Ben Moyer Senior Meteorologist National Weather Service Des Moines, Iowa

Basic Blizzard Ingredients Unmasked and a Review of the

5 April 2009 Non-Blizzard Through the Eyes of the SREF

Ben MoyerSenior Meteorologist

National Weather ServiceDes Moines, Iowa

National Weather Service - Des Moines, IA

The Why

• Verification of last 6 Blizzard Warning events in central Iowa lower than expectation• 5 April 2009 case was impetus

• Credible warnings important to many users• Study of past blizzard research did not discuss

basic meteorological parameters needed• Can a study of these events reveal any clues?


Snowcrystals.com Snowcrystals.com

Overview

• The six cases• Methodology

– Parameters studied• Analysis• SREF and 5 April 2009 event• Conclusions


Blizzard Warning Events UsedEvent Date Result Warning Duration Comments

1-2 Mar 2007 Hit 12 hours (12Z 1 Mar – 00Z 2 Mar) Classic

29 Jan 2008 Hit 9 hours (15Z 29 Jan – 00Z 30 Jan) Light snow (1-2”)

17 Feb 2008 Bust 15 hours (15Z 17 Feb – 06Z 18 Feb)

20-21 Dec 2008 Partial 24 hours (00Z 21 Dec – 00Z 22 Dec) Ground blizzard

12 Jan 2009 Bust 12 hours (21Z 12 Jan – 09Z 13 Jan)

5 Apr 2009 Bust 24 hours (00Z 5 Apr – 00Z 6 Apr)

22 Jan 2005 ? ? Used for pattern comparison


Methodology

• Reviewed the 6 cases on the Weather Event Simulator (mimics AWIPS)

• GFS, NAM, and SREF model forecast data– For the model run forecaster was using to make

Blizzard Warning decision• Reviewed observational data

– Radar– Surface observations– RUC model initializations


Parameters StudiedSurface 850 mb 500 mb

Low pressure magnitude Low height magnitude Low height magnitude

6-hr low pressure magnitude change

6-hr height magnitude change

6-hr height magnitude change

Absolute difference between the min and max 6-hr surface pressure rise and fall couplet

Max wind over central Iowa

Trough tilt over the Upper Midwest (Positive or Negative)

Max surface pressure gradient across the state of Iowa

Max temperature change over central Iowa


Sample Spreadsheet


Surface Analysis

Common values for blizzards with falling snow• Low pressure center sub-990 mb• Deepening to less than 985 mb• Minimum 6-hour intensification of 5 mb• Max pressure gradient across Iowa of at least

18 – 19 mb (300-400 miles)


Max 6-Hour SFC Low Central Pressure Change and Iowa Gradient


1-2 Mar-07 29-Jan-08 17-Feb-08 20-21 Dec-08

12-Jan-09 5-Apr-09

-10

-5

0

5

10

15

20

25

Max 6 hr Change (mb)Max IA Gradient (mb)

H980 mb

H PH994 mb

1002 mb

1012 mb

983 mb

984 mb

850 mb Analysis

Common values for blizzards• Low height center deepening and preferably

less than 1300 meters• Minimum 6-hour intensification of 40 meters• Maximum wind at least 45 knots (mixing)

– 1-2 Mar 2007 as high as 60 knots• Cold advection at the rate of at least 10

degrees Celsius per 12 hours


500 mb Analysis

Common values for blizzards• Not as definitive• Rapidly deepening negatively tilted cyclone• Also open waves (22 Jan 2005) and positively

tilted waves (29 Jan 2008)


Best Correlations for Iowa Blizzards

• Surface low pressure– Intensity– Trends– Gradient

• 850 mb Height– Intensity and trends

• 850 mb Winds


5 April 2009 Winter Storm• Blizzard Watch issued 36-48 hours in advance• Blizzard Warning issued 24 hours in advance


00Z 4 Apr 2009 GFS 500 mb


00Z 4 Apr 2009 GFS 850 mb


00Z 4 Apr 2009 GFS Surface


5 April 2009 Winter Storm

• How did model data at warning decision time correspond to blizzard values shown in this study?


00Z 4 April 2009 Model RunsFor 24 hour period 00Z Apr 5 to 00Z Apr 6

Values GFS, NAM, SREF Average Values Met?

