Transcript Santa Ana Winds

Santa Ana Winds
Looking east toward
Santa Ana Canyon along US-91
WHAT IS A SANTA ANA WIND?
“Santa Ana” wind is a local name given to a weather
condition in which a migratory high pressure system
moving over the Great Basin (Nevada & Utah) forces a
hot dry foehn-type wind southward. These winds flow
from the Desert Southwest, funnel through the passes
and canyons of SOCAL and out over the SOCAL OPAREAS.
As the air is forced down through these passes and
canyons it heats due to atmospheric compression, at the
dry adiabatic lapse rate, at approx. 5°F per 1000 FT.
Weather conditions during a Santa Ana event are
characterized by an increase in temperature (well
above the seasonal average), a decrease in the dew
point (relative humidity, commonly < 10%) and strong
gusting winds during early AM hours (0400-1000L).
Gusts exceeding 60-70 KTS (70-80 mph) are common in
the inland valleys of SOCAL (aft sunset thru midmorning).
WHAT IS A FOEHN-TYPE WIND?
Definition: A warm, dry wind which flows down the lee
side of mountain ranges, adiabatic compression warms and
dries the air as it descends in the atmosphere. Santa
Ana winds are already relatively dry before they descend
the MTS. WHY?
Note: As discussed, the air will heat at approx. 5°F
per 1000 FT. The MTS ranges of SOCAL average about
3-5 KFT. Using this “rule-of-thumb”, air descending
these MTS will warm 15-25°F by the time they reach the
mesa and coastal regions.
4000 to 5000 FT
3000 to 4000 FT
SOCAL
WHERE IS THE GREAT BASIN?
AND WHY ARE LOOKING FOR A HIGH IN THIS REGION?
The Great Basin is the
high plateau east of
the Sierra Nevada MTS
and west of the Rocky
MTS including most of
Nevada and Utah.
H
From the NPMOC-SD
FCSTRS Handbook: The
primary synoptic
feature associated
with Santa Ana
conditions is the
Great Basin High.
BOTTOM LINE: WITHOUT THE OUTFLOW FROM A HIGH LOCATED
SOMEWHERE OVER THE GREAT BASIN, SANTA Ana CONDITIONS CAN
NOT EXIST.
WHERE IS
SANTA Ana CANYON?
WHAT EFFECTS DO SANTA Ana WINDS
HAVE ON DAILY OPERATIONS?
AIRCRAFT OPERATIONS: Santa Ana wind speeds usually
increase rapidly with height, creating STRONG LOW
LEVEL WIND SHEAR and TURBULENCE. Slant Range VSBY is
effected when haze/dust is lift aloft.
LOCALIZED HIGH WIND/SEAS: Over the Northern SOCAL
OPAREAS it is common to experience areas of High
Wind/Seas from 0400-1000L. Highest probability,
VCNTY KNTD.
HEAT STRESS: Due to the increase in temps and decrease
in relative humidity (dew point), the perceived heat
stress on the body will increase.
FIRE HAZARDS: Again, due to the decrease in the
relative humidity and an increase in wind speed, the
chance for wild fires increase. During the October
2003 San Diego Wild Fires, Santa Ana’s were the
initial cause for the rapid loss of containment.
October 2003
San Diego Wild Fires
WHAT TIME OF YEAR SHOULD WE BE MOST
CONCERNED ABOUT THESE CONDITIONS?
Santa Ana conditions can occur at anytime during the year
BUT are most frequent from early Fall to late Spring, with
a peak frequency during the Winter months.
NWS: Santa Ana winds commonly occur between October and
February with December having the highest frequency of
events.
From the NPMOC-SD FCSTRS Handbook: Winter is the time of
year when air masses move into the Great Basin most
frequently.
CLIMO: During the Summer, Santa Ana’s are most often
caused by the EASTPAC High (mT) ridging in over the Great
Basin. During the Winter, migratory highs (mP and cP)
generate these conditions. During the early/late Summer
there is still a chance that a migratory high could move
over the Great Basin if the conditions are right (i.e. a
weak EASTPAC High well off the coast and a strong push of
cold air over the Gulf of Alaska).
THE UPPER-LEVEL RIDGE
1. Nearly every Great Basin High has the vertical
support of a pronounced upper-level ridge situated
over or just off the West Coast (upstream) which feeds
subsiding air into the low level outflow.
2. The stronger the ridge, the stronger the SFC high
will be. This will enhance the out-flow from the high
and increase the winds along the coast.
3. When/if the long wave ridge becomes stationary over
the West Coast, short wave troughs moving through the
upper level flow may temporarily flatten it. These
short wave troughs ”sometime” support surface fronts.
4. The ridge and high usually rebuild sharply after
frontal passage resulting in the establishment of
Santa Ana conditions.
5. In addition to a strong ridge over the West Coast
a deep trough over Central USA (downstream), is
favorable for Santa Ana conditions.
