Thursday, January 17, 2019

Modest Winds Tonight, Stronger Winds Tomorrow Night

After a period of warm, benign weather, storms are now moving towards our region.  Tonight a modest, elongated low center is moving northward up our coast, as illustrated by the sea level pressure map at 7 PM tonight (see below).

The winds surged this evening, as illustrated by the maximum gusts at Seattle's West Point, where wind revved up to around 35 knots (about 40 mph).

24 hour from now (7 PM Friday), a  much stronger low-pressure center will be due west of Vancouver Island.  The pressure analysis shows an intense pressure gradient (change of pressure with distance) over the offshore waters, which will bring big winds over the water.

The wind gust forecast for the same time are impressive, with 60 knot gusts south of the low (wind gusts at 10 PM Friday are shown).  Note winds are NOT strong in the center of the low.

Looking closer, as the storm approaches later Friday, winds will increase along the coast and over NW WA (see 6 PM gust forecast below)--as high as 50-60 mph in places.

Winds will increase later over central Puget Sound, strongest after midnight.

By 4 AM Saturday, the low has moved NW of Vancouver island, with a very large pressure change along the Vancouver Is. coast.

Wind gust are even stronger at that time, but shifted NW with the low.

Some big waves will be forced by the low, up to 9-10 meters high (see plot)
The winds here in Puget Sound will pick up again late tomorrow night as the second low moves by, but there is a lot of uncertainty, depending on exactly the path of the low.  Here is a plot of the UW many-forecast (ensemble) system.  Winds rev up after 10 PM Friday, but forecasts are all over the place.....will have to wait to have more certainty about the forecast.

Tuesday, January 15, 2019

Is the U.S. Government Shutdown Hurting Weather Model Accuracy?

My phone has been ringing from media asking whether the partial government shutdown is degrading the skill of the National Weather Service model, the GFS. 

What is driving this interest?    First, a number of media outlets, including the well-known Washington Post Capital Weather Gang, have made the claim of worsening U.S. forecasts (see examples below).

These stories describe a situation in which important observations, the input data streams for numerical weather prediction, are not being used or are degraded because of changes in coding of data formats.  As a result, the initialization (starting point) of U.S. global forecasts are degraded, lessening the skill of the predictions.

Then there is the U.S. model forecast skill itself, which appears to have worsened (compared to the European Center or the UK MET office model) just after the shutdown (on December 22nd).

The graph below from the National Weather Service illustrates this. Skill is plotted against time for the 5-day, upper level forecast for day 5 over the northern hemisphere (1 is a perfect forecast).  The black line shows the skill of the U.S. GFS model and the red dotted line presents the skill of the world-leading ECMWF model.  U.S. skill seems decline, both in absolute sense and relative to ECMWF right after December 22nd.

So are these claims of shutdown forecast problems well founded?   Is U.S. forecast skill drifting because of a lack of skilled NWS personnel minding the shop?

I think these claims are baseless.  

One of the first things I did was to check with some very well connected colleagues in NOAA and they confirmed these stories are nonsense.  Yes, many NOAA/NWS employees are not working, but since the models are considering essential for national security some staff are working--monitoring the global forecast system doing whatever it takes to keep it working smoothly.  Those NOAA/NWS staff are a dedicated lot!

But being a fact-driven type of guy, I decided to see if I could PROVE that the degradation claims are false with real data.

There are other explanations for the forecasts getting worse that have nothing to do with poor initial data.  For example, certain weather regimes may be more difficult for the U.S. model and perhaps that is all that we are seeing.

If the initial data was bad the short term forecasts would be less skillful.  So let's look at the skill of the 1-day forecast in the middle troposphere (500 hPa) for the U.S. GFS model (black line) and several others (the European Center is red).   No hint of any shutdown changes.

What about quality of the wind initialization at 500 hPa when compared to radiosondes (balloon-launched weather instruments)?  Little evidence of a shut-down effect (see below).

I have looked at many other fields and the answer is the same:  there is NO EVIDENCE that the initialization of the U.S. global model has been degraded as a result of the partial government shutdown.

In contrast, the predictions of the University of Washington regional prediction systems HAVE been hurt, because one of the important data streams we get from NOAA--the NOAA/NWS RAP model grids--has been cut  off during the government closure.  The shutdown is a disaster for weather prediction and weather research, but degradation of the U.S. global forecasts is really not an issue.