# Presentation: 028003

## Topographic Forcing and Front Range Winter Storms

- 24-25 October 1997 Front Range Blizzard and Blowdown in WY/CO/NE/KS
- Doug Wesley
- Winter Weather Workshop, August 2002

## OUTLINE

- Topographic forcing in the context of a strong Front Range winter storm, 24-26 Oct. 1997
- Eta and Meso-Eta forecasts
- RAMS forecasts
- Advantages of improved NWP resolution in these types of storms
- Influence on precipitation fields of small-scale terrain features and larger-scale wind fields

## CYS WFO, evening of 24 Oct. 1997

## Cheyenne, WY 25 Oct. 1997

## Cheyenne, WY 25 Oct. 1997

## I-25 Corridor, 25 Oct. 1997

## Terrain reliefj:/shared/for_wesley/canada/NorthAmerica....bmp

- Rocky Mountain barrier; Colorado region has probably the highest continuous N-S barrier in North America
- Relatively lower “plains” to the north
- East-west ridges on the “plains”
- Source region of modified arctic air masses

## Eta and meso-Eta model terrain

- AWIPS: 2 grids; 40 and 80 km grid spacing, although model is currently running at 12 km
- GARP: 80 km plotted grids utilized for Eta in 1997 (model data computed at 48 km)
- Misses many major terrain features
- Smoothness of N-S cross section

## Eta terrain 1997

## West-east cross section, Eta

## North-south cross section, Eta

## North-south Eta x-section, 2002

## Meso-Eta terrain (1997)

## 12 km Eta terrain (AWIPS)

## Sample omega field 2002, Eta

## J. Steenburgh’s findings

- http://www.met.utah.edu/jimsteen/imtn2001/imtn_wkshp_2001_files/v3_document.htm

## Slide 18

## RAMS model terrain

- 3 grids: 15, 5, 1.7 km

## Geographical grid setup

## RAMS grid 1 terrain

## RAMS grid 2 terrain

## RAMS Grid 3 terrain

## RAMS 5km grid S-N cross section, through approx. DEN & CYS

## Storm overview/synoptics

- Upper levels: deep, intensifying cutoff low pressure system over the four corners region and eastward
- Influx of cold low-level air from north
- Timing of above 2 factors favorable
- Result was intense upslope flow for 12+ hours over the Front Range of the Rockies

## 700 mb analysis

## Platteville profiler data

## Observed snowfall

## Satellite analysis of snow coverage

## Eta 18hr forecast, valid 0600 25 Oct.

## Eta 36 hr. accum. precip. at 00Z 26 Oct.; simulation began at 00Z 24 Oct.

## Eta 36 hr. accum. precip. at 00Z 26 Oct.; simulation began 12Z 24 Oct.

## Meso-Eta 24 hr. accum precip. at 00Z 26 Oct.; simulation began at 15Z 24 Oct.

## Slide 34

## CSU RAMS Mesoscale Model Characteristics

-         Non-hydrostatic
-         Nesting capable
-         Terrain-following vertical coordinate
-         Microphysics: Mixed-phase; predicts cloud droplets, raindrops, pristine ice crystals, snow, aggregates, graupel, hail.
- .      (Generalized gamma distribution used for each)
-         Eta (forecast) boundary conditions for this study
-         Grid spacing for this study: 15, 5 and 1.67 km

## RAMS QPF

- Grid 1
- Grid 2
- Grid 3
- Precip. rates compared with observations at Jeffco... “R1” means full microphysics package

## RAMS grid 1 total precip., 36 hr

## Grid 2 RAMS total precip, 36 hr.

## Grid 3 RAMS total precip., 36 hr.

## Point comparison, precip., at Marshall site

## Northeast of Steamboat Spgs., CO(sometime after 26 Oct. 1997)

## Slide 42

## Blowdown regions

## Aug. 2002 “Hinman” fire

## Slide 45

## Wind predictions/verification

- Observed: mountain wind gusts to 50 m/s on western slope
- Meso-Eta: 25 m/s aloft, no amplifying wave (less than 10 m/s at lowest levels)
- RAMS 5 km grid: ~25 m/s @ 50m; amplifying mountain wave
- RAMS 1.7 km grid: ~30 m/s @ 50m and > 40 m/s aloft; generally ~33% greater speeds; evidence of rotors and non-linear wave dynamics

## 24hr Eta forecast winds, theta, w at 1200Z 25 Oct.

## RAMS E-W cross section, theta

## RAMS forecast of winds

## Wind direction issues

- In an upslope-type of event, direction is critical on the eastern slope; simultaneous downslope winds need strong cross-barrier component
- Difference of 20 deg. can have serious forecast implications, e.g. heavy snow vs. light snow
- Importance of winds in low- and mid-levels

## Meso-Eta height/wind forecast

## Slide 52

## SUMMARY

-  This case demonstrates that in a situation strongly forced by orographic lift, improved grid spacing can enhance usefulness of mesoscale QPF
-  Dynamics also appear to be handled better by RAMS in this case for a 30-hr. forecast
-  Mountain wave development dependent on model grid-spacing and possibly vertical coordinate
-  As grid spacing continues to decrease, clearly the usefulness of mesoscale QPF will increase
-  Research on local mesoscale model operational guidance in complex terrain continues to address operational challenges

## Summary (cont’d.)