Cold Air Damming Forecast Tips

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Cold Air Damming Forecast Tips

• A Training Presentation from
• NWS Raleigh, NC
• and
• North Carolina State University
• Presentation by Gail Hartfield Scott Sharp, NWS
  RAH

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Whats the big deal?

• All CAD events are NOT created equal
• Some have a big impact but some dont (doesnt
  necessarily depend on high strength)
• CAD effects are HUGELY precip-dependent
• High-impact events typically contain precip near
  onset
• Models dont do well
• Capture onset but do poorly with erosion
• Erode CAD bottom-up rather than from above
• QPF must be right!

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Whats the big deal?Your forecast is at stake,
thats what!

• During CAD, grid population with model data just
  wont do the job!
• Particular signals can help guide you to a more
  correct solution than what the models depict
• Recognizing these signatures ( having a thorough
  understanding of the processes represented) can
  improve your forecast of temps, precip type,
  wind, etc. (very important if wintry weather is
  involved!!)

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The Testimonial(Sing it, brother!)

• It worked for me it can work for you, too
• Need to monitor real data
• Know your air mass
• Know what to do with MOS and the models

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Part 1 Surface Features
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A good understanding of the basic process of
damming is essential.
In the forecast Make sure all forecasters
understand how CAD develops.
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(2) A wedge signature doesnt always mean cool
and cloudy.

• Highs can (and do) ridge down the east slopes
  with little effect on sensible weather
• Skies can be sunny, with near-normal highs
• A big key is what is happening upstairs

Surface
In the forecast Dont jump on CAD conditions
with every ridge that noses down the East Coast.
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(3) Fast-moving highs with little sensible
weather impact most often appear as long, skinny
north-to-south oriented ridge axes.
Surface

• Why?
• High often supported by a ridge atop it
• Strong southward extension of high suppresses
  moisture to south thus less impact

In the forecast Lean toward shorter-duration CAD
with temps 2-5F below normal in the damming
region.
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(4) More rounded, west-to-east oriented ridge
axes often indicate more prolonged CAD with a
greater impact.

• Why?
• High often supported by (and elongated by)
  confluent flow aloft
• Less ridging ( stronger srn stream low) at 850
  mb low level jet brings moisture

In the forecast Lean toward longer-duration CAD
with temps 10F below normal in the damming
region.
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(5) The coldest, deepest, and driest air is
located beneath the surface ridge axis.
Unadjusted
(from Bell and Bosart, 1988)
In the forecast Adjust your temp grids spatially
according to climatology for CAD events.
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(6) The wedge signature typically begins as a
dry air ridge in eastern VA/NC, which then
slides west toward the east slopes 6-24 hours
later.
06Z 03/17/02
In the forecast Recognize this sign of an
impending CAD event and adjust 1st period temps,
dewpoints, etc. accordingly.
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(7) Highs centered SOUTH of 40N are less likely
to produce CAD with significant sensible weather
effects (needed moisture is suppressed south).
In the forecast In the absence of significant
lift mechanisms, keep high temps less than 8F
below normal when the CAD high is south of 40N.
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(8) Canadian highs that build east into New
England are more likely to produce CAD than those
that build south (toward TX) then move to New
England.
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Surface maps, September 20-23, 2000
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Surface maps, September 20-23, 2000
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Surface maps, September 20-23, 2000
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Surface maps, September 20-23, 2000
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Surface maps, September 20-23, 2000
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Surface maps, September 20-23, 2000
In the forecast Check the characteristics of the
potential CAD high the air mass may modify
before it builds into NC. This may result in a
short-duration and/or less-intense event.
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Part 2 Features Aloft
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(9) Anchors for CAD high include confluent 500
mb flow and a strong 500 mb ridge.
500 mb

• This allows for prolonged cold low-level dry air
  advection
• If precip is present, a persistent feed of dry
  air will allow continued evaporative cooling a
  longer event

In the forecast Beware of model forecasts of
fast-moving surface ridges when an anchor aloft
is present. If the anchoring mechanism will lead
to prolonged dry air advection, increase the
events duration.
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Dual upper jets enhance CAD. Pay attention to
your southern stream!

• Polar jet helps to anchor surface high
• Subtropical jet provides additional dynamic lift
  for supporting precip

In the forecast Watch for a favorable upper jet
structure that may enhance CAD. In particular,
track observed jets and any lift (precip) they
are producing.
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(11) Southwest flow just above the surface can
greatly enhance CAD.

• Usu. a trough over southern plains
• Warm air advection atop cold dome enhances
  inversion
• Flow brings moisture from gulf
• Lift can be produced by low level convergence
• Isentropic upglide can enhance precip

In the forecast If you see this regime
developing (especially in real time) with a CAD
supporting surface high, you can confidently
increase cloudiness and further reduce temps. If
the surface high is providing continuous dry air
advection to keep evaporative cooling going, then
increase the events duration.
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(12) A cold front aloft (CFA) can enhance CAD,
then can contribute to its erosion.

