Wednesday, December 17, 2014

Are the chances of a White Christmas improving?

   Since our last post, we have gotten a little more resolution on the prospects of seeing a White Christmas across the Inland Northwest. But before we answer that question we need to deal with a weak weather system on Thursday night and Friday and then a much stronger system for the weekend. As of our latest forecast (issued 3pm Wednesday) the Thursday night and Friday system is calling for light snow, mainly confined to the mountains. Valley snow chances will be reserved for the Cascade valleys, Okanogan Valley, and Waterville Plateau. These locations will generally see amounts ranging from 1-3 inches.
Thursday night-Friday snow forecast
The storm system for Saturday and Sunday stands a much better chance of producing significant snowfall as it will contain much more atmospheric moisture. The only problem is the snow will need to battle with warming temperatures as this moisture source will have origins in the sub-tropics. Our confidence is high that moderate to heavy snows will fall in the mountains near the Canadian border and in the Cascades. Amounts well over a foot are possible. Meanwhile, the forecast for the nearby valleys is a tougher call as temperatures will be critical. If they warm much above 32°F the snow accumulations could be less than forecast, in fact much less. Right now, we are forecasting temperature very close to freezing over most of these lower elevation locations.  

Weekend snow forecast
So by Sunday afternoon temperatures will warm significantly as the warmth associated with the sub-tropical moisture continues its northeastward surge. Snow levels will rise to 4000' or higher over the entire Inland Northwest. This will translate to melting snow over most of these valley locations. The big question is can we melt what falls? That depends on how much accumulates over these locations. The warming will be accompanied by breezy conditions which will help melt the snow especially when the dewpoint temperature and nighttime lows rise above freezing. And that's what we are expecting as of our latest forecasts. Here's a look at the high-temperature forecast for Sunday.
Sunday high-temperature forecast
 So based on the current forecast the only valley locations which stand a chance of snow before Christmas Eve would be near the Canadian Border, Cascades, or Waterville Plateau. How about the remainder of the region? Well, as we talked about in our last blog entry we are expecting a big pattern change, right around Christmas Eve. That's still true. Our mild west-southwest flow of late will take a decided cooler turn to the northwest. Here's what the upper-level pattern is going to look like (at least something like it anyway). The wind flow will be parallel to the yellow height lines. That means our flow will be riding through the Gulf of Alaska before it moves into the Inland Northwest. Meanwhile, the remnants of the weekend moisture stream will get hung up somewhere over the Pacific Northwest. The big question is where is that going to happen? Most likely it will occur immediately downstream or east of the kink in the yellow lines. In the picture below that suggests somewhere over the southern third of Washington and northern Oregon. 
500 mb pattern for Christmas Eve
So is there good model agreement in where that kink is going to form? That is the key to this forecast. As of the latest model runs, consensus is growing. Here's a look at four various model runs all looking at the afternoon hours of Christmas Eve. We have placed the precipitation forecast on top of the 500 mb yellow height lines. Notice, they all show a similar kink in the height lines or flow, however they vary on how far north to place it based on a trailing low-pressure system dropping through BC. The model in the lower right is the farthest north of the solutions due to the strength of the low (circular height line) over southern British Columbia. This places the band of heaviest precipitation (blue and purple shading) over the Idaho Panhandle and the eastern quarter of Washington. Meanwhile, the other solutions are not depicting a closed low and hence drive the moisture plume farther south.  The further south solutions have become more common and are beginning to raise our confidence for locations near the Oregon/Washington border such as Lewiston, Pullman, and Ritzville seeing the best chances of precipitation. However, even in those locations temperatures may remain just a little too warm for snow.  

4 model solutions to the Chrismas Eve storm with 500 mb heights and precipitation
So to answer our original question. Are the chances for a White Christmas improving? For locations such as Mazama, Republic, or Plain the answer is a decided yes. For Lewiston or Pullman the answer is a maybe. Folks in Spokane, Couer d'Alene,  Moses Lake, or Wenatchee the odds are not great. Of course, Christmas eve is still  a week away and much can change between now and then. So stay tuned and maybe just maybe the yellow kink will decide to set up over the entire Inland Northwest resulting in a White Christmas for all. 

