Monday, April 28, 2014

Big Warm up after a Cold Weekend

We just finished up a rather chilly weekend for late April.  Snow fell in the northern valleys on Sunday morning, and thunderstorms rumbled through the area on Sunday afternoon, following by a few snow flakes Sunday night.  But if you're looking for warmer weather, your wish is about to be granted.

Here's the forecast high temperatures for Monday:

The cool weather (blue and green colors) are largely confined to the western US, while the southern and eastern states have warmer weather (yellow and orange colors).  But then look at what will happen by Friday:

The map is flipped.  Now warmer weather is found in the western US, while the eastern US will experience a cool-down.  This isn't unusual.  What's happening in one half of the country is often opposite of the other half.

Here is a meteogram of forecast temperatures for the next 7 days at Spokane Airport:

The various colored lines are forecasts from different computer models.  The green line is the official NWS forecast.  As you can see, Monday through Thursday will see a warming trend of about 7 degrees each day.  So, Monday's high should be about 55, Tuesday 62, Wednesday near 70, and Thursday into the mid 70s.  The NAM model (in red) is actually going for a high on Thursday close to 80F.

Now technically Spokane hasn't seen it's first 70F day yet this spring.  On April 8th the airport topped out at 69F, while most other sensors in the metro area reached 70F or better.  While Wednesday will be close, Thursday is a slam dunk as far as having our first 70F day of the spring.  So you're probably asking yourself, is this first 70F day early or late this year?  Well, here's the statistics:

Average:  17 April
Earliest:      9 March 1889
Latest:      21 May 1896

Thus technically, we'll be later than normal for our first 70F day (by 15 days), but nowhere close to the record of 21 May.  And since you maybe wondering what the numbers are for the first 80F day at Spokane, here they are:

Average:  12 May
Earliest:     7 Apr 1977
Latest:      22 Jun 2011

So on the outside chance that we actually reach 80F at Spokane with this warm-up, it would be ahead of schedule by about 10 days.

Wednesday, April 23, 2014

Severe thunderstorms tomorrow? It's possible.

After a seemingly endless barrage of weather systems, yet another is already beginning to show its stripes. Take a look at the satellite picture below. This is a water vapor image.Notice the green area covering the western portions of Oregon and Washington. That's associated with a plume of sub-tropical moisture which should deliver rain (and high mountain snow) to most of the Inland Northwest between tonight and Thursday morning.  Also notice the dark area over the eastern Pacific and the kink in the blue lines located along 130w. This is associated with a fairly vigorous shortwave trough. This trough will become a crucial player for our weather across the Inland Northwest by tomorrow afternoon.

10pm PDT Water Vapor image and 500 mb heights

So as mentioned before we expect widespread rains (and mountain snow) tonight into early Thursday. This will likely prime the atmosphere full of moisture and could set the stage for some active weather tomorrow afternoon across portions of the Inland Northwest. Lets look at some model data for details. First we will start with a course resolution model, the 90km GFS. Here's a look at what that models is showing as far as the shortwave trough moving through the region. The first image shows the previously displayed 500 mb heights combined with what we term Q-Vector convergence. This is a fancy term that essentially shows upper level lifting. In this case the purple shading shows where the lifting in the upper levels is strongest. The top image shows the trough extending from the central Washington Cascades to the SE corner of Washington around 11 am with the best upper level lifting focused over most of the region. The second image shows the same thing only at 5 pm. Notice by this time the trough is focused along a line from Sandpoint to Missoula, however the strongest lifting by that time has shifted into eastern Montana and SE British Columbia. That's all well and good but what does that translate to weatherwise?

11am PDT Thursday 500 mb heights and Q-vector convergence

5pm PDT Thursday 500 mb heights and Q-vector convergence

Lets take a look at one more 90km GFS image before moving on. This time we will look at CAPE values. CAPE or Convective Available Potential Energy is simply the amount of potential energy that can be released should we be able to lift an air parcel. You can read more about it here if you desire. Suffice it to say, the higher the CAPE values there more energy or explosiveness there is in the atmosphere. So what is explosiveness tomorrow afternoon? Its actually fairly impressive at least for this region and this time of year. Notice the axis of the highest CAPE values extend from the NE corner of Washington southeast toward the Clearwater Mountains southeast of Lewiston. Keep in mind this area of instability coincides with the passage of the shortwave trough and upper level lifting...a good thing for producing thunderstorms.

