CAD Forecast Tips
Many of the following forecast tips were derived through observations gathered from
the nearly 50 case studies conducted during the CSTAR project. Routine hand analysis
of surface and standard level upper air data along with an acute awareness of trends in
observed data are vital for recognizing and following CAD evolution. Using real time
data to monitor the evolving meteorological features, their impact upon sensible
weather, and their responses to physical processes are critical to forecasting CAD and
for noting potential errors in the NWP solutions.
CAD Surface features:
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Awedge@ signature is not always associated with CAD sensible weather. Skies
may be mostly sunny and daytime temperatures near seasonal norms.
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long & skinny, north to south high pressure ridge axis are most often associated
with fast moving migrating surface highs whose impact on sensible weather is
limited.
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large concentric shaped surface highs whose ridge axis extend west to east and
then north to south down the eastern seaboard are typically associated with the
more prolonged CAD events & have a greater impact on sensible weather.
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CAD=s Awedge@ signature is typically first seen as a ridge of high pressure
(Adry air ridge@) building southward. into the eastern portions of Virginia and
North Carolina. The Awedge@ then develops as the Adry air ridge@ slides
westward, typically 6 - 24 hours later. Precipitation west of the dry air ridge
accelerates the wedge building process.
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Canadian high pressure systems first entering the US will build southward into
Texas or eastward toward New England. Those highs building eastward are the
ones most likely to produce CAD. Highs building southward may produce CAD, if
the center of the parent high migrates northeastward from the southern plains
through the Ohio Valley toward the northeastern states.
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the coldest, driest, and deepest cold air is located beneath surface high pressure
ridge axis.
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surface highs whose centers are located south of 40 degrees are less likely to
produce CAD, typically requiring sufficient precipitation falling into substandially
surface based dry air to do so.
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Miller type AB@ cyclogenesis (2 lows, 1 inland & 1 coastal) is associated with
strong CAD. This pattern is capable of producing corridors of predominant p-
types in VA/NC/SC.
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A APiedmont-Foothills Low@ in the lee of the Blue Ridge Mountains of the
Carolinas results in part from low level convergence associated with the Adry air
ridge@ to the east and westerly winds advecting cold air around the southern end
of the Appalachian Mountains. Ohandley and Bosart in Mon Wea Rev showed
how the Smokies often splits an eastward moving cold front into a AS@ shape
pattern while infrequently a surface low develops in northwest SC and/or
southwest NC along the split front. This infrequent pattern of cyclogenesis is
seen with a strong parent surface high located far to the northwest (e.g.,
Dakotas). Well defined axises of surface high pressure extend eastward and
southward producing a confluent flow in the lee of the Appalachians. This small
scale low can enhance wintery precipitation amounts, especially in the Upstate of
SC and the foothills and far western Piedmont of NC..
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Some CAD events are prolonged due to secondary centers of high pressure
building into the same geographical area occupied by the original parent high
before it migrated offshore.
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Surface highs whose centers are located offshore and are south of 40 N do not
contribute to supporting the associated Awedge@ connected to them.
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Weak surface highs north of 40 N can potentially contribute to the support of a
Awedge@ which resulted from diabatic cooling. In some of these cases as CAD
develops, the surface winds back sufficiently to advect cold air into the
preexisting Awedge@.
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CAD Features Aloft:
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confluent flow at 500 mb anchors CAD=s parent surface high and prolongs the
CAD event
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dual upper level jets enhances CAD. A low level ageostrophic circulation
between the two jets increases cold air advection into the damming region. An
active southerly jet is a source of moisture transport and lift which also enhances
CAD.
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southwest flow just above the surface based cold air inversion is an effective
means of moisture transport strengthening CAD=s surface based inversion. The
southwest flow is typically associated with a well defined mid level trough in the
southern plains upstream of an axis of high pressure extending from the Ohio
Valley southeastward to off the coast of the Carolinas.
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warm season CAD is often characterized by a northwest flow aloft.
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cold front aloft (CFA) will first enhance CAD if the precipitation it generates falls
into subcloud dry air; once saturated a CFA contributes to erosion via latent heat
release and cold advection aloft.
Impact on CAD Sensible Weather:
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typically cool season CAD produces 10-15 degree F departures from normal
daytime high temperatures; the stronger CAD events result in 15-20 F degree
departures or even 30 degrees F when accompanied by snow; weak CAD events
typically result in departures of 3-5 degrees F from daytime normals.
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warm season CAD may also produce 10-20 F degree departures from normal
daytime high temperatures.
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warm season CAD diminishes torrential rains by suppressing deep tropical
moisture and its associated surface boundary south and east of the CAD region
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strong CAD well entrenched is associated with Miller Type AB@ cyclogenesis (2
surface lows, 1 inland, 1 coastal) and often results in well established corridors of
predominant p-types.
