Four-Panel Progs from the Penn State Tropical e-Wall

This is a sample lesson page from the Certificate of Achievement in Weather Forecasting offered by the Penn State Department of Meteorology. Any questions about this program can be directed to: Steve Seman


Upon completion of this page, you should be able to interpret the forecast variables on the four panels of the progs on the Penn State Tropical e-Wall. In particular, be sure to take note of standard thresholds of vertical wind shear and sea-surface temperatures that are relevant for tropical cyclone development.


In order to introduce you to some of the forecast variables that are important to tropical forecasters (and which may be new to you), I want to take a brief tour of the four-panel forecast progs available on the Penn State Tropical e-Wall. You may encounter other web pages with model graphics that include many of these same variables, but the four-panel display on the e-Wall provides a convenient way to look at them simultaneously.

The tropical e-wall allows you to keep tabs on tropical cyclones in the major basins around the world, and includes satellite imagery and various specialized model fields. The four-panel progs (like the one below) are the most frequently used item on the page, so I'll take a little time to discuss their format because they're not the standard progs you've seen before. Below is a sample of a four-panel tropical prog from the GFS over the Northwestern Pacific basin. The initialization for this 12-hour forecast was 00Z on June 7, 2005, so the prog was valid at 12Z on June 7.

Four-panel forecast prog from the PSU tropical e-Wall, initialized at 00Z on June 7, 2005 and valid at 12Z the same day
A sample GFS prog from the Penn State tropical e-wall. For the record, the initialization for this 12-hour forecast was 00Z on June 7, 2005 (the prog was valid at 12Z on June 7).
Credit: Penn State Department of Meteorology

I'll briefly touch on each panel, with the understanding that we'll discuss all the concepts that I mention in much greater detail later in the course. Let's start with the upper-left panel. It shows the predicted mean wind speed (color-coded in knots) and wind direction (streamlines) for the tropospheric layer between 850 mb and 250 mb. Since the mean flow in this layer is a primary contributor to the steering of strong tropical cyclones, this panel essentially shows the predicted steering currents in the basin.

The upper-right panel shows the predicted mean sea-level isobars. The shades of blues and oranges/red represent areas of anticyclonic (negative) and cyclonic (positive) relative vorticity at 925 mb (standard height is 750-800 meters). The units are x10-7sec-1. Tropical forecasters use this field to forecast the development of tropical cyclones, which depends, in part, on a source of low-level cyclonic vorticity. For example, the bull's eye of closed isobars and strong cyclonic relative vorticity off the east coast of Asia is the footprint of Typhoon Nesat.

Onto the lower-left panel, which displays vertical wind shear (the change in wind speed and / or wind direction with increasing altitude). Specifically, the lower-left panel displays the predicted east-west component (called the "zonal" component) of the wind shear between 850 mb and 250 mb (color-coded in meters per second). Westerly wind shear appears in shades of red and orange while darker shades of blue mark easterly shear. Streamlines indicate the direction of the total shear vector (the resultant from the vector difference between the wind directions at 250 mb and 850 mb).

Why do forecasters assess the predicted "deep layer" vertical wind shear? In a nutshell, when vertical wind shear is too strong, tropical cyclones can't maintain organized thunderstorms around their cores. Thus, vertical wind shear between 850 mb and 250 mb (or a similar layer) must be relatively weak for tropical cyclones to form and develop. Basically, tropical forecasters look for wind shear values to be less than 10 meters per second (about 20 knots) as an indication of favorable conditions for genesis and development of tropical cyclones.

There's one subtle point to keep in mind about vertical wind shear: A hurricane can create its own vertical wind shear. Indeed, the cyclonic circulation of winds around a hurricane sometimes produces a narrow swath (or swaths) of vertical wind shear on the periphery of the storm. As a general rule, you should ignore this swath (or swaths). Rather, focus your attention on the overall pattern of the surrounding environmental vertical wind shear in which the tropical cyclone is embedded. For example, check out this forecast prog showing Hurricane Igor in 2010. Note, on the lower-left panel, the swaths of relatively high westerly and easterly zonal wind shear on the northern and southern flanks of Igor (respectively). These swaths of high zonal shear were associated with the storm's cyclonic circulation, and were not part of the overall environmental shear (and, thus, did not hinder the storm).

