Snowfall Amount Forecasting

1
Snowfall Amount Forecasting

• Current Weather Discussion
• 11 January 2008

2
Overview

• In the coming weeks, we will get into a number of
  specialized winter weather topics
• Precipitation Type Forecasting
• Lake Effect Snows
• Mesoscale Structures Banding
• But first, how do we actually forecast snowfall
  and snowfall amounts?

3
Outline

• Precipitation Type Basics
• Snow Forecasting Checklist
• Synoptic-Scale Conditions Favoring Snow
• Moderate-Heavy Snowfall Forecasting
• Forecasting Snowfall Amounts

4
Precipitation Type Basics

• Precipitation type forecasting is entirely
  dependent upon the tropospheres vertical
  temperature profile.
• Temperatures need to be sub-freezing throughout
  the troposphere to see snow.
• Sleet, freezing rain, and rain are cases where
  one or more layers are above freezing.
• What atmospheric situations lend themselves to
  seeing snow?

5
Cyclone Structure Overview

• Classic wave cyclone
• Cold conveyor belt (CCB) behind cold front
• Colder airmass (cP)
• Strongly descending
• Warm conveyor belt (WCB) ahead of cold front
• WCB rises over gently sloping warm front
• Diffluent aloft in nature in cool sector
• Isentropic upglide prevalent

(http//www.aos.wisc.edu)
6
Cyclone Structure and Precipitation

• Precipitation occurs primarily in the warm sector
  and atop the cool sector associated with the
  WCB.
• In the warm sector, unless sub-freezing air is
  being transported north by the WCB, thermal
  profiles generally only support rain.
• It is in the cool sector where frozen
  precipitation is generally most prevalent.

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Example Thermal Profile

• Albany, NY 16 Dec 2007
• Cold air mass in the boundary layer
• Warm tongue from WCB atop upper-level warm front
• Narrow near-freezing layer between 750-800 hPa
• Completely saturated up to the tropopause at 170
  hPa
• Precipitation at time of sounding light snow

(Univ. of Wyoming Sounding Archive)
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Precipitation Type

• The warm tongue shown on the previous sounding
  becomes critical when forecasting precipitation
  type in winter.
• Below freezing snow
• Shallow above freezing layer sleet
• Deep above freezing layer with sub-freezing
  surface temperatures freezing rain
• Well cover non-snow precipitation type
  forecasting in a couple of weeks.

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Snowfall Forecasting

• Assuming that snow is possible, what factors do
  we want to look at to help make this
  determination?
• Our key considerations are two-fold
• Vertical thermal profile
• Presence of moisture
• and both must be met for snows to be likely!

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NWS WFO Springfield Snowfall Forecast Checklist
The first seven of the eight listed criteria come
from a document by John Gordon, a former
forecaster at NWS WFO Springfield, MO. The latter
is an additional prerequisite for sufficient ice
growth to favor snowfall. More yes answers
suggest a better chance you have for snow.

• Is the surface temperature below 35F?
• Is the freezing level greater than 1200 ft?
• Is the 850 mb temperature less than 0C?
• Is the 700 mb temperature less than -4C?
• Is the 1000-500 mb thickness less than 5400 m?
• Is the temperature below freezing from 1200 ft to
  700 mb?
• Is the surface-700 mb layer reasonably moist?
• Is the temperature at the base of the cloud less
  than -10C?

11
Synoptic-Scale Patterns Favoring Snowfall

• Typical wintertime synoptic-scale pattern that
  generally leads to snows in the northeast US

Systems tilt toward cold air with increasing
altitude. Developing systems often have the tilt
at left. Mature systems are often stacked.
(Base map from Google Maps)
12
Synoptic-Scale Patterns

• We previously said that snowfall was most likely
  in the cool sector, or that north of a warm
  front

The progression of the entire synoptic-scale
pattern directly influences snowfall!

• Locations thermally and moisture dependent
• Amounts duration and track dependent

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Synoptic-Scale Example

• Noreasters intense, often rapidly developing
  low pressure systems

• Initial low over Ohio Valley (the primary)
  transfers to the coast
• Greatest baroclinicity due to Gulf Stream
• Cold high over Canada serves as an impediment to
  the primarys progression
• Coastal low (the secondary) takes over and
  deepens/travels along the coast

(Base map from Google Maps)
14
Synoptic-Scale Example

• With noreasters, our critical factors manifest
  themselves in several ways
• Low-level temperatures
• How far north does the primary travel?
• How strong is the surface high over Canada?
• How close to the coast does the secondary
  travel?
• Warm tongue
• How intense is the low-level jet in the warm
  sector?
• How far north does the primary travel?

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Snowfall Amount Forecasting

• So, were expecting snow.
• Were given a series of synoptic charts.
• How can we pinpoint areas that will likely
  receive the heaviest snows?
• How can we actually use these pieces of
  information to predict snowfall amounts?

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Moderate-Heavy Snowfall

• We again turn to the NWS WFO Springfield, MO
  winter weather forecast guide.
• We are going to look at 6 atmospheric levels to
  try to pinpoint regions of heavier snows
• Surface
• 850 hPa
• 700 hPa

• 500 hPa
• 300 hPa
• 200 hPa

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Heavier Snows Surface

• 2-2.5 latitude left of the track of the low
• 5 latitude ahead of the low
• Favored with strengthening surface cyclones
• Culminates when low begins to weaken
• Enhanced when surface ridge is located to the
  north or northwest
• Optimal surface temperatures between 27-32F.

