Using The Froude Number to Improve
Orographic Snow Forecasts in the Green
Mountains of Vermont
Michael J Muccilli
NOAA/NWS Burlington, VT
NROW XV 2014
1
Top 3 Take-Aways
2
 The Froude Number is a useful tool for determining
the characteristics of Orographic Snow Events
 How and Why the Froude Number works for
orographic snow forecasts
 How to use in Operations
NROW XV 2014 – Michael J Muccilli
Outline
3
 Motivation
 Synoptic Overview
 Define the Froude Number
 Green Mountain Study
 Operational Use at WFO BTV
NROW XV 2014 – Michael J Muccilli
4
Motivation
NROW XV 2014 – Michael J Muccilli
January
2nd
-
3rd,
2012
5
Source: Weather Prediction Center
NROW XV 2014 – Michael J Muccilli
December
28th
-
29th,
2011
6
Source: Weather Prediction Center
NROW XV 2014 – Michael J Muccilli
7
December 28th - 29th, 2011
Froude = 4.4
January 2nd – 3rd, 2012
Froude = 0.91
NROW XV 2014 – Michael J Muccilli
8
Synoptic Overview
NROW XV 2014 – Michael J Muccilli
Terrain of WFO Burlington County Warning Area
9
4395 feet
100 feet
15 Miles
NROW XV 2014 – Michael J Muccilli
Synoptic Overview
10
 Upper Level Trough or
Closed Low Progressing
through the Region
Source: NOAA ESRL
Composite of 25 Upslope Cases
NROW XV 2014 – Michael J Muccilli
Synoptic Overview
11
 Surface & Low Level
Pressure System exiting
the Region
Source: NOAA ESRL
Composite of 25 Upslope Cases
NROW XV 2014 – Michael J Muccilli
Synoptic Overview
12

Increasing west to northerly flow in low levels along with
Source: NOAA ESRL
lingering low level moisture
Composite of 25 Upslope Cases
NROW XV 2014 – Michael J Muccilli
Synoptic Overview
13
 St. Jean et al. (2004) found specific important factors to the
development of significant upslope snow events:
 Near-Saturated Conditions from surface to
ridge-top level
 Strong low level winds (>10 m/s) with
significant cross-barrier component
 Equivalent potential temperature decreasing
with height in the low levels
 Event duration of at least 12 hours
NROW XV 2014 – Michael J Muccilli
14
Objectives

Make it easier to identify significant orographic (upslope) snow
events and placement/orientation of heavy snow using:
I. The Froude Number
II. Low/Mid Level Humidity Profiles
III. Vertical Profile of Wind Speed & Direction
IV. Low level Stability
NROW XV 2014 – Michael J Muccilli
The Froude Number


Potential Temperature (Surface & Mountain Top)
Mountain Height

Speed of Wind Perpendicular to the Barrier
Froude Number Equation
15
Brunt-Vaisala Equation
***RESULT: A Unit-less expression that represents the flow of
air when it comes in contact with a barrier (Green Mountains)
NROW XV 2014 – Michael J Muccilli
The Froude Number

Subcritical (Blocked), Froude < 1
 Precipitation likely to fall
upwind of barrier

Critical, Froude =~1
 Precipitation likely to fall
16
along barrier

Supercritical (Unblocked), Froude > 1

Precipitation likely to fall on
lee side of barrier
NROW XV 2014 – Michael J Muccilli
Data & Methods - Green Mountain Study

25 cases (2007 – 2012)
 12 “Blocked” (Froude < 1)
 13 “Unblocked” (Froude >1)

11 Stations Used
 2 Champ Valley
 5 West Slopes
 3 East Slopes

17
1 East Valley
NROW XV 2014 – Michael J Muccilli
Data & Methods - Green Mountain Study
18
 Froude Number calculated for each event, in the
mid-point of each event
 Used archived NAM/RUC Soundings at KBTV
 After calculation, grouped by calculated Froude
Number into 8 bins
 0.25, 0.25 - 0.49, 0.50 - 0.84, 0.84 - 0.99
 1.0 - 1.33, 1.34 - 1.75, 1.76 - 2.0, >2.0
NROW XV 2014 – Michael J Muccilli
Results
19
1.34-1.75
0.25-0.49
1.0-1.33
0.85-0.99
0.50-0.84
<0.25
Froude
Fr 1.76-2
>2
NROW XV 2014 – Michael J Muccilli
Results – 3 Types of Events
20
 Consistent with theory behind the Froude number
 Can then be separated into 3 Types of Events
 Froude > 1: Spine and eastward (Unblocked)
 Froude 0.5 – 1: Heaviest Western Slopes (Classic
Blocked)
 Froude < 0.5: Champlain Valley (Very Blocked)
NROW XV 2014 – Michael J Muccilli
Results – 3 Types of Events
21
Fr <0.5
>1
0.5-1
NROW XV 2014 – Michael J Muccilli
Results – 3 Types of Events


Unblocked
Froude #: 4.4


Low level RH 80-90%
WNW winds
throughout low levels
Well Mixed to

22
Mountain Height
NROW XV 2014 – Michael J Muccilli
Results – 3 Types of Events
23
Unblocked
Froude #: 4.4
NROW XV 2014 – Michael J Muccilli
Results – 3 Types of Events

Classic Western Slopes




Blocked
Froude #: 0.98
Low level RH ~90%
West winds veering to

Northwest throughout
low levels
Isothermal layer below
24
mountain height
NROW XV 2014 – Michael J Muccilli
Results – 3 Types of Events
25
Blocked
Froude #: 0.98
NROW XV 2014 – Michael J Muccilli
Results – 3 Types of Events






