|PREDICTING HIGHS, LOWS|
AND TEMPERATURE TRENDS
METEOROLOGIST JEFF HABY
(1) Study surface analysis charts:
Take note of:
a) Thermal advection (remember thermal advection is a function of the thermal gradient, the wind speed
through the thermal gradient, and the angle the wind is through the temperature gradient)
b) Keep in mind the soil temperatures, cloud cover / lack of cloud cover and snow cover. These three
parameters will modify air temperature as an air mass advects.
c) Wind speed/ direction
d) Pressure pattern
analysis chart. Take note of same items as above.
(3) Study surface and 850 mb model output
/ GFS) for these same items above
LATEST SOUNDING. Ask yourself how the sounding will be modified throughout the day. An approximation
of the afternoon high can be made by taking the DALR
from the top of the PBL to the surface. This works best
on clear, moderate wind days in a barotropic atmosphere.
TEMPERATURE CHANGE FUNCTIONS
*Thermal advection (strength of WAA and CAA)
*Local heating/ cooling (ISR, OLR)
*Daylength (less daylight = less ISR = lower temperatures)
*Atmospheric albedo (i.e. clouds, snow cover, suspended particles)
*Dewpoint (sets a quasi-limit for overnight cooling in a barotropic
*Latent heat release or absorption (i.e. wet bulb cooling, latent heat release from dew)
*Vertical temperature movement (adiabatic ascent, descent; downdraft from storm)
*Wind speed (determines how much heat will build at surface by day and how much cool air will build at night;
strong winds keep the air well mixed)
*Mesoscale effects (i.e. topography, urban heat island, water bodies)
High and low temperatures are a primary function of thermal advection, wind speed, cloud cover, dewpoint and
the number of daylight hours.
*Highs can be less than expected due to CAA, high wind speed, increased cloud cover, higher dewpoint and shorter
*Highs can be greater than expected due to WAA, low wind speed, decreased cloud cover, lower dewpoint and longer
*Lows can be less than expected due to CAA, low wind speed, decreased cloud cover, lower dewpoint and
*Lows can be greater than expected due to WAA, high wind speed, increased cloud cover, higher dewpoint
and shorter nights.
*Mesoscale effects such as urban heat islands, differential vegetation, topography, nearby lakes / rivers / oceans
and altitude must also be taken into consideration!
*Low wind speed on sunny days will result in warmer temperatures than if the winds were stronger. With light winds,
heat can build right at the surface without being significantly mixed with cooler air aloft. This can form what is
known as the superadiabatic lapse rate. This
is the opposite case on a clear night. Light wind at night does
not allow radiationally cooled air at surface to mix with warmer air aloft.
Fronts may cause highs and lows to occur at untraditional times during the day. In association with a strong
cold front, the high will occur before frontal passage and the low will occur at midnight (assuming CAA and
temperatures continue to decrease throughout the day). The timing of the front is critical in determining what
the high will be before the front passes and how much CAA will occur when the forecast period ends
Sloped terrain produces downsloping and upsloping wind especially when the wind direction is perpendicular to
the slope of the terrain. A wind direction forecast is critical in determining how much adiabatic warming or
cooling will occur along the slope. Upslope cooling in the cool season can produce adiabatic cooling and snow
while downslope flow produces adiabatic warming and a chinook wind.
Wet-bulb cooling will occur over
regions precipitation occurs. If the precipitation forecast is incorrect, odds
are the temperature forecast will suffer also. Afternoon thunderstorms can cause the high temperature to be cooler
than predicted. Any rain during any time of the day will cause evaporational cooling at the surface. The low
level dewpoint depression determines
how much surface temperatures will cool. A high dewpoint depression will
result in a greater evaporational cooling. Convective thunderstorms also transports air from higher in the
atmosphere to the surface. This will alter surface temperatures greatly in these downdraft regions.
*Temperature accuracy diminishes rapidly after 3 days
extended forecast temperatures and your knowledge of the
to make extended temperature forecast
*Underneath trough--- cooler than normal
*Underneath ridge--- warmer than normal
GFS / NAM MOS data and
after coming to your own forecast consensus. Tweak forecast if data is reading a process you did not consider. Learn
the MOS, meteogram biases and
PITFALLS for your
zones and state forecast discussions
(7) Debate in your mind why you agree or disagree with the NWS and computer models.
(8) Make your temperature forecast (lows, highs). Keep in mind the
starting and ending time of your forecast period. Note also how the
forecast verifies. This will prevent the same forecasting mistake
from being made in the future. Experience adds greatly to making
correct temperature forecasts.