Wednesday, January 28, 2015

Sad Skiing in the Northwest

We go from blizzards in the northeast to an extraordinary lack of snow in the northwest. Here are the most recent snowpack estimates from the NRCS Snotel sites:

Instead of our booming snowpack we had last year, the Washington and Oregon Cascades are well below normal for this time of year---many locations only have around 25% or less of the snow they usually have.  It improves somewhat as you move east into Montana and Wyoming.  However, despite having little snow, this does NOT mean we've had little precipitation.  Here's the total precipitation percentage of normal for the same sites:
Right at 90-110% for most of Washington and Oregon!  Despite some earlier storms that brought flooding, California hasn't seem much since.  The low snow amounts in the Sierras have a lot to do with low precipitation amounts in general (for the third year in a row....).  But it's a different story up north.  Here in Washington we've had the precipitation---it has just been too warm for it to be snow.

We can look for a baseline to compare against by going to the Storm Prediction Center's new sounding climatology page.  They've basically gone through the entire record of radiosonde (weather balloon) launches over time and computed daily statistics about what various sounding parameters should look like at each site throughout the year.  We're going to look at the 850hPa temperatures from the Quillayute sounding out on the western Washington coast (KUIL).  The 850hPa temperatures give us an idea of what the temperatures have been like in the lowest part of the atmosphere.  Here's what the climatology looks like:

The smooth, solid black line running down the middle is a smoothed mean 850hPa temperature throughout the year.  The golden line above represents the 90th percentile of the 850hPa temperatures and the red lines above that represent the record maximum 850hPa temperature for each day (with the thicker red line a running mean of the daily maximum).  You can see that starting in October the 850hPa temperature is, on average, around 6 Celsius, with it dropping to just below freezing (0 Celsius) by the time we get into December and January.  On average.  I grabbed the actual 850hPa temperatures from KUIL since October 1st and here is what we have:

The solid black line is an estimate of the black mean line in the above climatology. The horizontal blue dotted line is at 0 Celsius and the red dashed line is the average over the past four months.  You can see that since about the beginning of November we've been above average, for the most part.  In fact, even though the mean 850hPa temperature should be about -1 Celsius during January, our January average to date is 4.7 Celsius---5.7 degrees warmer than normal and, notably, above freezing.   In fact, based on the climatology, our average January 850hPa temperatures are close to the 90th percentile in the climatology, with some days actually setting new warm temperature records.  

The long-term forecast has these warm conditions continuing for at least the next few months.  The CFS ensemble has high probabilities of 850hPa temperatures being above normal over the northwest.  Here's their ensemble mean forecast for February-April:
Does not look promising for more snow on the slopes out here!

Tuesday, January 27, 2015

Stumbling about in the snow

Let's talk a bit about the forecasting of this blizzard event in the northeast yesterday and today.  A lot of people are writing about what went into this forecast and why New York and Philadelphia did not get nearly as much snow as was forecast (thought Boston and points east did!).  The common theme?  Forecasters didn't adequately communicate the uncertainty regarding this forecast.

Just how uncertain was the forecast?  Here's an example of the predicted snowfall accumulation at JFK airport in New York City from several of our model systems---the NAM (the reds and greys), the GFS (the blues) and the Rapid Refresh (green) starting at 12Z yesterday (yesterday morning).
The numbers drop over time after reaching their peak because this forecast factors in melting and compaction of the snow which will decrease its depth over time.  You'll notice that all of the higher forecasts (20-34") are all red, orange or grey---these estimates all were based on forecasts from the NAM model.  On the other hand, the lower forecasts (6-12") are all based on the GFS (or RAP) forecasts.  In total, this is a lot of uncertainty---as few as 6" or as much as 34".  6" is a normal, everyday winter snow.  36" will keep a city shut down for days.  And this is the morning before the event, when leaders and other interested parties have to make decisions about committing resources, closing things down, etc.  What is a forecaster to do?

It turns out that the GFS had the low pressure center moving further east.  Here's the 30 hour GFS surface pressure forecast (the black contours) from the 12Z run, so this this a forecast for where the low would be around the middle of the day today:
Note the center of low pressure is forecast to be southeast of Martha's Vineyard, with a longitude east of Cape Cod.  This GFS forecast kept the heavier snow bands east over Long Island, but not over New York City, hence the lower snow totals that we saw above.  Here's the exact same forecast, but from the ECMWF model (thanks to Cliff Mass for grabbing these):
Here the low pressure center is further to the west---centered straight south of Cape Cod at this time.  The ECMWF had snow bands further west as well---over New York City.  The NAM happened to agree more with the ECMWF as to the position of the low.