Monday, December 15, 2014

Looking for significant snow? Is there any hope soon?

  Well, another week into December and much of the Inland Northwest has yet to see any significant snow. The only exceptions have been near the Cascades and a small part of the Okanogan Valley. As of this afternoon, here what the snowpack summary looked like. Notice there is very little if any snow in the valleys. If you wanted to see significant snow, you'd have to trek into the mountains north and west of Wenatchee and Omak  or north of Sandpoint.
Snow depth as of 5pm Monday 12/15/14
Although these areas were shaded in purple indicating anywhere from 20" to 50" of snow on the ground, that pales vs. where we are supposed to be this time of year. Here's a look at the amount of water that's in the current snowpack. Generally speaking, it's right around half of where it should be this time of year (orange shading) and is actually far worse across most of the Cascades!

Snow water equivalent vs. normal

So is there any hope we can add some more snow to the forecast this week? Actually there is some hope however it won't add up to much. We have several weak weather systems set to impact the Inland Northwest this week. The first will arrive late tonight and into Tuesday. For now, here is our forecast of snow.
Snow forecast for late tonight-Tuesday
As you can see we have light snow forecast across much of the region. We are most confident about the snow totals near the Cascades and northern Valleys. Elsewhere, it looks like the snow might begin too late in the day so it might make accumulations difficult to come by. But that's not the only hope for snow. A couple weak disturbances look like they will impact the region through the remainder of the workweek. Here's our latest forecast of snow for late Thursday through Friday night. Confidence is not high as these will be weakening and warming winter storm systems. Temperatures away from the Cascades and northern valleys could be marginal for snow, especially accumulating snow.

Snow potential for Thursday afternoon-Friday night
Even if the snow materialises, it won't likely stick around long as we will see a yet another round of very warm and wet weather. This weather will be brought to us by another atmospheric river or Pineapple Express. Here is the model depiction of the latest plume. The moisture is represented by the stream or river of greens colors extending across the eastern Pacific to the Washington Coast. Notice the plume originates to the west of Hawaii (lower left corner of the picture), suggesting it will be a very juicy airmass. Temperatures should surge well above freezing over most locations which will melt most of not all of the valley snow which falls this week.
Atmospheric river forecast for Saturday

So this warm air is only forecast to persist through Monday night or so which doesn't bode well for a White Christmas. But is all hope lost? First off this would be a good time to show what the climatological odds are of seeing a White Christmas. There is a very large variability across the Inland Northwest ranging from near 100% for the northern valleys of Washington, and most of the Idaho Panhandle to less than a 25% chance by the time you go south toward the Tri-Cities and Lewiston. For a more detailed look of the map below check out this link from the National Climate Data Center.

Odds of a white Christmas. 
So enough about climatology, what about this years weather? There are actually some good indications that we could see a moderate to major winter storm on Christmas Eve. Well, why is that? It looks like we will see a very good setup where the moisture from early next week sticks around and is intercepted by much cooler arriving from the northwest.
500 mb pattern showing northwest flow from BC and Gulf of Alaska

Notice the kink in the flow setting up near the Washington/Oregon border. Where this sets up gives us a clue for where we can expect the heaviest snow.While there are model uncertainties as to where this kink sets up most of the model solutions are showing this. Here's a look at several solutions for Christmas Eve. Notice they are all showing a band of moderate to heavy precipitation (purple and red shading), they differ significantly on where to place the band.
Various model solutions for precipitation on Christmas Eve
 As this forecast period nears, we should be getting a better handle on where the band of precipitation sets up. The good news is whatever falls should easily fall as snow as we rid ourselves of the above freezing temperatures. So if you are wishing for a White Christmas this year, your wishes just might come true!

