Numerical Models
An Overview and Tutorial
Types of Models
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Short range models
 These tend to be more suitable for more specific features such as
fronts, temperatures, and convection. They are considered nonhydrostatic.
 Forecasting for as little as the next 1 hour to as long as 3 ½ days.
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Long range models
 These are hemispheric or global models and are highly skilled at
wave patterns within the jet stream. However they also have skill
at synoptic features and an outperform the short range models at
times! These are usually hydrostatic or isentropic.
 Forecasting out to as far as 15 days
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For an understanding of terms such as hydrostatic or
isentropic, we encourage you to look outside this tutorial
(google it!) to get an overview.
So many models!
► RUC
– Rapid Update Cycle
► NAM – North American Mesoscale
 WRF-NMM, WRF-ARW and WRF-HRW
► GFS
– Global Forecast Systems
► ECMWF – European Center for Medium
Range Weather Forecasting
► NGM – Nested Grid Model (being phased
out.)
So many models!
► GEM
– Global Environmental Multiscale
(Canada)
► UKMET – United Kingdom Meteorological
Model
► NOGAPS – Navy Operational Global
Atmospheric Prediction System
► Hurricane models such as the HWRF, GFDL,
and WW3
Let us narrow it down
► For
the purposes of this tutorial we will only
concentrate on the models most used here at
NEXLAB.
► And they are ….
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RUC
WRF
GFS
And to a certain extent the ECMWF and GEM
Oh, and there is ensemble forecasting, too. But we’ll get
to that later 
Rapid Update Cycle (RUC)
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The RUC is a very short range model with forecasts as short as 1 hour.
Many versions of the RUC run at places such as NCEP, FSL, local
weather offices, and even colleges. The operational model comes out
of NCEP and is run every hour.
Every three hours (0Z, 3Z, 6Z, etc.) it runs a forecast out to 12 hours.
During the hours in between it forecasts only for the next 3 hours. So,
at 0Z you get forecast through 12Z but at 1Z you get a forecast valid
through 4Z
Generally used for the 1, 2, 3, 6, 9, and 12 hour forecast period but as
mentioned before, many versions and suites of the RUC are run so this
is not always the case. But this is what you see on our website here at
COD.
RUC Analyses often used for “analysis” data such as the SPC mesoanalysis page so understanding the RUC can be vital in modern day
forecasting.
Rapid Update Cycle (RUC)
► The
lastest version of the RUC (as of
September 2008) has a horizontal resolution
of 13km and 50 layers through the depth of
the atmosphere.
► Much information can be found about the
RUC at http://maps.fsl.noaa.gov/
Rapid Update Cycle (RUC)
This is the domain of the RUC-13km model. Data is taken a grid point
spaced every 13km throughout this entire region (but only this region!)
Rapid Update Cycle (RUC)
This is the 12Z RUC, 3 hour forecast
of surface theta-e, valid at 15Z.
North American Mesoscale (NAM)
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The NAM is the operational short range model run by NCEP.
The actual model itself is called the WRF (Weather Research and
Forecasting), and more technically the WRF-NMM (Nonhydrostatic
Mesoscale Model).
The WRF, like the RUC, is run different ways by different places. You
may see references to the WRF-ARW (Advanced Research), too.
With all that being said, the NAM is the operational run and uses the
WRF-NMM model for the basis of operational forecasting. If you see
NAM or WRF, it simply means you are looking at the WRF-NMM model.
Confused yet? Don’t worry because the models are always changing
names. 
I encourage you to look at the WRF homepage at http://www.wrfmodel.org/index.php
North American Mesoscale (NAM)
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The NAM/WRF is run four times a day at 00Z, 6Z, 12Z, and
18Z.
The NAM/WRF does a forecast for every 6 hours out to 84
hours from the time it is run, or 3 ½ days.
The NAM/WRF is set up as a non-hydrostatic model run at
12km of horizontal resolution and has 60 layers through
the depth of the atmosphere.
Further learning about the NAM resolution can be found at
http://www.meted.ucar.edu/nwp/pcu2/namhres1.htm
North American Mesoscale (NAM)
This is the domain of the WRF-12km model. Data is taken a grid point
spaced every 12km throughout this entire region (but only this region!)
