Winter Weather Forecasting Primer

Lamont BainOWL 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/thicknesscriteria/

“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 GrowthFreezing Rain? Sleet? Snow? Just a cold rain?

Ice Crystals• For Snow to occur, ICE MUST BE PRESENT

IN 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 proxy) collocated within the Dendritic Growth Zone?

Negative Omega-Upward Vertical MotionPositive 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 28-29th 2010 Ice Storm Saturated mid/low levels of the troposphereSub Freezing Temperatures at the SFCWarm Air AloftConvection!?

12Z & 15Z OUN RAOBs

18Z & 21Z OUN RAOBs

0Z OUN RAOBs

Questions? Concerns? Comments?