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Isohyets of snowfall (in.) for the 24-h period ending 1200

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The Role of Coupled Jet Streaks in a Midwestern Heavy Snow EventChad M. Gravelle1,3, Scott M. Rochette1, and Thomas A. Niziol21Department of the Earth Sciences, SUNY College at Brockport2NOAA/NWSFO Buffalo, NY3Current Affiliation: Department of Earth and Atmospheric Sciences, Saint Louis University

Introduction

- Band of heavy snow developed north of an area of weak low pressure from eastern North Dakota southeastward into the lower Great Lakes on 21 January 2005.
- 8-12 inches of snow fell over a 12-h period.
- Strong gradient of snowfall on southwest side of snowband from Minnesota through northern Illinois created a difficult forecast challenge.
- 24-h Eta forecast of precipitation amounts were accurate (~ 8 in.), however the strength and position of the southern gradient was too weak and too far south.

Isohyets of snowfall (in.) for the 24-h period ending 1200

UTC 22 January 2005, based upon cooperative station data

from NCDC.

80 km Eta 00-h forecast (1200 UTC 21 January 2005)

80 km Eta 06-h forecast (1800 UTC 21 January 2005)

80 km Eta 00-h forecast (0000 UTC 22 January 2005)

Top left: Isobars (red solid lines, hPa), 300-hPa heights (pink solid lines, dm), 300-hPa Isotachs (shaded >90 kts). Top

right: 500-hPa heights (green solidlines, dm), 500-hPa abs vorticity (shaded). Bottom left: 500-hPa omega (red solid lines

Î¼b s-1, red dashed lines Î¼b s-1), 300-hPa heights (pink solid lines, dm), 300-hPa isotachs (shaded > 90 kts). Bottom right:

2 km WSI NOWrad.

Ageostrophic Circulations

Enhancing Heavy Snow Ingredients

a

b

~1900 km

~1570 km

80 km Eta 500-hPa omega (red solid lines -Î¼b s-1, red dashed lines

Î¼b s-1), 300-hPa heights (pink solid lines, dm), 300-hPa isotachs

(shaded > 90 kts): (left) 1200 UTC 21 Jan 00-h forecast, (right)

0000 UTC 22 Jan 00-h forecast. Shaded blue region on right is

Area of heavy snow at 0000 UTC 22 Jan.

80 km Eta 850 Î¸e advection (blue solid lines +K 12 h-1), blue

dashed lines -K 12 h-1), 300-hPa heights (pink solid lines, dm),

300-hPa isotachs (shaded > 90 kts): (left) 1200 UTC 21 Jan 00-h

forecast, (right) 0000 UTC 22 Jan 00-h forecast. Shaded blue

region on right is area of heavy snow at 0000 UTC 22 Jan.

c

d

Ingredients for Winter Precipitation

80 km Eta 500-hPa omega (green/red solid lines -Î¼b s-1, green/red dashed

lines Î¼b s-1), 300-hPa heights (pink solid lines, dm), 300-hPa isotachs (shaded

> 90 kts): (left) 300-hPa ageostrophic wind vectors (red) 0000 UTC 22 Jan 00-h

forecast, (right) 850-hPa ageostrophic wind vectors (red) 0000 UTC 22 Jan 00-h

forecast.

~1400 km

(Wetzel and Martin 2001)

~970 km

- Forcing for Ascent
- Moisture
- Temperature (Precipitation Type)
- Instability
- Efficiency

Eta omega (shaded warm colors -Î¼b s-1, shaded cool colors Î¼b s-1),

isotachs (black solid lines, kts), isentropes (tan, K), ageostrophic vertical

circulation (blue arrows): (a) 1200 UTC 21 Jan 00-h forecast, (b)

1800 UTC 21 Jan 06-h forecast, (c) 0000 UTC 22 Jan 00-h forecast,

(d) 0600 UTC 22 Jan 06-h forecast. Inset figures provide the orientation

of the cross-section with respect to the isotach field (shaded, > 90 kts).

White shaded area in topography of cross-section is area of snow at

that time and distance between ageo circulations is marked.

80 km Eta 850 to 500-hPa 2-D frontogenesis

(pink solid lines +K m-1 10-10 s-1), pink dashed

lines - K m-1 10-10 s-1), 300-hPa heights (blue

solid lines, dm), 300-hPa isotachs (shaded

> 90 kts): 0000 UTC 22 Jan 00-h forecast. Shaded blue

region is area of heavy snow at 0000 UTC 22 Jan.

Enhancers

Quasi-geostrophic Forcing

Conclusions

- Snowband intensified and became defined as ascending branches of two upper-level jet streaks and associated ageostrophic circulations merged.
- Coupled jet streak circulation enhanced and intensified two of the main ingredients required for heavy snowfall.
- Forcing for Ascent

- Moisture

- Coupled jet streak circulation strengthened the QG forcing for UVM (Hakim and Uccellini 1992).
- Area of Differential Positive Vorticity Advection

- Strong Warm Air Advection

- Synoptic scale QG concepts may be applicable in the meso-Î±scale.
References

Hakim, G.J., and L.W. Uccellini, 1992: Diagnosing Coupled Jet-Streak Circulations for a Northern Plains Snow Band from the Operational Nested-Grid Model. Wea. Forecasting., 7, 26-48.

Wetzel, S.W., and J.E. Martin, 2001: An Operational Ingredients-Based Methodology for Forecasting Midlatitude Winter Season Precipitation. Wea. Forecasting., 16, 156-167.

Composite

0000 UTC 22 January 2005

80 km Eta temperature advection

(shaded warm colors K 12 h-1,

shaded cool colors â€“K 12 h-1),

isotachs (black solid lines, kts),

isentropes (tan, K), ageo vertical

circulation (blue arrows): 0000 UTC

22 Jan 00-h forecast. Inset figure

provides the orientation of the

cross-section with respect to the

isotach field (shaded, > 90 kts).

80 km Eta 500-hPa heights

(green solid lines, dm), 500-hPa

abs vorticity (shaded ): 0000 UTC

22 Jan 00-h forecast. White shaded

area is location of heavy snow and

white arrows are the jet cores at

300-hPa.

80 km Eta positive vorticity

advection (shaded 10-9 s-1),

isotachs (black solid lines, kts),

isentropes (tan, K), ageo vertical

circulation (blue arrows): 0000 UTC

22 Jan 00-h forecast. Inset figure

provides the orientation of the

cross-section with respect to the

isotach field (shaded, > 90 kts).

80km Eta 925 to 700-hPa QG

frontogenesis (blue solid lines

+K2 m-2 10-15 s-1), blue dashed

lines -K2 m-2 10-15 s-1), 300-hPa

heights (pink solid lines, dm),

300-hPa isotachs (shaded > 90 kts):

0000 UTC 22 Jan 00-h forecast.

Shaded blue region is area of

heavy snow at 0000 UTC 22 Jan.