Transcript Synoptic-Scale Weather Systems of the Intermountain West

Synoptic-Scale Weather
Systems of the
Intermountain West
Dr. David Schultz
NOAA/National Severe Storms Laboratory
Norman, Oklahoma
[email protected]
http://www.nssl.noaa.gov/~schultz
What Might Be the Forecasting
Challenges in February 2002?
• 1–2 February 1989: SLC Airport greatest 24-h Feb
snowfall of 11.9”, associated with strong cold
front. 40–60-mph south winds ahead of front.
• 10 February 1975: 65-mph winds derailed chair lift
at Park City Ski Resort. Gondolas shut down.
• 12–13 February 1986: 3 feet of snow in the
Wasatch Mts. Avalanche in the Sundance
area.
• 13–19 February 1985: Dense fog caused multiple
vehicle accidents (30 car and semi pileup on
13th) and 3 deaths.
What are the Winter-Weather
Problems in Northern Utah?
• low pressure systems and frontal passages
• snowstorms (mountain and valley)
• winds (synoptic-scale and canyon winds)
• fog due to synoptic-scale ridging
• bitter cold from arctic outbreak
What are the Winter-Weather
Problems in Northern Utah?
• low pressure systems and frontal passages
• snowstorms (mountain and valley)
• winds (synoptic-scale and canyon winds)
• fog due to synoptic-scale ridging
• bitter cold from arctic outbreak
TO BE DISCUSSED IN THIS TALK
Planetary-Scale Climatology
In the wintertime, a planetary-scale ridge and subtropical
high are usually positioned over the western US, implying
that the passage of cyclonic storms usually is inhibited.
(Lackmann et al. 1996)
Leads to high incidence of
anticyclones, cyclolysis,
frontolysis
In this regime (positive
PNA pattern), forecast
models are usually more
predictable (Palmer 1988),
probably due to
persistence.
Zishka and Smith: anticyclones
Zishka and Smith (1980)
Zishka and Smith: cyclones
Zishka and Smith (1980)
Nevada Lee Cyclogenesis
Favored Feb.–May, with a secondary maximum in Nov.
Lee (1995) identified two kinds of Nevada lee cyclones:
SW (75%) and NW (25%)
SW cases begin with lee troughing, then when forcing
aloft overspreads trough, cyclogenesis occurs and
cyclone becomes mobile away from lee of Sierra
Nevada.
Tiros Lee: Nevada lee cyclogenesis
L
SW Nevada Lee
Cyclogenesis
–24 h
0h
Composite
500-mb height,
QGPV and sfc
highs/lows
L
L
+24 h
L
+48 h
(Lee 1995)
H
Tiros Lee: Nevada lee cyclogenesis
L
NW Nevada Lee
Cyclogenesis
–24 h
0h
Composite
500-mb height,
QGPV and sfc
highs/lows
H
H
L
+24 h
L
+48 h
(Lee 1995)
Frequency of
700-mb
vorticity
maxima
passages
through
western U.S.
Atallah and
Bosart (1996)
British Columbia track
Columbia River Valley track
Frequency of
700-mb
vorticity
maxima
passages
through
western U.S.
Arizona and
California
Central
Valley tracks
Atallah and
Bosart (1996)
Tracking Cyclones and Upper-Level Forcing
• Lows typically don’t move through the West continuously.
• Schultz and Doswell (2000) suggested that tracking the
occurrence of a mobile pressure minimum (a signal
of the upper-level forcing) may assist in analysis.
L2
L1
primary low
Fraser River
trough
lee low
L3
Tracking Cyclones and Upper-Level Forcing
• Look for pressure-check signatures in time series of SLP
or altimeter setting, or the location of the zero isallobar
Frontal Passages in the West-I
• Upstream topography tears fronts apart: Steenburgh and Mass (1996)
• Fronts passing through the west can be poorly defined at the surface
for many reasons.
TEMPERATURE:
- trapped cold air in valleys masks frontal movement aloft
- diurnal heating/cooling effects
- different elevations of stations (use potential temperature)
- frontal retardation/acceleration by topography
- precipitation (diabatic) effects
- upslope/downslope adiabatic effects (e.g., Chinooks)
PRESSURE:
- diurnal pressure variations
- sea level pressure reduction problems
WINDS:
- diurnal mountain/valley circulations
- topography channels the wind down the pressure gradient,
therefore the wind is not nearly geostrophic
Modification of Geostrophic Balance
by Topography
Rossby radius of deformation (lR) is a measure of the
horizontal extent to which modification of the force
balances takes place.
lR=Nh/f
lR is about 100–200 km for the Wasatch.
