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Utilizing satellite imagery to help analyze / forecast Winter Storms Part 2
1.
29-30 October 2003 case. We will be forecasting for the 12-36 hour time period
beginning on the morning of 29 October 2003.
2.
Case objectives
3.
GOES-10 Water Vapor imagery from 01:00 UTC – 23:45 UTC 29 October, 2003.
Broad trough moving south out of western Canada. Strong jet over
Washington/Oregon drifts slowly southeast towards Wyoming. Notice the
cellular cloudiness entering Oregon on the morning of the 29th. This was
associated with relatively cold temperatures aloft.
As the jet moves eastward into our area of interest, colder cloud tops are observed
in northwest Wyoming. NLDN lightning data showed some strikes in the Teton
range indicating convection. Meanwhile a long narrow dry slot combined with
upper cloud deck moving slowly southward from Canada helps identify a shear
zone. One vort max can be seen on the southwestern most portion of the
deformation zone in southwest Canada, the other moving from southern Alberta
into southern Saskatchewan.
At around 03:00 UTC on the 29th a long narrow band of cold tops becomes
stationary over east central Wyoming. These are mountain wave clouds. Several
other ranges develop wave clouds until about mid-day on the 29th. This signals a
change from subsidence over central Wyoming to a less stable environment.
4.
Eta 500 mb Height and Vorticity from 12:00 UTC 29 October through the 36 hour
forecast period. Main vort max associated with low moves east while another
weaker one to the west moves southward. A more significant vort max is
developing in the CA/NV/OR region where height gradient is forecast to
strengthen. This vort max is not forecast to move over Wyoming, however the
height gradient strengthens over Wyoming. As we continue to look at more
products we will monitor these features.
5.
Eta 300 mb Height and Isotachs from 12:00 UTC 29 October through the 36 hour
forecast period. The upper jet is moving southward over the RIW CWA as
shown on the satellite imagery. Notice the two jet maxima in the 18 hour forecast
chart. Shortly thereafter the westernmost jet is forecast to strengthen (becoming
the primary jet) and move over the RIW CWA. As the time gets closer we will
look for indications of this jet on the satellite imagery.
6.
Terrain for the RIW CWA with Eta sfc wind 30 hour forecast valid 18:00 UTC
Oct 30 (fade enabled). Terrain plays a critical role in this region. Riverton is
located in a basin between the Wind River mountians to its southwest and the
Bighorn mountains to its northeast. Which low-level wind direction would be
most favorable for upslope and which low-level wind direction would provide
easiest access for the arrival of cold frontal air masses? Which areas would likely
see less snow than the Eta QPF is showing based on the terrain?
7.
MesoEta MSLP, winds and QPF from 12:00 UTC 29 October through the 33
hour forecast period. The Eta is slow to move the cold front through, a known
bias in the high plains. Eta forecasts precip along and just behind the cold front.
What effect will the terrain have on the forecast precip given northeast flow and
the limited resolution of the model? The QPF fields in northwest Wyoming show
precip associated with the jet over the higher terrain. As the jet moves southward
the QPF field in the Teton range moves south. Note the areal coverage of precip
expands as the primary jet is forecast to arrive into the area (around 09:00 UTC).
Sfc wind speeds picked up between 09:00 – 12:00 UTC, corresponds to increase
in QPF. Notice pressures are rising throughout the region leading to a stronger
pressure gradient and therefore stronger winds (and upslope flow in certain areas).
8.
MesoEta 500-700 mb QG Frontogenesis shaded, 600 mb deformation (yellow
contours), 600 mb temperature (white when toggled on) from 12:00 UTC 29
October through the 33 hour forecast period. We are analyzing for the potential
for mesoscale banding. The most favorable area for mesoscale banding to occur
is where the maximum in mid-level deformation is juxtaposed with the southern
edge of the mid-level frontogenesis maximum. Since we are looking at a high
elevation location we chose the 500-700 mb level. At lower elevations you
should choose a lower layer (e.g. 850 – 700 mb). Note that the most favorable
conditions begin to setup around 03:00 UTC. By 06:00 UTC the signature is
favorable just south of Riverton. Therefore forecast snowfall amounts should be
boosted due to potential for mesoscale banding. By overlaying the 600 mb
temperatures we see the forecast band orientation is ENE-WSW.
