Tuesday, August 19, 2014

Pattern Shift - How Long will it Last?

The weather pattern across the Inland Northwest so far this summer has been hot and dry.  This has dried out fuels across the region.  Any moisture that has crept up into the region has at times sparked off thunderstorms.  Lightning from these storms have ignited numerous fires across the region.  We have also seen multiple severe weather episodes with damaging winds and large hail reported across the region from July into early August.  The weather pattern has sounded like a broken record with these spells of hot and dry weather conditions intermixed with rounds of thunderstorms sweeping across the region.  This pattern can be attributed to a persistent area of high pressure building up into the region out of the Four Corners area in the Desert Southwest.  This ridge has supplied the heat and potential energy, while weather impulses off of the eastern Pacific have supplied the forcing and moisture to spark off thunderstorms.  This large scale pattern is going to change over the coming week.  The question is: how long will our pattern shift hold over the area? And, can we expect more summer like weather to return before the fall season tightens its grip over the region.

Before we dive into some of the details of what can be expected over the next seven to ten days, let's have a little review of the summer season thus far.  The figure below is a daily mean composite of the 500 MB height pattern from July 1st through August 17th.
This is a fairly typical summer pattern that is observed over the western U.S.  High pressure tends to build in over the Four Corners area in the Southwest (indicated by the warmer colors).  This acts to trap in the heat bubbling up from the equatorial region.  This heat will tend to push up into the Inland Northwest as this area of high pressure strengthens and builds northward.  We have also seen a persistent area of low pressure over Alaska and into the Gulf of Alaska (indicated by the cooler colors).  This is a rather amplified large scale weather pattern from the eastern Pacific into the contiguous U.S. and has been a entrenched for much of the summer thus far.  Although this pattern is nothing unusual, the strength of this pattern has been a bit anomalous (as reflected in the graphic below).
This plot shows similar information as in the first, but represents the mean 500 MB height anomalies.  The warmer colors indicate positive height anomalies and the cooler colors represent negative height anomalies for the same period examined before (July 1st - August 17th).  What is apparent from this graphic is the large negative height anomaly located over the Gulf of Alaska and the large positive height anomaly over the Northwest into southwestern Canada.  Heights have also been anomalously low over the eastern U.S., but less in magnitude compared to the area of low pressure over the Gulf of Alaska so far this summer.

So, how has this pattern affected our local weather in July through the first half of August?  Well temperatures have been hot.  Temperatures at the Spokane International Airport have reached 90 degrees in 26 of the 48 days during this period and the average high temperature was 89 degrees.  In comparison, the average number of 90 degree days Spokane will see at the airport is 14 and the average high temperature is 84.2 over this same period dating back to 1881.  That means we have seen almost double the number of 90 degree days than what is typically observed.  July and August are typically dry across the Northwest, but the current water year (beginning and ending on October 1st) has been below normal across the Inland Northwest; through August 17th, Wenatchee is 3.06 inches below its normal value of 7.58 inches, Spokane is 3.83 inches below its normal value of 15.63 inches and Lewiston is 2.80 inches below its normal value of 11.32 inches just to name a few locations.  This deficit has directly contributed to the current drought status in Washington state (see figure below).
This is the current drought status through August 14th.  The yellow area indicates abnormally dry conditions, light orange indicates moderate drought and dark orange indicates severe drought.  Notice that the central portion of the state has been particularly dry through the water year so far.  This area, not surprisingly, is where vegetation has been driest and where wildfires this season have been the most intense.  It is unlikely that the drought situation will improve much until we transition into our wetter seasons from fall into winter.

Now that we have summarized a little bit of what has happened through the summer so far this year, it is time to look at what the rest of the summer has in store for us...well we will at least look at the next ten days.  It appears as though the ridge over the Four Corners area is going to be suppressed and actually slide eastward a bit.  This is going to allow a long wave trough to dig across the Rocky Mountains with a ridge of high pressure building into the Gulf of Alaska.  This is in direct contrast with what we have seen throughout this summer.  The next series of images is a model forecast off of the GFS, which was ran at 4:00 PM PDT on Monday.  Displayed is the 500 MB heights starting with Tuesday afternoon and continuing out at 48 hour increments through Thursday afternoon August 28th.

Notice that the western U.S. is under a trough of lower pressure through at least early next week.  High pressure over the Four Corners area becomes suppressed and moves more over the southeast portion of the U.S.  Meanwhile, high pressure builds and strengthens over the eastern Pacific.  The image below is a rough sketch of the mean 500 MB pattern that can be expected the rest of this week and how temperatures will compare to normal.

