Tuesday, February 25, 2014

A Terrible Forecast

If you live in the vicinity of the Spokane metro area, you're probably asking yourself "what happened to the forecast?"  In a word, it was a terrible forecast.  Here's the forecast we issued on Sunday afternoon for Monday:

COEUR D`ALENE AREA-SPOKANE AREA-
 INCLUDING THE CITIES OF...COEUR D`ALENE...POST FALLS...HAYDEN...
 WORLEY...SPOKANE...CHENEY...DAVENPORT...ROCKFORD
 309 PM PST SUN FEB 23 2014
 
 ...WINTER WEATHER ADVISORY IN EFFECT UNTIL 11 PM PST THIS
 EVENING...
 
 .TONIGHT...SNOW IN THE EVENING. SNOW ACCUMULATION OF 1 TO
 2 INCHES. LOWS 19 TO 21. NORTHEAST WIND 5 TO 15 MPH WITH GUSTS TO
 AROUND 25 MPH. CHANCE OF PRECIPITATION NEAR 100 PERCENT. 
 .MONDAY...MOSTLY CLOUDY. HIGHS IN THE LOWER 30S. NORTHEAST WIND
 5 TO 15 MPH. 
 .MONDAY NIGHT...MOSTLY CLOUDY. A 20 PERCENT CHANCE OF SNOW IN THE
 EVENING. LOWS 18 TO 21. NORTHEAST WIND 5 TO 10 MPH. 


After Sunday's snow wound down, we expected that Monday would be dry, with just a 20% chance of snow on Monday evening.  

We updated the forecast at 338am Monday morning.

COEUR D`ALENE AREA-SPOKANE AREA-
 INCLUDING THE CITIES OF...COEUR D`ALENE...POST FALLS...HAYDEN...
 WORLEY...SPOKANE...CHENEY...DAVENPORT...ROCKFORD
 338 AM PST MON FEB 24 2014
 
 .TODAY...MOSTLY CLOUDY WITH A 20 PERCENT CHANCE OF SNOW. HIGHS IN
 THE LOWER 30S. NORTHEAST WIND 5 TO 15 MPH. 
 .TONIGHT...MOSTLY CLOUDY WITH A 20 PERCENT CHANCE OF SNOW. LOWS
 18 TO 21. NORTHEAST WIND 5 TO 15 MPH. 


We now had a 20% chance of snow for Monday and Monday night.  But still, when we have probabilities that low, we don't even include the expected snow amounts.  Even if we had, it wouldn't have been anywhere near what actually happened.

Most people understand that a forecast a few days into the future is a bit iffy and subject to change.  But a forecast only a few hours into the future?  Surely we can get those right.  So how could we bust a forecast less than 8 hours into the future?

First, let's take a look at the weather pattern.  Here's the surface analysis at 4am Monday:


The red and blue line south of Washington is called a stationary front.  This front moved down from Canada through Spokane on Sunday, bringing us our Sunday snow.  But it stalled over northern Oregon.  Note that there's no big areas of low pressure off the Washington/Oregon coast. 

Next, we'll look at the satellite image at this time:



We realize that this may not mean a great deal to most of you.  But trust us, the satellite doesn't show any big storm over the ocean, headed our way.  In fact, the satellite shows a rather disorganized atmosphere.

But perhaps the radar was showing something.  Here's a image from 330am


The radar doesn't show any big areas of precipitation headed our way.  There is a couple of bands of precipitation north of Seattle, and some snow showers over the southern Idaho Panhandle.

So what did the computer models show?  Here's the forecast 6-hour precipitation for Monday afternoon from 3 of our models.


NAM Forecast Precipitation 1-7pm

GFS Forecast Precipitation 10am-4pm


HRRR Forecast Precipitation 9am-3pm


As you can see, 3 different computer forecasts all put an west-to-east band of precipitation over southeast Washington into the southern Idaho panhandle, well south of the Spokane area.  And this made sense, given how far south the stationary front was.

Bu 830am, surely we knew the forecast wasn't panning out, right?  Here's the radar image at that time.