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insitu or weakening hybrid CAD events can be associated with severe weather
provided there is an active southerly jet, moisture transport, & instability. CAD=s
inland frontal boundary is the source for local helicity and shear.
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CAD events without moisture transport features often result in mostly sunny
skies and near normal daytime high temperatures.
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CAD sensible weather (i.e., overcast skies, low stratus, fog, drizzle, and below
normal daytime temperatures) can linger in the western and central portions of
the CAD region 12-36 hours after the wedge signature losses definition and is
replaced by the appearance of a broad Abaggy@ ill defined surface high.
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CAD erosion via a coastal low is marked by little change in sensible weather.
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CAD erosion via a scouring cold front is usually accompanied by a rise in surface
temperatures.
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Strong and deep CAD followed by a digging upper level short - wave without
significant isentropic lift in advance may limit precipitation amounts and coverage
in the western sections of the Carolinas and Virginia east of the mountains.
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CAD Erosion:
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seldom do CAD events erode via a surface low moving northeastward through
the CAD region.
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surface lows typically move around the peripheral of the CAD wedge. An
exceptional strong low and/or a quickly retreating CAD high is required for
erosion via a warm frontal passage/southwest low scenario.
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CAD erosion via a strong and deep southwest low will also likely be
accompanied by severe weather.
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CAD erosion via the inland movement of a coastal front seldom occurs west of
the central Piedmont of North Carolina. Areas to the west are usually not eroded
until CAD is scoured out by a cold frontal passage from the northwest.
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the loss of a Awedge@ signature followed by the appearance of an ill-defined
lingering Abaggy@ high signals a very shallow surface based CAD airmass is
persisting.
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with a surface low passing northwest of the CAD region, erosion in eastern
Carolinas will likely occur as the coastal front moves inland; erosion in the
western Carolinas most often must wait until CAD is scoured out by a cold frontal
passage
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CAD Model Errors;
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NWP models typically capture the onset of CAD events more accurately than
CAD erosion.
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MOS is most often premature (usually by 1-2 days) in warming daytime CAD
temperatures back to or above seasonal norms.
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Historically, NWP models have been prone to erode CAD via warm frontal
passage ahead of a southwest low moving northeastward through the CAD
region.
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The principal model error associated with warm season CAD has been in its qpf
placement - too much rainfall within the cool air dome.
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MOS errors have historically been largest for erosion via bottom-up by solar
heating as seen with the Awedge@ evolving to a Abaggy@ high scenario. Recent
revisions to the eta may improve its Asunshine@ bias.
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Recent changes to the eta may decrease its tendency to force coastal/wedge
fronts too far inland and warm fronts too far north into the CAD region.
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Models tend to handle erosion via cold frontal passage and coastal low passage
better than erosion via surface low passing to the northwest, or a surface low
approaching the CAD region from the southwest. With the northwest low, the
inland movement of the coastal/wedge front is typically overdone, while
southwest lows are too often driven through the teeth of the CAD wedge
preceded by warm frontal passage that is forecast too far north.
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CAD Forecast Process Using Observed Data:
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monitor the characteristics of the potential CAD=s parent high well upstream;
single digit dew points are especially indicative of an airmass subject to
producing a CAD event as well as the potential for wintry precipitation types.
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closely follow RAOBS to evaluate the depth of surface based cold air, the vertical
extent of the dry air (potential for evaporative cooling), and the strength of
CAD=s inversion.
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3 hourly hand surface analysis provides continuity and detail for following the
evolution of CAD - location of parent high, strength of parent high, development
of Adry air ridge@, development of Awedge@, changes to the wedge@,
development of meteorological features signaling erosion, etc.
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routine hand upper level analysis provides continuity and detail for following key
upper level features important to CAD - (e.g., confluent flow aloft, location of 850
mb ridge axis, dual or single upper level jets, upstream upper level short-waves,
etc).
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using objective analysis, monitor the lifted indices (e..g., MSAS) to see CAD=s
boundary and changes to CAD=s location and strength.
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closely follow visible satellite imagery for possible cloud breaks and low level
CAD boundary movement.
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wind profiles near CLT and RAH can show hourly changes in CAD=s wind profile
signaling changes to the strength of CAD=s inversion or other possible erosion
mechanisms (e..g. shear).
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monitor radars for precipitation returns. CAD induced or enhanced by diabatic
cooling will show precipitation returns which lowers surface temperatures,
increases surface pressure followed by backing winds at reporting stations.
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monitor the airmass for saturation; typically most CAD air masses become
saturated after 0.10 - 0.25 inches precipitation has fallen.
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timing of short-waves are crucial for insitu CAD; precipitation must arrive quickly
falling into the dry air deposited by the retreating parent high before dew points
are modified by a return onshore flow.
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Use TRENDS= predominant p-type nomogram to analyze CAD=s evolution.
When used with model data, TRENDS= is a quick and efficient means to
diagnose the physical processes driving the models solutions for CAD and
cyclogenesis.