CAPE forecast for 5pm Thursday
The parameters shown above are what we meteorologist have looked at for many years, however we are now also getting more refined and specific data from finer resolution models and simulated radar data. So what is this newer model data showing? Its actually quite interesting. Here a look at simulated radar for 5 pm Thursday that was run 42 hours previously (at 11 pm Tue). Notice over NE Washington and the SE corner of Washington there are a pair of bright orange and yellow cells. This model was suggesting that there could be thunderstorms over this area. We see this a lot from the finer resolution models but often can't put a lot of faith in them unless there is some consistency.
Simulated 42hr Composite reflectivity (radar) from 11pm Tue Model run
 So has there been consistency? Lets see the next model run. This one was run 6 hours later and sure enough there are the two yellow and orange areas over NE Washington and a slightly larger one over SE Washington.
Simulated 36hr Composite reflectivity (radar) from 5am Wed  Model run
So that gives us some confidence. Now how about the latest model run? If you look at the image below you can see that the information isn't  quite as convincing as the previous two but it still showed two yellow and orange areas in the same general vicinity. Interesting. So is this enough information to convince us that we will see severe thunderstorms tomorrow afternoon. Probably not, but it does at least hint at a possibility of it. Our confidence is much higher that we will see some thunderstorms...a few of which will produce hail and possible some gusty winds. If the latest models were showing what we see depicted from northern Louisiana toward northern Illinois our confidence would be much higher!
Simulated 24hr Composite reflectivity (radar) from 5pm Wed  Model run

Saturday, April 12, 2014

Will the Weather let us see the Lunar Eclipse?

In case you hadn't heard, there's a lunar eclipse coming up.  It will occur on Monday night.  You can read more about it at:

Here's a great figure from  that shows the lunar eclipse times for those of us in the Pacific Time Zone:

So as you can see, the moon will start to disappear around 11pm PDT.  Complete eclipse will take place between 12:07am and 1:25am, or 0707 to 0825 UTC.  The moon will be high in the sky for this eclipse. 

The question then becomes, will we be able to see it, or will clouds get in the way?  The answer isn't a slam dunk either way.  Here's one graphic that we look at when making forecasts for a single point.  It shows the forecast relative humidity in the atmosphere for the next 3 days at Spokane.  The horizontal axis is time (increasing time goes from right to left), and the vertical axis is height.  We call it a time-height chart.

NAM Time-Height of RH at Spokane, WA
The time window of the eclipse is shown with the red lines.  Higher RH (green shading) means clouds, while brown shading indicates dry cloud-free air.  As you can see, we'll have some clouds on Saturday and Saturday night.  But then Sunday will be very dry with abundant sunshine.  For Monday, a few high clouds will move over.  By the time of the eclipse, more clouds are moving into the region.  In other words, it's gonna be close.

Let's look at another kind of forecast from the University of Washington WRF model.  In these graphics, they attempt to take the computer forecast and make it look like what a satellite picture would look like.  Areas of white are clouds, dark colors are the ground.  Here's the forecast valid at 8pm Monday evening:

WRF cloud cover forecast for 8pm 14 April 2014

Note the well defined band of clouds extending from southern BC, across eastern Washington and Oregon.  But then there's a break in the clouds over central Washington, with more clouds over western Washington.  Now let's look at the forecast for 11pm, which  is when the eclipse starts:
WRF cloud cover forecast for 11pm 14 April 2014
We can see that the features in the previous forecast have all translated eastward.  Now the Idaho Panhandle has the clouds, eastern Washington is in the clear slot, with more clouds coming over the Cascades.  And the forecast for 2am, at the end of the eclipse, looks like this:

WRF Cloud Cover forecast for 2am 15 April 2014
Most of eastern Washington has clouds, while the Idaho Panhandle is now in the clear slot.

