Appalachian ColdAir Damming: Climatology and Sensible Weather Impacts

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Appalachian Cold-Air DammingClimatology and
Sensible Weather Impacts

• NWS-NCSU Collaboration
• CSTAR Regional Workshop
• Thursday 16 October, 2003

2
Outline

• 1.) A word about collaboration
• 2.) CAD Process Refresher and Classification
• 3.) CAD Climatology
• 4.) An Example
• 5.) Synoptic Composites
• 6.) Sensible Weather Impacts
• 7.) Hourly Trends

3
1.) Collaboration

• The NWS-NCSU collaboration
• Understanding and accepting the others
  reality constraints, responsibilities,
  priorities
• Discovering and complimenting each others
  strengths, helping with the others challenges
• Creative thinking, flexibility, and patience
• The more you put in, the more you get back

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2.) CAD Process Refresher and CAD Classification
Number of freezing rain observations September
through April 1982-1990. From Robbins and
Cortinas 1996, 15th AMS conf. on Weather Analysis
Forecasting.
5
Upper-level Processes Conducive to Building
Parent High

• Confluence aloft, jet entrance
• Upper-level ridge west of parent high

Kocin and Uccellini (1990)
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CAD as a Geostrophic Adjustment Process
Mass depletion west of barrier reinforces trough
Mass accumulation creates pressure ridge
Stable atmosphere terrain impede flow, alter
force balance
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Processes Contributing to Blocking

• Adjustment process occurs with stable flow
• Blocked vs. Not Blocked? Really, degrees of
  blocking
• Dry adiabatic layer deeper than mountain depth
  no blocking
• Strong inversion intersecting mountain complete
  blocking
• And everything in between
• Processes that enhance stability, blocking
• Differential thermal advection (WA aloft, CA
  below)
• Differential evaporational cooling
• Radiational cooling and stable winter air mass
  seasonality
• Solar sheltering due to clouds

Bell and Bosart (1988)
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Four Processes Contributing to Pressure Rises in
CAD Ridge

• Upwind high, lee trough Adiabatic
  cooling/warming, hydrostatic pressure
  rises/falls enhanced when stable
• Geostrophic adjustment Coriolis deflection
  leads to mass accumulation on upwind side of
  barrier
• Along-barrier near-surface cold advection
  stabilizes air east of barrier, strengthens ridge
  (hydrostatically and otherwise)
• ? Upslope flow on east side favors formation of
  clouds, precipitation there DIABATIC PROCESSES

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Diabatic Effects on CAD

• Precipitation falling into dry air evaporational
  cooling

Result increased stability, blocking pressure
rises ? Low-level wind adjusts to diabatic cool
pool northeasterly, upslope flow CA ? Clouds
shelter cold dome from solar heating, mixing ?
NWS forecasters in damming region noted different
CAD flavors variability in diabatic
contribution
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ClassicalDry synoptic forcingMay have
diabatic contributionStrong, northerly parent
high
In-Situ Diabatic processes essential High
location unfavorable Little/no CA

• Hybrid
• Equal contributions from
• dry synoptic diabatic Parent high
• Right place, too weak
• Progressive

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The Original CAD Spectrum
Classical
CAD Intensity
Hybrid
In-Situ
ltlt1
?1
gtgt1
Diabatic
Synoptic
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3.) NWS/NCSU CAD Climatology Research Objectives

• Objectives
• Reproduce NWS sub-type climatology, objectively
• CAD detection algorithm
• CAD classification criteria
• Seasonal sub-type statistics
• Distinguish high-impact cases from non-events
• Upper-air patterns
• Other characteristics?
• Develop baseline climatology for sensible weather
  impact

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CAD Detection Algorithm (Bailey)

• Based solely on surface station data
• Detection keys on characteristic pressure ridge,
  cold dome, ageostrophic northeasterly flow

• Laplacians calculated for SLP, ? along 3
  mountain-normal lines
• Laplacian quantitative measure of cold dome,
  ridge
• Barrier-parallel line (line 4) is surrogate for
  NE ageo. Flow
• Thresholds set to allow detection of weaker events