9.
Terrain with a line indicating the cross section to be used in the following slide.
10.
Eta Cross section of temperature, omega (shaded), winds from 12:00 UTC 29
October for the 30 hour forecast time. The cold front has allowed a deep
northeasterly flow to setup a cold airmass on the eastern side of Wind River
range. By this time, the Eta had forecast precipitation to occur and forecast the
boundary layer to deepen along the eastern side of the mountains. Note that the
temperatures have cooled to around -16 (favorable dendritic growth) in the region
of upward vertical motion. There is northeasterly flow in the boundary layer east
of the mountains and southwesterly flow above it. The shear allows the
precipitation bands to remain quasi-stationary. The depth of an upsloping air
mass is an important factor in deciding whether precipitation will occur. The
depth is also a contributing factor to the intensity of the precipitation.
11.
Surface observations from 12:00 UTC – 20:00 UTC 29 October. Cold front can
be seen moving southward east of the Bighorn Range (west of Sheridan)
beginning to advect into the RIW area from the northeast by late in the period.
Dewpoints are in the mid-30’s just behind the front, however keep in mind the
elevation for this area is high. Note the spotty snowfall obs in Montana. The cold
front passage corresponds with the dissipation of mountain wave clouds seen on
water vapor imagery earlier.
12.
GOES-10 Water Vapor imagery with Eta 300 mb isotachs overlay from 16:30
UTC 30 October – 01:15 UTC 31 October, 2003. Notice how fast the clouds are
moving over our CWA, further support of the intense jet. Also note as the
shortwave seen earlier over Oregon arrives in northwestern Wyoming / south
central Montana a baroclinic leaf develops. The western edge of the baroclinic
leaf is near Billings with another area of colder cloud tops west of there. This is
an indication of the lift associated with the shortwave. When we overlay the 300
mb isotachs we can see that the colder cloud tops are in the left front quadrant of
the jet. These higher clouds are probably associated with the ageostrophic
transverse circulation but remember that there is also vertical motion taking place
in association with the front and the local terrain. By the end of the loop a cold
conveyor belt (CCB) developed from northeast Idaho into Montana. Note the
trough axis over Oregon.
13.
KRIW 0.5 degree reflectivity from 22:25 UTC 29 October – 01:32 UTC 30
October, 2003. Frontal passage with precipitation starting in earnest around 00:00
UTC. Casper precipitation begins as rain but changes over to snow by 01:00
UTC as temperatures/dewpoints drop. Also, note the obs in north central
Wyoming show the deeper cold air is now advecting into the region. The first
signs of the transverse bands appear just after 00:00 UTC south of RIW, the
precipitation becomes enhanced as the bands go through. Beam blockage
prevents seeing much of the precip southwest of the Wind River range (SW of
RIW), light snow was reported at KBPI (Big Piney) despite no reflectivity
showing up.
14.
GOES-10 IR imagery overlaid from 16:00 UTC 30 October – 01:00 UTC 31
October, 2003 with MesoEta 300 mb isotachs. Recall that a 300 mb jet was
expected to arrive during the morning hours of the 30th. Here we can see the jet
arriving in southwest WY at the expected time. Note the transverse waves that
develop by mid-afternoon as the jet streak enters the region. These transverse
waves often indicate where the jet streak is located. By overlaying the model
output with the imagery we can judge how well the model is doing.
15.
GOES-10 Visible imagery from 14:45 UTC 30 October – 22:45 UTC 30
October, 2003. Southwest-northeast oriented mesoscale bands evident over the
RIW CWA. Note the change in character of the cloud field by 21:00 UTC (from
northeast-southwest oriented bands in the morning to east-northeast to westsouthwest oriented) transverse bands by late afternoon. Heavy snowfall
developed as these bands became more well defined. The transverse bands are a
signature of the upper jet streak.
