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National Centers for Environmental
Prediction (NCEP)
Hydrometeorlogical Prediction Center
(HPC) Forecast Operations Branch
Winter Weather Desk
Dan Petersen
[email protected]
301-763-8201
Goals of Presentation
• Review HPC winter weather
desk forecasts and
verification
• Review verification of snow
to liquid ratio forecasts
• Review Short Range
Ensemble Forecast
Temperature Biases and
Upcoming Changes
HPC Winter Weather Desk
• Issues heavy snow and icing forecasts and collaborates
these forecasts with local Weather Forecast offices
September 15- May 15
– Internal 24-hour Snow and Ice accumulation probability
graphics for days one through three.
– Public and technical versions of low tracks graphic
depicting synoptic scale surface low tracks associated
with heavy snow or ice.
– Heavy Snow and Ice Discussion
• Forecasts and discussions posted at
http://www.hpc.ncep.noaa.gov/wwd/winter_wx.shtml
HPC Products and Services
Winter Weather Desk
Probabilistic Snow Forecasts are issued for 4, 8 and 12
inches, and icing for 0.25 inches
HPC Winter Weather Desk Day 3
probabilistic snow forecast verification
2006-8
30
HPC forecaster
improvement
over Brier skill
score for all
NCEP model and
ensemble
members
(2007-2008 top,
2006-2007
bottom)
2007-8
29
28
27
26
4"
8"
12"
25
24
15
10
5
0
2006-7
4"
8"
12"
HPC Winter Weather Desk Days 1-3
probabilistic 0.25” ice forecast
verification 2006-8
78
HPC forecaster
improvement
over Brier skill
score for all
NCEP model and
ensemble
members
(2007-2008 top,
2006-2007
bottom)
77
76
75
74
2007-8
Day 1
Day 2
Day 3
73
48
2006-7
46
44
42
40
38
36
Day 1
Day 2
Day 3
HPC Products and Services-Winter
Weather Desk Low Tracks Graphic
The displayed model
and ensemble low
positions show the
uncertainty of the low
positions- the black
line represents the
HPC preferred track
HPC Products and Services
Low track verification 2007-8
250
200
ECMWF had
lowest rms
errors 48-72
hours, followed
by UKMET
150
100
50
0
12
HPC
24
NAM
SREFMN
36
GEFSMN
48
60
GFS
CMC
72
UKMET
ECMWF
HPC Winter Weather Forecast Process
• Snow accumulations obtained by multiplying experimental
snow to liquid ratios times quantitative precipitation forecast
(QPF)
• Experimental ratios blend the UWM (Roebber et al) neural net
technique, climatology, and fixed snow to liquid ratios
HPC Winter Weather Desk Forecast Process
Climatology of Snow to Liquid Ratios (courtesy Marty
Baxter)
http://www.eas.slu.edu/CIPS/SLR/slrmap.htm
The Morphology of Snow Crystals and Crystal Density
Snow to Liquid Ratio
References
Libbrecht, http://www.its.caltech.edu/~atomic/snowcrystals/
Snow to liquid ratio studies
Roebber et al (2003)
Snow ratios determined by temperature, relative humidity, solar
radiation (month), and compaction (via wind speed and weight of liquid
forcing snow to compress)
10-member ensemble of artificial neural networks assigns snow ratio in
one of three classes: heavy (ratio < 9:1), average (9:1 to 15:1), and light
(ratio > 15:1).
Real time Web page interface
for calculating SLR
http://sanders.math.uwm.edu/cgi-bin-snowratio/sr_map_main.pl
Role of Vertical Velocity in
determining snow ratio?
• The maximum growth rate is expected to
occur near the level of maximum upward
air motion within the cloud, where the
greatest water vapor delivery occurs (Auer
and White 1982).