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CAD Detection Algorithm Criteria
1.) Mountain-normal SLP Laplacian negative,
gt 1 standard deviation above mean negative
value 2.) Mountain-normal ? Laplacian must be gt 0
3.) SLP must be greater at center stations 4.)
Line 4 pressure ? 1.5 mb higher at RIC than GSO,
or GSO to GSP 5.) Must meet requirements for at
least 6 hours (for at least one mountain-normal
line)
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Climatological Results
Total Number of Events 353 Cold Season Events
180 Warm Season Events 173

• September - Highest frequency of events (?!!)
• July - lowest frequency of events
• Highest frequency of strong events in Winter
  months
• Lowest frequency of strong events in summer
  months (June - August)

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Climatological Results

• 353 events identified (1984-1995)
• Original damming subtypes emerged, but...
• Many cases that did not fit the original subtypes
  were also identified
• Parent high too far south or too weak for
  classical label, no precipitation, damming
  still evident
• Weak, short-lived warm-season cases not
  accompanied by precipitation at onset, but
  maintained in part by solar sheltering
• Events in which a cyclone to the south provided
  the along-barrier pressure drop
• Extended CAD spectrum (perhaps broadened
  definition of CAD to include cases that were
  previously lookalikes)

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4.) An Example CAD Sensible Weather Impact

• Period from 5-20 December 2001 very active in
  terms of CAD
• Examples taken from period to illustrate
  challenge in forecasting sensible weather impact
• Consider Eta SLP analyses at 12-h intervals from
  11-18 December 2001

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12 UTC 11 December, Eta Sea-Level Pressure
Analysis
Dec. 11th, TMAX/TMIN/PRE IAD 50/38/.35 RIC
50/45/.76 ROA 47/39/.23 ORF 56/52/1.33 RDU
49/42/.71 CLT 56/42/.02 CAE 57/45/T
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00 UTC 12 December, Eta Sea-Level Pressure
Analysis
Dec. 11th, TMAX/TMIN/PRE IAD 50/38/.35 RIC
50/45/.76 ROA 47/39/.23 ORF 56/52/1.33 RDU
49/42/.71 CLT 56/42/.02 CAE 57/45/T
2
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12 UTC 12 December, Eta Sea-Level Pressure
Analysis
Dec. 12th, TMAX/TMIN/PRE IAD 50/35/.04 RIC
50/46/.01 ROA 47/45/.06 ORF 56/52/T RDU
52/49/.01 CLT 52/49/.02 CAE 55/50/.04
2
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00 UTC 13 December, Eta Sea-Level Pressure
Analysis
Dec. 12th, TMAX/TMIN/PRE IAD 50/35/.04 RIC
50/46/.01 ROA 47/45/.06 ORF 56/52/T RDU
52/49/.01 CLT 52/49/.02 CAE 55/50/.04
2
