factors contributing to a heavy snowfall event across northern illinois december 2009 nathan...
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FACTORS CONTRIBUTING TO A HEAVY FACTORS CONTRIBUTING TO A HEAVY SNOWFALL EVENT ACROSS SNOWFALL EVENT ACROSS
NORTHERN ILLINOIS DECEMBER 2009NORTHERN ILLINOIS DECEMBER 2009
Nathan Marsili/Stephen Rodriguez
2010 GLOM Workshop
Toronto, Ontario
Introduction
Synoptic Background and Forcing Mechanisms
Mesoscale Features/Forcing Mechanisms Snow/Liquid Ratios
Equally spaced west-east oriented snow bands (max reflectivity 35-40 dBZ).
Bands short-lived but multiple bands affect same areas to produce 2-4 inches of snow accumulation between midnight and 6am.
Occurred in differential negative vorticity advection region, seem to feed off shallow low level convective instability.
Phase 1: 06Z-12Z December 26th
Horizontal Convective Rolls (HCRs)
Schultz (2003)
- Develop from thermal instability.- Enhanced by wind shear, but shear minimized in cross-roll direction.- Noted with large surface/air heat fluxes (warm ground/cold air).
Horizontal Convective Rolls (HCRs)
A
B
A B
KLOT 0.5 base reflectivity 0841Z 0-2.5 km shear vectors
Reflectivity cross section
Phase 2: 12Z-00Z Synoptic Forcing
Persistent warm advection associated with slowly retreating cold dome
12Z NAM 280K
Phase 2: 12Z-00Z Mesoscale Factors
Moderate mid level frontogenesis
12Z NAM 6-hr forecast 700 mb frontogenesis 12Z NAM 6-hr forecast 700 mb Fvec dvg (color-filled)
KLOT 0.5 deg base reflectivity ~1800Z
Phase 2: 12Z-00Z Mesoscale Factors
Bands become more enhanced when entering mid level frontogenesis region
In addition to area of apparent enhancement due to frontogenesis, smaller scale linear elements also noted with locally more intense snowfall rates.
Complex banding structure shows up early in the event during the overnight hours on KDVN radar.
Phase 2: 12Z-00Z Mesoscale Factors
West-east banding appears to be convectively driven – horizontal convective rolls
North-south oriented bands do not necessarily line up nicely with DPVA, frontogenesis, or mid level warm advection?
Enhanced reflectivities where bands intersect (and deeper reflectivity signatures)
CSI evaluated and found not to be playing a role in this case
What else could be forcing these bands?
Phase 2: 12Z-00Z Mesoscale Factors
North-south bands may have been enhanced by gravity waves?
Enhancement of updrafts due to HCR/gravity wave interaction.
Christian 87
Conclusions Up to 1 foot of snow due to several
synoptic/mesoscale factors. Upper level low and associated
isentropic lift with departing cold dome provided extended duration snowfall.
Thermal instability led to HCR development early morning producing storm total forecasted amounts by 12Z over localized areas.
Conclusions Mid level frontogenetical forcing
providing greatest snowfall rates midday.
Frontogenetical forcing co-located with deep DGZ leading to highly anomalous SLR.
Prolonged forcing combined with rare SLR’s led to very challenging forecast.