The Northwest will be sandwiched between the ridge of higher pressure to the west and the trough of lower pressure over the Rockies.  This will place the region in a northwest flow pattern (or the flow of the atmosphere will be from northwest to southeast).  Since the trough and coolest air will be displaced further east, we are not expecting temperatures much below normal, but we should at least see a relief of above average temperatures for the rest of this week.  Here is what the Climate Prediction Center is predicting over the next 8-14 days for temperatures:

This graphic indicates that the trough of lower pressure that digs in over the Rookies will shift slightly to the east next week.  This is also reflected in the model forecast off of the GFS (refer back to the last image in the series of GFS forecasts shown earlier).  If this idea does come to fruition, then we could be in line for temperatures warming back above average by the middle of next week.  However, this would be in response to the ridge of high pressure nudging eastward off of the Pacific and not from the Four Corners area.

Sunday, August 10, 2014

Thunderstorm Potential for Monday and Tuesday

After 2 damaging severe thunderstorm episodes this summer, is it possible to have another one?  At this point, the answer is "yes, it is possible".  That said, it is too early to tell the exact timing, intensity, and location of any strong thunderstorm.  So at this point, there is no reason to panic.  Just pay attention to the forecasts today and Monday as they become more detailed.  There is still a good chance that all of the ingredients don't come together just right, and the event winds up being rather benign or localized.  So let's take a look at some of the ingredients.

First we want to see instability in the atmosphere.  There's lots of things that go into this, more than we can cover here.  But essentially, we want to see an atmosphere that is very hot at the surface or cold aloft, or the combination of the two.  Also, moisture in the low levels is needed.  We measure these parameters by calculating the Convective Available Potential Energy of the atmosphere, or CAPE.  Values of 1000 for CAPE are fairly large for our area.  Below is the probability of CAPE > 1000 on Tuesday.  As you can see, the shaded areas have a 70% chance or greater of having CAPE > 1000.  So we will have instability.

15Z SREF Probability of CAPE > 1000 J/kg on Tuesday 12 August

Moisture is another key ingredient.  A dry atmosphere can be unstable, but it won't form any clouds.  Again, there's several ways to assess this.  One way is what we call "precipitable water".  Despite the name, it is not a measure of how much rain you'll get.  Below is the forecast precipitable water.  A "normal" value for this time of year is about 0.70".  So a forecast of 1.25" is nearly twice what we normally see in August.

15Z SREF Precipitable Water on Tuesday 12 August

Another ingredient is what we refer to as "upper level dynamics".  That is, a larger scale feature in the atmosphere that will support and enhance thunderstorms.  The satellite image below shows a large swirl of low pressure off the northern California coast.  That low is forecast to move over our area on Tuesday.

Water Vapor image at 10 August 2014 at 2pm

There are other ingredients that are more subtle, but can still have a big impact on the event.  For example, if there are too many clouds on Tuesday, we won't heat up as much as forecast.  That decreases the instability and would lessen the event.  On the other hand, a bright sunny day could result in hotter-than-expected temperatures and more instability.

So what do we expect to happen?  Here's a basic breakdown of what to look for:

Thunderstorms will fire over central Oregon on Monday.  The remnants of these storms will drift northward into eastern Washington Monday night.  These storms could create what we call a "haboob", which is a strong wind storm, often with blowing dust and very little if any rain.  This would be most likely in southeast Washington in the evening, moving into the Spokane metro area late in the evening.  Here's a computer forecast of what the radar might look like.  
NAM reflectivity forecast for Monday evening

This activity will gradually weaken overnight.  Tuesday will see more thunderstorms.  The difference is that the low we previously talked about will be moving into our area on Tuesday, helping to support the storms.   Storms will fire over the Washington and Oregon mountains.  The Oregon storms will move into Washington from the south.  The low will keep these storms going, not letting them die.  Here's the computer forecast of what the radar might look like for Tuesday evening.

NAM reflectivity forecast for Tuesday evening

The one missing ingredient in all this is something we like to call "vertical wind shear".  There will be some, but not a lot, and we don't expect that to change.  That could hinder storms from becoming very strong.