North American Mesoscale (NAM)
This is the 12Z WRF, 6 hour forecast of
QPF (precipitation), valid at 18Z
Global Forecast System (GFS)
► The
GFS is a short, medium, and long range
model with forecasts out to 384 hours.
► The GFS forecasts for every 6 hours (00Z, 6Z, 12Z,
and 18Z) out to 180 hours, then every 12 hours
out to 384 hours.
► At 180 hours the resolution is not as fine so the
user will see less detail in products beyond 180
hours.
► The GFS has 64 layers throughout the depth of
the atmosphere with a horizontal resolution of
30km (though 180hrs).
Global Forecast System (GFS)
The GFS is a global model run for the northern hemisphere (the domain is shown
above) and for the southern hemisphere. This is one reason the GFS is very
skilled at wave patterns and overall jet stream flow.
Global Forecast Systems (GFS)
This is the 00Z GFS, 9 day forecast of
500 speeds, valid at 00Z of day 9
The Products
► Speeds
(jets)
► Vertical Velocity and vorticity products
► Temperature fields
► Humidity and moisture
► Thicknesses and precipitation (QPF)
► Instability and shear fields
The Products -- Speeds
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GFS (in this case)
500mb speeds
Jet maxima
Heights
Upper lows
Ridge
Wind barbs
Color bar (key)
Products like this can
be found on just about
every model for major
layers of the upper
atmosphere such as
250mb, 500mb,
700mb, and 850mb.
Some models will
include a few layers in
between, as well.
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The Products – Vorticity
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Vorticity is a complicated thing to understand. For the basis of this
tutorial we will only discuss the basics. You are encouraged to learn
more about vorticity on your own!
Essentially, vorticity is the spin of the air. The more the air is sheared,
or spun, the higher the value of vorticity.
Vorticity is generally displayed on a 500mb map due to the fact that
500mb is in the middle of the troposphere.
When looking at a vorticity map, the center of vorticity (the maximum
value) is generally associated with the heart of a shortwave trough. It
might be referred to as a “vortmax”
Areas downwind of this vorticity center (where vorticity is being
advected positively) can be understood to be an area of upward
vertical motion. Consequently, areas upwind of the shortwave trough
where vorticity is decreasing can be understood to have sinking air.
Again, this is a simplified understanding of vorticity but does have
practical applications.
The Products - Vorticity
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GFS (for this case)
500mb Vorticity
500mb Heights
Vorticitiy Maximum
(vortmax)
Shortwave troughs
Color Bar
X
X
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X
X
The Products – Vertical Velocities
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The Vertical Velocity product (or VV’s or UVV’s for upward
vertical velocities) is simply a measure of how much the air
is rising or sinking.
This is a measure of overall rising air (synoptic) so even
the fastest of vertical velocities is very small. This is not a
measure of strong forcing as in a thunderstorm. The
measure is done in microbars/second which is close to
centimeters per second.
Two main ingredients go into measuring vertical velocities
– temperature advection and vorticity advection. Warm
advection and positive vorticity advection are associated
with lift, or rising air. Cold advection and negative vorticity
advection are associated with subsidence, or sinking air.
The Products – Vertical Velocities
Like the other products,
we are using the GFS
model here.
► Areas of rising air
► Areas of strong sinking
► Also, as with previous
maps and slides, the
heights and color bar are
located on here.
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Products like this can be
found on most models and
at many layers such as
500mb, 700mb, and
850mb. The default, or
standard layer, is usually at
700mb for this product.
The Products - Temperatures
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are three main types of temperature
products found on most model outputs.
 Basic air temperatures (T)
 Wet-bulb temperatures (Tw) – used for
precipitation type.
 Theta-e (Θe) – a measure of total heat
(basically a combination of air temperature and
moisture)
The Products - Temperatures
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GFS (in this case)
Surface (2 meter)
temperatures
Mean sea level
pressure
Highs and Lows
Wind barbs
You can find fronts,
troughs, and ridges
Tropical systems, too
All models have some
form of temperature
forecast plot. Generally
you will find this for the
mid and low levels
(500, 700, 850, 925)
and the surface.
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The Products – Temperatures
(Theta-e)
An understanding theta-e is more
easily attainable when one has studied
soundings and the concept of parcels.
What one should realize, though, is
that theta-e is a measure of the total
heat. The higher the value of theta-e
(measured in Kelvin in this picture) the
higher the heat content. In this picture
to the right it is easy to see that a very
cool, dry, continental air mass is taking
hold of the Great Lakes region.