(Base map from Google Maps)
18
Heavier Snows 850 hPa

• 1.5 latitude left of the track of the low
  (closer than surface)
• Notable when cooling occurs in rear quadrant of
  low
• More likely for storms moving northeast
• Heaviest snows not likely for storms moving
  southeast
• -5C isotherm bisects heavy snow region -2 to
  -8C for moderate snow
• Significant WCB moving in

(Base map from Google Maps)
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Heavier Snows 700 hPa

• Heaviest snows between -6 and -8C isotherms and
  south of -10C isodrosotherm
• Heaviest snows along the lows path
• Heaviest snows found in 850-700 hPa
  frontogenetical region
• Heaviest snows located where best moisture is
  located
• Occur north of any closed 700 hPa height contour

(Base map from Google Maps)
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Heavier Snows 500 hPa

• Occur about 7 latitude downstream of the
  vorticity maximum
• Also occur slightly left of the track of the
  vorticity maximum
• Greater than 90 m height falls
• Heaviest snows occur in temperature band between
  -20 and -25C
• Found between 500 hPa ridge axis and the
  diffluent region of the 500 hPa trough

(Base map from Google Maps)
21
Heavier Snows 300 hPa

• Heaviest snows in regions of jet favored for
  rising motion left exit and right entrance
• Heaviest snows occur in region between two
  coupled jet streaks
• Look for strongest Q-vector convergence
• Typically occurs with deep and/or deepening long
  wave troughs

(Base map from Google Maps)
22
Heavier Snows 200 hPa

• Stratospheric warming, both in space and in time,
  tends to lead to more intense snows (Note that
  with these systems, 200 hPa is often in the lower
  stratosphere.)
• Heaviest snow occurs north of 1640 dm height line

(Base map from Google Maps)
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Snowfall Amount Prediction

• Now that we have pinpointed regions of relatively
  significant snows, how much snow is going to
  fall?
• Well cover two forecasting methods
• Model QPF and projected liquidice ratios
• The easy and often less accurate way
• Ingredients-based checklists
• The better yet still limited way

24
Model QPF and Liquid Ratios

• QPF (quantitative precipitation forecast) maps
  show how much liquid precipitation is expected to
  occur within a certain time interval.
• Model skill at QPF forecasting is dubious at best
• Short-term skill scores are generally fair to
  good
• Medium-range skill scores are generally poor
• Base ratio is 10 of snow to every 1 of liquid
• Colder, drier conditions lead to higher ratios
• Can see ratios of 301 or higher
• Use with extreme caution!

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Model QPF Example

• Focus on Minnesota in this example
• Assuming we are dealing with snow, how much is
  there?

• Raw model output 0.10-0.25 in six hours
• 850 hPa temps between -10C and 0C
• Likely 101 to 121 ratios (not overly cold or
  dry)
• Resulting snowfall forecast 1-3

(NCEP Central Operations)
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Snowfall Amount Prediction

• There are many ingredients-based methodologies
  used by various forecasters
• Lemo Technique (e.g. NWS WFO Chicago)
• Magic Chart
• Garcia Method
• Cook Method
• Well cover the Garcia method in detail.
• If interested in the others, search for them
  online

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Garcia Method

• Relevant references Garcia (1994) Cook (1996)
• Uses mid-level moisture along with persistence to
  determine maximum snowfall amounts.
• Recall that 700 hPa moisture was one of our key
  factors for heavy snowfall!
• Assumes that sufficient conditions for snow
  already exist (or will exist) method is not
  applied blindly!
• Does not take vertical motion into account
• Increase projections slightly for stronger rising
  motion
• Decrease projections slightly for weaker rising
  motion

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Garcia Method

• The methodology of the Garcia Method is as
  follows
• Find the isentropic (constant potential
  temperature) level that intersects the 700-750
  hPa surface in focus region
• http//weather.cod.edu/analysis/analysis.isentropi
  c.html
• Note the mixing ratio (in g/kg) in that region
• Use the winds on that surface to determine
  moisture source region
• Determine the highest value of mixing ratio that
  could make it from there into the region of
  interest in 12 hr
• Average the two mixing ratio values together

29
Garcia Method

• Match that averaged value to the column on the
  left to get the maximum projected snowfall

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Garcia Method Example

• Focus on Washington in this example, using the
  295 K surface in the vicinity of 750 hPa
• Assuming weve identified this as a snowfall
  region, how much snow do we expect?
• Mixing ratio 2 g/kg
• Potential mixing ratio 3 g/kg
• Maximum snowfall potential 4-6 inches

(College of DuPage)
31
Summary

• Snowfall amount forecasting is a multifaceted
  process
• Need to determine if precipitation will occur
• Next, determine what type of precipitation will
  occur
• Only then can snowfall amount predictions be made
• Snowfall amounts are heavily dependent upon
  mesoscale and synoptic-scale structures
• E.g. path of low pressure systems, mesoscale
  bands
• Unless nowcasting, one is often limited by the
  skill of the models (not just with QPF, either)!