Cobb 21st Conference on Weather Analysis and
Forecasting
A simple physically based snowfall algorithm
http://ams.confex.com/ams/WAFNWP34BC/techprogram/paper_94815.htm
Cobb (Caribou) Method
• SLR is
computed for
each cloud
layer and
then weighted
towards the
layer with the
greatest
vertical
velocity (UVV)
Verification of the Snow to Liquid Ratio
Forecast Tools
Winter 2006-7 and 2007-8 Snow to Liquid Ratio Verification
Summary for the SLR methods run using GFS model output and
verified against cooperative observer and CoCoRahs reports
5
4
Day 1 Caribou
3
Day 1
Climatology
Day 1 UWM
2
1
0
RMS Error
Bias
Community Collaborative Rain, Hail & Snow Network www.cocorahs.org
Verification of the Snow to Liquid Ratio
Forecast Tools
Winter 2006-7 and 2007-8 Snow to Liquid Ratio Verification
Summary for the SLR methods run using NAM model output
5
4
3
Day 1 Caribou
2
Day 1
Climatology
Day 1 UWM
1
0
-1
RMS Error
Bias
Short Range Ensemble
Forecast (SREF) Temperature
Forecast Issue and Upcoming
Changes
• Non Hydrostatic Mesoscale Model (NMM)
• Advanced Research Weather Research
Forecast (ARW)
SREF NMM Member 2m
Temperature Bias Dec. 1, 2007Feb. 29, 2008
0
-0.5
-1
-1.5
-2
-2.5
-3
-3.5
-4
-4.5
-5
0
6
12
18
24
30
36
NMMCTL
42
48
NMMMN1
54
60
NMMP1
66
72
78
84
SREFMean 2m temperature
bias Dec. 1, 2007-Feb. 29, 2008
GFS
NAM
SREFMEAN
SREFMean
(Blue) often
the coldest bias
vs. the NAM
(black) gfs
(red), bias
corrected
srefmean
(green)
SREF 2m Temperature upgrade
impact:
• Increase in WRF to ten members
- Non Hydrostatic Mesoscale Model (NMM)
increase to five members (if temperature
bias uncorrected, would have greater
impact on mean)
- Advanced Research Weather Research
Forecast (ARW) increase to five members
Mean forecast error of 2m Temp (C) of
parallel SREF (left), current operational
SREF (right) June 16-26 2008
0.5
0.4
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
0
6 12 18 24 30 36 42 48 54 60 66 72 78 84
NMMCTL
NMMMN1
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
-1.2
-1.4
-1.6
-1.8
0
6
12
NMMP1
18
24
30
NMMCTL
36
42
48
NMMMN1
54
60
66
72
78
NMMP1
The results from the parallel experiment show a slight positive bias
the first 48 hours, trending to a slight negative bias 54-84 hours
(green line is zero bias)
84
SREF Mean 2m temperature analysis
03z Sun 10/26/08
Area of concerncentral to
northern
Rockies and
high Plains
2m SREF Mean temperature 84
hr forecast at 03z 10/26/08
84 hour
operational
forecast shows
minima of 0C
(red) where no
areas analyzed
less than 5C
2m SREF Mean parallel
temperature forecast
84 hour parallel
SREF forecast
shows minima of
5C (still a cold
bias but
improved over
prior SREF by
3-4 degrees C)
12z Tue. Oct 28 2008
SREF operational (green), parallel (red) 27 hour
forecast sea level pressures on left screen, 12z HPC
surface analysis (right)
Sea level pressures 4-6 mb too high in much of NY-PA-NJ-New England in both
operational, parallel sref
12z Tue. Oct. 28 2008 500 mb 27 hour forecast sref operational
(green), parallel (red) left screen, initial global data assimilation
analysis (right)
500 mb low verified near PA-NJ border with both SREF versions showing low near PA-MD border
• Questions/comments?
• [email protected]
(301)763-8201
Upcoming SREF changes
December 2008
• Upgrade both NMM and ARW to version 2.2 and increase their
model horizontal resolution from 40/45km to 32km
• Upgrade RSM model version and increase resolution from 45km
to 32km
• Increase WRF membership from 6 to 10 (5 NMM and 5 ARW)
and reduce Eta membership from 10 to 6
• Enhance physics diversity among RSM members: replace Zhou
cloud scheme with Ferrier scheme for 3 SAS members
• Increase output frequency from 3hrly to hourly for the 1st 39hr to
meet the demand from aviation and convection forecasting
• Add aviation and wind-variance fields to ensemble products
• Breakout “big” SREF bufr output into individual stations
Short Range Ensemble Forecast (SREF)
derived probabilities for snowfall rates,
amounts, and precipitation type
• http://www.spc.noaa.gov/exper/sref/
• Select most recent model run, then winter
weather option on left
Probability of
snow as
precipitation
type for 8 am
EDT Friday
April 11 2008
SREF Derived Winter Weather Impact
Graphics-derived probabilities for snow fall
rates, visibilities, and event duration
• http://www.hpc.ncep.noaa.gov/wwd/impactgraphics/
SREF
probability of
one inch per
hour snowfall
00z Friday
April 11 2008
SREF Temperature Bias Dec. 1
2007-Feb 29 2008
HPC Products and Services
Low track verification 2007-8
250
200
12 hr
24 hr
36 hr
48 hr
60 hr
72 hr
150
100
50
0
HPC
NAM
SREFMN GEFSMN
CMC
UKMET
ECMWF