16.
KRIW 0.5 degree reflectivity from 22:25 UTC 29 October – 00:01 UTC 31
October, 2003. Remember the MesoEta output found banding conditions setting
up by 03:00 UTC and becoming more favorable by 06:00 UTC. The radar shows
that banding actually sets up around 00:00 UTC (3 hours earlier than Eta
suggested) with well defined bands by 03:00 UTC (again, 3 hours earlier than Eta
suggested). The air behind the cold front becomes so cold and dry that the snow
ends for a time at RIW right after frontal passage then fills in again as upslope
flows increases relative humidity. Heaviest snowfall rates are occurring on the
upslope favored areas of the ranges as well as within the mesoscale bands.
Note that KBPI by 09:00 UTC is reporting snow but no reflectivity (beam
blockage). The visible loop shows the extension of the snowbands well into
southwest WY
17.
Snowfall observations for the event. Areas that had the preferred upslope winds
and persistent mesoscale bands received the most snow.
18.
Summary
19.
Medford CWA. Case is from 28-29 December, 2003. We are forecasting for the
next 48 hours beginning the morning of 28 December 2003.
20.
Case objectives
21.
Water Vapor imagery from 11:00 UTC 27 December – 14:00 UTC 28 December,
2003. Basic, or perhaps in-stream, cyclogenesis developing in the eastern Pacific
moving towards the west coast of the US. Notice the vorticity center moving
through southwest Canada towards the northwest coastel region of the US.
22.
Eta 500 mb Height and vorticity forecast from 12:00 UTC 28 December. Strong
northwest flow over Oregon is forecast to move east during the day. Meanwhile,
the trough associated with the developing cyclone moves in. Vorticity maximum
associated with developing cyclone looks intense around 06:00 UTC 29
December as it approaches the coast, but seems to weaken in time. The vort max
moving southward from Washington intensifies and seems to play a more
dominant role in amplifying the trough.
23.
IR with GFS Boundary Layer wind barbs analyses from 14:30 UTC 27 December
– 17:30 UTC 28 December. The boundary layer (BL) winds should be well
correlated with the low-level cloud streets. Notice the streets in the warm sector
off the California coast as well as the cellular clouds off the British Columbia
coast. The satellite data combined with the model BL winds can give us an idea
of how well the model is doing.
24.
Medford terrain. Noting the location of mountain ranges and valleys, where are
the most likely and least likely areas for heavy snow. The east-west mountain
range on the north side of the Shasta valley is called the Scott Bar range.
Typically these mountains receive relatively high amounts of precip while the
areas downwind in the Shasta valley get very little precip. If the cold air remains
in place over the Shasta valley, what effect would this have on this typical
scenario? This is not a terrain exercise, but rather we are using a terrain problem
to illustrate how to deal with the multi-layered cloud decks that occur frequently
in winter.
25.
Surface observations from 05:00 UTC 28 December through 14:00 UTC
December 28. We see relatively cold temperatures and calm winds in the
Sacramento and Shasta valleys. Dewpoints are below freezing in various
locations in our CWA.
26.
Eta MSLP / QPF and winds 18 hour forecast from 12:00 UTC 28 December valid
06:00 UTC 29 December. The low pressure associated with the large cyclone
developing off the coast is begins affecting the CWA at this time. Note the high
pressure over central CA. This is in the valleys where the temperatures were cold.
27.
Fog product from 03:00 – 09:30 UTC 28 December 2003 with METARs overlaid.
This product is made by differencing the 10.7 um and 3.9 um channels.
Characteristics of emitted radiation from water clouds in the 3.9 um channel are
different from the 10.7 um. By differencing these two channels we can identify
water clouds. In this color table stratus and fog are white, ice clouds are black.
Notice the light area which develops in the Shasta valley in northern California.
