Thursday, January 16, 2014

Does a dry winter lead to a bad fire season?

We have received many questions from the public inquiring whether our dry start to the winter will equate to a bad fire season this summer. Common sense would suggest that if the fuels begin the fire season on a dry note that should increase the risk of fires later in the year. But is that necessarily true? Before we address that notion, lets review the weather pattern thus far this water year and what the weather looks like for the remainder of the winter. For climate purposes in the western U.S., the water year is defined as the 12-month period beginning in October. Since October, the western U.S. has generally been affected by a persistent ridge of high pressure fixed just off the coast. Meanwhile, the remainder of the U.S. has seen a rather persistent trough of low pressure. This scenario is reflected in the mean 500 mb (~18k ft MSL) seen below. Notice the large buckle in the flow centered just of the western coast of North America and the dip over the eastern half of the continent.
500 mb pattern October 1-January 12 2014

This is a rather unusual flow pattern. The typical flow pattern for this time period generally features swift west-southwest flow into the region as seen on the map below. This southwesterly flow often draws deep moisture from the sub-tropics and slams it into the Pacific Northwest (think Pineapple connection or atmospheric river). This setup produces frequent and heavy precipitation, especially west of the Cascades. 
Typical 500 mb pattern October-early January.

When we compare the top image (October 2013-early January 2014) to the bottom were are left with some pretty sizable departures from the normal which is termed an anomaly. The biggest anomaly this wet season was the huge red bulls-eye centered over the Gulf of Alaska. This is the culprit responsible primarily for our unusually dry winter thus far. 
500 mb anomaly (red=higher than normal heights, blue=below normal heights)

So climatologically speaking, how has that translated to the precipitation departures from normal across the area? As you can probably guess, the precipitation this wet season has been quite meager.  Most locations across eastern Washington have seen less than half the normal amount of precipitation with locations near Wenatchee, Omak, and Moses Lake seeing less than 25% of the normal precipitation (dark red). As of January 13th, both the Omak and Wenatchee areas have received less than an inch of precipitation since October 1st, and both were the 2nd driest on record, only trailing an even drier 1976. 
October-December 2013 precipitation percentages of normal. 

An obvious implication of this lack of precipitation was an even more impressive lack of snow pack and the water contained in that pack. Needless to say our snow pack is running much below normal, especially in the Cascades where totals were hovering between 50-75% of normal (yellow and orange shading). Conditions were significantly worse across Oregon and California (not shown).

Current snow water equivalent 
So being as dry as it's been can we expect to recover to near normal conditions by the end of winter. Based on past winters with similarly dry beginnings, the answer is no. See this previous blog link for more details. So does the weather in the next 8-14 days look promising for ending or at least lessening this dry winter pattern? The answer to that is also no, as as the strong eastern Pacific ridge is expected to rebuild and remain fixed over the region (note ridge axis right off the coast) for the next couple weeks. Below is a map of the mean 500 mb pattern expected over North America through the end of the month. 

Mean 500 mb pattern expected through end of January

This translates to little promise for appreciable precipitation (dark brown area over Washington is a 60-70% chance of below normal precipitation) 

Precipitation probability Jan 24-30, 2014
Based on the current dry conditions and little improvement expected anytime soon, can we assume that will translate to a busy fire weather season? To determine that answer we took a look at annual fire acreage across the Inland Northwest since 1970 and compared it to the winter precipitation conditions that preceded it. We first took a look at the precipitation measured at the Spokane Airport between October 1st and January 15th and ranked those values. We then compared these precipitation rankings to the amount of acreage which burned in the following fire season. Below are the results. 

Of the 10 driest October-January 15th periods in Spokane,  4 of the following fire seasons resulted in above-average fire acreage (average burned acreage is 61,000), while 5 saw well below-average fire activity. That alone would suggest there isn't any correlation between a dry winter and a bad fire season.  In fact, the driest year, 1976-77 only resulted in the 26th ranked fire season. But note that 4 of the top 6 fire seasons are in this list (2001, 2012, 1994, 1970).

Top 10 driest Oct-Jan 15th since 1970  in Spokane and the following amount of burnt acreage
  If we rank the seasons based on precipitation in Wenatchee, the message is still muddy. Four of the following summers were active fire years, but the other 5 were below normal.

Top 10 driest Oct-Jan 15th periods since 1970 in Wenatchee and the following amount of burnt acreage
This suggests that a dry beginning to winter doesn't necessarily lead to a busy fire season. What about the opposite? Does a wet season lead to a quiet fire season? 

Top 10 driest Oct-Jan 15th periods since 1970 in Spokane and the following amount of burnt acreage

Similar to the dry winters, the numbers are about evenly split. Four summers following wet winters had active fire seasons, and 5 were less active.  In fact, the wettest Oct through mid-January (2006/2007) was followed by the largest fire season since 1970.

Top 10 driest Oct-Jan 15th periods since 1970 in Wenatchee and the following amount of burnt acreage
In Wentachee the 10 wettest winters since 1970 led 6 of the quietest fire seasons (ranks of 35 or higher). So perhaps a wet winter could yield a fair prediction to a slow fire season, especially when considering the Wenatchee precipitation. After all, most of the fire acreage in the Inland Northwest typically burns near the Cascades. This all makes sense, since a wet winter will typically yield a large snow pack which will take longer to melt.  So the higher elevations have no chance of wild fires until much later in the summer, if at all, which reduces the amount of forest available to burn during the summer.

So what does all this mean?  Most large fire seasons are the result of the combination of lots of lightning and dry fuels.  Just because you have one, doesn't mean you'll get the other.  So even if you have a dry winter, a quiet lightning season will result in a quiet fire season.  However, as we saw from the tables above, a dry winter lead to 4 of the top 6 fire seasons.  Meaning, if you do happen to have an above-average fire year, it has the potential to be a really bad year.

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