ben moyer senior meteorologist national weather service des moines, iowa
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Basic Blizzard Ingredients Unmasked and a Review of the 5 April 2009 Non-Blizzard Through the Eyes of the SREF. Ben Moyer Senior Meteorologist National Weather Service Des Moines, Iowa. The Why. Snowcrystals.com. Snowcrystals.com. - PowerPoint PPT PresentationTRANSCRIPT
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?
National Weather Service - Des Moines, IA
Snowcrystals.com Snowcrystals.com
Overview
• The six cases• Methodology
– Parameters studied• Analysis• SREF and 5 April 2009 event• Conclusions
National Weather Service - Des Moines, IA
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
National Weather Service - Des Moines, IA
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
National Weather Service - Des Moines, IA
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
National Weather Service - Des Moines, IA
Sample Spreadsheet
National Weather Service - Des Moines, IA
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)
National Weather Service - Des Moines, IA
Max 6-Hour SFC Low Central Pressure Change and Iowa Gradient
National Weather Service - Des Moines, IA
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
National Weather Service - Des Moines, IA
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)
National Weather Service - Des Moines, IA
Best Correlations for Iowa Blizzards
• Surface low pressure– Intensity– Trends– Gradient
• 850 mb Height– Intensity and trends
• 850 mb Winds
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5 April 2009 Winter Storm• Blizzard Watch issued 36-48 hours in advance• Blizzard Warning issued 24 hours in advance
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00Z 4 Apr 2009 GFS 500 mb
National Weather Service - Des Moines, IA
00Z 4 Apr 2009 GFS 850 mb
National Weather Service - Des Moines, IA
00Z 4 Apr 2009 GFS Surface
National Weather Service - Des Moines, IA
5 April 2009 Winter Storm
• How did model data at warning decision time correspond to blizzard values shown in this study?
National Weather Service - Des Moines, IA
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
National Weather Service - Des Moines, IA
SREF Winter Weather Fields• Focus on period when forecaster was making
the Blizzard Warning decision• 21Z 3 Apr 2009 model run (from SPC)
National Weather Service - Des Moines, IA
Fields Applicable to Blizzard Conditions / Impacts10 Meter Maximum Wind
Likely Precipitation Type
Mean Depth Dendrite Growth Zone >= 50 mb
Mean Depth Dendrite Growth Zone >= 100 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
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25 to 30 mph
10 Meter Maximum Wind (MPH) From Any Member
F045 valid 18Z 5 Apr 2009
F039 valid 12Z 5 Apr 2009
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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
F030 valid 03Z 5 Apr 2009
National Weather Service - Des Moines, IA
Best near MN border, but before probability of snow became likely.
Mean Depth Dendrite Growth Zone >= 50 mb
F042 valid 15Z 5 Apr 2009
National Weather Service - Des Moines, IA
Max shifted east and probability decreased. (Turned out to be too far north.)
Mean Depth Dendrite Growth Zone >= 50 mb
F030 valid 03Z 5 Apr 2009
National Weather Service - Des Moines, IA
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
F039 valid 12Z 5 Apr 2009
National Weather Service - Des Moines, IA
Mean 32F contour (dashed blue) not even into central IA until now – 12 hours into valid time of Blizzard Warning.
Calibrated 6 Hour Probability New Snow or Ice on Roads
Probability only 25%
F045 valid 18Z 5 Apr 2009
National Weather Service - Des Moines, IA
After 12Z, 32F contour (dashed blue) moved a little farther southeast, but then retreated by 18Z.
Calibrated 6 Hour Probability New Snow or Ice on Roads
Probability decreased through the morning.
F039 valid 12Z 5 Apr 2009
National Weather Service - Des Moines, IA
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
F042 valid 15Z 5 Apr 2009
National Weather Service - Des Moines, IA
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
F036 valid 09Z 5 Apr 2009
National Weather Service - Des Moines, IA
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
National Weather Service - Des Moines, IA
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.
National Weather Service - Des Moines, IA
00Z-18Z 5 Apr 2009
National Weather Service - Des Moines, IA
Regional Radar Loop
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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
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Final Snowfall Map
National Weather Service - Des Moines, IA
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
National Weather Service - Des Moines, IA
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
National Weather Service - Des Moines, IA
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
National Weather Service - Des Moines, IA