|NOWCASTING WINTER PRECIPITATION|
ON THE INTERNET
METEOROLOGIST JEFF HABY
A. Winter Weather Radar
Online location: Click here
for winter weather radar
(can be zoomed by clicking mouse on a region of the country, check valid time to make sure site is updating)
1. Winter weather radar uses a variety of information to determine the precipitation type that is reaching
the surface. The primary sources of data are surface reports. Other high tech means include model forecast
soundings and changes in radar reflectivity with height. Winter weather radar divides precipitation into rain,
mix, and snow. The mix is used to denote sleet and freezing rain. It is a good idea to compare the winter radar
with station observations since there is interpolation error/mesoscale variation on the dividing lines between
2. An example winter weather radar image is located at:
Click here for winter radar image
*Polygon shape of some of the precipitation type transition boundaries
*Bands of heavier precipitation (precipitation is moving toward the NE)
*Back edge of winter precipitation extending from OKC to SPS to ABI
3. Radar can be used to notice precipitation trends. As a general rule, when an area of precipitation over
the last hour has continued to increase in areal coverage it will continue to increase in areal coverage.
When an area of precipitation over the last hour has continued to decrease in areal coverage it will continue
to decrease in areal coverage. This trend can help the forecaster determine regions receiving lighter and
heavier winter precipitation totals.
4. Look for training and banding of wintry precipitation on radar. Winter precipitation tends
to fall in distinct bands. These bands may have 2 or more times the winter precipitation than surrounding
locations. The locations where these bands are setting up will show as training echoes of higher
reflectivity on radar.
5. Once the back edge of winter precipitation passes a location, the winter precipitation event is usually over.
This helps the nowcaster predict more accurate ending times for the winter precipitation event.
6. If the PBL is dry (dewpoint depression greater than 10 F), light precipitation detected on the
radar will have a difficulty reaching the surface. Light precipitation falling into dry air will
become virga and will cool the air due to evaporational cooling.
7. Compare radar echoes to precipitation progs from the
FORECAST MODELS. This enables the forecaster
to know immediately if the location of precipitation forecasted by the models is accurate. The region
of greatest accumulation can be adjusted by using these trends. The radar will let the forecaster know
the regions the heaviest precipitation bands are setting up better than the forecast models.
B. Satellite Imagery
Online satellite imagery available at:
1. As with radar, look for trends. A whitening of clouds indicates vertical expansion of cloud tops. As a
general rule, when a cloud band or a cloud shield over the last hour has continued to increase in whiteness,
it will continue to increase in whiteness. When a cloud band or cloud shield over the last hour has continued
to decrease in whiteness, it will continue to decrease in whiteness. This trend can help the forecaster
determine regions receiving lighter and heavier winter precipitation totals.
2. Upper levels lows can be tracked very successfully using satellite imagery. Again, a whitening
indicates the upper level low is strengthening. Upper level lows are responsible for many of the
"surprise" snowfalls because often the forecast models have a difficulty handling them.
The following image shows an upper level low on water vapor imagery in Oklahoma. The center of
rotation is in NE Oklahoma. Notice the whitening of clouds and moisture to the south of the upper
level low. Upper level lows in the presence of a "winter weather sounding" can produce significant
mesoscale snowfall or wintry precipitation.
Click here for upper level low
C. Surface Observations
1. The most important elements to observe on surface observations in a winter weather situation
nowcast are temperature, dewpoint depression, and wind speed/direction.
With the ASOS/METAR system in place, updates on surface conditions are available continuously. Upper air
data is only updated twice a day and it is not real time. A website that has surface observations updated
several times per hour can be found at:
Click here for Surface METAR US map
NOTE ABOUT THIS SITE: Regional hourly surface observations are available by clicking on a region of the country.
For the most updated surface observations, click the
"THIS JAVA TOOL" button. With
your mouse you can
CLICK and DRAG to outline an area of the country of your choosing. Placing the mouse over an observation
will give the latest and most up to date METAR readings. At the bottom you can choose which weather
elements to display.
2. Temperature-- Notice the temperature gradient and the freezing line (you may have to "mentally" draw
the freezing line while examining the observations). Some type of frozen precipitation is guaranteed for
locations colder than the freezing line. Watch the progression of the freezing line over time.
3. Dewpoint depression-- Determines wet-bulb cooling potential. I have found the (1/3 - 5) rule to work
successfully for temperature near freezing. When the dewpoint depression is less than 5, very little
cooling takes place when precipitation begins because the air is so close to saturation. The 1/3 -5 rule
is as follows: Take the dewpoint depression, subtract 5 from it, divide this number by 3, subtract this
number from the temperature. This gives the temperature that will result from evaporational cooling.
For example suppose the temperature is 35 and the dewpoint is 18. The dewpoint depression is 35 F - 18 F = 17 ,
17 - 5 = 12, 12/3 = 4. This results in evaporational cooling giving a temperature of 35 - 4 = 31 F.
Wet-bulb cooling can change rain to freezing rain, sleet or snow. Wet-bulb cooling will move the freezing
line into regions that were originally above freezing even the absence of CAA. WAA and wet-bulb cooling
will partially or completely offset each other.
4. Wind speed-- Determines the rate of advection. WAA or CAA can change precipitation type.
5. Wind direction-- Determines the movement of the freezing line and which regions will experiences
WAA or CAA. See if the wind direction is continuing to feed cooler air and lower dewpoints toward the
forecast area or if a WAA pattern is in the process of developing. In winter precipitation events, wind
direction can change dramatically over the course of a few hours. A wind direction shift that
brings WAA will often spell a quick ending to the winter precipitation event (especially if temperatures
are already near freezing).
6. Air mass modification of polar air is an important consideration as the air mass moves out
of the source region and toward lower latitudes
*Shallower polar air masses tend to warm more quickly than deep polar air masses
*Warm soil temperatures will warm a polar air mass
*If a polar air mass moves over a large snowfield, it will not warm as quickly
*Precipitation into a polar air mass will chill and moisten the polar air mass
*Sunshine and higher sun angles will warm a polar air mass
*Night and lower sun angles do not allow the air mass to warm substantially
*Clouds significantly reduce polar air mass warming (especially in lower latitudes)