So here's a breakdown of what the potential impacts might be from these storms::
  • Flash Flooding: Precipitable water values will be up to 200% of normal for August. Storms will not be moving fast, probably about 15 mph. Debris flows are possible for recent burn scar areas. 
  • Wind: The main threat will be strong gusty winds. The previous two events this summer have shown the potential problems this poses. It's impossible at this point to say where this will occur, so stay tuned. 
  • Fire starts: Given our very dry fuels, any lightning will likely start fires. Even wet storms can start fires. 
  • Hail: Instability over the mountains would certainly support large hail. The SREF has 70% probability of CAPE > 1000 J/kg. However, we are lacking strong atmospheric wind shear, so that could limit hail production.
How does this event compare to our previous 2 events this summer?  This event is similar to 23 July in that there is a strong low aloft to provide forcing.  The August 2nd event was lacking this.  In that case, the thunderstorms provided all of the lift themselves.  This event is also similar to 23 July in the track of the storms, moving from south-to-north.  The August 2nd event saw storms that tracked from west-to-east.  But July 23rd had a stronger low and more instability.

Similar to the August 2nd event, this one will feature a drier lower atmosphere initially.  This favors more wind than hail.  Also, the lack of vertical wind shear is similar to August 2nd.

All of these ingredients have to come together just right for a large convective event.  If the low is a little slower and arrives at night, we won't see as much thunderstorm activity.  Or the moisture may be over-forecast. So please, keep up to date on the forecast.  

Monday, July 28, 2014

July 23 2014 Severe Thunderstorm Review

On July 23rd, very strong thunderstorms moved across the Inland Northwest.  These storms produced weather that is uncommon in our part of the world.  But possibly not as uncommon as you might think.

Two years earlier, 20 July 2012, a very similar weather event occurred in the Inland Northwest.  Folks in Ferry county will remember it for the massive blow-down of trees and power lines that crippled much of that county for weeks.  And people north of Lewiston will remember the baseball-sized hail that fell from a thunderstorm.  But for the major population centers (Spokane/Coeur d'Alene, Lewiston, Wenatchee), this event wasn't very impressive.

But the recent July 23rd event brought a direct hit to parts of the Spokane metro area as well as locations to the north.  As such, it has received a great deal more attention.  

Before we get to the thunderstorm that hit the metro area, there were some other big storms that were also impressive.  The first developed in southeast Washington near Pomeroy.  It moved to the northeast, dropping hail the size of golf balls.  Here's the radar image of it as it was crossing the Snake River.

Radar image of a severe thunderstorm at 3pm 23 Jul 2014.  The yellow box is the NWS Severe thunderstorm warning area
By the time the storm reached Pullman, the hail was still ping pong ball sized.  Meanwhile, another thunderstorm was developing to the south that also prompted a warning.  You can see both storms in the image below.  The first storm is just north of Pullman, while the second storm is to the south of it.

Radar image of two severe thunderstorms at 335pm.

This storm weakened a bit before reaching the Pullman/Moscow area.  Hail was a bit smaller, the size of half-dollars.

As this was going on, another strong thunderstorm developed near Kettle Falls.  It wound up producing the largest hail of the day.  Two inch hail stones left dents in cars and cracked windshields along Highway 395 just south of Kettle Falls.

Radar image of severe thunderstorm near Kettle Falls at 327pm.

These storms were all similar: high reflectivity cores indicating large hail and heavy rain, along with some strong winds.  Below is the radar showing a different storm about to hit Spokane.

Radar image at 4pm of the storm that hit Spokane (left side of the image). 

 Doesn't look all that impressive, does it?  The storm over Benewah county (right side of the image) is the storm that previously went through Pullman, and is still putting down large hail.  The storm on the left that's about to hit Spokane pales in comparison.  Or does it?  Here's the radar velocity image at the same time.

Radar Velocity image at 4pm
Do you see that area of bright green colors?  That is showing very strong winds (about 60 mph) heading for the west plains of Spokane.  The NWS office measured a gust of 61 mph while the airport recorded a 67 mph gust. 

Here's a video of the wind and rain at the NWS office.

As the gust front went by the radar, the color switches from green (which indicates winds blowing toward the radar) to red (winds blowing away from the radar). 

Radar Velocity image at 420pm

The light orange color in the center of the image indicates winds of 70 mph.  On Five Mile Prairie, a wind sensor measured a gust to 65 mph.  The storm continued to track to the northeast.

Radar Velocity image at 434pm
By this time, the strong winds had reached the Deer Park and Chattaroy area.  As you can see from these three images, the strongest winds followed a line from the West Plains, across northwest Spokane and Nine Mile Falls, and up to Deer Park.  

