functions as a hard thermodynamic lid on the marine layer.

If you want to see ACARS data for yourself, you are in luck. We happen to give it out for free!  Learn more here.

I should make and post a video demonstrating how to request this data. Until then, follow these instructions:

  1. Register for a RDA account (registration is free).
  2. Search for dataset 351.0, and click on the Data Access tab. 
  3. Click on "Get a Subset" under the "Customizable Data Requests" column.  
  4. Enter your desired time range.
  5. Use the pull-down menu under "Select Spatial Subset Preference" to "Select region via Google map". Click "Submit Data Request"
  6. You will receive robo-email from my work email address when your data is ready for pickup.  The data will be in human-readable ASCII.

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