Monday, December 8, 2014

Where's the Snow?

We're now into the second full week of December, and most of us in the Inland Northwest haven't seen much snow yet.  The notable exception is the folks in the Methow Valley, which received up to 18" from a storm just before Thanksgiving.  Outside of that, most locations have seen a dusting here, half an inch there, but nothing that survived for very long.  

So the first question often is "Has this ever happened before?"  Our climate records for this area generally go back into the early 1900's with some sites having data into the late 1800's.  And some sites have good, continuous data, while others are a bit spotty.  So lets take a look at a few sites that have good data and are representative of the area.

First we have to define how to look at the early season snow.  The first measure is fairly obvious: "How much snow do we normally have by this date?"

Location
Avg Snow through   7 Dec
Total Snow 
7 Dec 2014
Lowest Snow through 7 Dec
Spokane Airport
9.6”
0.9”
0.3” in 1954
Wenatchee WP
3.8”
2.3”
0” in 2012
Moscow
7.6”
2.0”
0” in 1904
Priest River
13.9”
4.6”
0” in 2002
Republic
9.3”
10.7”
Trace in 1943
Winthrop
15.9”
20.0”
Trace in 1969
Kellogg
7.4”
1.7”
0” in 1920

As you can see, in most locations, we're behind our Average snow fall through December 7th.  But we're still doing better than other bleak years.  So no records there.

Another way to look at it is the average date of the first inch of snowfall. 

Location
Avg Date First 1” Snow
2014 First 1” Snow
Latest First 1” Snow
Spokane Airport
Nov 19th
Not Yet
Dec 23rd 1976
Wenatchee WP
Dec 6th
Nov 22nd
Feb 1st 1963
Moscow
Nov 22nd
Nov 29th
Jan 4th 1990
Priest River
Nov 14th
Nov 25th
Dec 15th 1926
Republic
Nov 13th
Nov 22nd
Dec 23rd 1976
Winthrop
Nov 12th
Nov 22nd
Dec 11th 1936
Kellogg
Nov 20th
Dec 4th
Dec 25th 1954

Again, most locations got a late start, but nothing that would break any records.  The exception is the Spokane metro area, which has yet to even have it's first inch of snow.

So what's been the cause of this lack of snow?  Here's the average temperature for the Nov 1st - Dec 5th period.


As you can see, for the past 30 days it's been actually colder than normal for the Inland Northwest.  Of course, this is an average of 2 cold snaps with some rather mild weather in between.  Here's the daily temperature data for Spokane:


The blue bars are the daily temperatures, while the red and blue shading shows the daily records.  Spokane hasn't had any record highs or lows during this period, but they've come close.  The mild temperatures are certainly not conducive to snow.  But what's frustrating is that the cold snaps didn't equate to much if any snow.  Typically, cold air that moves in from Canada is very dry and doesn't provide much snow, which was the case with both of these systems.  But then eventually a Pacific storm will bring moisture into the area and it will fall as snow until the cold air can be pushed out of here.  But in both of these events, that didn't happen.  Instead, the Pacific storm went south of our area, leaving the Inland Northwest largely dry.

The percent of average precipitation shows that after a promising start to the wet season in late October, we've been fairly dry over the past 30 days.



So what has this meant for the mountain snow pack?  It's not good.  Here's the Snow Water Equivalent (SWE) analysis (i.e. how much water is in the snow that's on the ground).  The color shading represents the comparison of this year to how much is typically in the snow pack by December 8th:



It shows that most of our area is lagging behind normal at this point.  While this is bad news for ski enthusiasts, it's way too early to worry about it from a water supply standpoint.

Is there any hope of a change in the weather?  Not really.  Here's the 8-14 day outlook for temperatures and precipitation from the Climate Prediction Center (CPC).






The folks at CPC see warmth for next week across just about the entire US, while precipitation is expected to be near to below normal for mid-December.  This is an average outlook for a 7 day period.  So there can still be a weak snow event buried in there, but it's not likely.  And since this outlook ends on December 22nd, the issue of a White Christmas is starting to come into play.



