1. The Role of Coupled Jet Streaks in a Midwestern Heavy Snow Event Chad M. Gravelle 1,3, Scott M. Rochette 1, and Thomas A. Niziol 2 1 Department of the Earth Sciences, SUNY College at Brockport 2 NOAA/NWSFO Buffalo, NY 3 Current Affiliation: Department of Earth and Atmospheric Sciences, Saint Louis University 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) Eta omega (shaded warm colors -μb s -1, shaded cool colors μb 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. ~1900 km ~1570 km ~970 km ~1400 km Ageostrophic CirculationsEnhancing Heavy Snow Ingredients Quasi-geostrophic Forcing 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 μb s -1, red dashed lines μb s -1 ), 300-hPa heights (pink solid lines, dm), 300-hPa isotachs (shaded > 90 kts). Bottom right: 2 km WSI NOWrad. a b c d 80 km Eta 500-hPa omega (green/red solid lines -μb s -1, green/red dashed lines μb 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. 80 km Eta 500-hPa omega (red solid lines -μb s -1, red dashed lines μb 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 θe 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. 80 km Eta temperature advection (shaded warm colors K 12 h -1, shaded cool colors –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 +K 2 m -2 10 -15 s -1 ), blue dashed lines -K 2 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. 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. Ingredients for Winter Precipitation (Wetzel and Martin 2001) Forcing for Ascent Moisture Temperature (Precipitation Type) Instability Efficiency Enhancers 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- α 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 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.