Friday, October 31, 2014

How's tonight's trick-or-treat weather looking?

Yesterday we discussed what the odds of a wet Halloween would be and whether or not the holiday revelers would need to accessorise with umbrellas or additional rain gear. Well, now that the event is nearing our confidence in where rain will fall during the evening is growing. First lets look at the latest satellite picture. The feature of concern is the north-south band of clouds and moisture (while and green band) associated with a cold front (blue dotted line). This cold front is slowly working its way to the east and will likely it's trek through the evening.

3pm Water Vapor imagery with approximate cold front position in blue

So while the satellite was showing the cold front moving slowly to the east, what did that translate to in regards to precipitation? As you notice on the radar loop below, much of the preciptation has shifted east of the Cascades and Yakima (aside from some isolated showers over western Washington and Oregon) and was making a slow northeast path toward Spokane and northern Idaho. Based on this, it seems certain that rain will fall this evening over these locations, however the front is still expected to weaken during the evening. So where do we expect to see the best chances of rain during the evening between 6 pm and 9 pm?

Radar Mosaic from 130pm-300pm

To answer this question we will refer to the same ensemble forecast as yesterday (SREF model) as well as some hourly model guidance (HRRR model).

According to the SREF model, the best chances will occur over the extreme northeast portions of Washington and adjacent portions of north Idaho (areas shaded in purples and reds). This would impact trick-or-treaters in Sandpoint, Bonners Ferry, and possibly Colville. However notice the SREF isn't expecting much chance of measurable precipitation over Spokane or Coeur d'Alene(greens are 20% chance of less). This is because the front is still expected to stretch and weaken as it moves to the east this evening. The other area of concern for rain is over the extreme southeast corner of Washington, near the Blue Mountains and Pomeroy. Based on current radar trends, this seems overdone, but bears watching. Let's now check on the hourly HRRR guidance to see if it agrees with the SREF.
SREF measurable rain chances from 5pm-11pm

So below you will see the hourly HRRR model data for the period between 6pm-9pm. The images represent what the radar might be showing during those times. Greens and blues show where the model thinks there will be rain. Generally speaking it agrees quite well with the SREF, by keeping most of the rain to the north and east of Spokane. In fact, most of the rain generally remains fixed over northeast Washington and doesn't really hit north Idaho in force until after 8pm, and by then the front looks quite weak. Also notice that over southeast Washington the forecast looks dry until 9pm and then later into the night (not shown here).

6pm simulated radar from the SREF

7pm simulated radar from the SREF

8pm simulated radar from the SREF

9pm simulated radar from the SREF
Does this mean we can count on a dry evening over Spokane and Coeur d'Alene? We don't think so (the HRRR is showing a few tiny specks of green and blue around the area), The front has enough moisture and lift to produce some light rain in this area, but it won't likely amount to more than a few sprinkles and thus rain gear will be optional.

Rain or not, one thing is certain, this will be another mild Halloween. Much more so than what the region endured back in 2002. That evening, the holiday revelers had to endure temperatures in the teens. 2003 was not much warmer. However, since 2007, every Halloween evening has been fairly mild. Here's a look at trick-or-treat temperatures since 2000.

Halloween temperatures at the Spokane Airport since 2000

Thursday, October 30, 2014

Will the Halloween revelers need umbrellas this year?

Aside from planning where to gather candy and what costume to wear on the evening of Halloween, many folks are likely wondering if they need to pack an umbrella as well. The answer to that question will be fairly straightforward for a good portion of the Inland Northwest, for other sections, the outlook is a little less clear. Before we get to that answer though, let's look at what a typical Halloween day brings from a climatological perspective.

The last day of October can actually be quite wet depending on your location. In Spokane and Lewiston, measurable precipitation is reported around 1 out of 3 years (the exact number is 36% in Spokane and 37% in Lewiston) while in locations such as Wenatchee, Moses Lake, and Omak rain is expected about 1 out of every 4 years.  Here's a table of the Halloween rainfall frequency from sites across the Inland Northwest.

