A Compare and Contrast Study of Two Banded Snow Storms Part I – January 6th, 2002.

Part I – My presentation A brief review of banded snowstorms January 6th, 2002 (A classic case?)

A mechanism for banded snowstorms (Nicosia and Grumm)

Novak examined several years of data, defined single band events as events with >25 mm or rain or >15 cm melted snow, >250 km in length, 20-100 km in width, >30 dbz, > 2 hours.

Composite maps for “single-band” banded snow-storms (from Novak) (>25 mm rain or >15 cm melted snow, >250 km in length, 20-100 km in width, >30 dbz, > 2hrs).

Typical cross section of theta-e, omega and fgen (Novak)

Conceptual model from Novak Conceptual model from Novak. Band formation is favored in the deformation zone, along the axes of dilitation.

Summary Banded snowstorms typically associated with major, deepening cyclones. Bands typically form in an area of mid-level deformation and frontogenesis. Storm-relative flows associated with the storm act to favor slantwise instability in the area of deformation.

January 6th, 2002

Water Vapor imagery at 1815 UTC on 1/6/02

Water Vapor imagery at 0015 UTC on 1/7/02

Eta 12 hour forecast 300 mb height and windspeed 00z 7th.

12 hr Eta forecast 700 mb heights verifying 1/7 00 UTC

12 hour Eta forecast pmsl and 850 mb temperature 1/7 00 UTC.

12 hr Eta forecast 500:400 divergence of Q 1/7 00 UTC.

12 hr Eta forecast 600:500 layer avg rh and omega verifying 1/7 00 UTC

12 hour Eta forecast sounding at AVP displayed on BUFKIT 1/7 00z. 10 kft 0 C -10 C

Composite of base reflectivity 1/6 2050 UTC

Composite of base reflectivity 2350 UTC 1/6/02

Composite of base reflectivity 0100 UTC 1/7/02

Observed snowfall 1/6 – 1/7 2002. Heavy snow

12 hr Eta forecast 800 mb deformation and wind 1/7 00 UTC

12 hr Eta forecast frontogenesis and omega

12 hr Eta forecast frontogenesis and negative EPV 1/7 00 UTC. Snow bands Snow bands

12 hr Eta Theta-e and geostrophic momentum 1/7 00 UTC. Snow bands

12 hr Eta forecast 700 mb absolute geostrophic vorticity 1/7 00 UTC.

12 hr Eta forecast theta-e and non-geostrophic momentum 1/7 00 UTC.

What is physically happening that is producing convective snow bands in this case?

Explanation 1) Inertial instability, resulting in horizontal (northward) accelerations, with individual parcels being forced upward along the sloping front zone. These accelerations act to remove the inertial instability. N Parcel becomes sub-geostrophic, PGF > Coriolis, parcel accelerates to the north. N S

700 mb heights Another way of looking at it: The air parcel can’t make the sharp anticyclonic turn, so it accelerates down the height gradient. Key: the existence of the sharp, small-scale downstream ridge.

Explanation 2) The real momentum surfaces of the real atmosphere are not sloping downward, like in the Eta forecast of geostrophic momentum, but are flatter than the Eta forecast of non-geostrophic momentum. Slantwise convection can occur. Theta-e Momentum

Summary - January 6th – “Classic” Aspects A rapidly deepening storm with strong, widespread forcing. Banding developed in an area of mid-level frontogenesis and lower to mid-level negative EPV.

SUMMARY – January 6th “Non-Classic” Aspects The system was associated with a compact short-wave located well downstream from the long-wave trough. The atmosphere was not close to convectively unstable above the frontal zone. Model forecasts indicated the presence of inertial instability in the geostrophic wind field.

SIMILARITIES BETWEEN 1/06 AND 1/19 Significant under-forecast bands of snow in each case. The bands of heavy snow were located to the northwest of the strongest large-scale forcing for upward motion. The bands were located near the northern edge of sloping regions of frontogenesis. Instability, identified by negative regions of EPV on cross-sections, became co-located with the frontogenesis.

DIFFERENCES BETWEEN 1/06 AND 1/19 The snow band associated with 1/6 was very narrow and intense, and located within a large area of light to moderate snow. The band on 1/19 was broader, with little snow occurring outside the band. Large-scale forcing was much stronger and broader on 1/6. Thermodynamic profile included a warm layer on 1/6, no warm layer on 1/19.

DIFFERENCES BETWEEN 1/6 AND 1/19 (cont). The instability associated with 1/19 appeared to be conditional instability. By contrast the atmosphere above the frontal zone on 1/6 was statically stable, may have been inertially unstable.