Blazek thesis
Steenburgh
and Blazek
(2001)
Frontal Passages in the West-II
• Warm-frontal passages are often not well defined at the surface, although
regions of warm advection are likely to be occurring aloft. (Williams 1972)
• “The strength of the potential temperature gradient associated with the front is
strongly modulated by differential sensible heating across the front. An
estimate of the contribution to frontogenesis from differential diabatic heating . . .
shows that it is several times greater than the contribution from the surface winds
alone.” (Hoffman 1995)
• Advection of postfrontal air through the complex topography is difficult to
accomplish. Therefore you may not see classic frontal passages at the surface,
but the baroclinic zone may be advancing aloft. The temperature decrease (if
any) behind the cold front may be a result of downward mixing of the colder air.
Isallobars may be useful to follow these elevated frontal passages through the
west.
• Larry Dunn has described some frontal passages in the West as split fronts.
This concept may be useful and is in qualitative agreement with the results
described above. In these cases, the precipitation may be out ahead of the
surface position of the front.
Failure of the Norwegian Cyclone Model
• lack of warm fronts
• occluded fronts sometimes act as cold fronts
• deformation of fronts by topography
• precipitation is often unrelated to surface
features
• disconnect between upper-level systems and lowlevel systems (e.g., IPEX IOP 3)
IPEX IOP 3: 1800 UTC 12 Feb 2000
Cross Section
NSSL 4 sounding
NSSL 5 sounding
P-3 flight track
trough at 700 mb
Mt. Ogden
W
Great Salt Lake
E
(Courtesy of Justin Cox)
Forecasting Snowstorms in Utah
• Favorable track: Nevada cyclogenesis with track of
surface low through SLC or just north of SLC
• Track of surface low south of SLC favors downslope flow
along Wasatch, holding snowfall down
• Well-defined shortwave trough aloft
• Difference between 5–10-inch and >10-inch snowstorms:
DURATION, either by a slow-moving trough or multiple
shortwaves in a long-wave trough
• Be aware of warm-advection snowstorms from southwest,
with stationary/cold front draped across state.
• Synoptic-scale banding (“warm seclusion”–Dunn): snow
in valleys>=snow in mts. (e.g., IPEX IOP 5)
IPEX IOP 5: 17 February 2000
•
•
•
•
Surface cyclone south of SLC
Weak flow field at all levels
Snowband northwest of cyclone
4–12 in. snow in Tooele Valley
SURFACE
500 hPa
6-h median
reflectivity from
KMTX
yellow maxima
are 20-25 dBZ
(Horel)
700-hPa FRONTOGENESIS
500-hPa omega
L
700-hPa theta
shading
700-hPa frontogenesis
700-hPa winds
RUC-2: 1500 UTC
Using MesoWest to Aid
Synoptic Analysis
• Use of multiple stations to confirm frontal passage
• Use of multiple elevations for interpreting vertical
structure of weather systems (e.g.,
Promontory Point is 2700 feet above SLC)
• Generating time series of a particular station
(e.g., looking for pressure minima, frontal
passages)
Even if you were able to predict the
liquid equivalent perfectly
• . . . you’d still have to know the snow
density.
• Usually this is assumed to be 10 inches
of snow to 1 inch of liquid water
(snow ratio), higher for “the greatest
snow on earth”
• The following graph is snow ratios from
2273 snowfall events greater than 2 mm
liquid from 1980–1989 for 29 U.S.
stations.
percent
10 to 1 ratio
ratio of snow to liquid equivalent
(Roebber, Bruening, Schultz and Cortinas)
Number of events
119 events
SLC 1980-89
ratios of 5–15
account for
57% of events
(Roebber, Bruening,
Schultz and Cortinas)
ratio of snow to liquid equivalent
REFERENCES
Hill, C. D., 1993: Forecast problems in the Western Region of the United States. Wea. Forecasting, 8,
158–165.
Schultz, D. M., and C. A. Doswell III, 2000: Analyzing and forecasting Rocky Mountain lee cyclogenesis
often associated with strong winds. Wea. Forecasting, 15, 152-173.
Steenburgh, W. J., and T. R. Blazek, 2001: Topographic distortion of a cold front over the Snake River Plain
and central Idaho mountains. Wea. Forecasting, 16, 301-314.
Williams, P., Jr., 1972: Western region synoptic analysis--Problems and methods. NOAA NWS Western
Region Tech. Memo. NWSTM WR-71, 71 pp. [Available from NOAA NWS Western Region
Headquarters, 125 S. State Street, Rm. 1311, Salt Lake City, UT 84138-1102.]
http://www.wrh.noaa.gov/Saltlake/projects/indexWorkArea.html
http://www.nssl.noaa.gov/~schultz/ipex/refs.html
http://www.nssl.noaa.gov/~schultz/wwt/