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EZTip No. 36 - AWC vs WPC progs - What's the difference?

It's a common question. Are there any differences between the short range prog charts found on the Weather Prediction Center (WPC) website versus those found on the Aviation Weather Center (AWC) website? In fact, there are more similarities than there are differences...but the differences are quite important.

First let's sort out some terms. "Short range" refers to a forecast with a lead time of 6 hours through 60 hours based on the WPC definition. The "extended range" forecasts are those with lead times beyond 60 hours through 7 days...often referred to as the Day 3, Day 4, Day 5, etc. forecast. Whether you view these progs on the WPC site (or in the EZWxBrief progressive web app) or AWC site, the forecast for fronts/drylines, pressure centers, mean sea level pressure and other features (e.g., squall lines, surface troughs) will be identical since the WPC is the organization that issues these forecasts. In other words, the AWC doesn't issue such a forecast...that's the job of the WPC.

Both short range progs provide access to the National Digital Forecast Database (NDFD) precipitation type and likelihood (i.e., chance or likely) as shown by the colors and legends on both charts. This precipitation forecast is not issued by the WPC, nor is it issued by the AWC. In fact, it is issued by forecasters located at the 116 NWS local weather forecast offices throughout the U.S. that gets stored in the NDFD. On both the WPC and AWC progs, this forecast gets ingested from the NDFD and is rendered on the prog chart in reference to the valid time. Shown above is the forecast from the WPC.

Now, here's one difference. The NDFD grids for precipitation type and likelihood issued by those local weather forecasters are updated throughout the day. Notice that both the AWC prog (below) and the WPC prog (above) have a valid time of 1200Z, July 6th. This is the forecast with a 60-hour lead time. If you compare the two forecasts, you will notice that the isobaric, frontal system and pressure center forecasts are identical. However, if you look at the precipitation type and likelihood forecasts, there are differences. Specifically, in the magnified area in the Mid-Atlantic, notice on the AWC site (below) there is a fair coverage for a chance of rain in eastern West Virginia and northern Virginia. But on the WPC forecast (above), this isn't present. Also notice the red-hatched region in southeastern Virginia on the AWC forecast (below) that forecasts a chance of convective rain, but the WPC forecast (above) just forecasts this as a chance of nonconvective rain.

What is going on? Take note of the issue time of the two forecasts. The AWC forecast was issued at 0704Z, July 4th and the WPC forecast was issued at 1313Z, July 4th. Essentially, every three hours, the WPC will update the precipitation forecast that is rendered on the short range progs given that the NDFD forecasts are updated every three hours. However, the AWC does not update the NDFD forecasts despite new forecasts being available.

The second important difference occurs in the extended range progs. At Day 3 to Day 7, the AWC progs do not always provide a precipitation type and likelihood forecast for some portions of the day as you can see below. But that is not the case for the WPC extended range forecast.

When the AWC extended range forecast does provide a precipitation type and likelihood forecast like the one below, there are likely substantial differences.

In the case of the AWC extended range progs like you see above valid at 1200Z, July 7th, notice that the forecast for fronts/drylines, pressure centers and the mean sea level pressure forecast is identical to the WPC extended range forecast below also valid at the same time. That's the similarity. The differences are found on the precipitation type and likelihood forecast once again. The precipitation forecast on the AWC progs comes from the NDFD similar to the short range progs where the forecast is valid at the time on the chart, in this case, 1200Z, July 7th. So it represents a precipitation coverage forecast. The precipitation forecast for the WPC progs, on the other hand, does not use NDFD, but uses an internal WPC precipitation forecast that is valid over a range of time. While it is not documented on this chart, this precipitation type and likelihood forecast on the WPC extended range progs is always valid from 1800Z to 0000Z.

That means there will be difference due to the timing (morning versus afternoon) as well as the range of time versus a single time. This will affect where the precipitation is expected and also affect the likelihood as well. With a forecast over a six hour period, you will likely see more dark colors on the WPC forecast than you will with the AWC forecast.

Are you interested in taking your aviation weather knowledge to a new level? Then consider purchasing The Skew-T log (p) and Me: A Primer for Pilots, available in both softcover and eBook format.

Most pilots are weatherwise, but some are otherwise™

Dr. Scott Dennstaedt

Weather Systems Engineer

Founder, EZWxBrief™

CFI & former NWS meteorologist

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