National Weather Service Des Moines IA

2006 Winter Weather Workshop

The views expressed are those of the author and do not necessarily represent those of the National Weather Service.

Diagnosing Heavy Banded Snow

Precipitation Band Definition--an elongated region of accumulated banded precipitation-an instantaneous radar feature which may or may not result in a banded accumulation

Precipitation Band Definition--an elongated region of accumulated banded precipitation-an instantaneous radar feature which may or may not result in a banded accumulation

Ingredients for Mesoscale banded snow

(Banacos, 2003)

•Banding results from mechanisms that organize lift or moisture into narrow regions (Comet,2005)

Frontogenesis definedFrontogenesis defined

• (in general terms) refers to the change in the magnitude and orientation of the temperature gradient at a level or in a layer (e.g., 850-700 mb) due to directional and speed changes in the wind field.

• (in specific terms) refers to an increase in thehorizontal thermal gradient with time.

F > 0 frontogenesis, F < 0 frontolysis

(Funk, 2004)

Vector Frontogenesis FunctionVector Frontogenesis Function

(Funk, 2004)

snF sn FF

)(

pDt

DknsF

pDt

DnF

The F vector is the full wind version of the quasigeostrophic Q vector. It can be broken down into 2 components (natural coordinates): Fn and Fs.Fn:-Frontogenetical (dominant) component of F-Directed perpendicular to temperature lines-Refers to changes in magnitude of thermal gradient-Corresponds to vertical motion on the frontal scale (mesoscale bands)Fs:-Rotational component of F-Directed parallel to isotherms/thicknesses-Refers to changes in direction (orientation) ofthermal gradient with no magnitude change-Corresponds to synoptic-scale vertical motionon the scale of the baroclinic wave itself

(Moore, 2003)

Strong Surface Cyclone Conceptual Model

Common Synoptic Patterns for F to act

(Moore, 2003)

Weak Surface Cyclone / Stationary Front Conceptual Model

Forecast premise for mesoscale banding:

•Does NOT require a strong surface cyclone, only a low-mid tropospheric baroclinic zone

• Requires a strengthening baroclinic zone in the presence of sufficient moisture for precipitation (AND – for snow, the proper thermal stratification).

• Large-scale deformation zones are BY FAR AND AWAY the most common means of manifesting areas of frontogenesis within the 850-500mb layer.

(Banacos, 2003)

Use of Frontogenesis in Forecasting

Use of Frontogenesis in Forecasting

• Presence of F in 850-500mb layer can help diagnose and predict areas of heavy banded precipitation.

• Potential for banding can be assessed using F field in numerical models, with placement of banding refined in <12 hour period.

• New graphic forecast tools allow location of banded precipitation to be conveyed to the user.

• Presence of F in 850-500mb layer can help diagnose and predict areas of heavy banded precipitation.

• Potential for banding can be assessed using F field in numerical models, with placement of banding refined in <12 hour period.

• New graphic forecast tools allow location of banded precipitation to be conveyed to the user.

(Banacos, 2003)

Horizontal ConvergenceHorizontal Convergence Convergence (Divergence) acts frontogenetically (frontolytically),

always, irrespective of isotherm orientation. Convergence (Divergence) acts frontogenetically (frontolytically),

always, irrespective of isotherm orientation.

F<0

(Banacos, 2003)

Horizontal DeformationHorizontal Deformation

Flow fields involving deformation acting frontogenetically are prominent in the majority of banded precipitation cases.

Flow fields involving deformation acting frontogenetically are prominent in the majority of banded precipitation cases.

F>0

(Banacos, 2003)

Dynamics of Frontogenesis (vertical circulation)

Dynamics of Frontogenesis (vertical circulation)

Flow field dominated by deformation.

(Banacos, 2003)

Dynamics of Frontogenesis (cont.)Dynamics of Frontogenesis (cont.)

Ageostrophic circulation develops as a response to increasing temperature gradient. (Banacos, 2003)

Dynamics of Frontogenesis (cont.)Dynamics of Frontogenesis (cont.)

When we talk about frontogenetical forcing, it’s the resulting ageostrophic circulation we are most interested in for precipitation forecasting

(Banacos, 2003)

Sloping Continuity of FSloping Continuity of F

• Presence of parallel axes of positive frontogenesis sloping upward toward colder air is a common aspect of heavy banded precipitation areas.

600 mb

700 mb

850 mb

(Banacos, 2003)

850mb

700mb

600mb

Role of Deep-Layer Shear ProfileRole of Deep-Layer Shear Profile

(Banacos, 2003)

Using CSI/EPV to Measure Stability

Using CSI/EPV to Measure Stability

EPV = Equivalent Potential Vorticity

• A relatively simple, quick, and effective way to evaluate CSI. Gravitational instability may also be present.

• The closer EPV is to zero, the more responsive the atmosphere will be to a given amount of forcing.

• IF EPV<0 , then CSI is present. Overlaying EPV with theta-e is an effective

way to determine if convective (gravitational) instability also exists.

