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Early Winter Storm 9-11 October 2013

Early Winter Storm 9-11 October 2013. Matthew Aleksa National Weather Service 22 November 2013. Photo Credit: Dan Bender. Forecast Before the Storm. Two phases: Warm (pre-frontal) vs Cold (post-frontal) Were expecting thunderstorms Wednesday night into Thursday afternoon

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Early Winter Storm 9-11 October 2013

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  1. Early Winter Storm9-11 October 2013 Matthew Aleksa National Weather Service 22 November 2013 Photo Credit: Dan Bender

  2. Forecast Before the Storm • Two phases: Warm (pre-frontal) vs Cold (post-frontal) • Were expecting thunderstorms Wednesday night into Thursday afternoon • Issued Special Weather Statement Mon (10/7) and Watch Tue (10/8): • Significant snow above 7kft starting Wednesday night (10/9) into Friday with cold front Thursday afternoon (10/10). Rain below 6-7kft. Uncertainty of warm nature. On Wednesday (10/9) afternoon, time of watch extended from midnight until 9 pm Friday. • Winter Storm Warnings: issued Wed (10/9) through Thu (10/10) morning • Were forecasting 5-10” (northern CO mountains); 6-12” (central CO mountains & San Juans), higher amounts above 10kft (early season criteria) • SW Winds 20-40 mph with gusts 60-70 mph through early Thursday • Winter Weather Advisories: issued Wed (10/9) • Were forecasting 3-6” (Uncompaghre & Tavaputs Plateaus); 4-8” (eastern UT mountains) Photo Credit: Dan Bender

  3. Forecasted 12hr Snow AccumulationWarm Phase: 3 am – 3 pm 10 Oct 1-2” 2-6” 6-15”

  4. 3 am (09Z) 10 October IR Satellite Imagery with 1 hr Lightning and 700 mb heights Temperatures, Winds and Heights at 700 mb (10kft MSL)

  5. 9 am (15Z) 10 October HEAVY MORNING RAINS FLOOD ADVISORIES ISSUED IR Satellite Imagery with 1 hr Lightning and 700 mb heights Temperatures, Winds and Heights at 700 mb (10kft MSL)

  6. 12 pm (18Z) 10 October IR Satellite Imagery with 1 hr Lightning and 700 mb heights Temperatures, Winds and Heights at 700 mb (10kft MSL)

  7. 3 pm (21Z) 10 October IR Satellite Imagery with 1 hr Lightning and 700 mb heights Temperatures, Winds and Heights at 700 mb (10kft MSL)

  8. Forecasted 12hr Snow AccumulationCold Phase: 3 pm 10 – 3 am 11 Oct 4-6” 2-3” 4-8” 2-4”

  9. 6pm (00Z) 10 October IR Satellite Imagery with 1 hr Lightning and 700 mb heights Temperatures, Winds and Heights at 700 mb (10kft MSL)

  10. 9 pm (03Z) 10 October IR Satellite Imagery with 1 hr Lightning and 700 mb heights Temperatures, Winds and Heights at 700 mb (10kft MSL)

  11. Observed Precipitation • Rain most valleys and below 8kft • 0.30 to 1.00+ inches, up to 2” in Durango (mostly rain with 1” snow) • Snow levels dropped as low as 6500 ft in southern CO valleys with 1-3” in Bayfield, Durango and Pagosa Springs vicinities; 4-5” between 7-8kft • Snowfall above 8kft; higher above 9-10kft (mostly WARM PHASE) • Central CO mountains: 1-5” around Aspen; 5-10” around Crested Butte; 10-16” on Grand Mesa above 8500 ft level (rain below) • Southern CO mountains: 3-7” above 8kft on Uncompaghre Plateau with 12” on Columbine Pass; 8-20” in SW San Juansup to 23”; 9-16” in NW San Juans above 9kft with 4-8” in the 7500-9000 ft range • Eastern UT mountains: 3-9” in Uintas; about 10” in La Sal and Abajos • COLD PHASE: Northern CO mountains: 3-10”; 19” at Tower (outlier) Photo Credit: Dan Bender

  12. Observed Snowfall: Warm vs Cold Phase

  13. Storm Summary • WARM PHASE • Thunderstorms and Heavy Rains Wednesday night (10/9) into Thursday morning (10/10) causing some flooding issues. • Heavy snow above 10kft with lowering to 8kft with cold front Thursday morning-afternoon (10/10) during the warm phase of storm. Southern CO valleys received unexpected snow. Lower snow levels south slopes. • Plenty of moisture for 10% (10 to 1) snow ratios (valleys and mountains upwards of 1-2” liquid) • COLD PHASE • Much less moisture, forcing and lift. Energy directed towards Plains. • Little additional accumulation post-frontal in cold phase except for northern CO mountains (Tower) Thursday evening through Friday still under wraparound and NW flow. • Led to early cancellations of remaining highlights by Friday afternoon. Photo Credit: Dan Bender

  14. Future Work Considerations • Models tend to predict Warm Air Advective (WAA) precip more often than it actually occurs. Why does this happen? WAA precip does not often occur. What are the triggers to distinguish these events? • When WAA precip occurs, the cold phase of the storm seems to be over-predicted by the models. Why? • Why did we have heavy precip in the downslope situation with strong winds? Precip was more uniform in this event when typically the downslope areas are shadowed. • Rule of thumb? Snow levels tend to be 1-2kft lower on the south-facing slopes vs the north-facing slopes in these WAA events. This was similar to the December 2010 event. Photo Credit: Dan Bender

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