It is important to note that theta-e
values are often detailed at the
surface, 850mb (as in this picture),
and 700. For severe weather, we look
for theta-e to decrease with height. In
other words, very warm/moist at the
low levels and much cooler/dryer air
aloft.
Concepts of theta and theta-e also
involve motions of air such as
isentropic lift. We encourage all
students to look further into these
concepts on their own (or in a class!)
Cool and dry air
advecting southward
Southeasterly winds
advect warmer and
more moist air
Moisture Parameters
► Relative
Humidity – generally done for
upper air products, 250mb-850mb.
► Dewpoints – generally done for the lower
layers of the atmosphere (850mb, 925mb)
and near the surface.
Moisture Parameters - RH
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Relative Humidity plots are done all upper levels of the atmosphere.
 250 – One might look for areas of RH to see where thick cirrus is being forecast
 500 and 700 – The model world’s version of looking at a water vapor satellite. One
can see dry intrusions, areas of vertical ascent and subsidence. And a careful
observer can note areas of convection blowing up within a dry intrusion.
 850 and 925 – Forecasters can use this to see areas of thick stratus and warm
advection as well as general areas of vertical motion (which creates or impedes
cloud development.)
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Streamlines, a snapshot of the wind flow, are also plotted along with the RH to
help the user in the analysis.
Moisture Parameters – Dewpoint
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Dewpoint temperatures (Td) are generally plotted for the lower levels
of the atmosphere such as 850mb to the surface.
Surface dewpoint plots can be calculated in different ways of which
the user needs to be aware. Here at COD, we do it two ways.
 2m Td – The “skin” layer of the surface. This is the model prediction of
what the surface observations will record the dewpoint to be at each hour.
 0-30mb Td – An average dewpoint of the lowest 30mb. This takes into
account some mixing that goes on in the lowest part of the boundary layer.
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Do not be surprised to see other parameters plotted along with the
dewpoints such as streamlines, wind barbs, lifted indexes, C.A.P.E, etc.
Dewpoint temperatures plots are vital to severe weather forecasting
hence the severe weather overlay plots and wind plots.
Dewpoints are also very important in winter forecasting, temperature
forecasts, general forecasting (clouds, etc.), among other things.
Moisture Parameters – Dewpoint
This is a great example of
“moisture return”. At
image one (at 12Z Nov
20) one can observe a
surface high pressure
sitting along the coast of
Louisiana and Texas. This
is noted by the
anticyclonic (clockwise)
flow of air. With time, this
high pressure moves to
the east, allowing the
southerly flow on the
back side of the high to
bring moisture from the
Gulf of Mexico back into
the eastern half of the
US.
Thicknesses and Precipitation
Most sites that provide model data, including here at NEXLAB, tend to display thickness
plots with a QPF (Quantitative Precipitation Forecast) overlay.
► What is thickness?
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To properly understand thickness plots, students are encouraged to further look into the
hypsometric equation. A basic understanding is as follows: “The thicker the warmer”. Air
expands when it is heated, and the atmosphere responds accordingly. Higher thickness values
mean a higher average temperature within that layer.
A standard thickness plot is done for the 1000 – 500mb layer. However there are many others
and can be seen on our models’ page.
The “540” line is a first guess, rule of thumb, for rain vs. snow. 540 is short for 5400 meters.
That can, often, represent the thickness associated with an average temperature from 1000500mb that is cold enough for snow or ice. But beware – it is not very precise.
What is QPF?
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Be careful to understand that precipitation forecasts are based on precipitation that has fallen
and not precipitation that is falling !
The precipitation plot is for precip that has fallen either for the previous 3 or 6 hours. Be aware
of which one you are looking at!
Just because a model says it will have precip doesn’t mean it will have it. Model world does not
equal reality. Models are tools, not gospel! Do not get caught up in looking at QPF and basing
your forecast completely on those plots. They are good indicators but not perfect
prognostications.
Thicknesses and Precipitation
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QPF along with
color bar.
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1000-500mb
thicknesses
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The “540” line
(5400 meter
thickness) is solid
yellow
Using thicknesses to forecast
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There are 6 thicknesses
plotted, as well as the
850mb, 0 degree line.
Also the average RH from
850-500mb.
