Weather and Climate Lecture 14 28 April 2008 Forecasting If you know where the low [or the high] is now You should be able to guess your future weather You should be able to guess your future temperature It moves to your north or west Winds will shift to the south
28 April 2008
Winds will shift to the south
Any frozen precipitation will likely turn to rain or drizzle
By the time the cold air arrives, moisture is gone
It moves to your south or east
Winds will shift from easterly to northerly to northwesterly
Frozen precipitation will continue
Cold air will arriveA Low is Approaching
Which way will this storm move?
Weather forecasts play a role in the everyday decisions that an average person makes.
Many entities need
The individual satellite paths are obvious!
infer a lot from its structures on a satellite image
(This storm is likely extremely strong with strong winds)
Note the grid – values are defined at every point on the grid
Different models make different assumptions about aspects
of the weather. Or, they have different resolutions. Result: different forecast outcomes. Overall agreement is usually okay
If sequential model runs show similar results, you should forecast that event with more confidence
Don’t forecast with a lot of confidence in that case:
Emphasize what you know, and mention the possibilities
stronger than analyzed? How will that affect the
evolution of errors in the subsequent forecast?
Sometimes, the computer simulation dies (or is obviously wrong), or it cannot be transmittedIf there are problems with numerical weather prediction models, what other methods can a forecaster use?
Persistence assumes tomorrow is a carbon copy of today
Things are changing at a fixed rate that doesn’t change with time
You could use the
information on this
chart to predict
weather on the
20th or 21st by
assuming that the
evolution of the
Still -- use ‘old’ path to
forecast weather during ‘new’ storm
This is the focus of most current advances in NWP
If you’re forecasting for someplace that’s not predictable,
you change the definitiveness of your wording!
You can also use different models, with the same initial fields, and see how each model describes the evolution of the atmosphere
Think about Phoenix, AZ.......if you had to make a forecast for tomorrow, would you forecast rain or sunshine?
A climatological forecast is based solely on what the climate records indicate for that location.
Chance of a White Christmas
What is a “rule of thumb”?
Clouds affect the overnight low – cloudy nights are usually warmer
Cloudy days are usually cooler than clear days
High to your west? Cooler
High to your east? Warmer
Are south winds moving in warmer air?
Snow cover leads to cooler high temperatures
These are the 'normal' forecast parameters
But some companies might want specific
things forecast: When will the RH exceed
90%? What is the wind direction?
THEN TRY TO PREDICT THE CONDITIONS OF THAT ATMOSPHERE OVER A SMALL AREA WITHIN A 5 MILE RADIUS FOR A PERIOD OF ONE TO FIVE DAYS IN ADVANCE!!
What the meteorologist sees!
All of the above get the same data from NOAA/NCEP! That’s why many forecasts sound the same!
Did it verify?