Tuesday, May 12, 2015

The high desert gets soaked, but not as bad as Oklahoma

After days and days of high-precipitation thunderstorms with embedded tornadoes and large hail, Oklahoma is finally getting a bit of a break.  Just how wet was it there?

The Oklahoma Mesonet recently published rainfall totals for the past two weeks.  Here's what it looked like:

Huge rainfall totals across the state.  Over 12 inches in some locations in only 2 weeks.  The annual percentage of normal rainfall has spiked through the roof.  So far through the roof, in fact, that it's maxing out the colorbar on the Oklahoma Climate Survey plot, with most places greater than 180% of their total rainfall.
Needless to say, the latest monthly outlook from the Climate Prediction Center has drought conditions in Oklahoma and Texas "improving" over the next month...
But drought persists elsewhere---California is seeking no help and the persistent troughing has kept most of the heavy rain-makers to the south of the upper midwest, causing drought conditions to develop in Minnesota and the Dakotas.

Eastern Washington and Oregon have also been running a bit low on their precipitation and are in "drought" conditions as well.  A warmer-than-average winter has allowed the western side of the Cascades to see about normal precipitation, but without much snowfall accumulating in the mountains.  This is bad news for the eastern sides of the states, where they don't see as much precipitation, but rely on meltout from the snowpack to supply them with water for agriculture.

Here's a plot showing the snow water equivalent normals and what we've had this year at Stampede Pass, which is in the Cascades southeast of Seattle.
The light blue line is the normal Snow Water Equivalent, or the mass of water in the snowpack.  The dark blue is this year.  You can see that by this time of year we should be just dropping down from our peak snow in mid April, but this year we're already at zero snow.  Looking at a map of percentage of average snowpack as of last week, we can see pretty much everyone is basically melted out (or at least below 25% of where they should be).
But there is some hope for the eastern side of the mountains!  An upper-level low is churning its way through Oregon at this time.  It's moving slowly and a bit further south than usual, and this is allowing a fair amount of moisture to get to the eastern sides of the Cascades.  Here's the latest water vapor satellite image.
We can see the swirl of the low with its attendant moisture streaming up through Nevada and into Idaho.  That low is forecast to slowly lift north-northeastward over the next couple of days.   With cold air underneath that trough, we're expecting some destabilization and a few rounds of thunderstorms to develop.  All of this in an area that does not get much rain to begin with.  Eastern Washington and Oregon are basically deserts; here's a map (from the PRISM group) of annual average precipitation across Oregon.  Much of the high desert east of the Cascades sees 10-15 inches or less of rainfall per year.
Here's a similar map for Washington, with the same story away from the higher terrain:
Our middle- to long-range forecasts are calling for several inches of rain by the end of the week east of the Cascades.  This is an unusual setup.  Here's the total rainfall accumulation by Friday evening from this morning's local WRF model run.
Large swaths of area getting 1-3 inches of rainfall.  Great for farmers!  It may seem like these numbers are small in comparison to Oklahoma, but keep in mind that these are places that are lucky to get a tenth of an inch of rain with a good convective shower.  There are usually a few thunderstorms that occur throughout the year that will give a more thorough soaking; this will be one of those periods.

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