SFC Low sub-990 mb 993 mb 1001 mb 999 mb No

Deepening to < 985 mb? No No

Min 6-hr intensification 5 mb +8 mb first 12 hr / -2 mb last 12 hr No

Max IA pressure gradient (18-19 mb) 20 mb Yes

850 mb low height < 1300m 1310m 1330m 1340m No

Min 6-hr intensification 40m -25m No

Max wind >/= 45 kts 38-47 knots Possibly - Barely

Cold advection 10 C/12 hr 9C No

500 mb low rapidly deepening? 5420m 5440m 5430m No

Negative tilt? No No


SREF Winter Weather Fields• Focus on period when forecaster was making

the Blizzard Warning decision• 21Z 3 Apr 2009 model run (from SPC)


Fields Applicable to Blizzard Conditions / Impacts10 Meter Maximum Wind

Likely Precipitation Type

Mean Depth Dendrite Growth Zone >= 50 mb


Calibrated 6 Hour Probability New Snow or Ice on Roads

Chance S/IP/ZR Detection on Roads

Mean Estimated Snow to Liquid Equivalent Ratio

Probability Snowfall Rate >= 1”/Hour


25 to 30 mph

10 Meter Maximum Wind (MPH) From Any Member

F045 valid 18Z 5 Apr 2009



Snow becomes likely precipitation type 09-12Z 5 Apr 2009 – 9 to 12 hours into the valid time of the Blizzard Warning!

Likely Precipitation Type



Best near MN border, but before probability of snow became likely.




Max shifted east and probability decreased. (Turned out to be too far north.)




Never more than 10 to 30% and before snow became likely.

50 mb and 100 mb charts infer than an abundance of heavy snowfall was unlikely in Blizzard Warning area.

Mean Depth Dendrite Growth Zone = 100 mb



Mean 32F contour (dashed blue) not even into central IA until now – 12 hours into valid time of Blizzard Warning.


Probability only 25%



After 12Z, 32F contour (dashed blue) moved a little farther southeast, but then retreated by 18Z.


Probability decreased through the morning.



Equal to above normal chances of snow accumulating on the roads. But, remember it is early April. Already somewhat anomalous to have snowfall.

Chance S/IP/ZR Detection on Road

Note Mean 32F contour



Barely got to 10 and only after 12Z.

Wetter than normal snow – not likely to blow around much.

Mean Estimated Snow to Liquid Equivalent Ratio



Only 10 to 30% over northern Iowa and again before snow became likely.

Again, heavy snowfall seemed unlikely.

Probability Snowfall Rate >= 1”/Hour

SREF Summary

• Several signs in SREF output showed attaining true blizzard conditions would be difficult– Warm ground and air temperatures– Wetness of snow– Unlikelihood of sustained heavy snowfall– 10m winds of less than 35 mph


What Happened?

• It eventually snowed in central Iowa.• Surface temps were too warm for light snow

to accumulate much.• A heavy band did set up…but mainly outside

of Blizzard Warning area• Wet snow precluded any true blizzard

conditions.• Any 35 mph winds were very short duration.


00Z-18Z 5 Apr 2009


Regional Radar Loop


Max Potential Blizzard Time?10Z 5 Apr 2009

• Significant portion had very little snow

• What did fall had little chance to accumulate

• SREF was on to something!• Heavy band of snow

farther south had brief 35 mph winds

• Low pressure 1001 mb north of St. Joseph, MO


Final Snowfall Map


Conclusion

• Model forecasts were in fairly good agreement• Some magnitudes off slightly, but trends verified

well• Basic thresholds for blizzard potential established• SREF output an excellent resource for corroboration• Other parameters like mixed layer wind (gust

potential), character of falling snow or antecedent snow cover, and depth of snow already on ground, not included in this study


Conclusion

• Using a combination of basic meteorological parameter thresholds, and

• Observing trends in the SREF winter weather fields and impact graphics,

• Forecasters should be able to make higher confidence forecasts and more easily discern the validity of issuing a Blizzard Warning


Acknowledgements

• Karl Jungbluth, SOO – NWS Des Moines, IA• Dan Baumgardt, SOO – LaCrosse, WI• SPC SREF website:

http://www.spc.noaa.gov/exper/sref/frames.php?run=latest


Documents

Ben Moyer Senior Meteorologist National Weather Service Des Moines, Iowa