As time goes on this area becomes larger and more well defined. Note that none
of the METARs are within this region. We looked at the cloud base observations
for the nearest stations and found measured cloud bases of 700 feet. Therefore
we are looking at stratiform cloudiness which increases in areal coverage
throughout the night.
28.
Loop of visible imagery from 16:30 – 23:15 UTC 28 December, 2003. First we
look at the loop at the default speed. We see embedded convective cloud tops
moving into northwestern CA. Based on the position of the large cyclone, the Eta
seems to be doing a reasonable job. Also notice the well-defined warm front east
of the low by late in the loop.
Next, we will increase the speed and rock the loop. Frequently, this technique
allows us to identify persistent low-level features that are partially covered by
higher clouds. Here we can clearly see the areas of stratiform cloudiness in the
Shasta valley. Also, note the stable wave clouds east of Medford. Both
observations suggest a stable air mass which is trapped in the valleys, and is not
mixing out.
29.
10.7 um imagery from 21:00 UTC 27 December through 20:30 UTC 28
December. Cyclone is developing with a well defined WCB. Appears to be
occluding near the end of this loop. Note cellular convection (cool air) just off the
Washington / British Columbia coast. This cold air never makes it onshore but is
offset by low-level warm air advection caused by the approaching cyclone. Note
the surface low is still well offshore.
30.
KMAX 0.5 degree tilt reflectivity from 18:40 UTC 28 December through 05:07
December 29, 2003. Strongest reflectivity mostly southwest of Medford during
this period. Heavy snow over the Scott Bar Mtns. And over the town Yreka, CA
(all time record of 23”) which normally is a snowfall minimum due to downslope
winds. In this case the cold air noted above remained in place throughout the day
preventing the downslope drying to take place.
31.
KMAX 0.5 degree tilt reflectivity from 05:13 UTC 29 December through 23:54
December 29, 2003. Note that the precipitation over the Shasta valley continues.
The precipitation was slow to end partially because there is a secondary maximum
in precipitation which begins on the early morning of the 29th. This may be a
small unanalyzed shortwave that we noted as the subtle feature on the water vapor
loop.
32.
Water Vapor imagery from 14:00 UTC 28 December – 23:00 UTC 29 December,
2003. WCB eventually moves east of Medford’s CWA. Shortwave moving
southward from Canada merges with initial trough and acts to deepen it further
and slowing it down as well. Notice the small disturbance that ejects from the
main cyclone. This feature enhances as it comes onshore on the early morning of
the 29th.
33.
Snowfall map with terrain. Deep snowfall over both high terrain and open
valleys.
34.
Summary
35.
Intro for 8-10 December 2003 case. We are forecasting for the 12-36 hour time
period based on data available at 12:00 UTC 9 December 2003.
36.
Case objectives
37.
Eta forecast 500 mb HT/VOR from 12:00 UTC 9 December 2003. Large
amplitude trough located over the Plains is forecast to move eastward and deepen
with time. Notice that the vorticity center over the southwest TX panhandle is
forecast to move eastward, then northeastward, towards Missouri. The PVA
associated with this vort center seems to correspond with the developing
convection (secondary WCB) in the water vapor loop.
38.
GOES-12 water vapor imagery for 09:15 UTC December 8 – 16:59 UTC
December 9, 2003. An upper low can be seen developing over the Rockies. This
system moves into western Texas by the end of the loop. It is cold air
cyclogenesis at this time. About 07:00 UTC we see a jet streak developing
rapidly ahead of the upper low in western Oklahoma. This feature was not
resolved well on the Eta.
There is also a hint of convection developing in eastern Texas in between the
upper low and WCB. This is the development of a secondary WCB which would
suggest instant occlusion cyclogenesis. Remember this convective development
implies unstable air advecting towards the surface low, and means that this low is
deepening rapidly.
39.
MesoEta forecast MSLP/T/QPF/SFCT and winds from 12:00 UTC 9 December
2003. The sfc low talked about in the water vapor loop is forecast to move from
northeast OK towards St. Louis and deepen with time. Note the strong thermal
advection on either side of the low. The freezing line moves across Missouri just
behind the surface low. There is precip both ahead of, and behind, this low.