Rainfall frequency on 10/31

So as you can see, rain on Halloween isn't that rare, especially over the Idaho Panhandle and extreme eastern Washington. Now that we have the climatological background, lets delve into the specifics for this year's festivities. First we will take a look at the regional satellite imagery below. The water vapor image below shows the current weather will  be dominated by a cluster of low pressure areas off the coast combined with an elongated cold front (blue dotted line) stretching from British Columbia south to off the California coast. There also was a weak warm front extending into Washington (red dotted line).
Water Vapor Satellite-- 12 pm 10/30/14

These fronts were producing a widespread blanket of rain covering much of Washington as seen on the radar image below. All indications are that this rain will persist through most of today and into tonight. But what happens to this rain shield tomorrow, and most importantly, what will become of the rain by Halloween night? To best answer this question, we will want to track the cold front, and try to place it by tomorrow evening. 

1200 pm Radar Image
So looking at most of the model guidance below, the front (represented by green shading extending from BC to northern California) is placed near the Cascades by early Halloween morning. This is where we'd expect most of the precipitation to occur. Rain chances look very good in these areas.

5am Halloween 
By midday, the front continues it slow march to the east, nearly reaching the Idaho/Washington border. However, during the morning, the front begins to weaken and stretch apart. Notice how much narrower the green shaded band is compared to the image from 6 hours earlier. This is a sign that the chances of rain associated with the front will begin to decrease. 

11am Halloween

By 5pm, the front continues it trend of stretching and weakening as it crosses the Washington/Idaho border. Although the front is still weakening, the rainfall chances will likely be bolstered somewhat by the upslope wind flow pushing into the Panhandle mountains. Thus, we expect rain to be found over most of north Idaho and extreme eastern Washington at this time. Amounts may not be heavy where most people live though. But with most trick-or-treating occurring after dark we really are concerned with what's expected in the window from 6pm-9pm (sunset is around 530 pm). 

5pm Halloween

During this time, the front really begins to lose its eastward momentum. Most of this is due to the deepening of the offshore trough (orange lines). When troughs deepen, they typically slow the forward (eastward) progression of any fronts found ahead of them. This is certainly the case here which then poses a problem with the weather forecast. How fast will the front really move?

8pm Halloween
By 11pm, the front has barely moved as the offshore trough continues to deepen (you can better visulise the movement of the front and deepening of the trough in the second movie image below). Notice locations from southeast Washington and across most of the Idaho Panhandle remain covered by the green shading. Does this translate to wet conditions over these areas?

11 pm Halloween

Loop of  frontal progression between 8am 10/30 through 11pm 10/31

To answer that question, let's take a look at the amount of precipitation expected between 5pm-11pm. Here's a look at 4 different models for that period and the amount of rain expected (green shading represents light rain while blues equate to moderate rain). For trick-or-treaters in Wenatchee and Omak the news is good with little if any threat of evening rain. Meanwhile holiday revelers in Sandpoint, Kellogg and Lewiston may want to think about incorporating an umbrella into the costumes. So what's our advice for locations on the western edge of the precipitation shield such as Spokane, Colville, Ritzville  and Newport? This is where the forecast gets tougher. A little slowing of the front will equate to persistent rain in these locations, but what are the odds of such slowing?

That's not an easy question to answer at this time. It will become clearer tomorrow, but until then we can rely on such tools as ensemble forecasting. For some background on ensemble forecasting, you can refer to this earlier blog entry. Anyway when utilizing ensemble model input we can see that most of the model perturbations or variations place the bulk of the precipitation threat (between 5pm-11pm) from Northport through the Idaho Panhandle, and south-soutwest toward Pullman, and Walla Walla (purple shading). This is where we have high confidence in a rainy forecast and this largely reflects the 4-panel of precipitation amounts seen above. Meanwhile, there is some agreement the front could still deliver some precipitation to locations such as Spokane, Pomeroy, and Colville (represented by blue shading). The blue shading simply states that 30-40% of the various model variations are producing measurable rain in these locations. 
Ensemble chances of measurable rainfall between 5pm-11pm Halloween

So for now our advice is pack an umbrella if you are trick-or-treating in Idaho or southeast Washington. If your plans are in east-central or northeast Washington, stay tuned for later forecasts and be prepared for the possibility of a slowing front and some rain drops. Even if rain does occur, it should be quite light meaning Halloween celebrations will not be a complete rain out.

Friday, October 24, 2014

An Update on Windy Weekend and Hurricane Ana

The Inland Northwest will see some interesting weather over the next 4 days.  First, we have a strong Pacific storm moving through the area this weekend, then the potential for remnants of Hurricane Ana.  First, let's update the forecast for the weekend wind.

