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:

"THE 00Z ECMWF REMAINED VERY CONSISTENT WITH ITS
PREVIOUS RUNS. IT DID LOWER QPF SLIGHTLY...BUT THE OVERALL EVOLUTION
OF THE SYSTEM REMAINED SIMILAR TO ITS 12Z RUN. DID NOT WANT TO MAKE
DRASTIC CHANGES TO THE EXPECTED SNOW AMOUNTS AND HEADLINES WITH
JUST ONE MODEL CYCLE. IN FACT...THE LATEST 06Z NAM HAS COME INTO
CLOSER AGREEMENT WITH THE 00Z ECMWF."

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:

"THE STORM APPEARS ON TRACK. THE 12Z GFS IS AN OUTLIER FROM THE 12Z
NAM AND 12Z ECMWF. HAVE UPPED SNOWFALL TOTALS ACCORDINGLY WITH
MOST AREAS IN THE 20-30 RANGE FOR THIS EVENT."

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?

Strong winds and rain in the east; snow in the northwest?

In my last blog post I talked about the blast of arctic air that was forecast to move across the country last weekend and into this week in association with a cut-off low that would slowly creep across the southern US.  Here we are on Monday and today's 500mb pattern from the ECMWF still shows that cut-off low over Texas and New Mexico.

The colder air and lift that are accompanying this storm have already contributed to snow and ice throughout New Mexico, Texas and Oklahoma.  In fact, the weather map from the Oklahoma Mesonet today shows several sites that are not able to report the wind speed or direction (the red dots on the map below) because the anemometers and wind vanes are frozen!

This storm is expected to  slowly chug eastward over the next few days before ramping things up along the east coast.  Look again at the 500mb chart above.  See that other trough in the northern Great Lakes / northwestern Ontario?  That trough is forecast to merge with the cut-off low sometime during the next few days.  One one hand, this means that the cut-off low will no longer be cut-off---back in the normal flow, it will accelerate and move on out of here.  Before it does, though, the cold air it's bringing behind it is going to run into the warm Atlantic Ocean, and that means rain--heavy rain--for the eastern US.  Here's the ECMWF forecast rainfall around 15Z Wednesday morning:

Heavy rain up and down the east coast on a day when lots and lots of people will be travelling.  Not good.  Fortunately by Thanksgiving day itself, it appears that the precipitation will be on its way out (as this storm accelerates off the coast).  Here's the ECMWF precipitation forecast for Thursday morning at 15Z:

Only light precipitation for parts of the northeast.  A lot of this near the lakes may be lake effect snow as strong northwesterly winds will occur behind this storm.  See the tight gradient in the blue contours on the above map?  Those are the surface pressure contours, and the tighter the pressure gradient the stronger the winds.

This brings up another question that I know many people are dying to hear the answer to--will the Macy's Thanksgiving Day parade balloons be able to float with this weather?  I'm told that if the winds gust higher than 35 mph then the balloons are a no-go for the parade.  So what are the models saying?

It's probably going to be close.  Our models mostly forecast sustained winds, that is, the average wind speed over a certain amount of time (usually a few minutes).  Wind gusts are instantaneous wind speeds, and the wind gusts can be significantly higher than the sustained winds.  Here is the ECMWF sustained wind forecast over New York City for 15Z on Thursday while the parade is going on:

You can see the winds really pick up speed off the coast.  But over the city itself, the bluish-greenish colors indicate winds of 15-18 knots or 17-21 mph.  Again, those are the sustained winds--gusts will be higher.  The GFS and NAM have slightly stronger sustained winds, getting up to more like 25-28 mph--again, with higher gusts.  So, as of right now, it's going to be borderline.  We'll have to watch the model runs as we get closer to get a better idea of how strong the winds are going to be.

And finally, just as a teaser for everyone out here in the Pacific Northwest, the really long range models for next Sunday-Monday have a strong cold front coming through in association with a sharp trough coming down the Pacific coast from Alaska.  Here's the ECMWF 156 hour forecast (!) of 850mb heights and temperatures for next Sunday.

A blast of cold air headed our way.  The ECMWF has some low-level instability following this cold front and is throwing out some convective snow showers in its wake.  Though we should never, ever believe models this far out into the future, for fun here is the ECMWF snowfall accumulation forecast for next Monday morning:

Two inches of snow in the lowlands east of Everett?  Eh...maybe.  Don't take my word for it yet.  But it's definitely something to keep an eye on...

From calm to cold with cut-off lows

Looks like we're in for a major synoptic pattern change over the next week or so.  This morning's 12Z analysis from the ECMWF model at 500mb finds us with a nice zonal (east-west) pattern.  There are a few minor shortwave troughs but nothing very large in scale.

By tomorrow morning (Thursday), that is forecast to change.  The weak troughing over the eastern Pacific and western US is forecast to deepen rapidly into a strong trough.

And by Friday morning, the trough has deepened to become cutoff low--completely separated from the main jet stream winds (shown by the enhanced colors on the map) that are blowing to the north.

It's pretty amazing how over the course of 48 hours the upper-level pattern can change so quickly.  As I noted in previous blog posts, cut-off lows like this tend to stick around for a while.  Because they are separated from the main jet stream winds, there's nothing to really force them to move along from west to east.  So they usually just sit there.  This low will hover around for just a few days though, before transitioning back to an open-wave trough by next Monday as seen here:

But then, yet another cut-off low is forecast to re-develop off the west coast by next Wednesday!

I should also note that this particular upper-air pattern forecast for next Wednesday is bad news for the east coast--you have a deep trough covering the northeastern quarter of the country and then another shortwave trough racing through the southeast US and about to emerge over the Gulf Stream.  Nor'easter type storms are often born with this sort of setup as cyclogenesis occurs off the Carolinas.  This is still a long way out in the forecasts (168 hours), but it's something to watch...

Another aspect of this cut-off low moving across the US this week into next is that it will open the door for colder air currently over Canada to sneak down into the US.  Compare this morning's 12Z surface temperature analysis from the GFS:

With the forecast for Friday morning's low temperatures:

Much colder air moving in.  Pretty strong front at the leading edge of it too.  There are chances for some severe weather tomorrow as the front crashes through the plains, followed by a round of wintry precipitation. We're going from tornadoes this past Sunday to winter weather by the end of the week.  Such is autumn...