500 mb Chart
ANAL
Location of
SFC High
564
570
H
1028
576
Look for a pronounced upper-level ridge
located just off/over the West Coast. This
ridge is the key support for the Great Basin
High. If the ridge begins to move eastward
during your forecast period, expect
decreasing Santa Ana Conditions. WHY?
500 mb Chart
24 HR PROG
Location of
SFC High
564
570
H
1032
576
582
As the upper-level ridge builds, what should
happen to the SFC high and what will happen
to the conditions along the SOCAL Coast?
500 mb Chart
48 HR PROG
Location of
SFC High
564
570
576
582
As the upper-level ridge begins to shift to
the east what should happen to the SFC high
and along the SOCAL Coast?
H
VARIATIONS IN SANTA Ana EVENTS
The sustained wind speeds and gusts of Santa Ana
winds are highly variable! The intensity and
trajectory of the high moving into the Great Basin,
orientation of pressure gradients and the effects of
local topography greatly effect conditions along the
coast.
CLIMO: KNZY extreme gust occurred in January, 55 KTS.
As already stated, along the SOCAL coast Santa Ana’s
usually generate a NE-E wind, variations can occur
based on the above effects depending where you are
located in the OPAREAS.
Over El Centro, CA, winds are frequently N-NW.
WHY?
KNJK
FRONTAL SYSTEMS
(FROM CLIMO AND NPMOC-SD FCSTRS Handbook)
Normally, by the time a front reaches SOCAL, it has
weakened to the point that it is difficult to define the
SFC location except for a few clouds with little/no
rain.
Although fronts may be weak and non-discernible, an
upper-level trough in the westerlies often marks the
forward progress of the new air mass.
“DRY” Santa Ana: A “DRY” Santa Ana is characterized by
a cP (or VERY modified mP or mT) air mass moving into
the Great Basin, RH approx. < 10%.
“WET” Santa Ana: A “WET” Santa Ana is when a mP or mT
air mass moves in over the Great Basin, RH around 30%.
Weakening cold fronts
usually move in from
the NW-WNW.
This is indicative of
a NW to W flow aloft.
The lag BTWN FROPA
and onset of Santa
Ana’s is normally 1236 HRS, depending on
the speed of the
front and strength of
the high.
This is the amount of
time needed for the
high to move into the
Great Basin and a
sufficient pressure
gradient to setup.
H
L
The lag BTWN fronts MOVG
over SOCAL from N to S is
usually 6-18 HRS.
The associated air mass is
usually mP or cP.
Highs MOVG into the Great
Basin from the north are
usually stronger and
produce more intense
conditions. These highs
take less time to build a
pressure gradient. WHY?
The timing of onset of
Santa Ana’s is more
complicated because these
fronts move slower and
have less “visible”
weather along the front.
H
Fronts that approach
SOCAL from the east
are referred to as
"Backdoor" fronts.
Although infrequent,
they occur when the
front runs along the
backside of the
Sierra Nevada MTS.
Associated high
pressures are almost
always cP.
By the time the
front moves over
SOCAL, the high has
already setup over
the Great Basin,
onset begins with
FROPA.
H
Occasionally the high
sets up a pressure
gradient strong enough
to push the thermal
trough out over the
SOCAL OPAREAS.
It is during this
scenario that the SOCAL
Coastal Regions and
OPAREAS would experience
strong sustained NE
winds. WHY?
These winds tend to
inhibit the Sea Breeze
from setting up along
the coast.
After sunset, as the
land cools, the thermal
trough weakens and it
may or may not
reestablish over land.
H
“RULES-OF-THUMB”
KDAG
4-6 MB BTWN
KLAX & KDAG
10-12 MB BTWN KNZY
& GREAT BASIN
KLAX
2-5 MB BTWN
KNZY & KNJK
KNZY
KNJK
“RULES-OF-THUMB” (cont.)
Max wind speeds occur most frequently in early to mid
morning as the Santa Ana’s are enhanced by the land
breeze.
Average max gusts are approx. x2 the maximum sustained
wind speed (other local effects may come into play).
Conditions usually last about 24 to 30 hours with
longer than average periods occurring in NOV - JAN.
A blocking high over the East Coast will prolong the
event and may allow for stronger pressure gradients to
setup BTWN the Great Basin and SOCAL.
The strongest potential for Santa Ana’s to begin is
BTWN 0300 to 0900L.
The most frequent time for Santa Ana’s to end is from
1100 to 1300L.
“RULES-OF-THUMB” (cont.)
Santa
above
above
least
Ana’s cause daily MAX TEMPS to be about 15°F
the average during the Winter and about 15-20°F
normal during the early Fall and Spring with at
one occurrence each Fall of MAX TEMPS of > 90°F.
Santa Ana conditions are almost always preceded by a SFC
front or mid-level trough, usually it is a trough.
Care should be taken in predicting Santa Ana conditions
beyond the Inner SOCAL OPAREA and along the extreme
SOCAL Coastal areas and Baja California.
NOTE: The Inner SOCAL
OPAREA is defined by a
line drawn from PT
Conception thru the
Channel Islands to
Ensenada, MX.
WHAT HAPPENS AFTER A SANTA Ana?
