weather 101 how to read computer model

Weather 101: How To Read Computer Model Graphics

One of the most common maps you see on Phillyweather.net are maps that

are generated by NCEP (National Centers for Environmental Prediction),

who generate free distribution maps for the GFS and NAM computer

models, among others. Both of these computer models use the same

graphics and run four times a day.

This edition of Weather 101 will take some of the basic points that

are on a run of the NAM computer model and show you what it is saying

and tell you what a possible forecast outcome will be based on this

model. We are using one map to illustrate the points of what one map

is showing. However, it's important to look at the whole context of

what the model is showing and not just one map as multiple maps will

show the direction a storm is moving, how fast it is moving, and

changes in temperature associated with weather patterns. Additionally,

one model's interpretation of upper air data may be different than

another so looking at multiple computer models in analyzing what

happens is important as well. In the case of the NAM it forecasts out

to 84 hours from its start time, or 3 1/2 days away. The GFS, for

reference purposes, forecasts out to 384 hours or 16 days away.

Feel free to click on the maps below to see them in a larger size.

Some of the most common questions that I run into are how to

understand and develop a sense of reading these maps, what they

entail, and what the map is showing in Philadelphia.

Based on this computer model run up above, the air 5000 feet above the

surface is forecast by the NAM to be below -10 Celsius, which would

equate to a temperature in the high 20's to low 30's in Philadelphia

if this model were specifically correct regarding temperature. Since

Philadelphia is north of the second blue line above the red line, this

correlates to temperatures at 5000' (or the 850 millibar level of the

atmosphere) of below -10 Celsius. A common equation to use with

temperatures at this level when figuring out surface temperatures is

to add between 5 and 15 degrees Celsius to the 850 temperature, with

the lower numbers being used in winter and higher numbers used in the

summer. A good 850 rule explanation is available at Wxrisk.com.

Why is the temperature at 850 mb important? Many times it is the

indicator of what precipitation type falls from the sky weather it is

liquid or frozen. There can be sleet if there is a warmer layer above

the 850 level but it can be a good indicator of understanding where

there is potential warm layers to deal with. In this case, since

temperatures are as cold as they are the NAM would be indicating a

likely snow scenario if it were correct for the time frame it is

forecasting.

The low pressure system on this specific run of the NAM is broad (no

distinct L in the Atlantic near the lowest pressure number on the

graphic above) and high pressure is located over Illinois on the map

(the black H up above). The closer the black lines are to each other

and the closer low pressure is to high pressure the stronger winds

will typically be. In this case, it may be breezy since high pressure

is strong and there is some tightening of the wind gradient over

Western Pennsylvania) but winds should be under 20 mph at the time of

this forecast.

It's important to note how much precipitation falls as well. In this

run of the NAM for Philadelphia we can see there will be at least .10"

of liquid precipitation over a six hour period. Keep in mind that

precipitation output on the NAM and GFS is always for the PRIOR six

hours. In the case of this particular storm, the NAM itself is

forecasting about 1/4" of liquid altogether but you can see that it is

at least 1/10" of an inch as Philadelphia is in the second shade of

green on the chart on the left. In South Jersey, the NAM is

forecasting more precipitation in a six hour period as they are in a

darker shade of green.

The time is important as well. In the case of NCEP model graphics, all

times are in UTC or Greenwich Mean Time. The NAM map above indicates a

forecast time of 7 PM EST for Thursday night. Model graphics will have

times of 00 UTC (7 PM EST), 06 UTC (1 AM EST), 12 UTC (7 AM EST), and

18 UTC (1 PM EST). If we are in daylight savings time you want to add

an hour to the times that are forecast.

On the bottom you can see two sets of times and dates. The 18 UTC is

the run time of the model (meaning it was run at 1 PM)...that time is

on the left next to the date of the model run (if you click on the map

you can see the date is 01/22/08 -- that was the date the model

forecast was made and the time the model ran was 18 UTC. On the bottom

right is the time the forecast is valid for -- 01/25/08 at 00 UTC --

or 7 PM EST on Thursday night.

Now that we have collected all the pieces of data above, we can add

each piece up to get an idea of what one computer model is suggesting

for Philadelphia. The NAM in this case is thinking that about 1/4" of

liquid precipitation will fall from 1 PM to 7 PM on Thursday

afternoon, with temperatures that are likely below freezing, and

breezy conditions. Without looking at all of the other layers of the

atmosphere, we can deduce that we will likely see snow from this if

this model is correct. Of course the computer models are not always

correct but this is one computer's idea behind what will happen.

As weather conditions warrant over the coming weeks I will continue to

do posts that will highlight different computer model maps and what

they show as well as how to read upper level weather forecast maps

that are put out by the GFS, NAM, and other computer models.

Tags: Weather 101, NAM, NCEP, model discussion, modelology

written by Tom @ 4:12 PM |