<p>The equal-area projection used to create the continental U.S. grid for weather forecasting does not represent lines of latitude and longitude as straight lines.  Instead, they are curved (note the Canada-USA border curvature below, which is actually along the 49th latitude parallel).</p>

<p>An equal-area projection is often used in regional weather modeling because it is important for each cell to represent the same amount of physical space during computation.  The map images posted in the question are using the Mercator (or Mercator for web) projection, where lines of latitude are horizontal and lines of longitude are vertical, but area is distorted. </p>

<p>The grid-cells from the regional model are also designed in a way that maximizes cost-benefit of computational power.  You want to minimize the number of grid cells modeled while maximizing the number of grid-cells in your domain that are relevant to your study area.  This ensures that you aren't wasting computational power and is often achieved by changing the orientation of the domain a little.  So, the grid cells do not align nicely on the mercator map used on web-app maps like google, bing, etc.  However, the grid-cells line up perfectly if visualized using the native projection for the image frame (e.g. below).</p>

<p>The reason that Hawaii does not follow the pattern is that the data for Hawaii is obtained from a model simulation for a different grid.  The nesting of grids can be seen in the image below.</p>

<p>I suggest you read <a href="http://cdn.intechopen.com/pdfs-wm/43438.pdf" rel="nofollow noreferrer">Grids in Numerical Weather and Climate Models
</a> for a nice look at the factors involved when choosing a grid definition.
You could also <a href="https://www.ncdc.noaa.gov/data-access/model-data/model-datasets/north-american-mesoscale-forecast-system-nam" rel="nofollow noreferrer">look here</a> for more info about NOAA prediction.
<a href="https://i.sstatic.net/d1Uok.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/d1Uok.jpg" alt="http://www.ncep.noaa.gov/newsletter/january2012/"></a></p>

<p><strong><em>North American Mesoscale (NSM) forecast model domains used by NOAA.</em></strong></p>

CC BY-SA 3.0
2016-05-09T00:57:16.000Z
I don't quite follow... Surely the lines of latitude are still parallel? Do you mean 'straight'? The answer would also benefit from some references.
Matt Hall
CC BY-SA 3.0
2016-05-09T03:07:29.587Z
Yes, good point.  I made a few changes.
f.thorpe
CC BY-SA 3.0
2016-05-09T00:43:35.793Z
2016-05-11T04:20:38.220Z
2016-05-11T04:20:38.220Z
f.thorpe
<p>I get my weather forecasts from good ol' <a href="http://www.noaa.gov" rel="nofollow noreferrer">NOAA</a>'s <a href="http://www.weather.gov" rel="nofollow noreferrer">weather.gov</a> site. On the detailed site weather pages, there's a map that shows the forecast area as a green square (for Boston at least, the square is about 1.5 miles on a side). You can click on nearby areas to change the presented forecast to that new location.</p>

<p>Here's the question: why is the square shown as the forecast area not oriented north/south/east/west? For some reason, near the US East Coast it's twisted clockwise, and on the West Coast it's twisted counter-clockwise. I assume it's something about the easy representation of weather information in a GIS system, but that's only a guess.</p>

<p>Here are some examples, from East to West. First, <a href="http://forecast.weather.gov/MapClick.php?lon=-71.0586870876267&amp;lat=42.36143032801277" rel="nofollow noreferrer">Boston</a>:</p>

<p><a href="https://i.sstatic.net/Hh1cV.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Hh1cV.png" alt="Boston, MA forecast area"></a></p>

<p>Second <a href="http://forecast.weather.gov/MapClick.php?lat=39.768376841000475&amp;lon=-86.15803708099969" rel="nofollow noreferrer">Indianapolis</a>:</p>

<p><a href="https://i.sstatic.net/L14nG.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/L14nG.png" alt="Indianapolis, IN forecast area"></a></p>

<p>Third, <a href="http://forecast.weather.gov/MapClick.php?lon=-104.97121589355471&amp;lat=39.78560985063913" rel="nofollow noreferrer">Denver</a>:</p>

<p><a href="https://i.sstatic.net/Zv898.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/Zv898.png" alt="Denver, CO forecast area"></a></p>

<p>Fourth, <a href="http://forecast.weather.gov/MapClick.php?lon=-122.42891992340103&amp;lat=37.75566584959775" rel="nofollow noreferrer">San Francisco</a>:</p>

<p><a href="https://i.sstatic.net/WHkJ7.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/WHkJ7.png" alt="San Francisco, CA forecast area"></a></p>

<p>Note that, if you go even further west to <a href="http://forecast.weather.gov/MapClick.php?lon=-155.08789&amp;lat=19.71275" rel="nofollow noreferrer">Hawaii</a>, the pattern breaks, and the square is perfectly north/south/east/west:</p>

<p><a href="https://i.sstatic.net/ZhnjT.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/ZhnjT.png" alt="Hilo, HI forecast area"></a></p>

<p>So, any ideas?</p>

CC BY-SA 3.0
2016-05-08T21:45:41.973Z
Possibly, the actual model grid cells are shown. In the projection that is used for the map, the grid cells are represented as trapezoids which angles are close to 90°. Hence, the grid cells look like squares.
daniel.heydebreck
CC BY-SA 3.0
2017-01-31T16:56:27.907Z
I assume they are square windows aligned on a satellite orbital path.
John
<meteorology><weather-forecasting><gis>
Why do forecast areas on weather.gov appear as twisted squares?
Daniel Griscom
# Why do forecast areas on weather.gov appear as twisted squares?

