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.

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