From reading the National Weather Service forecast discussions, this seemed to be what led to the decision to go with the higher snow totals.  This is from the 630 AM EST forecast discussion yesterday at the New York Weather Service Office:


Earlier in the discussion, though, they suggest that the spread in the models, particularly at such short forecast lead times, was more than they would like.  And again in the 1:34 PM EST discussion after the 12Z models shown above had come in:


So that was the logic.  And, in the past, this logic has served us well.  The European model (the ECMWF) has long been considered the "gold standard" of our weather prediction, usually surpassing the US GFS model in terms of forecast skill.  Indeed, it was the ECMWF's superior performance to the GFS in predicting the path of Hurricane Sandy that recently led to a large investment from Congress to improve the GFS.

What's more, even by 00Z yesterday evening, it was hard to distinguish which model is doing better.  The 12-hour forecasts from the GFS, ECMWF and NAM had the low in virtually the same place at that time, so it was hard to determine which way it would go.  It's also challenging when the low is off the coast, as it was here, to get an accurate fix on what's going on...there are limited surface observations over the ocean, so we have to rely on satellite data which can be tricky to interpret.

So what ended up happening?  The low actually moved to the east, as the GFS had been predicting.  Here's a map put together by @anthonywx showing the analyzed position of the low today at 15Z (the black L) and where the ECMWF forecast it would be 3 hours later (the red L, same as the position shown in the maps above):
So the GFS solution ended up playing out---the low was further east as was a lot of the heavy snow.  This didn't stop Boston and Massachusetts from getting heavy snow, but it definitely pulled things away from New York and Philadelphia.

Note the difference in the position of the low---120 miles.  At a quick glance, as far as a 30-hour model forecast goes, that really isn't too bad.  It just so happened that there was a strong gradient in snowfall on the western edge of this storm, at that gradient happened to pass through the largest city in the country.

There was a lot of uncertainty, but forecasters used knowledge that has served them well in the past to refine their forecast in support of decision makers.  Personally, I think that this is a good case of "better safe than sorry".  Would giving the public better estimates of the actual forecast uncertainty (which was large in this case) have helped people make better decisions?  Should we have put up graphics that said the snowfall could be anywhere from 6"-36" in New York City?  What was the actual forecast uncertainty? Again, I think that it was right of the forecasters to use their experience to try and refine that number, even if it may not have been the best refinement here.  Could an automated system have done better?  Much will be debated in the days to come.

And, for what it's worth, the GFS brings another Nor'easter through the area next Monday.  Who (or what) will we trust then?

Thursday, January 22, 2015

A Western Washington Tornado

Well, with a new year I decided it was time for me to resume my blogging activities.  So, I'll try to have regular updates on a weekly/biweekly basis at least as new weather topics come to me.

On Sunday afternoon, January 18th, most people in the Puget Sound area were glued to the television to watch the Seahawks game.  Passing outside their windows was a series of strong showers following a cold front, not at all unusual for the mid winter here.  Just before 2:30 PM PST the National Weather Service started getting reports of a tornado near Gig Harbor, Washington.  Here is the 0.5 degree base reflectivity radar image from the time.  Several strong, but compact cells across the southern Puget Sound lowlands.  Nothing that screams "tornado"...

It wasn't even the most intense cell that ended up attracting the attention here.  That cell produced hail later on, though at the sub-severe level.  No, it was the little blip to the west that produced an EF-1 tornado on the southeastern Kitsap Peninsula.  Here's the National Weather Service Seattle WFO storm survey results:
We joke about some of the storm damage markers we get from tornadoes in western Washington.  "Skylight cover sucked off".  "Kayaks moved around."  Though it's too bad that all tornadoes can't be this tame.

This has gotten me thinking about tornadoes in western Washington.  We get a few episodes with waterspouts every year, but tornadoes themselves are not very common.  I went back to the Storm Prediction Center's Severe Weather Database that I used in a previous blog post.  This includes a record of all known tornadoes since 1950.  It turns out in Washington state west of the Cascade Crest there have been 46 tornadoes since 1950 (47 after Sunday's event).  Here's a map of the tornadoes in the immediate Puget Sound area, labeled by year:
Those without red dots had path lengths too short to show up.  Given the short paths, relatively weak strength and the large amount of rural/undeveloped area in this region, there are probably more that escaped detection.  But there are actually more than I thought there would have been.

Another unusual feature of these tornadoes in the seasonal climatology.  Here is a histogram of the number of tornadoes per month (sample size of 46).
There's an odd sort of bimodal distribution here, with a rapid increase in number from March through June, then NONE in July, followed by a secondary ramp up in the fall through November before none in December and February.  Granted, this is a small sample size, but it's interesting that the tornado probability seems to crash so rapidly at the end of Spring and Fall.