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

OBSERVED SNOWFALL TOTALS

FORECAST SNOWFALL TOTALS

2 to 4 INCHES

1 to 3 INCHES

AROUND AN INCH

Radar Overview (06Z-00Z)

Synoptic Overview

300 MB 12/26/09 12Z

Synoptic Overview

700 MB 12/26/09 12Z

Synoptic Overview

850 MB 12/26/09 12Z

Synoptic Overview

SFC 12/26/09 12Z

Synoptic Overview

12/26/09 12Z ROAB KDVN

DGZ

Radar Evolution (KLOT) 06Z-12Z

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 Radar Storm Evolution 12Z-00Z

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.

Radar Imagery (KDVN)

KDVN 0.5 base reflectivity (~ 0900Z-1130Z)

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

Phase 2: 12Z-00Z Mesoscale Factors

Stable ducting layer

Convectively unstable capping layer

Phase 2: 12Z-00Z Mesoscale Factors

~1820Z

Snow to Liquid Ratios

Images from SLU Snow:Liquid Research webpage (Marty Baxter)

Snow to Liquid Ratios

Snow to Liquid Ratios

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.

Thank you

Any questions?