Thursday, November 13, 2014

Winter 2014/15 Outlook

One of our most asked questions is "What is the winter going to be like?"  In this blog, we'll try to answer that question as best we can.  As most people know, looking this far into the future is more of an "Outlook" than a "Forecast".  

Most everyone has heard of El Nino and La Nina, especially when we're talking about the winter outlook.  We often refer to this as ENSO (El Nino Southern Oscillation)  There are other "oscillations" in the atmosphere that affect our weather.  However, ENSO is the only one that is linked to the ocean temperatures.  This is important for two reason.  First, we have some skill at forecasting ocean temperatures. Second, the ocean temperatures don't suddenly change.  These two facts allow us to predict the atmosphere several months into the future.  Most of the other oscillations are purely atmospheric.  As such, we can predict them only 1-2 weeks into the future with any skill.  What does all of this mean?  ENSO is just one of the influences on our winter weather, and the only one that we can predict with any skill.  The other oscillations are less predictable, and could alter or even override the effects of ENSO.  So let's first take a look at what is the state of ENSO.

The area of the Pacific Ocean that is monitored for El Nino is shown in the figure below.
ENSO Monitored Regions

The Nino 3.4 is the most critical area, although the other regions also play a role.  What is monitored is the Sea Surface Temperatures (SSTs) in this area.  And the main way to look at these is to observe how the temperature compares to what we "normally" observe in this area.  This comparison is referred to as an "anomaly".  For El Nino, the anomaly must be 0.5C warmer than normal for 5 months.  La Nina is defined as an anomaly of 0.5C cooler than normal.

Currently, the SST anomaly in the 3.4 region is very close to 0.5C.  It's actually been close to that value since early summer, but decline a bit in August before warming this autumn.


Sea Surface Temperature Anomalies in the 4 NINO regions

So the Nino3.4 region is already close to the 0.5C criteria for El Nino.  So what is the forecast for the SSTs?  Here's a figure that shows a number of numerical predictions from several different countries.



This graph shows that the majority of forecasts lie between the 0.5C and 1.0C lines through the winter and next spring.  This would equate to a "weak" El Nino.  There are a few that expect a somewhat stronger El Nino, as well as a few that are predicting no El Nino.  

Here's the prediction from a number of runs from the U.S. CFS model:


Again, the majority (but not all) of the forecasts are between 0.5 and 1.0C, with additional warming during the summer.  The European climate model has a similar forecast:

So the take-away from all of this is that the majority of computer forecast are expecting a weak El Nino to persist through the winter and into the Spring of 2015.  The latest NOAA Climate Prediction Center discussion states that there is a 58% chance of an El Nino.  

The affects of El Nino on North America weather are fairly well understood.  And these are reflected in the official winter outlook from NOAA:






For the Pacific Northwest, the expectation is for warmer and drier conditions than normal.  And this fits what we commonly see in our area during an El Nino winter.

Here's a bar chart showing the observed temperatures at Spokane for the winters since 1949/50.  The bars are colored in red (El Nino), blue (La Nina), and white (neutral).  The black horizontal line represents a "normal" winter.







As you can see, just about every red bar (El Nino winters) is above the black line, meaning that the El Nino winters are warmer than normal.  

A similar graph for precipitation is found below:


In this instance, you can see more variability with the red bars.  Some El Nino winters are wetter than normal, others are drier than normal.

The third graph shows the occurrence of snow.



This is a fairly clear signal.  We haven't seen an above-normal snowfall El Nino winter since 1977/78.  Most El Ninos bring below normal snowfall to the Inland Northwest.

So the outlook for this winter in the Inland Northwest is for above-normal temperatures, with below-normal snowfall.  But as was earlier stated, this is by no means a done deal.  The other atmospheric oscillations could still alter the weather beyond the affects of ENSO.  Additionally, this is an outlook for the entire winter.  So there could still be a frigid or snowy week or two.  But when the winter is all said and done, we'll probably look back at it as a milder winter.



















