Using Frontogenesis in Winter Weather Forecasting Greg Patrick WFO FWD Nov 13, 2008 Parts of this presentation derived from presentations by Dr. David Schultz (NSSL) and Pete Banacos (formerly SPC) 1

Topics Motivation Frontogenesis Review Definition Interpretation Diagnosing Frontogenesis Conceptual Models Example 2

Motivation Frontogenesis was a significant contribution to forcing during two of the most significant winter events across north TX in the past ~ 5 years (2/24/03 & 3/6/08) Winter weather events with large geographic variations in impacts can result from events where Fgen forcing is dominant 3

Feb 24-25, 2003 Motivation Mar 6, 2008 Frontogenesis produced Banded pcpn 4

Frontogenesis Review Conceptually, F is the local change in horizontal temperature gradient near an existing front, baroclinic zone, or feature as it moves. When we talk about frontogenesis forcing, its the resulting ageostrophic circulation we are most interested in for precipitation forecasting 5

Frontogenesis Review Frontogenesis is an intensification of a temperature gradient at the surface or aloft Frontolysis is a weakening of the temperature gradient at the surface or aloft The 2-D scalar frontogenesis function (F ) – quantifies the change in horizontal (potential) temperature gradient following air parcel motion : F > 0 frontogenesis, F < 0 frontolysis 6

Petterssen (1936) Frontogenesis F = d/ dt | | F = 1/ 2 | | ( E cos2 - D) potential temperature E = resultant deformation = angle between the isentrope and the axis of dilatation D = divergence 7

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Frontogenesis Review Diagnosis of frontogenesis results in a diagnosis of the forcing for vertical motion on the frontal scale. Ascent occurs on the warm side of a maximum of frontogenesis and on the cold side of a region of frontolysis 9

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Horizontal Deformation Flow fields involving deformation acting frontogenetically are prominent in the majority of banded precipitation cases. F>0 11

Deformation – 2/24/03 Event 12

Deformation – 3/6/08 19Z mb 13

Deformation – 3/6/08 19Z mb 14

Deformation – 3/6/08 19Z mb 15

Conceptual Models 16

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Displaying Fgen Fields WFO only : AWIPS workstation Web: HPC Model Diagnostics page Web: SPC SREF page Web: Others? 18

Field is fgenslope 19

Look under Winter Weather or Lift 20

Example – Feb 24, 2003 Convection developed in a zone of strong frontogenetical forcing across western and northern parts of north TX, resulting in a mixture of moderate-heavy sleet and snow in some areas. Models (particularly Eta) focused UVM and QPF across southern parts of the FWD CWA, closer to surface front and stronger elevated instability 21

Cross section line taken perpendicular to frontal zone COLDER WARMER 22

Eta 3 pm Monday - Cross section taken across front – frontal circulation highlighted Cold Air KSPS Warm Air KGLS 5000 Feet 10,000 Feet 23

FEB 24-25, 2003 Feb 24-25, 2003 Event Totals 24

24 hour Low level Fgen Forecast (Eta) STP mosaic ending at 00Z 25

Operational Forecasting Summary Frontogenesis fields should be assessed anytime a strong frontal zone affects north TX Look for banded QPF in numerical model output or large values of +VV in bands parallel to front as clues that Frontogenesis may be a factor Look for sloped continuity of Frontogenesis Must also assess moisture and instability parameters along with vertical temp profile 26

References Dr. David Schultz NSSL Pete Banacos SPC Link to his banding/Fgen conference paper Reference to dynamic explanations of F and UVM H. B. Bluestein, Vol II, Synoptic-Dynamic Meteorology In Midlatitudes. Pages