1. FLURRIES OR FEET? An Analysis of QPF Errors in an Eastern North Carolina Snowstorm December 2-4, Jason Caldwell South Carolina State Climate Office

2. WINTER WEATHER FORECASTING
CSTAR Project
National Weather Service Offices (NC, SC, VA)
Focus on improving forecast skill in CAD, CF
Evaluation of the MM5 Real-Time Forecast System
NCSU, NC SCO, NCSC, Capitol Broadcasting
Focus on sources of model error on the synoptic and mesoscale related to CAD, CF
Examine model performance relative to Eta

3. SYNOPTIC SET-UP VALID 12Z 2 DECEMBER 2000 EDAS Surface Analysis
EDAS 850mb Analysis
EDAS 500mb Analysis
EDAS 250mb Analysis
VALID 12Z 2 DECEMBER 2000

4. MM5 vs. ETA Quantitative Precipitation Forecasts (24-30 hour forecasts valid Z on 3 December 2000)
Insert MM5 24h Forecast here

5. TOTAL ACCUMULATED SNOWFALL 2-4 DECEMBER 2000
(graphics courtesy NWS Raleigh)

6. What we know:
Model forecasts missed a precursor vorticity maximum at 500 mb
Models predicted coastal front too close to coast (site for surface cyclone formation)
Models under-predicted intensity of cold-air damming
Models produced heavy precipitation with a thermodynamic profile that was characterized by shallow cloud
Operational sea-surface temperature analyses were too warm according to NCEP, could have been tied to coastal front errors

7. HYPOTHESIS A
Lead vorticity disturbance at 500 mb(under-forecast by models) pushed the coastal front seaward, strengthened CAD, dried atmosphere over central NC before main vorticity maximum arrived through evaporative processes and subsidence
EVALUATE MODEL ERRORS IN 500mb VORTICITY
COMPARE VERTICAL CHARACTERISTICS BEFORE & AFTER VORT PASSAGE
TEST FOR EVIDENCE OF SURFACE BASED RESPONSE
HOW DOES THIS AFFECT QPF?

8. ETA 500mb Height/Vorticity 12h Forecast
EDAS 500mb Height/Vorticity Analysis
valid 00Z 3 December 2000
MM5 500mb Height/Vorticity 12h Forecast
valid 00Z 3 December 2000
valid 00Z 3 December 2000

9. VERTICAL CROSS-SECTION DIAGRAM
Yellow section indicates Central NC region; Black dot indicates location at max QPF gradient

10. EDAS Vertical Analysis (Omega,RH) EDAS Vertical Analysis (TAdv,Theta)
valid 00Z 3 Dec 2000
valid 00Z 3 Dec 2000
EDAS Vertical Analysis (Omega,RH)
EDAS Vertical Analysis (TAdv,Theta)
Drying aloft due to Subsidence
CAA increasing w/time
In the wake of lead vort max, AVA/NVA signals convergence at the 500mb level
Subsidence indicated below 500mb by drying over central NC and CAA increasing with time
Reflection of accumulation of mass seen with increased depth of CAD, seaward extension of CAD
Subsidence also indicated offshore by 06Z in response to CAA in low levels
OMEGA strengthens slightly around 700mb attributed to Differential Temperature Advection ( mb) and drier air over moist air  INCREASED BUOYANCY TERM
Temperatures rapidly decreasing w/height
UVV as response to DiffThermAdv
Low-level drying as CAD strengthens
CAA increasing w/time
valid 06Z 3 Dec 2000
valid 06Z 3 Dec 2000

11. EDAS FRONTOGENESIS AND SLP ANALYSES (PRE- & POST- 500mb VORT MAX)
PRE-VORT
POST-VORT
EDAS FRONTOGENESIS AND SLP ANALYSES (PRE- & POST- 500mb VORT MAX)
valid 00Z 3 Dec 2000
valid 06Z 3 Dec 2000
1000mb frontogenesis indicates CF pushed off-shore between 00Z and 06Z in wake of lead vort max
SLP between 1028 and 1032 over central NC at 00Z----- intensifies to 1030 to 1034mb by 12Z
valid 00Z 3 Dec 2000
valid 12Z 3 Dec 2000

12. HOW DOES THIS AFFECT THE QPF?
The strength and southern extent of the cold air damming was under-predicted by model forecasts and led to errors in QPF.
HOW DOES THIS AFFECT THE QPF?
The decreased magnitude of CAD shifts the location of the coastal front and affects low-level thermal and moisture profiles inland
- thermal gradient (baroclinic zone) is positioned farther west due to restricted areal coverage of cold dome
- moisture availability is higher due to less airmass intrusion from parent high
- lower atmosphere is less stable and more conducive to vertical motion
- coastal front more likely to propagate inland as WAA overtakes weaker CAD

