The process of atmospheric cooling due to melting precipitation is examined
to evaluate its contribution to determining precipitation type. The " melt
ing effect'' is typically of second- order importance compared to other pro
cesses that influence the lower- tropospheric air temperature and hence the
type of precipitation that reaches the ground. In some cases, however, coo
ling due to melting snowflakes can emerge as the dominant agent of temperat
ure change, occasionally surprising forecasters (and the public) by inducin
g an unexpected changeover from rain to heavy snow. One such case occurred
on 3- 4 February 1998 in east- central Tennessee and surrounding areas.
Commonly applied considerations for predicting precipitation type had convi
nced forecasters that significant snowfall was not likely with this event.
However, real- time observations and a postevent analysis by forecasters at
the Storm Prediction Center led to the hypothesis that the melting effect
must have provided the cooling necessary to allow widespread heavy snowfall
. To test this hypothesis, the Pennsylvania State University- NCAR Mesoscal
e Model was used to generate a mesoscale- resolution, four- dimensional dat
aset for this event. Diagnostic analysis of the model output confirmed that
cooling due to melting snowflakes was of a sufficient magnitude to account
for the disparity between observed and forecasted lower- tropospheric temp
eratures in this case.
A simple formula is derived to provide a " rule of thumb'' for anticipating
the potential impact of the melting effect. In addition, guidelines are pr
ovided for identifying meteorological patterns that favor a predominance of
the melting effect.