(Asked by: username_0 on 2016-05-08)

I get my weather forecasts from good ol' [NOAA](http://www.noaa.gov)'s [weather.gov](http://www.weather.gov) site. On the detailed site weather pages, there's a map that shows the forecast area as a green square (for Boston at least, the square is about 1.5 miles on a side). You can click on nearby areas to change the presented forecast to that new location.

Here's the question: why is the square shown as the forecast area not oriented north/south/east/west? For some reason, near the US East Coast it's twisted clockwise, and on the West Coast it's twisted counter-clockwise. I assume it's something about the easy representation of weather information in a GIS system, but that's only a guess.

Here are some examples, from East to West. First, [Boston](http://forecast.weather.gov/MapClick.php?lon=-71.0586870876267&lat=42.36143032801277):

[![Boston, MA forecast area](https://i.sstatic.net/Hh1cV.png)](https://i.sstatic.net/Hh1cV.png)

Second [Indianapolis](http://forecast.weather.gov/MapClick.php?lat=39.768376841000475&lon=-86.15803708099969):

[![Indianapolis, IN forecast area](https://i.sstatic.net/L14nG.png)](https://i.sstatic.net/L14nG.png)

Third, [Denver](http://forecast.weather.gov/MapClick.php?lon=-104.97121589355471&lat=39.78560985063913):

[![Denver, CO forecast area](https://i.sstatic.net/Zv898.png)](https://i.sstatic.net/Zv898.png)

Fourth, [San Francisco](http://forecast.weather.gov/MapClick.php?lon=-122.42891992340103&lat=37.75566584959775):

[![San Francisco, CA forecast area](https://i.sstatic.net/WHkJ7.png)](https://i.sstatic.net/WHkJ7.png)

Note that, if you go even further west to [Hawaii](http://forecast.weather.gov/MapClick.php?lon=-155.08789&lat=19.71275), the pattern breaks, and the square is perfectly north/south/east/west:

[![Hilo, HI forecast area](https://i.sstatic.net/ZhnjT.png)](https://i.sstatic.net/ZhnjT.png)

So, any ideas?

**Tags:** meteorology, weather-forecasting, gis

> **Comments:**
> **username_1**: Possibly, the actual model grid cells are shown. In the projection that is used for the map, the grid cells are represented as trapezoids which angles are close to 90°. Hence, the grid cells look like squares.  
> **username_2**: I assume they are square windows aligned on a satellite orbital path.  

---

## 1 answers

### Answer by username_3 on 2016-05-09. Score: 13 | Accepted answer

The equal-area projection used to create the continental U.S. grid for weather forecasting does not represent lines of latitude and longitude as straight lines. Instead, they are curved (note the Canada-USA border curvature below, which is actually along the 49th latitude parallel).

An equal-area projection is often used in regional weather modeling because it is important for each cell to represent the same amount of physical space during computation. The map images posted in the question are using the Mercator (or Mercator for web) projection, where lines of latitude are horizontal and lines of longitude are vertical, but area is distorted.

The grid-cells from the regional model are also designed in a way that maximizes cost-benefit of computational power. You want to minimize the number of grid cells modeled while maximizing the number of grid-cells in your domain that are relevant to your study area. This ensures that you aren't wasting computational power and is often achieved by changing the orientation of the domain a little. So, the grid cells do not align nicely on the mercator map used on web-app maps like google, bing, etc. However, the grid-cells line up perfectly if visualized using the native projection for the image frame (e.g. below).

The reason that Hawaii does not follow the pattern is that the data for Hawaii is obtained from a model simulation for a different grid. The nesting of grids can be seen in the image below.

I suggest you read [Grids in Numerical Weather and Climate Models](http://cdn.intechopen.com/pdfs-wm/43438.pdf) for a nice look at the factors involved when choosing a grid definition.
You could also [look here](https://www.ncdc.noaa.gov/data-access/model-data/model-datasets/north-american-mesoscale-forecast-system-nam) for more info about NOAA prediction.
[![http://www.ncep.noaa.gov/newsletter/january2012/](https://i.sstatic.net/d1Uok.jpg)](https://i.sstatic.net/d1Uok.jpg)

***North American Mesoscale (NSM) forecast model domains used by NOAA.***

> **Comments:**
> **username_4**: I don't quite follow... Surely the lines of latitude are still parallel? Do you mean 'straight'? The answer would also benefit from some references.  
> **username_3**: Yes, good point.  I made a few changes.  

---