Strong winds also hit the Idaho Panhandle, from Silverwood up to the Lake Pend Oreille area.  The number of injuries sustained from falling trees was far more than we typically see in an Inland Northwest thunderstorm event.  A map of the hail and wind reports is shown below.   Or you can go to this site for a detailed look at the reports.  

Severe Weather Reports for 23 July 2014

Wednesday, July 16, 2014

Does a hot July mean that August will be hot?

So far, it has been a blistering July. We've seen day after day of scorching temperatures above 90°F with may locations topping 100°F. Today is the 9th day in a row that Wenatchee has seen temperatures at or above 100°F.  This sets the record for the longest consecutive streak of 100°F or higher in Wenatchee ever!  Records for Wenatchee Airport only go back to 1959, but this is still a an impressive mark.  Spokane’s streak of 90+°F days is now at 11 days and is expected to top off at 12 on Thursday before cooling down, making it the 4th longest streak in Spokane’s history. This heat has led to everyone asking one question, “What is August going to be like and will it be as hot as July?”

Well the simple answer is… we’re not sure yet. Is a hot July always followed by a hot August? Our past records have had some scorcher July's like this one and then very cool August's. For example, 1985 in Spokane saw a July much like our current one, at 5.1°F above average, only to be followed by an August that was 3.8°F below average. On the flip side of that, Wenatchee had a hot July in 2004, 3.9°F above average, only to be followed by an even warmer August that was 4.2°F above average. So there's no guarantee for August. However, based on history and our data we can make some predictions at what will happen. 

This graph shows the percentages of occurrence of a hot August, a cool August, and a normal August following a hot July. If you live in Spokane (green pyramid) and it's been a hot July, there is a 44% chance of a hot August.  But that means that there's a 56% chance of having a normal or cool August. In other words, just because it's been a hot July doesn't mean it's a sure thing that August will follow suit.

If you live in Wenatchee (blue pyramid), the numbers are a bit different.  A hot July has a 54% chance of being followed by a hot August.  However, Wenatchee also has a 27% chance for a cooler than normal August after a hot July. Those of you in the Lewiston area (red pyramid) also have about the same chance of a hot August (53%), with the next best chance being a normal August (37%). 

Even as we start off the month of July at near record high readings, it doesn't even guarantee how the rest of the month could go. July 1926 in Spokane was the hottest first half of July ever. The first 14 days were marked by temperatures in the 90s with 3 days above 100°F. However, as the month progressed the temperatures rapidly cooled off and returned to normal levels.  A few of the days failed to even warm into the 80s.  The thing to take away from this is even if the month starts off at record setting levels, there's no sure bet that it will end up the same way.  

Wednesday, July 2, 2014

What's the hottest day of the year?

The answer to the question of "what's the hottest day of the year?" depends on where you are.  And the differences may surprise you.

The National Climatic Data Center (NCDC) created a neat map that shows the average date of the hottest day of the year across the lower 48 states.  Now some people might guess that it would be the same no matter where you are located.  Others might think that latitude comes into play.  But the answer is much more complicated than that.

The first thing you notice is that there is quite a bit of variance.  For instance, if you live in El Paso (western Texas), your hottest day is usually in the latter half of June.  But over in Houston (eastern Texas), your hottest day isn't until mid August.  That's a big difference.  Which leads to the question:  what causes these variations?  They're actually rather explainable.  Let's look at a few.

The western U.S has the most drastic variability.  Much of the West has the warmest date around the 1st of August, give or take a week.  But there's a ribbon of purple colors along the west coast from Newport WA down to San Diego.  Those are late August and September.  What's up with that?  Well, most of the west coast of the U.S is blanketed by fog and low clouds for much of the summer.  

MODIS Visible Satellite image 20 July 2013

The cold Pacific waters are responsible for making the fog, and then the hot inland temperatures act to pull the fog onshore.  We all know about how cold it can be in San Francisco in summer.  

But as summer wanes, that "onshore flow" lessens.  So the fog isn't as extensive along the coast.  Also, we often see high pressure move into the western US behind a cold front.  This reverses the winds, and causes off-shore flow (winds blowing from east to west).  This not only pushes the fog away from the coast, but brings the warm air from the interior to the coastal towns. So some of the warmest weather on the west coast is actually in September.  Here's the normal temperatures for San Francisco.  

The peak of the brown shading (normal temperatures) is in early September.  And the hottest day ever at San Francisco Airport was 103F on Sept 14th, 1971.

Cliff Mass has a great blog and some of his entries do an excellent job of describing and explaining the effect of off-shore flow.