830am Radar

There's definitely more precipitation west of the Cascades than a few hours ago.  And a band of very light precipitation has begun to form south and west of Spokane.  But still, we expected the precipitation to stay south of Spokane, and so far it's looking good.

930am Radar
An hour later at 930am, the band near Spokane is starting to fill in.  At this point, the forecast was updated for the Spokane/Coeur d'Alene area for up to an inch of snow during the day, and possibly another inch in the evening.

 COEUR D`ALENE AREA-SPOKANE AREA-
 INCLUDING THE CITIES OF...COEUR D`ALENE...POST FALLS...HAYDEN...
 WORLEY...SPOKANE...CHENEY...DAVENPORT...ROCKFORD
 946 AM PST MON FEB 24 2014
 
 .REST OF TODAY...SNOW. SNOW ACCUMULATION UP TO 1 INCH. HIGHS
 AROUND 30. NORTHEAST WIND 5 TO 15 MPH. CHANCE OF PRECIPITATION
 90 PERCENT. 
 .TONIGHT...MOSTLY CLOUDY. SNOW LIKELY IN THE EVENING. SNOW
 ACCUMULATION UP TO 1 INCH. LOWS 18 TO 21. NORTHEAST WIND 5 TO
 15 MPH. CHANCE OF PRECIPITATION 60 PERCENT. 



By 1230pm, the radar is clearly showing a well-defined band of snow from around Omak to St Maries.  Additional echoes are now moving into south-central Washington.

1230pm Radar

The forecast is updated yet again, this time issuing a Winter Weather Advisory for the metro area.  

COEUR D`ALENE AREA-SPOKANE AREA-
 INCLUDING THE CITIES OF...COEUR D`ALENE...POST FALLS...HAYDEN...
 WORLEY...SPOKANE...CHENEY...DAVENPORT...ROCKFORD
 1249 PM PST MON FEB 24 2014
 
 ...WINTER WEATHER ADVISORY IN EFFECT UNTIL 10 PM PST THIS
 EVENING...
 
 .REST OF TODAY...SNOW. SNOW ACCUMULATION NEAR 2 INCHES FROM SPOKANE
 SOUTH...WITH UP TO AN INCH ELSEWHERE. HIGHS IN THE UPPER 20S TO LOWER 30S.
 NORTHEAST WIND 5 TO 15 MPH. CHANCE OF PRECIPITATION NEAR 100
 PERCENT. 
 .TONIGHT...MOSTLY CLOUDY. SNOW LIKELY IN THE EVENING. SNOW
 ACCUMULATION UP TO 1 INCH. LOWS 18 TO 21. NORTHEAST WIND 5 TO
 15 MPH. CHANCE OF PRECIPITATION 60 PERCENT. 


Even at this stage, the forecast was only for 2-3 inches at best.  

So how much snow actually fell?



You can see that the metro area received the most snowfall.  Locations to the north (e.g. Deer Park, Athol) as well as to the south (e.g. Palouse, Ritzville) received about half of what the Spokane and Coeur d'Alene area picked up.

In summary, this was not a typical snowfall event for our area.  A typical event is one characterized by a large storm moving in from the Pacific, bringing widespread precipitation to the area.  And that's the other aspect of this event: it was localized.  Those two attributes (atypical and localized) are the most difficult to forecast.  And they have the largest potential to be wrong.  Instead of the snow hitting the Palouse, it winds up 50 miles to the north, over the Spokane metro area.

Additionally, none of our computer guidance saw it coming either.  Add all of these factors together, and you get a busted forecast.

























2 comments:

  1. Thank you for all of your hard work! Things happen...

    ReplyDelete
  2. Great write-up! Post mortems are interesting. Going back and looking at SPC's mesoscale archive page, there was an area of 850mb frontogenesis over eastern WA starting around 17Z Monday, just prior to the event: http://www.spc.noaa.gov/exper/mesoanalysis/s11/8fnt/8fnt_14022417.gif

    Not sure if it would have made a difference in the forecast snow totals given the model data you had to work with.

    ReplyDelete