Polar Upper Low with a Strong Cold Front followed

by a significant Lake Effect Snow Band November 27-29 2006

Alexander Tardy

National Weather Service

Salt Lake City Utah

Introduction

A cold front moved across northern Utah during the afternoon and early evening hours on November 27^th 2006. This produced heavy prefrontal snow (5 to 14 inches) across the Wasatch Range and adjacent valleys, but generally 2 inches or less across the Wasatch Front. The front then strengthened across central Utah and resulted in 5 to 12 inches of snow from Fillmore to Sevier and SanPete Valleys with up to 2 feet across the central mountains in Utah . A secondary cold front ushered in an arctic air mass during the afternoon and evening hours on November 28^th. Subsequently a lake-effect snow band developed over the Great Salt Lake during the evening hours on November 28^th . The band of snow fluctuated and redeveloped during the initial stages of its life-cycle (Fig. 1) and the orientation veered (clockwise) with time before dissipating in the Tooele Valley and Oquirrh Mountains near sunrise November 29^th . This lake effect event produced as much as 7 inches of snow at the Salt Lake International Airport (3 inch per hour rate) with widespread 1 to 5 inches across Salt Lake County. The band did not extend far to the southeast and was shallow, therefore the southern Wasatch Range (Cottonwoods) received similar or less snowfall as the Salt Lake Valley . The combination of clearing skies, light wind and the fresh snow cover allowed temperatures to drop in the single digits early Wednesday morning November 29^th which created hazardous driving conditions during the morning commute.

The ingredients were in place for a significant lake-effect snowfall event (see images below), however subtle changes in wind direction in the lower levels of the atmosphere and sinking air (subsidence) just behind the very cold upper trough diminished the duration and hindered the organization of a single lake-effect snow band. Lake-effect and lake-enhanced snow is more complicated than just cold air within northwest flow moving over the warmer Great Salt Lake. The necessary ingredients include sufficient over-lake temperature differential (i.e. greater than 17C from the surface to 10,000 ft MSL or 700 mb), sufficient distance of air flowing across the lake (northwest or north flow provides this across the longer portion of the Great Salt Lake ), minimal change of wind speed or direction in the boundary layer, deeply unstable boundary layer (top layer above 600 mb level preferred for sufficient convective growth), adequate steering flow to develop banding, and over-water surface convergence which is often driven by a land breeze circulation under radiational cooling at night (air flowing toward the warmer water from the colder surrounding land). The lake bands can be further enhanced when they are sufficiently deep and extend well inland resulting in upslope flow of the moist air along the west slope of the Wasatch Range. For a given event the ingredients for lake-effect and lake-enhanced precipitation varies with respect to the potential for generating snow. This will determine if a lake band or lake enhanced area of precipitation will develop and how long and intense the snowfall will be. This event demonstrated a significant lake- effect event over a short duration from the Great Salt Lake.

Lake-enhanced snowfall can also produce significant snowfall. Weak lake enhancement occurred in the central Wasatch Front (near Ogden) during the afternoon of November 28^th (Fig. 2). This type of snowfall would occur without the presense of a warmer body of water, however due to the instability (air and water differential and decreasing temperatures up to 15,000 ft MSL or 500 mb) that was present over the Great Salt Lake this interaction created heavier snow showers. There was some evidence of this with South Ogden getting up to 2 inches of low density snow and generally 1 inch or less of snow occurred across entire southern Wasatch Front along a secondary cold front. The criteria for lake enhancement also varies greatly but the ingredients are similar. Differences would include that the over water fetch does not have to be as long and winds can change considerably in the lower levels. During a lake-enhanced event other precipitation is observed upstream or away from the body of water (see Fig. 2). The snowfall Tuesday night and early Wednesday morning across the Salt Lake Valley was lake-effect snow since it would likely not have occurred without the interaction with the Great Salt Lake.

The images below depict the event using satellite and radar loops. An analysis of the event used observed and model data depicting the ingredients that were present for lake-effect and lake-enhanced snow. Real-time numerical model forecasts are also provided to demonstrate their performance in simulating a small scale weather event.

Radar

Satellite

Observed Soundings

Observed Surface Data

Model Soundings

Model Analyses

Model Forecasts

Local Model Forecasts from 1200 UTC

Local Model Forecasts from 0000 UTC

Radar Imagery

Fig. 1. KMTX base reflectivity loop from 0002 to 1259 UTC (1702 MST 28 November to 559 MST 29 November) 29 November 2006.

Fig. 2. KMTX base reflectivity loop from 1603 to 2304 UTC (903 to 1604 MST) 28 November 2006. Note the west to east moving Radar echoes that enhance

over Willard Bay and the broken line of reflectivity (snow showers) that extends from SLC to Tooele County (Dugway) are not downstream of the Great Salt Lake in

this flow. A surface cold front associated with this band of snow showers is evident on the Mesowest plots.

