1. 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

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

3. 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

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

5. 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

6. 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

7. 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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9. 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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11. Horizontal Deformation Flow fields involving deformation acting frontogenetically are prominent in the majority of banded precipitation cases. F>0 11

12. Deformation – 2/24/03 Event 12

13. Deformation – 3/6/08 19Z 800 - 700 mb 13

14. Deformation – 3/6/08 19Z 800 - 700 mb 14

15. Deformation – 3/6/08 19Z 800 - 700 mb 15

16. Conceptual Models 16

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18. Displaying Fgen Fields WFO only : AWIPS workstation Web: HPC Model Diagnostics page http://www.hpc.ncep.noaa.gov/mdd/mddoutput/ http://www.hpc.ncep.noaa.gov/mdd/mddoutput/ Web: SPC SREF page http://www.spc.ncep.noaa.gov/exper/sref/ http://www.spc.ncep.noaa.gov/exper/sref/ Web: Others? 18

19. http://www.hpc.ncep.noaa.gov/mdd/mddoutput/ http://www.hpc.ncep.noaa.gov/mdd/mddoutput/ Field is fgenslope 19

20. http://www.spc.ncep.noaa.gov/exper/sref/ http://www.spc.ncep.noaa.gov/exper/sref/ Look under Winter Weather or Lift 20

21. 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

22. Cross section line taken perpendicular to frontal zone COLDER WARMER 22

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

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

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

26. 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

27. References Dr. David Schultz NSSL http://www.cimms.ou.edu/~schultz/http://www.cimms.ou.edu/~schultz/ Pete Banacos SPC Link to his banding/Fgen conference paper http://spc.noaa.gov/publications/banacos/F_conf_030415b.pdf Reference to dynamic explanations of F and UVM H. B. Bluestein, Vol II, Synoptic-Dynamic Meteorology In Midlatitudes. Pages 297-304 27