So what does all this mean?  All indications are that there will be some high wispy clouds on Monday evening around sunset.  There could be a break in these clouds at about the time of the eclipse, depending on where you live in the Inland Northwest.  Thicker clouds are expected to move into the region later in the night.

All of this said, it's a bit far into the future to trust the computer forecasts for this detail.  It won't take much for the weather system responsible for these clouds to wind up being 3 hours faster or slower than depicted here.  At this point, it's a safe bet that it won't be a completely clear sky, but the clouds might be sparse enough and thin enough to still get a good viewing of the eclipse.  And if it turns out that the clouds are too thick, well, we only have to wait until October 8th for the next lunar eclipse.

Wednesday, April 2, 2014

Unusually wet February and March

Greetings folks, sorry it's been a while since we last posted to this blog, but we have been focused on implementing a new computer system for our office used to compose our complete suite of products and weather grids. Now that we have gotten our feet wet, it's time to discuss the unusually wet February and March the Inland Northwest endured.

So the weather pattern that took a dramatic shift in February unexpectedly continued into March. What was the cause? Recall that a highly anomalous ridge was fixed over the extreme eastern Pacific through the first half of the winter. Here's what it looked like on a 500 mb chart (approximately 17-18k ft). Notice the strong buckling in the flow just off the west coast. This is rather unusual and resulted in almost a record breaking dry spell for the first half of a winter.
500 mb mean map for 10/1/13-1/15/14

Well as the calendars changed to February, the unrelenting ridge gave way and opened the doors to countless storms which surged through the Pacific Northwest for the following 2-month period.
The mean 500 mb charts for February and March exhibited this ridge flattening.
500 mb mean map for 2/1/14-3/29/14
Notice although the ridge is still present (albeit flatter), its actually a much more favorable setup for precipitation since the amplitude of the ridge allows sub-tropical moisture to be wrapped into passing weather systems. Here's is what the anomaly of the atmospheric moisture (called precipitable water) looked liked for the period. Notice the well defined connection of moisture extending from southwest of the Hawaiian Islands northwest to the coast from northwest California to southern Washington (red, orange and green shading).

Precipitable Water Anomaly 2/1/14-3/29/14

So how wet was the two month period? Very was the answer. The map below shows the departure from normal of precipitation for February and March. Every location saw wetter than normal conditions, with the Cascades and Idaho Panhandle mountains leading the pack with well over 10" more than the normal amount of precipitation.

Rainfall departure from normal February & March 2014

From a percentage standpoint vs. normal it was also an impressively wet period. Note the dark blues and purples over the Cascades and north Idaho mountains, indicating where precipitation amounts were 2 to 4 times the normal for that period.

Rainfall % of normal February & March 2014

The unusually wet period has also brought the snow water equivalent (amount of water in the mountain snow pack) to normal or just above normal over the entire region. Note that although conditions were moist over Washington, Idaho, and western Montana, things were still quite dry across Oregon and California (not shown).

Not only did a considerable amount of precipitation fall, but it also did so on an unusually high number of days. Here's a look at the numbers compared to the normals.

Days of measurable rainfall for various locations across the Inland Northwest Feb-Mar 2014

Over locations where the temperature was cold enough, most of this equated to snow. In Holden Village, WA the snowfall for the two-month period was 180.7". That shattered the old record of 166.7" (set in 1999). Not quite as impressive was the 14.0" which fell in Lewiston, ID. This was the most since Feb-Mar in 1985.

So will this unusually wet weather continue? Showers are still fairly common this time of year. In Spokane we typically see some precipitation about 50% of the time during the first half of April. That number drops to around 40% of the days by the end of the month.  In Wenatchee and Moses Lake, the percentage of wet April days is a mere 25%!

The 8-14 day outlook for precipitation shows near normal conditions for most of eastern Washington and north Idaho.

8-14 day precipitation outlook.