13. HOW DOES THIS AFFECT THE QPF?
Model forecasts predicted the placement of the coastal front west of the actual location off the coast of North Carolina and resulted in QPF errors
HOW DOES THIS AFFECT THE QPF?
Low pressure forms closer to the coast which extends the western boundary of precipitation into the Triad region
- enhanced upward vertical motion inland due to surface convergence at CF
- increased moisture transport with stronger and more easterly winds at low-levels
- prolonged duration of precipitation as a result of proximity
- prolific isentropic lift as warm air advection associated with cyclogenesis moves into NC over CAD at mid-levels

14. HYPOTHESIS B
Model microphysics were overactive in producing precipitation with dry air aloft and a shallow cloud
EVALUATE PRECIPITATION MECHANISMS PRESENT
DIAGNOSE VERTICAL TEMPERATURE/MOISTURE PROFILE OVER CENTRAL NC
HOW DOES THIS AFFECT QPF?

15. INGREDIENTS FOR WINTER PRECIPITATION FORECASTS
FORCING FOR ASCENT
Low/mid- level convergence
Upper-level divergence/diffluence
Differential Temperature Advection
AVAILABLE MOISTURE
*Skew-T profiles *Satellite Imagery
INSTABILITY
Temperatures decreasing w/height
Thermal and Moisture Advection
TEMPERATURE
Determines p-type and snow/water ratio
EFFICIENCY
Based on cloud Temperature and ice generation
*Max occurs at –15C w/strong forcing for ascent

16. MUCH DRIER THAN FORECAST
Cross-section of Omega/RH EDAS Analysis
valid 12Z 3 December 2000
MUCH DRIER THAN FORECAST
MUCH DRIER THAN FORECAST
Cross-section of Omega/RH ETA 24h Forecast
valid 12Z 3 December 2000

17. 24-hour MM5 and ETA Forecast Soundings and EDAS RAOB Raleigh-Durham International, NC (valid 12Z 3 December 2000)
Cloud top T ~ -10C
Shallow Cloud Layer
Models too moist below 850 mb
Cloud depth over-predicted by models
Less drying above 600 mb than in models

18. HOW DOES THIS AFFECT THE QPF?
The ETA model microphysics scheme allowed ice crystal growth at –10 degrees C which allowed precipitation generation to occur in shallow, super-cooled water droplet clouds.
HOW DOES THIS AFFECT THE QPF?
Dendrite growth occurred in an non-conducive thermal environment generating spurious precipitation
Increased moisture availability in the models through an extended depth over-quantified precipitation totals, when in reality the atmosphere was dry above 700mb

19. SUMMARY AND CONCLUSIONS
Classification of QPF Error Sources
(1) Synoptic-scale features (i.e. 500 mb vorticity)
(2) Circular feedbacks from multiple factors
(3) Model parameterizations
Model Inter-comparison
+ MM5 achieved better QPF forecasts than Eta yet MM5 also under-estimated the CAD and vort max
+ MM5 also out-performed the Eta in coastal front location, strength of CAD, and track of the cyclone

20. ( & OTHER CONSIDERATIONS)
DIRECTIONS FOR FUTURE RESEARCH
( & OTHER CONSIDERATIONS)
Sensitivity experiments using: - High-resolution SST - PBL and Microphysics options
Examine low-level moisture influx in model forecasts - Relationship to latent heat release
Statistically quantify the relative weight of each proposed hypothesis in generating model QPF errors

22. Vertical Motion & Moisture Cross-Section EDAS vs
Vertical Motion & Moisture Cross-Section EDAS vs. ETA 12h Forecast valid 00Z 3 Dec 2000
Low-level drier air as a result of subsidence in CAD region over central NC
RH values increase over eastern NC around 700mb in region of ascent where OMEGA= -7mb/s
Model indicates region of subsidence isolated west of RDU adjacent to the mountains
To the east, upward vertical motion (up to -11mb/s) predominates in the entire mb layer
RH too low over central NC at 500mb indicating drying and instability between mb
RH too high around 700mb level continues mid-level instability
12hr model forecasts indicate downward omega west of RDU over central NC/southern VA with UVV above 850mb east of the CAD region approaching –11mb/s in eastern NC in the mb level
RH forecasts low values over RDU at this time below 850 with higher RH over mts and aloft around 700mb

23. Vertical Motion & Moisture Cross-Section EDAS vs
Vertical Motion & Moisture Cross-Section EDAS vs. ETA 18h Forecast valid 06Z 3 Dec 2000
Positive values of OMEGA indicates subsidence at low-levels over central NC and offshore
Upward vertical motion (OMEGA=-9mb/s) strengthens near 700mb over eastern NC
RH values increased in the mb layer across all of NC
Substantial drying below 850mb and aloft above 700mb
Model indicates upward motion from SFC-500mb
Two OMEGA maxima (-12mb/s, -16mb/s) over central/eastern NC in the mb layer
RH too high at low-levels in CAD region (possibly due to erroneous precipitation)
Dry air at 500mb too far west over mountains; RH too high above 700mb east of mountains