The satellite shows a beautiful storm off the West Coast.  The warm front of this storm is the cloud band stretching from Washington into Montana.  The cold front is the cloud band that extends from western Washington southward off the California coast.  The warm front is bringing us rain this evening.  It will lift northward into BC.  Then we'll have a dry start to Saturday, before the cold front brings more rain Saturday afternoon/evening. 

Infrared Satellite at 730pm PDT 24 Oct 2014
Here's the UW WRF forecast of wind gusts as the low approaches the coast.

The dark blue colors along the southern Oregon coast are gusts around 60 kts (70 mph).  There's already a high wind warning out for that area.

As the low moves onshore and eastward across the US/Canadian border, winds will increase Saturday night over the Inland Northwest.  The cold front should move over the Cascades around midnight, and reach the Panhandle before sunrise.  That's when the winds will be strongest.  Below is the forecast for 5 am Sunday:

The green shading in southeast Washington is 35 kts (40 mph).  There's a bulls-eye of 60 kts over the Blue Mountains.

But this computer forecast looks too conservative.  The pattern of a deep low passing just to our north is a common pattern for high winds here.  Actually, the perfect pattern for really strong winds is where the low tracks just a little farther to the north.

So for the area from Spokane/Cd'A down to Lewiston and over to Tri Cities, expect wind gusts to 50 mph Saturday night into Sunday morning.

Then our attention turns to Hurricane Ana, churning northwest of Hawaii.

IR Satellite image 830pm PDT 25 Oct 2014

Here's the GFS forecast Friday evening.  The little white circle with the yellow shading is Ana.

GFS forecast of SLP (white lines) and precipitation (shading) for 11pm 25 Oct 2014

By Saturday afternoon, the Pacific low has picked up Ana and is moving her remnants to the north.

GFS forecast of SLP (white lines) and precipitation (shading) for 5pm 26 Oct 2014
24 hours later, the low is moving into the Gulf of Alaska towards the Canadian coast.

GFS forecast of SLP (white lines) and precipitation (shading) for 5pm 27 Oct 2014
And finally by Tuesday morning, the rain has reached the Inland Northwest.

GFS forecast of SLP (white lines) and precipitation (shading) for 5am 28 Oct 2014
Here's the track forecast from the Central Pacific Hurricane Center in Hawaii:

We can't remember ever seeing a situation quite like this.

So on Tuesday morning, as the rain falls on your windshield as you drive to work, you can thank Hurricane Ana for providing some of the moisture to the Inland Northwest.  

Once again, by the time this reaches the Pacific Northwest, it will not be a tropical storm, just the moisture remnants of it.  Still, it's kinda neat to think that our rain came in part from a Hurricane.

Thursday, October 23, 2014

A windy weekend expected

In yesterday's blog post, we talked about the potential for the remnants of Tropical Storm Ana to head to the Inland Northwest early next week.  In that blog, we briefly mentioned a possible wind storm on the weekend, before Ana arrives.  Let's take a look at some details of this event.

First, we'll look at the overall set up.  Here's the GFS model forecast for rain (shading) and sea-level pressure (red lines).

The forecast shows a deep low pressure center off the West coast Friday afternoon.  The precipitation extending across northern Oregon into central Idaho is associated with the warm front that will move into our area.  By this time, strong winds will be developing along the southern Oregon coast.

Overnight, the low deepens and moves north.  Here's the forecast for Saturday morning.

GFS Forecast SLP (red contours) and precipitation (shading) for Saturday morning 25 Oct
Now the low pressure is down to 990mb, and impressive low.  The warm front has lifted up to the WA/BC border.  The tightly packed pressure lines (red contours) indicate wind strength.  The tighter the packing, the stronger the winds.  Over the Inland Northwest, winds will be from the east on Saturday, blowing into the low.  Along the Oregon Coast it will be extremely windy.

By Saturday afternoon (below), the low moves onto the Olympic Peninsula and fills to 995mb.  

GFS Forecast SLP (red contours) and precipitation (shading) for Saturday afternoon 25 Oct

And by Sunday morning, the low will be in southeast British Columbia.

GFS Forecast SLP (red contours) and precipitation (shading) for Sunday 26 Oct

It's this Saturday night and Sunday morning period where the winds could be rather strong across the Inland Northwest.  On the one hand, the pattern for this storm is one that is common for strong winds, with the low moving by just to the north of the US/Canadian border.  On the down side, it's moving through at night, which tends to inhibit winds somewhat.  But in this case, that probably won't matter.