• CAD is enhanced if CFA precip falls into dry
  subcloud layer

February 12, 2000
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(12) A cold front aloft (CFA) can enhance CAD,
then can contribute to its erosion.
Image Eta 850 mb temp advection Valid 12Z/18Z
12/05/02

• CAD is enhanced if CFA precip falls into dry
  subcloud layer
• Once saturated, a CFA can help erode CAD via
  latent heat release and cold air advection aloft

In the forecast Models dont forecast CFAs
their effects well, so monitor upper air data and
examine existing precip to determine if a CFA
exists. Keep temps down if CFA precip is
arriving. Afterward, monitor cold air advection
atop the cold dome.
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(13) Warm season CAD often features northwest
flow aloft.
Surface analysis
May 17, 2001 12Z Max T at GSO 60 (Normal
high 77)
In the forecast In the warm season, with an
upper ridge axis to the west and strong trough to
the northeast, be alert for potential damming
highs and short-lived but strong CAD events.
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Part 3 Impact on Sensible Weather
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Sensible Weather Impacts

• Cool season CAD departure from normal daytime
  highs
• 10-15 F on average
• 15-20 F in strong events
• 3-5 F in weak/cloudless events
• Warm season CAD departure from normal highs can
  be 10-20F
• CAD-produced cloudy skies cool temps will
  linger in western portions of the damming region
  12-36 hours after the distinct wedge signature
  has evolved into a baggy ill-defined high
• CAD events lacking moisture transport generate
  little impact (abundant sunshine near-normal
  temps)

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Sensible Weather Impacts

• CAD featuring a Miller B pattern (two lows
  primary and coastal) is often very strong
  contains corridors of predominant precip type
• In situ and weakening events can be associated
  with severe weather, given an active
  southern-stream jet, wind shear in the low
  levels, and sufficient moisture
• CAD erosion via a coastal low may initially show
  little change in sensible weather (except a wind
  shift to the north)
• CAD erosion via a scouring cold front often
  features warmer surface temperatures behind the
  front

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Sensible Weather Impacts

• Warm season CAD limits or shuts off torrential
  rains by suppressing deep tropical moisture
  south/east of damming region

4/10/03 00Z Eta 300 mb analysis
4/09/03 00Z Eta MSLP 12-hr QPF valid 00Z
4/11/03
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Surface fronts radar summary, 12Z 7/30/03
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7/30/03 00Z Eta MSLP 6-hr QPF valid 18Z 7/30/03
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7/30/03 00Z Eta MSLP 6-hr QPF valid 00Z 7/31/03
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1955 UTC radar mosaic and surface observations
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2025 UTC radar mosaic and surface observations
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2055 UTC radar mosaic and surface observations
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Part 4 Model Errors
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Model Errors

• Models typically handle the onset of CAD better
  than its erosion
• Model output statistics (MOS) guidance is often
  premature in warming daytime CAD temps back to
  seasonal norms (miss the baggy high
  residual-CAD transition period)
• Model QPF is usually too high in the cold dome
  during warm-season CAD (dry air holds needed
  moisture out of the region)
• Eta problem of sun shining through clouds
  (which hastened bottom-up CAD erosion via solar
  heating) has been improved
• Historically, with a surface low moving southwest
  to northeast just west of the area, models have
  been too eager to bring a warm front north
  through the CAD dome

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CASE STUDY
Surface map Jan. 19, 2001 12Z
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CASE STUDY
Surface map Jan. 19, 2001 15Z
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CASE STUDY
Surface map Jan. 19, 2001 21Z
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CASE STUDY
Surface map Jan. 20, 2001 06Z
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CASE STUDY
Surface map Jan. 20, 2001 16Z
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CASE STUDY
AVN MOS high temperatures Valid Jan. 18 and 19,
2001
Actual Highs at GSO Jan. 18 - 42 Jan. 19 -
49 daytime Actual Highs at RDU Jan. 18 -
46 Jan. 19 - 73
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Eta analysis, valid Jan. 18, 2001 - 00Z
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Eta analysis, valid Jan. 18, 2001 - 12Z
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Eta analysis, valid Jan. 19, 2001 - 00Z
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Eta analysis, valid Jan. 19, 2001 - 12Z
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Eta analysis, valid Jan. 20, 2001 - 00Z
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Eta analysis, valid Jan. 20, 2001 - 12Z
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Water vapor imagery, Jan. 19, 2001
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Model Errors Erosion Types

• Models handle erosion via cold frontal passage
  and a coastal low better than they handle erosion
  via surface low passing to the northwest and
  surface low moving southwest-to-northeast through
  the area
• Low passing to northwest Can bring warm front
  too far NW
• Low moving southwest-to-northeast Often try to
  drive the low straight through the heart of the
  wedge a very rare occurrence

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Part 5 Monitoring Real Time Data
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Monitoring Real Time Data for CAD

• Surface maps hand-drawn analyses are important
  for pinpointing and tracking small-scale CAD
  evolution features (e.g. surface dry air
  advection)
• Upper air analyses determine degree of warm air
  advection ( reinforcement potential) atop cold
  dome (e.g. 850 mb) assess upper jet structure
  determine system movement identify any anchors
  for surface high
• Soundings, profilers check evolving thermal
  profiles (especially T vs. wet bulb to assess
  evaporative cooling potential) and wind profiles
  (monitor dome depth and shear atop dome)
• Satellite imagery visible/IR/fog channels to
  assess low cloud deck water vapor to assess
  strong mid level subsidence that may contribute
  to erosion, and to track shortwave troughs and
  upper jets
• LAPS MSAS monitor stability parameters
  (increasing/ decreasing), surface streamlines
  (confluent/ diffluent flow), boundaries
• Regional radar track precip that could
  strengthen cold dome

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Monitoring Real Time Data Indications of
Potential CAD Erosion

• Surface flow no longer uniform northeast
• High no longer in a favorable location to provide
  support (and dry air advection ends, thus ending
  contribution by evaporative cooling, even if its
  still precipitating)
• Warm front steadily advancing from south with no
  impediment (but make sure air mass to south can
  compete with air mass to north)
• Sounding shows a dropping inversion top
• CFA incoming with strong cold air advection at
  warm-nose level
• Coastal cyclone spinning up close to shore
• Cold frontal passage imminent
• Holes developing in cloud cover

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Thank you for your time and attention!
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Sensible Weather Impacts

• Warm season CAD limits or shuts off torrential
  rains by suppressing deep tropical moisture
  south/east of damming region

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(13) Warm season CAD often features northwest
flow aloft.