Correlations between observed snowfall and NAM “banded snowfall” forecast parameters Mike Evans...

Correlations between Correlations between observed snowfall and observed snowfall and NAM “banded snowfall” NAM “banded snowfall”

forecast parametersforecast parameters

Mike Evans and Mike Jurewicz Mike Evans and Mike Jurewicz WFO BGMWFO BGM

Banded vs. Non-banded events Banded vs. Non-banded events (from Novak et al.)(from Novak et al.)

Frontogenesis and Stability Frontogenesis and Stability (from Novak et al.)(from Novak et al.)

Frontogenesis (shaded) and saturated equivalent potential temperature (contoured)

Moderate Event Schematic Cross Moderate Event Schematic Cross Section (from Wagner)Section (from Wagner)

Schematic cross section through a cool-season moderate precipitation band showing frontogenesis (red ellipse), negative EPV* (dashed blue ellipse), WMSS (brown dotted region), saturation equivalent potential temperature (dark green contours), and transverse circulation (arrows).

Lift in the “dendrite zone” (from Lift in the “dendrite zone” (from Waldstreicher)Waldstreicher)

Questions…Questions…

Key factors appear to be magnitude and Key factors appear to be magnitude and persistence of frontal-scale forcing for lift persistence of frontal-scale forcing for lift and stability, plus favorable and stability, plus favorable thermodynamics.thermodynamics.

Can the significance of these factors be Can the significance of these factors be validated using 40 km AWIPS forecasts validated using 40 km AWIPS forecasts from a large number of heavy and from a large number of heavy and moderate snow events?moderate snow events?

Can we identify thresholds of these values Can we identify thresholds of these values that would be useful to forecasters?that would be useful to forecasters?

MethodologyMethodology Examine “synoptic” snow events in the BGM Examine “synoptic” snow events in the BGM

CWA since 2002 (throw out lake-effect; look CWA since 2002 (throw out lake-effect; look for events with at least 30 dbz reflectivity). for events with at least 30 dbz reflectivity).

29 events identified - maximum snow 29 events identified - maximum snow accumulations ranged from 4 to 34 inches.accumulations ranged from 4 to 34 inches.

Examine 6-24 hour NAM time-height forecasts Examine 6-24 hour NAM time-height forecasts of “banding factors” at these locations. of “banding factors” at these locations.

Examine data in time-height cross-sections Examine data in time-height cross-sections (to look at depth and persistence of features).(to look at depth and persistence of features).

Examine data in conventional cross-sections Examine data in conventional cross-sections (for a better look at structure). (for a better look at structure).

Results – Depth and PersistenceResults – Depth and Persistence

Looking for depth and persistence of Looking for depth and persistence of “favorable” conditions (areas on time-“favorable” conditions (areas on time-height diagrams).height diagrams).

Examine frontogenesis / Fn Examine frontogenesis / Fn ConvergenceConvergence

Examine “* Signatures” – omega < -8, Examine “* Signatures” – omega < -8, EPV < 0, RH > 80 percent.EPV < 0, RH > 80 percent.

Examine “** Signatures” = omega < -Examine “** Signatures” = omega < -12, EPV < 0, RH > 80 percent.12, EPV < 0, RH > 80 percent.

Correlations with event max snowfall - Depth and Correlations with event max snowfall - Depth and persistence of Frontogenesis and Fn convergence persistence of Frontogenesis and Fn convergence

(12 hour forecasts) (12 hour forecasts)

Co-located upward vertical Co-located upward vertical motion and negative EPV – 12 hr motion and negative EPV – 12 hr

forecastsforecasts25 of 29 events had a * Signature 25 of 29 events had a * Signature

(omega < -8 with negative EPV).(omega < -8 with negative EPV).17 of 29 events had a ** Signature 17 of 29 events had a ** Signature

(omega < -12 with negative EPV).(omega < -12 with negative EPV).

Results – 12 hour forecastsResults – 12 hour forecasts

Negative Geostrophic EPV Negative Geostrophic EPV correlations – 12 hour forecastscorrelations – 12 hour forecasts

Result – yes / no questionsResult – yes / no questions

Results – yes/no questionsResults – yes/no questions

Correlations with max snowfall Correlations with max snowfall associated with conventional cross-associated with conventional cross-

sections (structure)sections (structure)

Change in EPV over the band location from (T-Change in EPV over the band location from (T-6) to (T) (-0.89)6) to (T) (-0.89)

Maximum omega in the Dendrite Zone (-12 to -Maximum omega in the Dendrite Zone (-12 to -18 degrees C): (0.75)18 degrees C): (0.75)

Maximum omega within areas of negative EPV Maximum omega within areas of negative EPV and RH > 80 percent (0.65)and RH > 80 percent (0.65)

Magnitude of negative EPV (0.63)Magnitude of negative EPV (0.63) Maximum omega (0.60)Maximum omega (0.60) Magnitude of Fn vector convergence (0.48)Magnitude of Fn vector convergence (0.48)

AcknowledgementsAcknowledgements

Keith Wagner, SUNY AlbanyKeith Wagner, SUNY AlbanyLance Bosart, SUNY AlbanyLance Bosart, SUNY AlbanyDan Keyser, SUNY AlbanyDan Keyser, SUNY AlbanyDavid Novak, NWS ER, Scientific David Novak, NWS ER, Scientific

ServicesServices Jeff Waldstreicher, NWS ER, Scientific Jeff Waldstreicher, NWS ER, Scientific

ServicesServices