As the Great Basin high begins to weaken and move
eastward the Desert Southwest begins to heat, leading
to the reformation of the Thermal Trough, and the
offshore pressure gradients over SOCAL decrease and
then reverse to onshore.
Since the decrease is gradual, Santa Ana’s do not end
as dramatically as they begin.
With the return of the cooler, moist onshore flow at
the SFC, a strong inversion is immediately
reestablished along the coast. This is conducive to
dense fog formation along the SOCAL Coastal Regions.
Dense fog can be expected during the first 48 HRS
after the high moves out of the Great Basin UNLESS the
high rapidly moves off to the east, moves S-SE toward
SOCAL or a upper-level trough moves into the region.
QUESTIONS?
Lets see what YOU learned!
GOES 12 (WEST) VIS
DTG: 18Z 28MAR04
CASE STUDY
1. Based on this
Satellite image and
the following 2
charts, how would
you start your METOC
discussion?
NOTE: The arrow indicates
the FCST-ed MOVMT of the
high for the next 24 HR
period.
2. What “rules” are
you going to use?
3. What
impacts
foresee
next 24
METOC
do you
over the
HR period?
TAKE YOUR TIME!
Use what you have
learned.
500MB HGT/WND/VORT 24 HR PROG
VALID: 18Z 29MAR04
SFC 24 HR PROG
VALID: 18Z 29MAR04
GOES 12 (WEST) VIS
DTG: 18Z 28MAR04
(Blow-up)
Which “Rules” Work?
Remember these?
1. 2-5 BTWN KNZY & KNJK
2. 10-12 BTWN KNZY & G/B
3. 4-6 BTWN KLAX & KDAG
Where is the Thermal
Trough? WHY?
CASE STUDY
DISCUSSION: Strong high pressure over the
Oregon/Idaho board is generating off
shore flow over the SOCAL Coastal Region.
Why not Santa Ana Winds? The “Rules-of-Thumb” 10-12 MB
BTWN KNZY and the Great Basin worked.
DID YOU REMEMBER: “Santa Ana” wind is a local name
given to a weather condition in which a migratory high
pressure system moving over the Great Basin (Nevada &
Utah)...
The high is over the Oregon/Idaho board, this is too
hard north to be Santa Ana.
The reason this high is being described as “strong” is
because it has caused the Thermal Trough to be
repositioned over the Coastal Regions, those the clear
skies along the Coast of California.
WHAT METOC EFFECTS WOULD YOU FCST?
AIRCRAFT OPERATIONS: LGT (maybe OCNL MOD) TURB.
Remember the MAIN reason for TURB and Wind Shear is the strong NE
wind coming across the MTS. We are under the Thermal Trough.
LOCALIZED HIGH WIND/SEAS: NO
Again, being under the Thermal Trough the SOCAL OPAREAS will NOT
experience the strong NE winds that would cause these conditions.
HEAT STRESS: Would increase!
On 29MAR04, temps across SOCAL were approx. 20°F above average.
Most inland observation sites recorded temps above 90°F.
FIRE HAZARDS: Would increase!
Even with temps in the 90’s across most of the county, the Fire
Hazard only went from “Low” to “Moderate”. The reason was
because of the higher than average rainfall during the month of
February.
The NEXT DAY!
1600L 29MAR04, the 500 MB
ridge is continuing to
move off to the east.
0001Z 30MAR04
As the high continues to
move off to the east the
Thermal Trough is now
returned to being located
over the Central Valleys
and High Desert.
NOTE: Notice the
indication of
moisture in the
upper levels
SKEW-T 1200Z 30MAR04
The next morning this
is what the balloon
sounding over the SOCAL
OPAREA looked like.
What is your FCST based
on this sounding?
(SFC Based inversion?)
WHAT HAPPENS AFTER A SANTA Ana?
As the Great Basin high begins to weaken and move
eastward the Desert Southwest begins to heat, leading
to the reformation of the Thermal Trough, and the
offshore pressure gradients over SOCAL decrease and
then reverse to onshore.
Since the decrease is gradual, Santa Ana’s do not end
as dramatically as they begin.
With the return of the cooler, moist onshore flow at
the SFC, a strong inversion is immediately
reestablished along the coast. This is conducive to
dense fog formation along the SOCAL Coastal Regions.
Dense fog can be expected during the first 48 HRS
after the high moves out of the Great Basin UNLESS the
high rapidly moves off to the east, moves S-SE toward
SOCAL or a upper-level trough moves into the region.
KNTD 300755Z 32005KT 1/4 FG
VV001 12/12 A2993
KNZY 301140Z 32003KT 3/4 BR
BKN004 BKN250 14/12
KNUC 301055Z 22010KT 1/4 FG
VV004 14/12 A2997
SUMMARY
As you can see, even through migratory high pressures
may not move over the Great Basin and develop
conditions per climatology, it is still possible to
feel the same effects IF conditions are right, as in
the Case Study.
BUT in order to be labeled as a
true Santa Ana the stated
conditions need to be met.
NOW, Go forth and
do great things!