Finally, the lower-right panel displays the predicted mean relative humidity between 700 mb and 500 mb. RH values greater than 70% appear in shades of green, while RH values less than 30% are marked by shades of peach. Relatively moist air in the middle troposphere is favorable for the genesis and development of tropical cyclones, while very low relative humidity values in the middle troposphere are unfavorable. Keep in mind that it's not unusual to see a pocket of high mid-level relative humidity collocated with a hurricane (again, check out this forecast prog showing Hurricane Igor in 2010). That's because the updrafts that sustain showers and thunderstorms promote cooling, lowering mid-level temperatures and increasing relative humidity there. It's the really low relative humidity (the peach colors on the progs in the PSU Tropical e-Wall) that's the kiss of death for hurricanes. Meanwhile, the short slashes on the lower-right panel mark areas of the ocean with sea-surface temperatures in excess of 26 degrees Celsius since SSTs greater than 26 degrees Celsius tend to favor development.

That covers the four panels of the progs on the Penn State Tropical e-Wall. But, these progs aren't all the page has to offer. If you're interested in learning about a few of the other products available on the page, check out the Explore Further section below. In the meantime, you're not faced with weeding through all of the computer model guidance on your own. Professional tropical forecasters around the world are always keeping tabs on the tropics, and they regularly issue forecast products when tropical cyclones are lurking. In the next section, we'll focus on some of the main forecast products available from the National Hurricane Center. They can be a great learning tool!

Explore Further...

Besides the four-panel forecast progs described above, the Penn State Tropical e-Wall offers some other products that may interest you. For starters, there's a variety of satellite imagery available for various tropical basins, like this infrared image from 1845Z on March 4, 2014, showing tiny Typhoon Faxai in the Western Pacific.

The tropical e-wall also includes handy "Comparison Maps" which allow you to see, side-by-side, forecasts from some of the major global models for forecast fields like total precipitation, vertical wind shear and 12-hour shear change (so that you can see if wind shear is increasing or decreasing in particular areas), and "multi-layer steering." Why would we need to look at multiple steering layers? Note that above, I said that the mean winds in the layer from 850 mb to 250 mb are a main contributor to steering strong tropical cyclones. As you'll learn later, as tropical cyclones become stronger, a thicker layer becomes relevant for their steering. Weaker tropical cyclones are steered by winds in a shallower layer. So, the multi-layer steering graphics give you a look at mean winds in various layers (700 - 250 mb -- "deep layer" winds, 850 - 500 mb -- "middle layer" winds, and 850 - 700 mb -- "shallow layer" winds). The "middle layer" and "shallow layer" winds can be particularly useful when dealing with tropical storms and tropical depressions.

Finally, there's some additional ensemble model guidance available, such as the image below, which shows the 48-hour forecasts of mean sea-level pressure and 925-mb relative vorticity from members of the GFS ensemble initialized at 00Z on March 4, 2014 and valid at 00Z on March 6. Based on the tiny position of the tiny bulls-eye of high 925-mb relative vorticity (orange, red, and pink shadings) in each panel, we can tell that the GFS ensemble members were in good agreement about the position of Typhoon Faxai's low-level circulation at these progs' valid time.

GFS ensemble forecasts for mean sea-level pressure and 925-mb relative vorticity initialized at 00Z on March 4, 2014 and valid at 00Z on March 6
The 48-hour forecasts from GFS ensemble members for mean sea-level pressure and 925-mb relative vorticity initialized at 00Z on March 4, 2014 and valid at 00Z on March 6. The position of the tiny bulls-eye of high 925-mb relative vorticity (orange, red, and pink shading) on each member's forecast suggests good agreement in the predicted location of Typhoon Faxai's low-level circulation.
Credit: Penn State University

Viewing the ensemble member forecasts can give us a little more insight into those ensemble track and intensity forecast plots that you learned about previously (in terms of why the models are differing in their predictions). The tropical e-wall provides ensemble plots for mean sea-level pressure / 925-mb relative vorticity (like the image above) as well as 850 - 250-mb steering winds.

A few other "goodies" are scattered throughout the tropical e-wall, like large graphics showing ECMWF forecasts of sea-level pressure and 925-mb relative vorticity in the Atlantic Basin (check out the latest 00Z run or 12Z run, for example). Free ECMWF products on the Web aren't quite as numerous as those for some other global models, so these graphics are a pretty nice feature of the page. The webmaster of the e-wall periodically adds new products, so I encourage you to browse around the tropical e-wall to see what's available. Enjoy!