26
“Champlain Valley Powder”
Very Blocked
Froude #: 0.07
Low level RH 95-100%
Northwest winds veering to
North throughout low
levels (WAA)/(Less
Perpendicular Flow)
Strong Inversion below
mountain height (Very
Stable)
NROW XV 2014 – Michael J Muccilli
Results – 3 Types of Events
27
Very Blocked
Froude #: 0.07
NROW XV 2014 – Michael J Muccilli
Results – Blocked vs. Unblocked
28
Unblocked
Blocked
Source: NOAA ESRL
Composite of 12 Blocked/13 Unblocked Upslope Cases
NROW XV 2014 – Michael J Muccilli
Results – Snow Ratios

29
Snow Ratios were found to be much greater than average
synoptic snowfall cases in CWA



Long Term Average (Baxter et al 2005) – 13:1
Upslope cases averaged 28:1
Ranged from 7:1 to 71:1
NROW XV 2014 – Michael J Muccilli
Results – Snow Ratios

30
Snow Ratios were found to have some association with
850mb Temperatures
 Average 850mb temperature during events was -13C
 Highest ratios occurred when 850mb temperatures were
between -11C and -15C, 25-35:1
NROW XV 2014 – Michael J Muccilli
31
Results – Snow Ratios
Average Snow-Liquid Ratios vs. Average 850 mb Temps
50
45
40
Snow-Liquid Ratios
35
30
25
20
15
10
5
0
-8
-9
-10
-11
-12
-13
-14
-15
-16
-17
-18
-19
850 mb Temperatures (C)
NROW XV 2014 – Michael J Muccilli
R2O- Use at WFO BTV



32
Worked with SOO to
develop into GFE Smart
Tool
Calculates Froude Number,
wind speed/direction,
relative humidity, and QPF
Run off local WRF model,
NAM, & GFS
NROW XV 2014 – Michael J Muccilli
R2O- Use at WFO BTV

33
Also have a realtime calculation
based off vertical
temperature
profile along
mountain chain
NROW XV 2014 – Michael J Muccilli
R2O- Use at WFO BTV
34
 These two products have led to:
 An increased situational awareness of upslope
events
 Improved forecasts of location of heavy snow
band, snow intensity, and snow amounts
 Improved lead time and verification scores of
winter weather products
NROW XV 2014 – Michael J Muccilli
35
In Summary
 The Froude Number is a useful tool in determining
characteristics of orographic snow bands
 Events can be unblocked (Fr > 1), Blocked (Fr < 1), or
Very Blocked (Fr << 1)
 Can be transitioned into operations and used to
improve orographic snowfall forecasts
NROW XV 2014 – Michael J Muccilli
36
References
Baxter, M.A., C.E. Graves, and J.T. Moore, 2005: A Climatology of Snow-to-Liquid Ratio for the Contiguous United States. Wea. Forecasting, 20, 729-744.
Bell, G.D. and L.F. Bosart, 1988: Appalachian Cold Air Damming. Mon. Wea. Rev., 116, 137-161.
Chen, S-H., Y.L Lin, and Z. Zhao, 2008: Effects of Unsaturated Moist Froude Number and Orographic Aspect Ratio on a Conditionally Unstable Flow over a
Mesoscale Mountain. J. of the Meteor. Soc. Japan, 86, 353-367.
Chu, C-M. and Y.L. Lin, 2000: Effects of Orography on the Generation and Propagation of Mesoscale Convective Systems in a two-dimensional
conditionally unstable flow. J. of Atmos. Sci., 57, 3817-3837.
Lee, L.G. and H. Gerapetritis, 2012: The Northwest Flow Snow Event of 11 February 2012.
NOAA Earth System Research Laboratory (ESRL), Physical Sciences Division (PSD), 2012: Interactive Plotting and Analysis Pages. [Available online at
http://www.esrl.noaa.gov/psd/]
NOAA Air Resources Laboratory (ARL), 2012: Gridded Meteorological Data Archives. [Available online at http://ready.arl.noaa.gov/archives.php]
National Weather Service Burlington, VT, 2012: Daily Climate Maps. [Available online at http://www.weather.gov/btv/climatemaps]
Keighton, S., L. Lee, B. Holloway, D. Hotz, S. Zubrick, J. Hovis, G. Votaw, L.B. Perry, G. Lackmann, S.E. Yuter, C. Konrad, D. Miller, and B. Etherton, 2009: A
Collaborative Approach to Study Northwest Flow Snow in the Southern Appalachians. Bull. Amer. Meteor. Soc., 90, 980-999.
Rutledge, S.A. and P. Hobbs, 1983: The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. VIII: A
Model for the “Seeder-Feeder” Process in Warm-Frontal Rainbands. J. Atmos. Sci., 40, 1185-1206.
Sisson, P.A., D. St. Jean, E. Evenson, W.E. Murray, S.F. Hogan, L. Bosart, D. Keyser, and B. Smith, 2004: Applying local research to National Weather Service
operations: Forecasting heavy mountain snowfalls in Vermont and Northern New York. Preprints, 11th Conference on Mountain Meteorology and the
Annual Mesoscale Alpine Program, Bartlett, NH, Amer. Meteor Soc., 17.5.
St. Jean, D.P., and P.A. Sisson , 2004: Characteristics of upslope snowfall events in northern new York state and northern Vermont: Diagnostics and model
simulations of several northwest flow cases. Preprints, 20th Conference on Weather Analysis and Forecasting. Seattle, WA, Amer. Meteor. Soc., 18.4.
NROW XV 2014 – Michael J Muccilli
37
Questions?
NROW XV 2014 – Michael J Muccilli