Each thickness rule of
thumb rain vs. snow
involves a different slice
of the troposphere.
Again, line is based on
statistics that say “given
this thickness, for this
particular layer, the
average temperature
generally produces wintry
precipitation.”.
This is NOT the same as
doing an analysis and not
absolute fact. This is only
one of many tools for rain
vs snow.
Instability and Shear Products
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of these products involve a more in depth
understanding of meteorology.
► However, limited knowledge of these products can
allow a user to at least get a feel for the severe
weather conditions possible.
► One is encouraged to further study severe
weather paramaters such as shear, instability,
sounding analysis, hodographs, and other severe
weather analysis tools and concepts.
Instability and Shear Products
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Instability products – how fast can air rise?
 Convective Available Potential Energy (CAPE) – Measure in Joules of
energy per Kilogram of air. The higher the number, the more unstable.
Usually ranges from 25 (very low) to 5 or 6 thousand (very high).
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(SBCAPE) Surface Based – air lifted upward from the ground
(MLCAPE) Mixed Layer – accounts for mixing within the boundary layer
(MUCAPE) Most Unstable (parcel) – lifts many parcels from within the boundary
layer then gives the value for the most unstable lifted air.
 Lifted Index (LI) – the difference in the temperature within the cumulus, or
thunderstorm, vs. the temperature outside the cloud. The more negative
the value, the more unstable.
 Lapse Rates or Delta-T’s – shows how fast the atmosphere is cooling with
height. The higher the number, the faster the atmosphere is cooling off
with an increase in altitude.
 (CINH) Convective Inhibition – how much will the air be suppressed?
Values that are higher point to air that will be prevent air from rising due
to warm air in the mid levels of the atmosphere. Also referred to as a cap
or a lid on the atmosphere.
Instability and Shear Products
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Shear products – how much can the air spin?
 Helicity – measures shear. The higher the number, the higher the shear.
► 0-6 km – an idea of “total shear” within a storm
► 0-3 km – concentrates on the lower part of the atmosphere
► 0-1 km – can help forecast the possibility for tornadoes
One should never take these products as gospel. Shear and instability are
very difficult to forecast and models are not very skilled or precise when it
comes to the placement or exact numbers of instability or highly sheared
areas. One should do a very good analysis before ever looking at a model!
Other model packages such as BUFKIT (which we have here at the lab!) and
RAOB do a wonderful job of showing a point by point vertical analysis of the
atmosphere, and show CAPE and shear far better than just simple plots.
A Convective Forecast
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are three basic ingredients for
convection (showers and thunderstorms).
 Lift – fronts, mountains, sea breezes, shortwave
troughs, etc.
 Instability – high CAPE values, large negative
values of LI’s, strong lapse rates, etc.
 Moisture – High values of dewpoints or mixing
ratio in the low levels (the boundary layer).
A Convective Forecast
First, lets us understand this is the GFS and it is a 5 day forecast so there are reasons to doubt
this. For the sake of this tutorial, let us just assume this is going to be correct or what we call
“perfect prog” (perfect prognostication).
The above is a forecast for SBCAPE valid at 18Z and 00Z. This is a fairly low instability forecast, but
there is enough instability for thunderstorms and severe with CAPE’s above 1000 forecast. What
do I notice here? I see a bull's-eye of CAPE in the Texas Panhandle at 18Z, then in southwest KS at
00Z. Clearly, the area of instability is in a north-south line in the western high plains of the US. This
should make a forecaster aware that thunderstorms are certainly possible in that region.
A Convective Forecast
This is the MLCAPE and Delta-T’s for 700mb to 500mb (one takes the temperature at a point at
700mb then subtracts the 500mb temperature from that same point to get the Delta-T, or change
in Temperature.
In this case, there isn’t much difference from the SBCAPE and MLCAPE. Sometimes there is.
However, at 18Z you can clearly tell that the extent of CAPE above 500 J/kg is far smaller than in
the SBCAPE plot. By 00Z, the MLCAPE and SBCAPE are far more similar. Why would this be?
Advanced students should really being to think about why/how this could occur.
A Convective Forecast
Another way to look at instability is by using the LI forecasting. This plot also can easily show you fronts
based on where the LI gradient is located. Notice how packed the lines are of positive LI’s (stable air) in
Nebraska, for instance. Again, this plot can confirm where the instability is. You can see the bulls-eyes
and area of most unstable air. One can also note the bountiful southerly flow brining warm and moisture
air into the Plains.