Now let's look a bit closer at the details.  First, here's the University of Washington WRF model forecast for Saturday.

This shows forecast gusts of 70+ mph along the central Oregon coast.  Not good beach umbrella weather.

And here's the WRF forecast for wind gusts Saturday evening:

The area of red and orange in southeast Washington is a forecast of up to 57 kts, or 65 mph.  Now, before you say "glad I don't live in southeast Washington", this is just one model forecast.  The next model run (tonight) could move that area of strong winds to a slightly different area, such as Tri Cities, or Pullman, or Spokane.  At this point, it's too early to tell exactly where the strongest winds will occur, and just how strong they'll be.

As for Tropical Storm Ana, the GFS and ECMWF models are still showing solutions similar to yesterday's blog.  Here's the GFS forecast for Tuesday afternoon:

GFS Forecast SLP (red contours) and precipitation (shading) for Tuesday 29 Oct
We'll write more about this Friday evening.

Wednesday, October 22, 2014

A Tropical Storm for the Inland Northwest?

In the past we've mentioned how much of the moisture that falls on our heads in the Inland Northwest actually traces it's origins back to the tropics.  The rain that is currently falling on the Northwest US is actually a perfect example of this.  The image below shows a global view of atmospheric moisture. 

Global Precipitable Water - 22 Oct 2014

The red/orange colors show the highest moisture is located in the tropics.  But there are lines of moisture (yellow and light blue colors) that extend away from the tropics in both hemispheres.  These are now being called Atmospheric Rivers.  Storms moving across the globe in the mid-latitudes "tap" into the tropical moisture and bring it poleward.  This is all a part of the global atmospheric circulation.  The current imagery shows that the moisture hitting the West Coast of the U.S. is an east-west "river" of moisture that originated in the central Pacific tropics.

But there's a more subtle feature hidden in these images that may impact our weather early next week.  If you look near the Hawaiian Islands (center of the image), you'll see a swirl of orange.  This is Tropical Storm Ana.  She passed just south of the Big Island late last week and it now west of the Islands.

There's a large storm further to the west (look at the large swirl east of Japan).  The computer forecast models expect this storm to merge with Ana as it moves eastward across the Pacific.  Where will it go?

The GFS model has an interesting forecast.  First, here's the weather in the Pacific on Wednesday afternoon.

GFS forecast of Sea Level Pressure (contours) and Precipitation (shading) Wed 22 Oct 2014

You can clearly see Tropical Storm Ana, as well as the Pacific Storm that we referred to in the previous satellite image.  Also you can see the large area of rain that will impact the Northwest Wednesday and Thursday.

By Saturday (image below), the Pacific storm is starting to pull the remnants of Ana to the north.  Meanwhile, a rather strong storm is forecast to hit the Pacific Northwest for the weekend.  The Washington and Oregon coasts will see strong winds with this storm Friday night.  By Saturday night and Sunday, the strong winds will move into the Inland Northwest, so be ready for that.

GFS forecast of Sea Level Pressure (contours) and Precipitation (shading) Sat 25 Oct 2014

For early next week, the GFS has the remnants of Ana moving into the Pac NW.  

GFS forecast of Sea Level Pressure (contours) and Precipitation (shading) Tue 28 Oct 2014
Let's be clear.  This is just one model forecast, and it's still 7 days away.  And even if it comes to pass, this will not be a Tropical Storm by this point.  It is merely the remnants of Ana.  Even so, it could still be a very wet and windy storm.

Here's another model forecast.  This image show the forecast moisture.  The black arrows represent the movement of the moisture.  It's a nice depiction of the potential from this storm.  The first image is the forecast for Saturday. It shows the moisture from Ana merging with the Pacific storm on the left side of the image.

GFS Integrated Vapor Transport Forecast for Saturday 25 Oct 2014

And by the middle of next week, there's a lot of moisture heading for the West Coast.
GFS Integrated Vapor Transport Forecast for Wed 29 Oct 2014

However, as mentioned, the GFS is just one forecast model.  Here's the ECMWF model forecast for this storm Sunday.  The Pacific storm is entering the Gulf of Alaska, with the remnants of Ana to it's south.

But by next week, the ECMWF has the low much farther north, impacting the west coast of British Columbia.  We could still see some rain from this forecast, but not as much as what the GFS is forecasting.

So as usual, stay tuned to the forecasts.  As this situation evolves, we'll try to update this blog to give you a better idea of what to expect.