(Moore,2003)

CSI = Conditional Symmetric Instability

•Symmetric instability is a special combination of unbalanced forces in which parcels may be stable to solely horizontal or vertical displacements but still be unstable. •The release of CSI can result in Slantwise Convection (McCann, 1999)

Modulation of Band Intensity by Instability for a constant

value of F

Modulation of Band Intensity by Instability for a constant

value of FAs gravitational or symmetric stability decreases, the horizontal scale of the band decreases while the intensity of the band increases. Multiple bands become established in an unstable regime

(Banacos, 2003)

(Moore,2003)

Conceptual Model of Frontogenesis / EPV Regions

Enhanced vertical motion as Symmetric Stability decreases

(Comet, 2005)

Using Soundings to Identify InstabilityUsing Soundings to Identify Instability

•Near neutral or unstable lapse rates (with respect to a moist adiabat) implies multiple narrow and intense (maybe 5-10 km or so in width), bands. (Banacos, 2003)

700-500mb lapse rate: 6.7 C/km

TROWAL:

•Trough of warm air aloft resulting from cyclonically turning warm conveyor belt

•Heavy banded snow is favored along and north of TROWAL axis

(Comet, 2005)

ETA 12z / March 15, 2004•COL point aloft-- relative minimum in wind speed within 850-700 mb region

• should be a cue to investigate frontogenesis and banding potential, and COL location may approximate the area of heavy snow

(Banacos, 2003)

Model Diagnostics of TROWAL, COL, Deformation Zone

Snow Band Checklist Snow Band Checklist

Presence of (1”/hr):

•Sloped region of mid-level 2-D Frontogenesis / Deformation axis in 850-500mb range / TROWAL

•Relative minimum in wind speed (<20kt) within 850-700mb region (Col point aloft) and/or uniform deep-layer shear profile with absence of substantial hodograph curvature

•limited dry air advection in near surface.

•near saturated / high low-mid level RH present (east CONUS, 1000-500mb >85%)

•Favorable thermodynamic profile for snow (i.e. cloud top temp <-9C, no melting layers)

(Banacos, 2003)

Snow Band Checklist (cont.)Snow Band Checklist (cont.)

Enhancement of (1-3”/hr, 5”/hr in extreme cases):•Presence of negative EPV, elevated potential or slantwise instability-CSI (convective snow potential, band multiplicity)

•Saturation through dendrite growth layer (-12 to –18C) coincident with strong UVV (high precipitation efficiency)

•Isothermal layer just colder than 0 C above surface:suggests higher atmospheric moisture content andenhances aggregation (Banacos, 2003)

(Moore, 2003)

Heavy banded snow is favored…

•To the southeast of the zone of mid-level frontogenesis

•To the northwest of the zone of mid-level negative EPV (near CSI region)

•Along and to the north of the TROWAL axis

•In a region of substantial isentropic lift and high RH

(Moore, 2003)

LL L L L

Quick Event Review

March 15-16 2004 heavy snow

12Z/15

18Z/15

06Z/1612Z/16

00Z/16

1012 mb

1013 mb1013 mb

1012 mb 1006 mb

Surface Low Track

Frontogenesis DOES NOT require a strong surface cyclone, only a baroclinic zone (this fact can lull the forecaster into neglecting heavy snow potential).

(Banacos, 2003)

Snow to Liquid Ratios for March 15-16, 2004

10-12 12-14 14-16 16-18 18+

101014

14 8 171417 25

1616

10 24

1312 10

11

8

12

10

1712

1520 16

13

14 109

14 11 10

20 1113

811

14 17 1322

25

9 13 271920

132215 14

251514

19 24

20

17

NWS and Cooperative

Observer Obs

•SLR’s in red

• outliers in blue

Layer Frontogenesis Diagnostics

Banded Snowfall Diagnostics

Winter Forecasting Resources

•SPC Winter SREF

•Bufkit

•Canadian EPV

National Weather Service Des Moines IA

2006 Winter Weather Workshop

The views expressed are those of the author and do not necessarily represent those of the National Weather Service.

The End

References

Comet, 2005: Mesoscale Banded Precipitation Web Module. Comet’s Winter Weather page, [http://www.meted.ucar.edu/mesoprim/bandedprecip/index.htm].

Moore, J.T., 2003: Heavy Banded Snow Web cast. COMET’s Winter Weather page, [http://meted.ucar.edu/norlat/bandedsnow/].

Funk, T., 2004: Understanding Frontogenesis and its Application to Winter Weather Forecasting. [http://www.crh.noaa.gov/lmk/soo/presentations/frontogenesis_lmk2.pdf]

Banacos, P., 2003: Prediction of Frontogenetically Forced Precipitation Bands, presentation at WDTB Winter Weather Workshop IV, [http://www.wdtb.noaa.gov/workshop/WinterWxIV/index.html]

McCann, D., 1999: Comments on “Evaluation and Application of Conditional Symmetric Instability, Equivalent Potential Vorticity, and Frontogenetic Forcing in an Operational Forecast Environment”, Wea. Forecasting, 3, 470.