Transcript Slide 1

Low Level Cold Air
One factor often overlooked during a heavy snow
event, even when the event is going full throttle, is the
amount of low level cold air. Philadelphia is a major
boundary area due to its proximity to the Atlantic
Ocean, Gulf Stream Current, and Appalachian
Mountains. Thus, even if an arctic high pressure is in
perfect position, the precipitation can still change to
sleet, freezing rain, snow pellets, or rain. Thus,
temperatures throughout all layers of the troposphere
need to be analyzed in real time and by using
numerical model guidance.
Click here for local data from upper air soundings
Analysis by “usedtobe” Prior to February 1-2, 2007 Storm
Forecast Sounding (All Snow at DCA)
Forecast Sounding at DCA
“Looks like sleet or freezing rain mixed with
snow. Warm layer about 125 mb thick, about +2
C at the highest.” -TerpEast-
Forecast Sounding at IAD
“Probably still snow, maybe sleet mixed in. Warm layer about 75 mb thick,
+1 C at the highest. There might be a bit of drying between 400 and 550
hPa, so it could be a dry slot and it might be freezing drizzle mixed with
snow without the dendritic growth layer.”
Precipitation Type
First, the forecaster must determine what type of precipitation is
expected. Use Table I as a guide to determine whether you will
have liquid or frozen precipitation. In addition, you can also use
the Freezing Drizzle vs. Snow Checklist (Table II) developed by
Headquarters Air Weather Service, Technology Training
Division at Scott AFB, IL (1996). It will help you determine
whether to forecast freezing or frozen precipitation.
Predicting Moderate and Heavy Snow
If you think the weather situation has the possibility of moderate or heavy snow, continue with Table
V. This table has specific parameters and features to look for at mandatory levels from the surface
all the way up to 200 mb. This information was gathered from a multitude of sources, the bulk of
which came from McNulty (1991), Terry (1995), and Weber (1979). Other sources came from
academicians such as Djuric (1994), Moore (1989), and Ucccellini (1990).
With winter weather lasting 3 or 4 months in most locations, forecasters tend to forget what to look
for at various levels. The following information will ease that memory loss, and allow operational
forecasters quick and easy reference, to better forecast major snowstorms.
Precipitation Type and Cold Air Aloft
Precipitation Type and Cold Air Aloft