EVAP! How thirsty is our ground?

Have you been watering your garden or yard less this month?  Some of you may have turned one of these off this week.

Well there is one way to measure the grounds need for water, and that is evaporation measurements.  These are taken on a daily basis from April 1st - September 30th at NWS Spokane near Airway Heights.  We call it EVAP for short.  Here is a picture of one of our forecasters adding water to the pan as part of taking today's measurement.

NWS Forecaster Jeremy Wolf adding water into the EVAP pan.  Picture taken 1 PM PDT July 18th, 2019

The main idea here is that the more water that is being added to the pan, the more "thirsty" the ground will be for water.  Another way of stating it is the more water being added, the more evaporation that has taken place in the past 24 hours.  Hot, dry, and windy weather will increase amount of evaporation while cool, wet, and light wind weather will decrease amount of evaporation.  Occasionally on wet days, we actually have to remove water from the pan, with a negative evaporation value for the day.

So, curious about the numbers?  Let's start with this month's value so far.  We will look at this year and compare it to each year since 2011.  If we sum up all the daily measurements from July 1st through July 17th, here is what we get.

The purple bar is this July showing less EVAP compared to the average line.  And much less compared to 2015 shown in red.  Recall that season was our big fire year.

Curious about June?  Were you watering a lot last month?

Answer: YES!  June EVAP was above average, with only 2015 surpassing it.  Recall that June 2015 Spokane recorded its hottest temperature on record (105 on June 28th).

What if we sum up these two charts.  Here is the June 1st-July 17th EVAP.

So while watering needs have been down so far this July, values since June are slightly above average. 

Expect watering needs to increase into early next week.  Today and Friday will have increased winds, as well as low relative humidity across the Columbia Basin, Spokane area, and palouse.

In fact, the jet stream overhead this morning set a record not just for today but for the month of July and August as noted by the black dot on the image below.

Wind speeds measured from balloon releases at 250mb (~30,000 ft) from all soundings from GEG/OTX from 1948-2014.  Black dot is July 18th, 12z observation

Then temperatures will warm significantly by early next week.

Here is the pattern for Friday, July 19th followed by Monday, July 22nd.

GEFS Forecast of 500mb heights and anomaly (shaded) for 5 PM PDT July 19th, 2019

GEFS Forecast of 500mb heights and anomaly (shaded) for 5 PM PDT July 22nd, 2019

Big change as low pressure digs off the coast with a much milder southwest flow.  Here is the change in temperature anomaly between July 19th and July 22nd as modeled by the GEFS.

GEFS 2 meter temperature anomaly (degrees C) for 5 PM PDT July 19th, 2019

GEFS 2 meter temperature anomaly (degrees C) for 5 PM PDT July 22nd, 2019

Translating from Celsius (from what the image shows) to Fahrenheit yields temperatures of 10-15F degrees below normal Friday to 5-10F degrees above normal Monday.  Get ready for more watering!