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Diagnosis and Near-Term Forecasting of an Intense Mesoscale Snowband Feb 2 nd , 2014

Diagnosis and Near-Term Forecasting of an Intense Mesoscale Snowband Feb 2 nd , 2014. Nathan Marsili NWS Northern Indiana 2014 GLOM Workshop. SYNOPTIC SETUP. 700 MB. 300 MB. 700 MB. 300 MB. FEBRUARY 2/1200Z. HEADLINES. * A 2-phased winter storm impacted area Feb1-Feb2.

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Diagnosis and Near-Term Forecasting of an Intense Mesoscale Snowband Feb 2 nd , 2014

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  1. Diagnosis and Near-Term Forecasting of an Intense Mesoscale Snowband Feb 2nd, 2014 Nathan Marsili NWS Northern Indiana 2014 GLOM Workshop

  2. SYNOPTIC SETUP 700 MB 300 MB 700 MB 300 MB FEBRUARY 2/1200Z

  3. HEADLINES * A 2-phased winter storm impacted area Feb1-Feb2. * These different storm phases led to two distinct areas of warning or near-warning criteria accumulations:

  4. EVENT OVERVIEW 00Z-12Z KIWX 0.5 Base Reflectivity

  5. EVENT OVERVIEW 00Z-12Z KIWX 0.5 Base Reflectivity

  6. EVENT OVERVIEW Narrow band of heavy snow

  7. EVENT OVERVIEW • Forecasters struggled with phase 2 of system – large run to run variability in southward progression of mid/upper level frontal zone. • Precip type issues also complicated phase 2 of snowfall forecast across southeast portions of forecast area. • Forecast thinking: Small window of time with profiles conducive for snow and presence of mid/upper level frontogenesis would limit snowfall amounts to less than 2 inches across the southeast. • Upstream sheared waves in southwest flow aloft critical in determining placement of mid level baroclinic zone. • What were some of the important factors leading to the banding toward the end of the event across the southeast portions of forecast area?

  8. INGREDIENTS METHOD • Wetzel and Martin (2001) developed a qualitative forecast approach assessing 5 basic ingredients to heavy snowfall forecasting: • Forcing for ascent • Instability • Moisture • Efficiency (snow growth) • Thermal profiles (top-down considerations) PVQ set to 0 if: Q-vector divergence OR EPV positive PVF set to 0 if: F-vector divergence OR EPV positive

  9. INGREDIENTS METHOD • Pros: • Quick way to assess potential areas for further investigation. • Flexibility for number of different situations. • Cons: • Does not distinguish between types of instability. • Not useful when model forecasts are poor. • Qualitative in nature. • Heavy/banded snow can occur in higher stability regions. • Not a substitute for more in-depth analysis. • Magnitude of PVQ/PVF difficult to interpret (model resolution dependence).

  10. 00Z NAM 850-500 MB PVQ/PVF PVF F09 Valid 09Z PVF F06 Valid 06Z PVQ F06 Valid 06Z PVQ F09 Valid 09Z

  11. 00Z NAM 850-500 MB PVQ/PVF PVF+PVQ F09 Valid 09Z PVQ F09 Valid 09Z PVQ F09 Valid 09Z

  12. 00Z NAM CROSS SECTION 00Z NAM Omega (contours), Q-Vector convg (color-filled) F09/Valid 09Z

  13. INGREDIENTS MAP EXAMPLE 00Z FEB2 NAM F009/V09Z 550-500 mb negative EPVg (color-filled) Q-vector convergence (dashed yellow)

  14. INGREDIENTS MAP EXAMPLE 00Z NAM F009/V09Z 550-500 mbPVQ

  15. 00Z NAM Q-VECTOR PARTITION 00Z FEB2 NAM Qn vector and Qn convg F09/Valid 09z 00Z FEB2 NAM Qs vector and Qs convg. F09/Valid 09z

  16. Q-VECTOR PARTITION Martin (1999) linked Qs convergence to the development of thermal ridging in occluded/TROWAL portion of cyclone. Qs vectors pointing with cold air to the left represent counter-clockwise turning of thickness pattern, and with cold air to the right represents clockwise turning of pattern. Qs convergence associated with larger scale synoptic forcing through 1) vorticity adv. and 2) shearing deformation Martin 1999

  17. Q-VECTOR PARTITION F06/V06Z F09/V09Z F12/V12Z 00Z FEB2 NAM 600-500 mb Qs convg 550 mb temps

  18. MODEL/OBSERVATION TRENDS 00Z FEB1 NAM 700-500 mb vorticity, 600 mb wind speed F33, Valid 09Z/FEB1

  19. MODEL/OBSERVATION TRENDS 00Z FEB2 NAM 700-500 mb vorticity, 600 mb wind speed F09, Valid 09Z/FEB1

  20. MODEL/OBSERVATION TRENDS 00Z FEB1 NAM 550 mb temp/deform. F33/Valid 09Z 00Z FEB2 NAM 550 mb temp/deform. F09/Valid 09Z

  21. MODEL/OBSERVATION TRENDS 0.5 KIWX Base Reflectivity 06Z 06Z FEB2 NAM 600 mb F-vector dvg F00/Valid 06Z

  22. MODEL/OBSERVATION TRENDS 0.5 KIWX Base Reflectivity 09Z 06Z FEB2 NAM 600 mb F-vector dvg F03/Valid 09Z

  23. INSTABILITY • CSI ASSESSMENT 00Z NAM KFWA fcst sounding F09/Valid 09z Weismueller/Zubrick 1998

  24. INSTABILITY • CSI ASSESSMENT 00Z NAM KFWA Forecast Sounding (F09/Valid 09Z)

  25. INSTABILITY A A’ 0856Z 0.5 KIWX Base Reflectivity

  26. INSTABILITY 06Z NAM EPVg (color-filled), omega (green), theta-e (white) F003/V09Z A A’

  27. INSTABILITY EPVg Calculations for 2/2/14 Banded Case • Large run to run model variations in distribution from 36 to 6 hours before heavy snow. • Some utility in the 6 to 12 hours preceding heaviest snow (especially < 6 hours). • Provided some indication to the potential of heavy banded snow, but not very helpful in banding location more than 6 hours before heavy snow. • Models slow to latch onto rotational frontogenesis aspect of event, and thus inconsistent depiction of frontal instability. • Small model initialization errors with mid/upper level wind field will hurt reliability of interpretation.

  28. INSTABILITY 12Z FEB1 NAM 550:500 Negative EPV F21/V09Z 00Z FEB1 NAM 550:500 Negative EPV F33/V09Z 00Z FEB2 NAM 550:500 Negative EPV F09/V09Z

  29. SUMMARY • Mesoscale/synoptic forcing mechanisms not well modeled 18 to 36 hours in advance of heavy snow band. • Ingredients plots did provided some indication of banding potential. • “Smaller” scale synoptic forcing acted to reinforce/focus mesoscale banding. • Q-vector partition indicated that synoptic scale forcing likely played large role in rotational frontogenesis, extended duration mid/level forcing from • previous forecast thinking. • Strong indications of slantwise ascent/CSI. • QG analysis from later runs of NAM appeared to align well with mesoscale band. • Event serves an important reminder of the synoptic/mesoscale link.

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