So what about the desert Southwest?  Why is late June and early July the hottest time of the year for Arizona and New Mexico?  The answer is what we call the "Southwest Monsoon".  During May and June, the desert Southwest is generally sunny, hot and very dry.  But by mid-July, moisture from the south (i.e. Mexico) routinely moves into the Southwest US, resulting in frequent clouds, showers, and thunderstorms.  The satellite image below shows an example of all the thunderstorm activity over the desert SW on 1 Aug 2013.

MODIS Visible Satellite image 1 Aug 2013

All of this cloudiness tends to keep temperatures a little cooler.   Here's the average temperatures for Tucson.

The brown shading shows the "Normal" temperatures.  And you can see that they do indeed peak in late June.  It's still hot in July through September, just not quite as hot as late June. 

Friday, June 20, 2014

Hoopfest 2014

Hoopfest 2014:  How is the Weather Looking?

Weather Outlook for June 27-29, 2014

   With Hoopfest right around corner, here is a general outlook of the potential weather for the weekend of June 27-29, 2014. 

Here is a quick look at the previous five years of max temperatures and rainfall on these dates for the Felts Airfield near downtown Spokane.  This weekend has generally been pleasant with very little rainfall and warm temperatures.



The following images are of the GFS and ECMWF 500MB Heights and 850 Temperatures in Celsius.  A quick description these can be found at the bottom of the ECMWF site.  The images are indicating a low pressure system impacting the region beginning Friday and lasting through Sunday.  The important take away from these images is the cool 850 MB temperatures over the region during the weekend.  These temperatures will be around 11°C ( 51°F).  The 925MB high temperatures for the weekend are around the Low 70’s.    


The following images are the GFS 12 hour forecast for rainfall on Saturday and Sunday.  The Low will bring a lot of moisture with it.  Rain showers can be expected throughout the weekend with an occasional thunderstorm.  Models are pointing to Friday having the most potential for thunderstorms and decreasing chances throughout the weekend.

                In Summary, this Hoopfest has the potential to be the wettest and coldest in the past few years.  Please plan accordingly when making your plans for this weekend.  As always, this outlook is subject to change as more information becomes available.  Keep track of the latest forecast at http://www.wrh.noaa.gov/otx/.

Tuesday, June 17, 2014

When Does Summer Start in the Inland Northwest

OK, you're looking at the calendar and at the weather outside.  It's June 17, and the temperatures are in the 50s with cloudy/rainy weather.  In most locations, those weather conditions aren't typically found in mid-June.  But in the Inland Northwest, it's not that unusual.

Temperatures at 346pm on 17 June 2014

The culprit is a large area of low pressure that slowly moved across our area.  It's now east of us, but the moisture often wraps around these slow moving lows.  So our rain and clouds are actually coming from Montana.

Infrared Satellite image at 3pm 17 June 2014

This kind of weather leaves folks wondering if summer will ever arrive.  Which leads to the question:  When does summer start?

If you look at your calendar, you'll see that June 21st is the first day of summer.  But in reality, that's just the longest day of the year.  It doesn't mean that meteorological summer has arrived. Try telling folks in Phoenix, Arizona who have been seeing triple-digit temperatures since early May that it's still Spring.  It's kinda silly to think that a season will start on the same day for all locations, Miami to Anchorage.

Meteorological summer is defined as the months of June, July, and August.  This is a little better than the calendar definition.  But again, not very realistic to apply to all locations.

So how do we define summer in the Inland Northwest?  We could come up with any number of measures (e.g. average temperature, hours of sunshine, etc).  But in general, we define summer as being from the 4th of July to Labor Day.  Now of course, in any given year, we can see hot weather in May and June.  But these warm spells typically are only for a few days, and are often followed by a rather cool period.  July and August are more likely to be dominated by warm-to-hot weather, with a few cooler spells occasionally thrown in.

The chart below shows the percent of days that the daytime high temperature is colder than 70F.  As you can see, there's still a fair number of days in late June (20-30%) that don't reach 70F.  But by early July, those cool days become very rare, less than 10% of the time.  Conversely, by the end of August, we're already starting to see an increase in sub-70F days.  In other words, summer in the Inland Northwest is about 2 months long.

Percent of Days at Spokane Airport where Max Temperature < 70F

Our temperatures are going to rebound rather quickly though.  Here's the forecast for the next 7 days.  By Friday we'll be approaching 80F, and mid-80s by Sunday and Monday.  These past few cold days will be a distant memory.

7 day temperature forecast of Spokane, WA