Wednesday, November 25, 2020

An Extraordinary Front And The Thanksgiving Day Forecast

I received a half-dozen emails yesterday about a very odd feature apparent on the National Weather Service radar at Hoquium.   Let me just show you!

Here is the lowest elevation radar image at 1:30 PM yesterday (Wednesday).   Red indicates very, very heavy rain, with yellow and orange just plain heavy.   What a bizarre looking feature crossing the coast, with undulations and regular gaps between heavy precipitation segments!

We learned about this features during the 1970s when the first radars were placed on the coast during some weather field experiments.

The feature in question is a narrow cold front rainband, associated with an unusually strong cold front crossing the coast.

Impressively, our high resolution forecast models were able to predict this strong front, as illustrated by the predicted surface winds, sea level pressure, and temperatures around 800 m above the surface at 1 PM (see below).   A very strong pressure trough (area of low pressure) is evident as is a very sharp wind shift from southwesterly to northwesterly flow and a decline in temperature.   A front you can not ignore.

What I really couldn't believe is that a simulated radar image from the forecast model actually produced a realistic corrugated narrow cold frontal rainband just like the one observed by the radar.  We have come a long way.

Sharp fronts like this one can produce severe low-level turbulence.  During an atmospheric field program called COAST, a bunch of atmospheric scientists flew through such a front at low-levels (~2000 ft).  The plane experienced several g's up and down and the interior of the aircraft was left in disarray.  The pilots made it clear---never again.

But you won't to worry about such a front on Thanksgiving Day!

Tomorrow, an upper level ridge of high pressure/heights will develop over the region--and such features produce sinking air and dry conditions (see upper level map--500 hPa, roughly at 18,000 ft--at 1 PM Thursday)

The total precipitation for the 24h ending 4 AM Friday shows nothing over Seattle and 
eastern Washington and only a passing sprinkle for the rest.

Plenty of clouds on Thursday, with some passing sun breaks.  Highs in the upper 40s.

Good enough to get a nice walk or run before enjoying your holiday meal.

Monday, November 23, 2020

An Important Finding about the September Labor Day Wildfires

One reason why research is so much fun is that once in a while you learn something important that is unexpected.  A new and highly significant finding. 

I had such a "wow" experience recently regarding the September wildfires that caused so much damage and smoke on the western slopes of the Cascades.

Currently, I have a research grant form the National Science Foundation, and smaller grants with the USDA Forest Service and WA DNR, to work on wildfire/weather issues. Recently, our group has completed three papers on the intersection of weather/climate with wildfires:  on the Wine Country Fires of 2017, the Camp Fire of 2019, and Diablo downslope winds of northern CA.

I was working on a paper on the meteorology of the great western Oregon/Washington wildfires, when the big Labor Day Oregon wildfire siege occurred.  So naturally my group turned to understand the event.

Why did this major event happen?  How did it evolve? Was something unusual going on?  How well was it forecast?  These and other questions were on the table.

We knew the strong easterly (from the east) winds during the event were critical for initiating and/or spreading the fires.  In fact, my initial work suggested that ALL major fires on the western slopes of the Oregon and Washington Cascades are associated with powerful easterly winds.

I asked research meteorologist David Ovens to take a look at the upper air weather observing sites in the region, locations where balloon-lifted weather stations (radiosondes) are launched twice a day to give us winds and other weather variables aloft.    

Of particular interest was the Salem, Oregon radiosonde data at the first standard elevation above the surface (925 hPa--around 800 meters above the surface).   This elevation is very relevant to winds observed over the nearby western slopes of the Oregon Cascades.

The record at Salem goes back 64 years to 1956, long enough to tell us a great deal about how unusual the situation was this September.  It did not take Dave long to send me a figure with the requested information and I had my wow moment.

Below is the figure.  Let me explain it.  

I asked him to only plot the 925 hPa (again about 800 m about sea level) winds and to limit the analysis to August and September, since those are the months of historical big fires on the western slopes of the Cascades.  Each observation during the 64 years during those months was plotted, with the associated wind direction indicated by the x-axis and the wind speed on the y-axis.   

You will notice two major peaks in strong winds during those late summer months:  (1) northerly to north-northeasterly and (2) south to southwesterly.    The northerly wind peak occurs when high pressure builds over the eastern Pacific and the southwesterly powerful winds occur when a strong trough or low-pressure system approaches the coast.    The southwesterly winds are the strongest (up to 51 knots!), but they are associated with clouds and rain, so little fire danger from them.

I asked Dave to identify the observations taken during the Oregon fire storm period with red dots--- and that is when the wow moment came.
Look at the red dot for September 8th at 1200 UTC--5 AM (2020-09-08 12z)---just when the fires were accelerating over western Oregon (indicated by red arrow).   

Just extraordinary.  The winds at that time were THE STRONGEST EVER OBSERVED  at the site  during those months for any wind direction from the north, east, southeast, or northwest.  

 The stronger the winds the greater the potential for rapid fire growth, and the greater the potential for fire ignition by failing electrical infrastructure and other causes.

Importantly, these were the strongest winds by far from the east and thus downslope on the western slopes of the Oregon Cascades.   Downslope winds from the east are inevitably very dry and the air progressively became drier the longer they blow from that direction.

So why were the easterly winds so strong?   

Our research identified the reason:   unusually cold air and accompanying high pressure moving to the east of the Cascades.  

The pressure analysis 5 AM Sept 8th is shown below, with a measure of the difference from normal shown by colors. High pressure was centered over Idaho and extended into eastern Oregon.   The colors indicate the pressure was VERY unusual--up to 4-5 standard deviations from the mean for that date.  To give you an idea of how unusual, if the deviation was 4.5, this would indicte an event that would occur once in 147,000 times.

The air was so cold that it brought record cold and snow to the Front Range of Colorado and environs.

So the whole situation is ironic and interesting:  record cold to the east brought record fires to the west.
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