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.






Year
28-Jun
29-Jun
Rainfall
2013
89
78
0.09
2012
85
78
T
2011
86
77
0
2010
85
80
0
2009
83
83
0




Average
Max
85.6
79.2






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

Wednesday, June 11, 2014

Severe Weather Tomorrow??? It's a Possibility.

We have all heard of the calm before the storm, today could be just another chapter in that book. This afternoon will feature highs in the 70's and 80's under mostly sunny skies. Come tomorrow the chance for showers and thunderstorms will be on the rise as an area of low pressure approaches the Inland Northwest. Temperatures will continue on the warm side of normal, but increased moisture will push ahead of the low pressure center increasing our chances for convective activity or thunderstorms. In this post we will evaluate the chances for thunder around the region and the potential timing through various weather maps and products.

So lets start out with today and move forward from there. Our current pattern has a ridge of high pressure directly over the Inland Northwest keeping conditions calm and pleasant. The ridge also allows for warm southwesterly flow to advect into our area bringing above normal temperatures today. We will take a look at the current set up in the following image.
 11am PDT Infared satellite map with 500mb heights
From the image we can see the area of low pressure to our west and the current ridge over us. The ridge is also diverting much of the cloud cover to our north. As the low slowly moves onshore the ridge will continue to be shifted to the east allowing clouds to move east of the Cascades. Not only will this bring increased cloud cover for tomorrow afternoon, but also increased moisture through the atmospheric column allowing better instability. So lets take a look at the increased moisture moving in. This can be accomplished in a couple of manners, but we will look at Precipitable Water or PWATs in the atmospheric column.

Precipitable Water values from 11am today (left image) and 5pm Thursday (right image)
The images above show the increase in atmospheric moisture from today to tomorrow from the North American Model (NAM). For much of the Inland NW we will see an increase of nearly a quarter of an inch of precipitable water. Another method to address the amount of moisture would be the change in forecast soundings which show temperature and dewpoints up through the atmosphere. Next we will examine the forecast soundings to see the change in moisture from today to tomorrow.
NAM forecast soundings from 2pm today (left image) and 2pm Thursday (right image)
From the sounding you can see the major differences in the amount of moisture present. The red line would indicate the temperatures and the green indicates the dewpoint which reflects the amount of moisture in the atmosphere. For the left image a dry layer is noted around 500mb (where the green line moves away from the red) whereas for tomorrow (right image) much more moisture is present (green and red lines are closer throughout the image). So from these two examples the change in moisture can easily be noted, but will we have the instability to promote the thunderstorms? We will now at these parameters that promote thunderstorms.

For thunderstorms to occur, many things need to be present including lift (forcing), moisture and instability (atmospheric stability). We will first look at the lift for the area and the one way to do this is to compare from today's conditions to tomorrow. Below is an image looking at the Q vector convergence in the upper atmosphere or Div-Q. Div-Q is a generalized way to assess the lifting potential in a portion of the atmosphere.
Upper level Div Q from 11am today (left image) and 5pm Thursday (right image)
From comparing the images we can see the major differences concerning forcing. Tomorrow the low will move onshore bringing a good amount of forcing ultimately aiding in thunderstorm development. With the right image being for 5pm tomorrow, we will most likely be looking at a case that will unfold more in the late afternoon and evening hours rather than early in the afternoon. Would this be a good or a bad thing with it unfolding in the evening? For now it looks good. We have been hitting our high temperatures in the late afternoon/early evening which would be the best time for the stronger storms, so this would also would aid in thunderstorm development. We have already looked at the moisture profiles for the area and have concluded that higher amounts of moisture will be in place. Finally we will look at one of the convective parameters that are normally consulted to address thunderstorm potentials which is CAPE or the Convective Available Potential Energy. First we will once again look at the NAM model. It has been the most aggressive when it comes to CAPE values, but paints a similar picture to the others as to the areas with the best chances for seeing thunderstorms.

NAM CAPE values for 5pm Thursday
From the NAM we can see a broad area with values surpassing 1000J/kg and localized areas greater than 1500J/kg (blue and green shading). If you remember around a week ago on the afternoon of June 3rd we had thunderstorms for much of the area. These storms were also working with around 1000J/kg of CAPE. With the NAM being on the upper end of model perspective lets take a look at another one. Next we will examine the model often used by the Storm Prediction Center (SPC) in Norman, OK which is the SREF or the Short Range Ensemble Forecast.
SREF CAPE values for 5pm tomorrow
Although not as colorful as the NAM, the SREF is also showing higher values for eastern Washington and into the Idaho Panhandle. It also has peak values in the 1000J/kg to 1250J/kg range. So we do have some consistency among short range models of the potential for higher amounts of CAPE. So with all of this in mind, now comes the million dollar? will we see thunderstorms and if so, how strong will they be? Concerning the chance for thunderstorms, it is a given. We will see thunderstorms tomorrow. The next question is where? As we can see, the best forcing will be in the eastern third of Washington and the Idaho Panhandle so these are the locations with the best chances. We do not want to omit the chance for the east slopes of the Cascades or the Basin, but the threat will not be as great as the other locations. From the SPC, they create a calibrated outlook of the thunderstorm chances for a location so lets take a look at what they think.
SPC calibrated thunderstorm probabilities between 5am-5pm Thursday
So from the SPC, they highlight a 40% chance for much of northeast Washington into the Idaho Panhandle.  These are some of the higher values I have seen from this for our area so it would lead me to think the chance for thunderstorms is essentially a slam dunk.  Finally the big one.....will any of the storms be severe? Here at the office we seem to think the potential is definitely there for strong storms. Comparing to last week, we had similar CAPE values, but tomorrow we actually have better dynamics to support storms. The SPC also does a product for the chances of severe storms and here it is.
SPC severe thunderstorm probabilities for 5pm Thursday
While the chances for severe storms do not look very high, they still highlight the potential for portions of the area. This product usually only has slight chances for us when severe events do occur, so the potential is there. The region will want to keep an eye on the skies tomorrow as active weather looks to be a given.

