Early Winter Storm October 9-11, 2013

Early Winter Storm9-11 October 2013Matthew AleksaNational Weather Service22 November 2013Photo Credit: Dan Bender

Forecast Before the StormTwo phases: Warm (pre-frontal) vs Cold (post-frontal)Were expecting thunderstorms Wednesday night into Thursday afternoonIssued 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) morningWere 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 ThursdayWinter Weather Advisories: issued Wed (10/9)Were forecasting 3-6 (Uncompaghre & Tavaputs Plateaus); 4-8 (eastern UT mountains)Photo Credit: Dan Bender Forecasted 12hr Snow AccumulationWarm Phase: 3 am 3 pm 10 Oct

6-152-61-23 am (09Z) 10 October

Temperatures, Winds and Heights at700 mb (10kft MSL)IR Satellite Imagery with 1 hr Lightning and700 mb heights9 am (15Z) 10 October

Temperatures, Winds and Heights at700 mb (10kft MSL)IR Satellite Imagery with 1 hr Lightning and700 mb heightsHEAVY MORNING RAINSFLOOD ADVISORIES ISSUED12 pm (18Z) 10 October

Temperatures, Winds and Heights at700 mb (10kft MSL)IR Satellite Imagery with 1 hr Lightning and700 mb heights3 pm (21Z) 10 October

Temperatures, Winds and Heights at700 mb (10kft MSL)IR Satellite Imagery with 1 hr Lightning and700 mb heightsForecasted 12hr Snow AccumulationCold Phase: 3 pm 10 3 am 11 Oct

2-44-84-62-36 pm (00Z) 10 October

Temperatures, Winds and Heights at700 mb (10kft MSL)IR Satellite Imagery with 1 hr Lightning and700 mb heights9 pm (03Z) 10 October

Temperatures, Winds and Heights at700 mb (10kft MSL)IR Satellite Imagery with 1 hr Lightning and700 mb heights

Photo Credit: Dan Bender Observed PrecipitationRain most valleys and below 8kft0.30 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 7-8kftSnowfall 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 Juans up to 23; 9-16 in NW San Juans above 9kft with 4-8 in the 7500-9000 ft rangeEastern UT mountains: 3-9 in Uintas; about 10 in La Sal and AbajosCOLD PHASE: Northern CO mountains: 3-10; 19 at Tower (outlier)

Observed Snowfall: Warm vs Cold PhaseObservation SiteLocationWarm Phase Snowfall (Start until 21Z/3pm)Cold Phase Snowfall(21Z/3pm until end)Mesa LakesGrand Mesa16 inches4 inchesPark ReservoirGrand Mesa16 inches6 inchesColumbine PassUncompaghre Plateau17 inches4 inchesLone ConeNW San Juans10 inches4 inchesLizard Head PassNW San Juans12 inches3 inchesRed Mountain PassSan Juans18 inches3 inchesSpud MountainSW San Juans16 inches8 inchesBear TownSW San Juans16 inches4 inchesLa Sal MountainLa Sals (E Utah)8 inches2 inchesCamp JacksonAbajos (E Utah)10 inches1 inch

Photo Credit: Dan Bender Storm SummaryWARM PHASEThunderstorms 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 PHASEMuch 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 Future Work ConsiderationsModels 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.Radar: 2 am (08Z) 10 Oct

Radar: 4 am (10Z) 10 Oct

Radar: 9 am (15Z) 10 Oct

Radar: 3 pm (21Z) 10 Oct

Precipitable Water(amount of available moisture)

WARMCOLD