Satellite Imagery

Fig. 3. GOES-11 visible image at 1630 UTC (930 MST) 29 November 2006. This image depicted the new snowfall on the ground from Milford to the

Canyonlands as well as the Wasatch Front.

Fig. 4. GOES infrared loop from 1400 UTC (700 MST) 28 November to 1300 UTC (600 MST) 29 November 2006. Colder cloud tops depicted by the yellow

and red enhancements in northern Nevada and Utah . Note the enhanced clouds that developed along a secondary cold front during the afternoon November 28^th

and then the lake-effect clouds during the early morning hours on the 29^th. Snow continued to pile up along the I-15 corridor and the Pahvant Range near Fillmore

well into the 29^th. The first images in the loop depict the stalled cold front in southern Utah which brought heavy snow to central Utah valleys and mountains during the

early morning hours on the 28^th.

Fig. 5. GOES water vapor loop from 0900 UTC (200 MST)28 November to 1200 UTC (500 MST) 29 November 2006.

Observed Soundings

Fig. 6. KSLC sounding at 0000 UTC (1700 MST 28 November) 29 November 2006. Near dry adiabatic lapse rates to 700-mb in this unstable profile.

Fig. 7. 1200 UTC (500 MST) 29 November 2006 KSLC sounding. There was some low level directional shear (change in wind) near 750-mb and the

subsidence inversion had lowered to 550-mb.

Surface Plots

Fig. 8. Mesowest surface plot from 1800 to 2200 UTC (1100 to 1500 MST) 28 November 2006. Note the wind shift in the Salt Lake Valley where a band of

snow showers extended westward into the Salt Flats and Dugway Area.

Fig. 9. Mesowest surface plot from 0000 to 1300 UTC (1700 to 0600 MST) 29 November 2006.

KSLC (Salt Lake International Airport) observations

Tabular Listing: November 28, 2006 - 23:00 through November 30, 2006 - 00:00 MST

Time(MST)	Temperature	Dew	Wet Bulb	Relative	Wind	Wind	Quality	Pressure	Sea level	Altimeter	1500 m	Precipitation	Precipitation	Precipitation	Snow	Weather	Visibility	Low cloud	Mid cloud	High cloud	Ceiling
		Point	Temperature	Humidity	Speed	Direction	check		pressure		Pressure	1hr	6hr	24hr	total	conditions		height	height	height
	° F	° F	° F	%	mph			in	in	in	in	in	in	in	in		miles	ft	ft	ft	feet
3:55	16.0	9.0	14.1	73	9	N	OK	25.76	30.16	30.08	25.10	0.00				overcast	8.00	5500	9000		5500
3:40	17.6	10.4	15.6	73	12	N	OK	25.76		30.08	25.10	0.00				overcast	9.00	4100			4100
3:35	17.6	12.2	16.1	79	12	N	OK	25.75		30.07	25.09	0.00				lt snow	8.00	2700			2700
3:25	17.6	12.2	16.1	79	14	N	OK	25.75		30.07	25.09	0.00				lt snow	2.00	2000			2000
3:00	19.4	15.8	18.3	86	20	NNW	OK	25.75		30.07	25.09	0.00				lt snow, fog	1.50	1200	2200	2800	1200
2:55	19.0	15.1	17.8	84	15	NNW	OK	25.74	30.14	30.06	25.08	0.04				lt snow, fog	1.75	800	3300		800
2:20	19.4	15.8	18.3	86	10	NNW	OK	25.73		30.05	25.07	0.03				lt snow, fog	0.50	700	2900		700
2:05	19.4	15.8	18.3	86	8	NNW	OK	25.73		30.04	25.07	0.02				lt snow, fog	0.50	1300	2900		1300
1:55	19.0	16.0	18.1	88	7	N	OK	25.73	30.11	30.04	25.07	0.05				lt snow, fog	0.50	300	900	2900	2900
1:50	19.4	15.8	18.3	86	8	N	OK	25.73		30.04	25.07	0.05				lt snow, fog	0.50	300	900	2900	2900
1:40	19.4	15.8	18.3	86	8	N	OK	25.72		30.03	25.06	0.05				mod snow, ice fog	0.50	300	900		300
1:20	19.4	17.6	18.8	93	8	N	OK	25.72		30.03	25.06	0.04				hvy snow, ice fog	0.25	100	900		100
1:10	19.4	17.6	18.8	93	10	N	OK	25.72		30.03	25.06	0.03				mod snow, ice fog	0.50	100	900		100
1:00	19.4	17.6	18.8	93	8	N	OK	25.72		30.03	25.06	0.03				hvy snow, ice fog	0.25	100	900		100
0:50	19.9	18.0	19.3	92	5	NNE	OK	25.71	30.10	30.02	25.05	0.09				hvy snow, ice fog	0.25	100	900		900
0:35	19.4	17.6	18.8	93	0		OK	25.71		30.02	25.05	0.07				mod snow, ice fog	0.50	100	900		100
23:55	21.0	18.0	20.0	88	3	WNW	OK	25.70	30.08	30.01	25.04	0.03				lt snow, fog	0.50	700			700
23:35	21.2	17.6	20.0	86	0		OK	25.69		30.00	25.03	0.03				lt snow, fog	0.75				600
23:20	21.2	17.6	20.0	86	5	W	OK	25.69		30.00	25.03	0.02				lt snow, fog	1.00	900	1600		900
23:15	21.2	17.6	20.0	86	7	W	OK	25.69		30.00	25.03	0.01				lt snow, fog	0.50	900	1600		900
23:10	21.2	17.6	20.0	86	8	W	OK	25.69		30.00	25.03	0.01				lt snow	0.50	1600	3200