Tuesday, May 20, 2014

Memorial Day Weekends in the Inland Northwest...Nice or Not?


 Now that Memorial Day is approaching, our most common question we get at the National Weather Service is "what's the weather going to be like this weekend?" After all, this is the start of the summer camping season for many of the denizens of the Inland Northwest. Before folks hear the answer, they often have the preconception that weather in this part of the country really doesn't become conducive for prolonged outdoor activities until early July. With that notion in mind, we decided to take a climatological glimpse of what Memorial Day weekends in the past truly delivered.

Usually looking up weather over holidays is easy. The 4th of July always falls on the 4th day of July, and Christmas always falls on December 25th. For Memorial Day though, the days varied from year to year, however before 1971 and the passage of the Uniform Monday Holiday Act, Memorial Day always fell on May 30th. Consequently, what constituted the Memorial Day Weekend isn't exactly clear. To make things simple we decided to only look at Memorial Day Weekends from 1971 and beyond. These dates can range from May 23rd through May 31st.

First what is the normal weather like for this period? Lets begin by looking at the mean 500 mb (18k ft MSL) maps. Sure enough, moist southwest flow typically is still pointed at western Washington and the Cascades. Meanwhile there is a ridge that sets up just west of the Continental Divide along the Alberta-British Columbia border extending into central Montana.

500 mb mean map 5/23-5/31

That map alone suggests the region still might be prone to wet weather so what are the climatological temperatures and chances for precipitation for this time of year? We will start by looking at data for Spokane. But knowing that many people like to get away from the metro area and go camping in the woods, we also decided to look at the weather at the Priest River Experimental Station in north Idaho. Here's a map of where that station is located



And here are the averages:

So not bad. An average high around 70°F with rain expected on roughly 1 out of 4 days doesn't sound too terrible. But that's climatology. Now here's reality. Lets start on a positive note and look at the top 10 warmest Memorial Day Weekends. Here is the list for Spokane.

Top 10 warmest Memorial Day weekends for Spokane. 

For those who like warm and dry weather, the holiday weekend in 1983 was hard to beat. Average highs were above 90° and every day was dry. In fact of the top 10 warmest holiday weekends, 7 of them were bereft of precipitation. At Priest River, the results were similar as far as temperatures. 1983 was the year of the warmest and driest conditions. However note that only 60% of those weekends were dry, and one of those weekends (1985) saw rain on every day.

Top 10 warmest Memorial Day weekends at Priest River experimental station. 

Now let's look at the list of least desirable Memorial Day weekends. For Spokane there were 5 extended weekends which saw the average temperature stuck below 50°F or roughly 10°F colder than normal. The worst year (by temperature) was 1975, however the rainfall that year was light (there were trace amounts of rain which fell on 2 of the 3 days). Perhaps 1989 could be considered a worse year for outdoor activities considering an average temperature of 48°F and just over an inch of precipitation.

Top 10 coldest Memorial Day weekends for Spokane. 
For Priest River, the bad weather Memorial Day weekends were  worse than what was experienced in the Spokane area. 1977 was the coldest year, by far with an average temperature just below 45° although that year was dry. Campers in north Idaho certainly woke up to frost on their tents as lows dipped to 30° on two of the three mornings. From a comfort standpoint, the worst weekend might be the one in 1994 with an average temperature about 8° cooler than normal and measurable rainfall on each day.


Now lets look at a few daily extremes:

Hottest High Temperature: Spokane     94° (May 29, 1983)
                                              Priest River 91° (May 24, 2003)

Coldest Low Temperature: Spokane 35°* (May 26, 1991, *last of several occurrences)
                                               Priest River 29° (May 30, 2011)

Wettest Day:   Spokane      0.79" (May 28, 1988)
                         Priest River  0.96" (May 29, 2010)

Wettest Weekend: Spokane 1.20" (May24-26, 1997...it rained all three days)
                                 Priest River 1.58" (May 29-31, 2010...it rained all three days)

Percent of entirely dry Memorial Day weekends: Spokane 19%
                                                                                  Priest River 23%

This last tidbit is a little unexpected since Priest River is typically a wetter location than Spokane. The answer may lie in the fact that the Spokane observation is taken at least once an hour through the day, whereas the Priest River one is taken once a day. Thus if a trace of rain fell at sometime during the day and it wasn't witnessed it might be recorded as a dry day. Nonetheless, the odds of seeing an entirely dry Memorial Day weekend in the Inland Northwest (especially the eastern third of it) is not good.

Will this weekend be one that sees dry and warm weather? Looking at the latest weather maps, the answer is a resounding no. Here is our latest forecast issued at 3pm today (Tuesday 5/20)

For Spokane it looks like at least a 20% chance of precipitation each day with temperatures cooling to right around the seasonal normals.

Spokane Outlook

For north Idaho things look a little cooler and a little wetter.

Sandpoint outlook...Priest River graphs are not available
Based on the temperature and precipitation forecasts this does not look to be a top-10 Memorial Day weekend weather-wise.