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12 UTC 13 December, Eta Sea-Level Pressure
Analysis
Dec. 13th, TMAX/TMIN/PRE IAD 56/45/.05 RIC
63/48/.01 ROA 63/45/.03 ORF 67/53/0 RDU
61/50/.01 CLT 57/51/.02 CAE 68/55/.11 RDU
Climo 54
2
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00 UTC 14 December, Eta Sea-Level Pressure
Analysis
Dec. 13th, TMAX/TMIN/PRE IAD 56/45/.05 RIC
63/48/.01 ROA 63/45/.03 ORF 67/53/0 RDU
61/50/.01 CLT 57/51/.02 CAE 68/55/.11 RDU
Climo 54
2
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12 UTC 14 December, Eta Sea-Level Pressure
Analysis
Dec. 14th, TMAX/TMIN/PRE IAD 67/53/.12 RIC
71/56/0 ROA 60/54/0 ORF 75/61/0 RDU
73/60/0 CLT 68/56/.01 CAE 73/62/0 RDU Climo
53
2
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00 UTC 15 December, Eta Sea-Level Pressure
Analysis
Dec. 14th, TMAX/TMIN/PRE IAD 67/53/.12 RIC
71/56/0 ROA 60/54/0 ORF 75/61/0 RDU
73/60/0 CLT 68/56/.01 CAE 73/62/0 RDU Climo
53
2
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12 UTC 15 December, Eta Sea-Level Pressure
Analysis
Dec. 15th, TMAX/TMIN/PRE IAD ?/31/0 RIC
?/40/0 ROA ?/39/0 ORF ?/48/0 RDU ?/42/0 CLT
?/45/0 CAE ?/47/0 RDU Climo 53
2
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00 UTC 16 December, Eta Sea-Level Pressure
Analysis
Dec. 15th, TMAX/TMIN/PRE IAD ?/31/0 RIC
?/40/0 ROA ?/39/0 ORF ?/48/0 RDU ?/42/0 CLT
?/45/0 CAE ?/47/0 RDU Climo 53
2
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12 UTC 16 December, Eta Sea-Level Pressure
Analysis
Dec. 16th, TMAX/TMIN/PRE IAD ?/26/T RIC
?/33/0 ROA ?/35/0 ORF ?/41/0 RDU ?/32/0 CLT
?/38/0 CAE ?/46/0 RDU Climo 53
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00 UTC 17 December, Eta Sea-Level Pressure
Analysis
Dec. 16th, TMAX/TMIN/PRE IAD ?/26/T RIC
?/33/0 ROA ?/35/0 ORF ?/41/0 RDU ?/32/0 CLT
?/38/0 CAE ?/46/0 RDU Climo 53
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12 UTC 17 December, Eta Sea-Level Pressure
Analysis
Dec. 17th, TMAX/TMIN/PRE IAD ?/?/.10 RIC
?/?/.32 ROA ?/?/.33 ORF ?/?/0 RDU ?/?/.39 CLT
?/?/.54 CAE ?/?/.17 RDU Climo 53
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00 UTC 18 December, Eta Sea-Level Pressure
Analysis
Dec. 17th, TMAX/TMIN/PRE IAD ?/?/.10 RIC
?/?/.32 ROA ?/?/.33 ORF ?/?/0 RDU ?/?/.39 CLT
?/?/.54 CAE ?/?/.17 RDU Climo 53
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• Now, verify your predictions for TMax at your
  city
• The next slide will show verification numbers
• Sum the differences for the 3 days between your
  forecast and observed. If your 3-day total error
  magnitude is
• lt5 degrees Take the day off and go tailgate
• 5-25 degrees Stick around but feel free to
  doze off at times
• gt25 degrees Alright Lackmann, you didnt give
  us enough information! Whats your point??