Friday, November 7, 2014

Winter Arrives Tuesday

We've had unprecedented warmth this Autumn season.  But all that is about to end.  First, let's take a look at how warm it's been.  

October was a record warm month is some locations.  Here's a quick table showing the warmth:


  •  Wenatchee
    • Oct 2014: 56.6F - 2nd warmest October
    • Record: 57.3F in 1988
  • Spokane
    • Oct 2014: 53.3F - 5th warmest October
    • Record: 54.5F in 1952
  • Lewiston
    • Oct 2014: 57.9F - 3rd warmest October
    • Record: 58.7F in 1988
The warmth actually extends back into mid-September.  Here's a graph of the temperatures at Spokane Airport for the last 2 months:



The blue bars are the observed high and low temperature this year.  The brown shading is what we typically see for temperatures on these days.  And the blue and red lines are the coldest/warmest temperatures ever observed.  So you can see that we didn't set any daily records at Spokane.  But notice how the observed temperatures were consistently above normal, with just a few cool spells.  Add to that, Spokane didn't reach the freezing mark until Nov 2nd, which ties with 2005 as the latest ever for the Airport first freeze.  

But as we said, all of that is about to change, and in a big way.  Here's the weather pattern that is going to do it.  Below is a depiction of the temperatures at about 5000' above sea level for this afternoon (Friday 7 November):

850mb Temperatures Friday afternoon 7 Nov 2014

This image shows that the coldest air is over north-central Canada with mild air over all of the western US.  Below is a forecast for Monday morning:

850mb Temperatures for Monday morning 10 Nov 2014

Now the cold Canadian air has penetrated into the northern tier of the US.  You can see that the Inland Northwest is right on the edge of the cold air.  By Wednesday morning, even colder air has made its way into the lower 48:

850mb Temperatures for Wednesday morning 13 Nov 2014

By this time, cold air has completely moved into all of Washington and Oregon east of the Cascades.  Even Portland will probably see some cold air seep through the I-84 Gorge.  Also note how far south the cold air will move in the central US, all the way down to the Texas Panhandle.  The coldest air will be over Montana into the Dakota, but the Idaho Panhandle will still be plenty cold.

What's causing this pattern shift?  In part, it's Super-Typhoon Nuri in the western Pacific.  Nuri developed east of the Philippines on Halloween, rapidly strengthened, and "recurved" into the westerlies.  The CIMMS blog has some great satellite images of it.   

As Nuri moved northward, it converted to an extra-tropical cyclone, and has currently become a massive storm in the Bering Sea.  So why does this affect our weather?  First, he's a graphic showing the jet stream today.


Jet Stream analysis Friday 7 Nov 2014


The yellow line represents the jet stream.  As you can see, the jet stream is coming at us from a very southern latitude, just north of Hawaii.  But as Nuri's remnants develop and move into the Bering Sea, the weather pattern will shift.  

Jet Stream forecast Monday 10 Nov 2014

By Monday, the jet stream will be directed from the central Pacific into Alaska, where it will pick up cold Canadian air, and bring it southward into the lower 48 states.

So how cold will it get?  Here's the forecast high temperatures for Tuesday.



That's right.  High temperatures on Veteran's Day will be below freezing for much of the Inland Northwest.  And here's the forecast low temperatures for Wednesday morning:


Yes, you're reading that right.  Low temperatures in the teens in many places.  This is way colder than anything we've seen this Autumn.  If you've been putting off those Fall Chores (e.g. blowing out sprinklers, putting away hoses, etc), this weekend will be your last chance to do that.

And with these temperatures, people will naturally be wondering if there's any snow that will come with it.  About the only chance for snow will be in the northern Panhandle and northeast Washington on Sunday Night and Monday Morning.  Here's the current forecast:






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.