Early Winter Storm9-11 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• 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

Forecasted 12hr Snow AccumulationWarm Phase: 3 am – 3 pm 10 Oct

6-15”

2-6”

1-2”

3 am (09Z) 10 October

Temperatures, Winds and Heights at

700 mb (10kft MSL)

IR Satellite Imagery with 1 hr Lightning and

700 mb heights

9 am (15Z) 10 October

Temperatures, Winds and Heights at

700 mb (10kft MSL)

IR Satellite Imagery with 1 hr Lightning and

700 mb heights

HEAVY MORNING RAINSFLOOD ADVISORIES ISSUED

12 pm (18Z) 10 October

Temperatures, Winds and Heights at

700 mb (10kft MSL)

IR Satellite Imagery with 1 hr Lightning and

700 mb heights

3 pm (21Z) 10 October

Temperatures, Winds and Heights at

700 mb (10kft MSL)

IR Satellite Imagery with 1 hr Lightning and

700 mb heights

Forecasted 12hr Snow AccumulationCold Phase: 3 pm 10 – 3 am 11 Oct

2-4”

4-8”

4-6”

2-3”

6 pm (00Z) 10 October

Temperatures, Winds and Heights at

700 mb (10kft MSL)

IR Satellite Imagery with 1 hr Lightning and

700 mb heights

9 pm (03Z) 10 October

Temperatures, Winds and Heights at

700 mb (10kft MSL)

IR Satellite Imagery with 1 hr Lightning and

700 mb heights

Photo Credit: Dan Bender

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 Juans up 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)

Observed Snowfall: Warm vs Cold Phase

Observation Site

Location Warm Phase Snowfall

(Start until 21Z/3pm)

Cold Phase Snowfall

(21Z/3pm until end)

Mesa Lakes Grand Mesa 16 inches 4 inchesPark Reservoir Grand Mesa 16 inches 6 inchesColumbine Pass Uncompaghre

Plateau17 inches 4 inches

Lone Cone NW San Juans 10 inches 4 inchesLizard Head Pass

NW San Juans 12 inches 3 inches

Red Mountain Pass

San Juans 18 inches 3 inches

Spud Mountain SW San Juans 16 inches 8 inchesBear Town SW San Juans 16 inches 4 inchesLa Sal Mountain La Sals (E

Utah)8 inches 2 inches

Camp Jackson Abajos (E Utah)

10 inches 1 inch

Photo Credit: Dan Bender

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

Future Work Considerations

1. 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?

2. When WAA precip occurs, the cold phase of the storm seems to be over-predicted by the models. Why?

3. 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.

4. 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.