WEATHER WITH THE SKEW-T
METEOROLOGIST JEFF HABY
Skew-T's are handy forecasting tools for predicting winter precipitation type. If temperatures from a 1000
meters above the surface to the top of the troposphere are below freezing, precipitation type is likely to be snow.
It may, for a short period, fall as a cold rain or a wintry mix, but through evaporational cooling the
precipitation type will change to snow (unless
low level warm air advection is occurring or
inadequate evaporative cooling occurs in the
In some cases, temperatures in the PBL will be below freezing but an
inversion just above the PBL will have above
freezing temperatures. This inversion could be the top of a shallow cold front or a layer of warm air advection.
This situation is conducive to producing
sleet. If the below freezing temperatures near the surface are fairly deep
are are capped with above freezing temperatures further aloft, precipitation type will likely be sleet. Precipitation
type will be freezing rain if the warm air aloft is well above freezing and/or deep.
The set up for freezing rain is similar to that of sleet except the below freezing temperatures extend only a
short distance above the surface (ranging from below freezing temperatures just at the surface or extending to
as high as 500 or so meters above the surface). An inversion of above freezing temperatures will cap the below
freezing low level temperatures, but the inversion will be closer to the ground than inversions
associated with sleet.
Only two balloon soundings are launched each day, therefore soundings can change rapidly in just a few hours.
From studying the analysis and forecast panels, gain an insight into thermal and
moisture advections that will
change the soundings. A slight change in thermal advection can change the precipitation type from one to the
other (i.e. warm air advection... snow to rain, freezing rain to rain, sleet to freezing rain;;;; cold air advection.
... rain to snow, freezing rain to sleet, freezing rain or sleet to snow).
Sometimes the vertical depth of cold air
and the inversion of above freezing temperatures will be near the cusps of a change in precipitation type. This
produces a wintry mix, precipitation type changes from one type to another or even sleet, snow, and cold rain all
falling at the same time. Evaporative cooling also plays a key role. Monitor the
wet bulb temperatures from
the surface to the top of the inversion to monitor possible changes in precipitation type (if wet bulb is at or
below freezing at all levels, precipitation type will eventually change to all snow, until or unless warm air
advection again changes it back to another type.
Remember that winter precipitation is "elevated lifting". Parcels of air will begin their ascent from the top
of the inversion. Calculate indices using the top of the inversion as a base for the convection or lift.
Forecasting winter precipitation using Thickness values: http://www.theweatherprediction.com/winterwx/thicknesscriteria/