METEOROLOGIST JEFF HABY
The 850 model prog is a lower troposphere prog that can be used to forecast
upslope/downslope wind, cyclogenesis and
dynamic lifting or sinking due to WAA and CAA. This model prog can be found at:
The prog displays isotherms (in colored 2 degree increments), wind vectors (longer wind vectors indicate
stronger wind) and the lines we see on each upper level prog:
height contours. The number 1 item a forecaster
looks for on the prog is WAA and CAA. Strong WAA can enhance
instability and uplift especially when
moisture advection. Strong CAA, especially when associated with
dry air, causes a dynamic sinking of air and the
promotion of stability. Thermal advection is analyzed by using the wind vectors and the temperature gradient
at 850 millibars. If strong winds (long wind vectors) and blowing through a "tight"
(isotherms close together), significant thermal advection will occur. If warmer temperatures are moving
toward a fixed point then it is WAA; if colder temperatures are moving toward a fixed point then it is CAA.
In the cool season, an important 850 temperature is the zero degree isotherm. If the temperature at 850
is above freezing (especially if more than 2 C above freezing), precipitation is likely to NOT fall as snow
according to that forecast model. Freezing temperatures at 850 mb can support snow or some other form of
freezing rain at the surface can occur when the 850 mb temperature
is above freezing. Check the surface and 1000 mb temperatures (among other parameters) for this potential.
Downslope and Upslope winds are
common near mountainous locations (i.e. Rocky Mountains, Laramie Mountains, Bighorn Mountains, Appalachian
Mountains, etc.) and across the sloping elevation of the High and Great Plains. Downsloping winds will
lead to warmer 850 mb temperatures than surrounding regions. Strong downslope winds in the winter are
referred to as the
Chinook in the High Plains region of the U.S. Upslope winds can cause snowfall and/or cooling
temperatures in the cool season.
Cyclogenesis can be located on the prog by noticing a gradual curving of the
isotherm colors and the beginnings of a counterclockwise rotating 850 wind vector field. Low level cyclogenesis
occurs often in a region with closely spaced isotherms. It can occur along a frontal boundary and along or near
a coastline (often a strong thermal gradient will exist near a coastline, especially if the coastline separates
warm and moist air from a synoptic scale polar like air mass). The prog can be used to help forecast temperatures.
Strong WAA will lead to a warming trend in temperatures and strong CAA will lead to a cooling trend.