Performance of Various Operational and Experimental Numerical Forecasts for the March 2003 Colorado

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Performance of Various Operational and
Experimental Numerical Forecasts for the March
2003 Colorado SnowstormEd Szoke Brent Shaw
and Paul SchultzNOAA Forecast Systems
LabBoulder, Coloradoedward.j.szoke_at_noaa.govDave
Barjenbruch Boulder WFOIn collaboration
with the Cooperative Institute for Research in
the Atmosphere (CIRA), Colorado State University,
Fort Collins, Colorado
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Its not 40, but still my biggest storm (25.2)

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Its not 40, but still my biggest storm (25.2)

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Impressive snow in the foothills...87.5 at 9000
feet in Coal Creek Canyon southwest of Boulder

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The snow was quite heavy with lots of broken
limbs and roof collapses

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Its not 40, but still my biggest storm (25.2)

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Ski area snow reports

Loveland Ski Area...at Continental Divide on I70
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I70.closed for over 2 days

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The widespread snowfall boosted mountain snowpack
to normal

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• There are MANY interesting aspects of this
  massive storm
• Longer range (8-15 days) model forecasts
  indicated potential

240 h forecast from the GFS run initialized at
0000 UTC on 8 March and valid 0000 UTC on 18
March
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• Longer range (8-15 days) model forecasts
  indicating potential
• Drifting to poorer forecasts in the mid-range

GFS 132 h forecast initialized 0000 UTC on 13
March and valid 1200 UTC on 18 March. This
forecast produces a major storm in the Midwest
but very little snow for the Front Range of
Colorado.

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Use of GFS and Canadian ensemble forecasts as
well as multi-model ensembles to make a better
forecast. GFS ensemble shown below for same 132h
forecast.

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Then quite good NWP forecasts in the lt 2-3 day
range. This Eta 84h forecast issued 1200 UTC
Fri/14 Mar and valid 0000 UTC Tue/18 Mar is quite
good.

500 mb height, vorticity and wind
Surface pressure, thickness, and precip
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Outline

• Then quite good NWP forecasts in the lt 2-3 day
  range
• Including impressive 5 ft. model forecasts for
  the foothills
• Excellent 1-1.5 day NWS snowfall forecasts for
  historic event
• Included forecast of 3-8 ft. of snow for the
  foothills
• Smaller scale detailsincluding the Lyons snow
  hole

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Smaller scale detailsincluding the Lyons snow
hole

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Outline...continued

• A significant forecasting aspect of the storm
  was rain versus snow at the lower elevations
  where most of the population is located
• Although a Winter Storm Warning was issued for
  the Front Range cities as early as Sunday (valid
  from Monday night into Wednesday)...there were
  conflicting signals as late as Monday evening
  about when (if?) the rain would change to snow!
• The operational models (Eta, RUC) were
  indicating that temperatures would be too warm
  for snow perhaps until late Tuesday
• However, it was noted that actual surface
  temperatures were colder then forecast or even
  analyzed by the models (at 00z/18 Mar)
• The focus of the rest of this talk will be on
  this issue and examining the 0000 UTC/18 March
  analyses and forecasts

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But first...a quick overview of the storm. 500
mb analysis at 0000 UTC/18 Mar

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500 mb analysis for 1200 UTC on Tue/18 March

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500 mb analysis for 0000 UTC on Wed/19 March

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Focus on the 0000 UTC/18 March time period

• Critical forecast issue was when the rain would
  change to snow.
• First a look at some observations

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1800 UTC/17 March IR image and surface
observations
Most of the precipitation is north of CO with a
surface low taking shape over se CO.

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1800 UTC/17 March radar and surface observations
Mild conditions along the Front Range with some
light rain. Quite warm over eastern CO. Main
precip area is in WY with cooling conditions
there.

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2100 UTC/17 March radar and surface observations
Mi

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0000 UTC/18 March radar and surface observations
Mild conditions along the Front Range with some
light rain. Quite warm over eastern CO. Main
precip area is in WY with cooling conditions
there.

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0600 UTC/18 March radar and surface observations
Still raining along most of the Front Range but
barrier jet that had been forming to the north
producing strong northerly flow down the Front
Range and advecting colder air southward from se
WY.

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1200 UTC/18 March radar and surface observations
Precipitation has changed to snow with several
inches of accumulation in many areas along the
Front Range.

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1800 UTC/17 March radar and surface observations
Mild conditions along the Front Range with some
light rain. Quite warm over eastern CO. Main
precip area is in WY with cooling conditions
there.

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Composite radar with obs at 21z 17 Mar
Convection becoming widespread on the Plains
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Eta cross-section of RH (image), wind, theta, T,
18h forecast valid 06z 18 Mar
Still has the nly winds along the Front
Range. Interesting reverse flow w of the
Dividedoes this effectively raise the mtn height
and coupled with the very moist and deep ely flow
help yield the S west of the Divide?
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Comparison of radar and obs at 00z 18 Mar with
Eta forecast
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Closeup reveals the wind is captured well but
temps too warm
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Similar problem with the 6h forecast from the 18
UTC Eta
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Closeup of 18z run.
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Eta analysis of surface T and wind with radar for
00z 18 Mar
Even the analysis misses the colder temps near
the Front Range.
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Closeup of the 00z analysis of the Eta run
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00z/18 Mar Eta 6h forecast of surface T, wind and
pcpn with radar for 06z 18 Mar
The warmer temps near the Front Range in the
analysis carry over through 6h.
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Closeup.
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00z/18 Mar LAPS analysis of surface T and wind
with radar for 00z 18 Mar
The LAPS analysis did do a good job, so in theory
a fine scale model initialized with this analysis
should do better.
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Closeup of the 00z/18 Mar LAPS analysis
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Composite radar with obs at 12z 18 Mar