A Convective Forecast
The question now is … “Why is the CAPE so concentrated along the western high plains? What is
going on there? At this point one should have done an upper air analysis. For the sake of this
tutorial we will simply show you the overall pattern at 500mb. An analysis of this shows a very
large, somewhat closed, upper low centered in Montana, and lifting to the northeast. A strong jet
max extends from the desert southwest into the northern plains. One should being to get a
picture of what is going on. Notice the southern part of that jet is co-located with the area of
CAPE you have seen in previous slides. Now you know you have an area of maximum winds aloft
over an area of instability.
A Convective Forecast
Now that we see what is going on in aloft we should take a look at the surface. Based on the temperatures alone,
we can see some sort of frontal moving through Nebraska, Colorado, and New Mexico. An area of low pressure (a
fairly weak one with pressures above 1005mb) exists in southwest KS/southeast CO. A fairly elongated area of low
pressure extends from that low down into New Mexico and extreme southwestern Texas (a surface trough).
Temperatures are maximized at 18Z along the front back into the Low and in southwestern Texas. This front, low,
and trough are serving as an area of convergence where air is forced upward. In other words, the area where
CAPE existed, and there was a jet max, also has lift.
Do you notice the surface winds in the Texas Panhanldle? At 18Z they are mostly southerly but at 00Z they back
(shift in a counter-clockwise direction) and our now out of the southeast? This increases the shear as winds aloft,
as you saw in the previous slide, are from the southwest! Those winds are turning with height.
At this point, we now we have lift and instability. Now we need to look for moisture!
A Convective Forecast
Very strong southerly to easterly flow from the Gulf of Mexico and southern US is very evident on the
streamlines plotted on this image. On top of that, dewpoints above 50F exist in a large area (anything in
the brighter green is 50+ Fahrenheit.) Dewpoints exceeding 60F are actually quite abundant along the
area of convergence along the fronts and trough. It appears a feature known as a dryline is located
from southwest Kansas into southeastern New Mexico. Students are encouraged to look further into
what this feature is. Regardless, it is easy to tell where winds are “piling up” or converging, and that is
what is important. Those areas are located where the strongest instability exists, as well as under that
500mb jet stream maximum.
A Convective Forecast
Cinh is simply the measure of stable air, or negative-CAPE, that exists which will impede the progress
of upward moving air. You will notice that at 18Z the cap (areas where cinh exists) breaks in the
Texas pahandle into western Kansas. You will also notice this is where, as noticed in previous slides,
convergence at the low levels exists, there is plenty of moisture, shear, and instability. Is it possible
that we could get convection? Could it be severe? It appears that, while the instability is weak there
is at least the possibility of convection and could even be severe in nature. The cap gets stronger at
00Z but still shows some weakness with low values. Advanced students should be thinking about why
this might be going on.
So now we must find out how the model is handling all this information!
A Convective Forecast
We have seen in the previous slides all the ingredients for thunderstorms that exist in the western
high plains. Now we check to see if the model is indeed forecasting thunderstorms to develop. Can
you see what the model is doing here? Precipitation is already ongoing in Nebraska at 18Z
(remember that precip is accumulated from 12Z to 18Z). Remember that front moving through
Nebraska? Meanwhile, from 18Z to 00Z precip moves down the trough into Texas and New Mexico.
Knowing what you know, you can infer that this precipitation is likely in the form of showers and
thunderstorms – convection.
A Convective Forecast
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Convection that may be severe - S.L.I.M.
 Shear – a change of wind speeds and/or direction over some
distance. This is best done by some sounding (model or real data)
analyses. But forecast helicities, or just an analysis of the jets and
low level wind flow can be sufficient as an overview of the potential
shear.
 Lift – an ability to force the air upward. Fronts, sea breezes,
mountains, etc. Generally speaking with severe, lift will be
associated with fronts, drylines, surface troughs, and shortwave
troughs.
 Instability – conditions in place to allow air to rise on its own (air
lifted must be warmer than the surrounding preexisting air in
place). Don’t forget to look at CINH, too, which is an area of
stability supressing convection.
 Moisture – sufficient moisture in place in the low levels of the
atmosphere for storms.