Model Sounding Analyses

BUFKIT profile at KSLC from the 0600 UTC (2300 MST) 29 November 2006 NAM analysis. Note the over-lake instability indicated

on this profile. Heavy lake-effect snow was occurring at this time.

BUFKIT profile at KSLC from the 1200 UTC (500 MST) 29 November 2006 NAM analysis. Note the over-lake instability indicated on

this profile. The lake-effect snow band was dissipating rapidly at this time.

BUFKIT profile at KSLC from the 1200 UTC (500 MST) 29 November 2006 NMM-WRF analysis. Note the over-lake instability

indicated on this profile with a depth up to near 500 mb. Heavy lake effect snow was occurring at this time. This model depicted a northerly

component in the low levels and was more representative of the lake band movement at 1200 UTC.

Model Analyses

1800 UTC (1100 MST 27 November) 28 November 2006 NAM 4-panel analysis. Heavy pre-frontal snow was occurring in the Wasatch Range and

mountain valleys.

0000 UTC (1700 MST 27 November) 28 November 2006 NAM 4-panel analysis. The cold front was producing a narrow band of heavy snow and thundersnow

in the Salt Lake Valley.

0600 UTC (2300 MST 27 November) 28 November 2006 NAM 4-panel analysis. Heavy snow was occurring in central Utah. The cold front (dashed line)

had strengthened and slowed across central Utah.

0000 UTC (1700 MST 28 November) 29 November 2006 NAM 4-panel analysis.

0600 UTC (2300 MST 28 November) 29 November 2006 NAM 4-panel analysis.

1200 UTC (500 MST 29 November) 29 November 2006 NAM 4-panel analysis.

Model Forecasts (times in UTC)

1200 UTC 28 November 2006 NAM forecast at 0600 UTC 29 November 2006.

1200 UTC 28 November 2006 NAM forecast at 1200 UTC 29 November 2006.

1200 UTC 28 November 2006 NAM forecast at 1500 UTC 29 November 2006.

NMM-WRF 5-km 1200 UTC forecast

NMM-WRF 1200 UTC 28 November 2006 forecast valid at 0300 UTC 29 November 2006 . 3-hour precipitation ending at 0300 UTC. The lake-effect

snow was developing at this time.

NMM-WRF 1200 UTC 28 November 2006 forecast valid at 0600 UTC 29 November 2006. 3-hour precipitation ending at 0600 UTC.

NMM-WRF 1200 UTC 28 November 2006 forecast valid at 0900 UTC 29 November 2006 . 3-hour precipitation ending at 0900 UTC. The band

was forecast too far west (see radar loop in Figure1).

NMM-WRF 1200 UTC 28 November 2006 forecast valid at 1500 UTC 29 November 2006 . 3-hour precipitation ending at 1500 UTC. The band

had shifted to the forecasted location but was dissipating rapidly.

NMM-WRF 1200 UTC 28 November 2006 forecast valid at 1800 UTC 29 November 2006 . 3-hour precipitation ending at 1800 UTC. The lake band

dissipated faster than this forecast indicated.

NMM-WRF 5-km 0000 UTC forecast

NMM-WRF 0000 UTC 28 November 2006 forecast valid at 0300 UTC 29 November 2006 . 3-hour precipitation ending at 0300 UTC. The lake band

was developing as depicted in this forecast (see radar loop in Figure 1).

NMM-WRF 0000 UTC 28 November 2006 forecast valid at 0600 UTC 29 November 2006 . 3-hour precipitation ending at 0600 UTC. The lake band

was forecast too far west in this forecast (see radar loop in Figure 1).

NMM-WRF 0000 UTC 28 November 2006 forecast valid at 0900 UTC 29 November 2006. 3-hour precipitation ending at 0900 UTC. The lake band was

forecast too far west in this forecast (see radar loop in Figure 1). Note the separate forecasted convection (snow showers) in the west central deserts which

occurred and moved into the southwest mountains of Utah during the early morning on November 29^th.

NMM-WRF 0000 UTC 28 November 2006 forecast valid at 1200 UTC 29 November 2006 . 3-hour precipitation ending at 1200 UTC. The lake band

was forecast in the correct location in this forecast (see radar loop in Figure 1).