Transcript Winter Weather Forecasting Primer

Lamont Bain
OWL Director of Operations
Thickness
 Thickness- distance between two pressure surfaces,
usually denoted in decameters.
 Derived using the hypsometric equation:
 Knowing the thickness between two different pressure
surfaces can give you an idea of the mean temperature in
that layer.
 Diagnose Warm Air Advection (WAA) and Cold Air
Advection (CAA) at the surface using “the trapezoid”
method and diagnose possible boundaries.
 Suggested reading:
www.theweatherprediction.com/winterwx/thicknesscrit
eria/
“Critical Thicknesses”
Diagnose WAA/CAA
weather.ou.edu/~metr4424
Thickness Sources
•HOOT: 1000-500 mb
thickness with 500mb
heights and
temperatures
(expansion is
anticipated)
•Patrick Marsh’s Map:
Page 1000-500mb
thickness with MSLP
•College of Dupage:
Various Layers for
Critical Thickness.
Forecast Soundings: A top-down approach
 Not just for thunderstorms!
 Guide for thermal structure and thus “p-type.”
 Ice Nucleation and Snow Growth
 Freezing Rain? Sleet? Snow? Just a cold rain?
Ice Crystals
• For Snow to occur, ICE MUST BE PRESENT IN
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CLOUD
Plains -9C suggests that IN have become activated.
-15C ice is likely!
-12C to -16C layer is often refereed to as the Dendridic
Growth Zone or DGZ (per RAOBs)
How do I determine the temperature?
– RAOB or IR Satellite Imagery
Top-Down Method
 Is it cold enough to support ice crystals in clouds?
 Are UVVs and higher RH values (or other moisture
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proxy) collocated within the Dendritic Growth Zone?
Negative Omega-Upward Vertical Motion
Positive Omega-Downward Vertical Motion
Is there any WAA/CAA in the column?
Dry or Moist Advection at mid/upper levels?
Evaporative Cooling/Melting?
Will temperatures at the surface support liquid,
freezing, or frozen precipitation?
SPC Sounding
BUFKIT Forecast Sounding
BUFKIT Time Series Plot
January
th
28-29
2010 Ice Storm
 Saturated mid/low levels of the troposphere
 Sub Freezing Temperatures at the SFC
 Warm Air Aloft
 Convection!?
12Z & 15Z OUN RAOBs
18Z & 21Z OUN RAOBs
0Z OUN RAOBs