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• Verification

Dec. 15th, TMAX/TMIN IAD 56/31 RIC 63/40 ROA
55/39 ORF 68/48 RDU 63/42 CLT 63/45 CAE 68/47
Dec. 16th, TMAX/TMIN IAD 46/26 RIC 51/33 ROA
51/35 ORF 48/41 RDU 57/32 CLT 57/38 CAE 60/46
Dec. 17th, TMAX/TMIN IAD 58/41 RIC 65/35 ROA
52/35 ORF 70/37 RDU 70/37 CLT 63/44 CAE 75/43
Dry Classical CAD Bark can be worse than
sensible weather bite Two events, similar SLP
patterns, very different impacts Must look
beyond the wedge (in SLP) to assess CADs
sensible weather impact!
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Effects of Cold Air Damming

• Sensible weather impacts
• Sleet, freezing rain, mixed precipitation
• Clouds, fog, and drizzle
• Visibility and ceiling height
• Below-normal temps in damming region
• adiabatic cooling, cold advection, evaporational
  cooling
• Or, next to nothing why?

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CAD Classification

• Next step classify!

Scheme designed with NWS consultation Key
parameters 1.) Location, strength of parent
high 2.) Presence or absence of precipitation at
CAD onset
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Classification based on location, strength of
parent high

• Classical (dry onset)
• High gt 1030 mb
• gt24 h duration
• dry onset

In-Situ

• Classical (Diab. Enh)
• High gt 1030 mb
• gt24 h duration
• precip w/in 6-h of onset

• Weak Onset
• High lt 1030
• or lt 24h duration
• dry onset

• Hybrid
• High lt 1030
• or lt 24h duration
• precip. W/in 6h of onset

• Southern Onset
• high S of 40
• dry onset

In-Situ
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The Extended CAD Spectrum
Classical (Diabatically Enhanced)
Classical (Dry Onset)
CAD Intensity

• Weak/Southern Onset
• dry onset
• high lt 1030 mb or S of 40N
• or progressive

Hybrid

• Unclassified
• solar sheltering

In-Situ
ltlt1
?1
gtgt1
Diabatic
Synoptic
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Compositing

• Composites generated for 6-day period, centered
  at onset, peak, demise (defined by algorithm)
• NCEP Reanalysis Wont resolve CAD, will resolve
  synoptic pattern
• Separated cold season (Oct 15 - Apr 15) from warm
  
• Separated cases by sub-type classification
• http//www4.ncsu.edu/nwsfo/storage/training/cadcl
  imo/

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Classical (Diabatically Enhanced)T-48-T00
Onset-centeredT00 Peak-centeredCDEN
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Classical (Diabatically Enhanced) T-48 Onset
Centered
250-mb Geo. Ht., Isotachs
500-mb Geo. Ht, Anomalies
H
850-mb Geo. Ht., RH
SLP, anomalies
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Classical (Diabatically Enhanced) T-36 Onset
Centered
250-mb Geo. Ht., Isotachs
500-mb Geo. Ht, Anomalies
850-mb Geo. Ht., RH
SLP, anomalies
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Classical (Diabatically Enhanced) T-24 Onset
Centered
250-mb Geo. Ht., Isotachs
500-mb Geo. Ht, Anomalies
850-mb Geo. Ht., RH
SLP, anomalies
43
Classical (Diabatically Enhanced) T-12 Onset
Centered
250-mb Geo. Ht., Isotachs
500-mb Geo. Ht, Anomalies
850-mb Geo. Ht., RH
SLP, anomalies
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Classical (Diabatically Enhanced) T00 Onset
Centered
250-mb Geo. Ht., Isotachs
500-mb Geo. Ht, Anomalies
850-mb Geo. Ht., RH
SLP, anomalies
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Classical (Diabatically Enhanced) T00 Peak
Centered
250-mb Geo. Ht., Isotachs
500-mb Geo. Ht, Anomalies
850-mb Geo. Ht., RH
SLP, anomalies
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Classical (Dry Onset)T-48-T00 Onset-centeredT00
Peak-centeredCDRY
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Classical (Dry Onset) T-48 Onset Centered
250-mb Geo. Ht., Isotachs
500-mb Geo. Ht, Anomalies
850-mb Geo. Ht., RH
SLP, anomalies
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Classical (Dry Onset) T-36 Onset Centered
250-mb Geo. Ht., Isotachs
500-mb Geo. Ht, Anomalies
850-mb Geo. Ht., RH
SLP, anomalies
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Classical (Dry Onset) T-24 Onset Centered
250-mb Geo. Ht., Isotachs
500-mb Geo. Ht, Anomalies
850-mb Geo. Ht., RH
SLP, anomalies
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Classical (Dry Onset) T-12 Onset Centered
250-mb Geo. Ht., Isotachs
500-mb Geo. Ht, Anomalies
850-mb Geo. Ht., RH
SLP, anomalies
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Classical (Dry Onset) T00 Onset Centered
250-mb Geo. Ht., Isotachs
500-mb Geo. Ht, Anomalies
850-mb Geo. Ht., RH
SLP, anomalies
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Classical (Dry Onset) T00 Peak Centered
250-mb Geo. Ht., Isotachs
500-mb Geo. Ht, Anomalies
850-mb Geo. Ht., RH
SLP, anomalies
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CDEN vs. CDRY

• Classical (Diabatically Enhanced)
• E-W elongation of high
• confluence, little ridging _at_ 500
• cold front close by

• Classical (Dry Onset)
• N-S elongation of high
• confluence AND ridging _at_ 500
• cold front far offshore

20 cases
70 cases
Onset time
Onset time
Sea level pressure
Legend
500 mb height
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CDEN CDRY
850-mb Geo. Ht., RH
850-mb Geo. Ht., RH
SLP, anomalies
SLP, anomalies
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HybridT00 Peak-centeredHYBR
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Hybrid (HYBR) T00 Onset Centered
250-mb Geo. Ht., Isotachs
500-mb Geo. Ht, Anomalies
850-mb Geo. Ht., RH
SLP, anomalies
57
Weak/Southern OnsetT00 Peak-centeredWKDR
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Weak/Southern Onset (WKDR) T00 Peak Centered
500-mb Geo. Ht, Anomalies
850-mb Geo. Ht., RH
SLP, anomalies
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In-Situ T00 Peak-centeredINST
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In-Situ (INST) T00 Peak Centered
250-mb Geo. Ht., Isotachs
500-mb Geo. Ht, Anomalies
850-mb Geo. Ht., RH
SLP, anomalies
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CAD Impact Data
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-Departure from Average Maximum Temperature (at
GSO, 1st 2 days of event)
Cold Season Events
8.4
6.7
6.6
6.7
1.7
Stan. Dev. cases
19
71
17
31
15
6
63
Cold Season Events
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Average Event-Total Precipitation by Sub Type
65
Average CAD duration by Sub Type
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Composites Based on Impact?

• We took 90 CLASSICAL CAD events (CDEN CDRY)
• Two categories
• GSO Tmax gt 15F below climo (high-impact)
• GSO Tmax lt 2F below, or above climo (low)
• Built composites to seek differences
• What do you expect to be the difference?

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CAD is Not a Monolithic Phenomenon!
Sea level pressure, anomaly
HIGH-IMPACT CAD
LOW-IMPACT CAD
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Conclusions

• Sub-types consistent with NWS Classification
• 2 additional sub-types, lumped into WKDR
• Warm season CAD more frequent than expected
• Strong events more frequent in cold season
• CAD impact strongly dependent upon clouds,
  precipitation, forcing for ascent- key
  distinction?

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Conclusions (cont.)

• Sea-level pressure does not tell the whole CAD
  story!
• Key upper-air features
• Upper jet dominates precipitating high-impact
  cases
• 500-mb ridge west of CAD region in dry onset
  composite
• 850-mb ridge axis over CAD region in dry onset
  composite
• Sfc. High axis orientation consistent with upper
  dynamics
• For details, see Bailey et al. 2002 WAF paper,
  reprints available

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Conclusions (cont.)

• Bottom Line For High-Impact CAD look for
• Upper dynamics, forcing for ascent (jet
  entrance/exit circulations, warm advection,
  isentropic upglide, etc.)
• Moist lower atmosphere
• Lower-tropospheric ridge axis east of damming
  region
• High impact generally accompanied by clouds,
  precipitation at onset
• Consider observational signals, be wary of
  reliance on model QPF, soundings, etc.

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Thanks to

• Chris Bailey (now at HPC) did much of work for
  his M.S. thesis
• Kermit Keeter and others for original ideas,
  foundation of research
• Gail Hartfield for lots of work on sensible
  weather climatology
• Scott Sharp for case summaries, invaluable for
  algorithm testing
• Doug Schneider for careful examination of CAD
  composites
• Jonathan Blaes for web support, endless technical
  assistance
• NWS CSTAR offices (esp. Neil Stuart, Larry Lee)
  and others
• NOAA/NWS CSTAR program (HQ, ER), Jeff
  Waldstreicher
• NCSU Al Riordan, Lian Xie

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CDRY Upstairs CDEN
250-mb Geo. Ht., Isotachs
250-mb Geo. Ht., Isotachs
500-mb Geo. Ht, Anomalies
500-mb Geo. Ht, Anomalies
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Precipitating Vs. Dry CAD

• Original CAD sub-types identified in 10-yr
  climatological sample
• Major distinction dry onset versus
  precipitating CAD
• For details, see Bailey et al. 2002 paper (WAF,
  reprints available)

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Are we splitting hairs with these distinctions?

• Stratified classical CAD cases by impact on Tmax
• One composite contains cases with Tmax 15F or
  more below climatology at GSO (high impact)
• The other cases where Tmax was 2F or less below
  climo, or above climo at GSO (low impact)
• Can you tell which is which?