There is a large area of 0.5-1.0” per 6 hour forecast to fall in the below freezing
region. This suggests that the precip will begin as rain, and changeover to snow.
The precipitation can be seen to wrap around to the northwest of the developing
low. This will be shown to be a TROWAL which overrides the cold air and
brings snow to western Missouri.
40.
MesoEta forecast sounding near Columbia, MO from 12:00 UTC 9 December
2003 through 30 hours. Initially the lower levels of the sounding are well above
freezing and the sounding is unstable. This unstable air is where the secondary
WCB is filling in. Beginning at 06:00 UTC on the 10th winds shift to westerly
and at 09:00 UTC the temperatures become cold enough for the precipitation to
change from rain to snow. Early in the period we observed the winds veering
with height (WAA), after the wind shift the winds are backing with height (CAA).
41.
GOES-12 Water Vapor imagery from 14:15 UTC through 22:45 UTC 9
December, 2003 overlaid with Eta time matched 500 mb height and vorticity
(toggle). As noted earlier, when the short wave approaches the warm sector in
eastern Texas there is a sudden development of convection in the WCB. (Toggle
on 500 mb) This shortwave was indicated on the Eta 500 mb forecast. This
example illustrates the utility of combining satellite imagery with NWP output.
The NWP analysis in this case leads us to expect a shortwave, the satellite
imagery confirms the shortwave’s existence and gives us better definition in time
and space.
42.
GOES-12 10.7 um IR imagery from 02:45 UTC 9 December – 01:01 UTC
December 10, 2003. Now we will begin looking at the nowcast time period. We
can see the stratiform associated with post-frontal precipitation moving across
Oklahoma into Missouri. Early in the loop we see a cusp in western Kansas
associated with the cold air cyclone. Remember the surface low is just eastof this
cusp.
As instant occlusion takes place, a second cusp forms over central Oklahoma
(around 20:00 UTC ) and this becomes the dominant low. As this vorticity center
approaches the WCB of the primary system, convection begins to wrap around
back into northeastern Oklahoma. Stations in northeast Oklahoma reported 6-8”
of snow as this occurred.
43.
GOES-12 water vapor imagery from 18:30 UTC December 9 – 14:15 UTC
December 10: We can see the system moving rapidly to the northeast but the
cusp is still over Missouri by the end of the loop. The precipitation associated
with the CCB / TROWAL is still occurring. During the second half of the loop
we can see the WCB also wrapping back which maintains the feed of moisture.
44.
GOES-12 10.7 um IR imagery from 07:15 UTC– 15:59 December 10, 2003 with
metars overlaid. Let’s look at this with the 10.7 um IR channel. By overlaying
the METARs we can see that the rain/snow line is entirely associated with the
north/south band advancing eastward with the elongated cusp. This snow region
is situated entirely behind the surface low. This is not always the case but for this
example we can use this satellite feature to track the rain/snow changeover. There
was 10-14” of snow in southwest Missouri
45.
Radar reflectivity mosaic 00:48 – 20:46 UTC December 10, 2003 with metars
overlaid. Here is a review loop of the entire event, we can see the early
convection associated with the WAA and secondary WCB. We can also see the
wrap around precipitation that appears as the TROWAL develops. By utilizing
satellite imagery to define the larger picture in this case we can better understand
the evolution we observe on radar.
46.
Snowfall map.
47.
Summary
48.
Intro for 10 - 13 December 2003 case. We will be forecasting for the 12-36 hour
time period beginning on the morning of 12 December 2003.
49.
Case objectives
50.
GOES-12 water vapor imagery from 01:01 UTC December 11 – 15:45 UTC
December 12. The shortwave trough position is coming out of Arizona (stop near
end of loop to show that). We can also see an Artic trough dropping south
through the northern Plains. Sub-tropical jet over northern Mexico; Polar jet
(entering southwest Texas by the end of the loop) rotates around the base of the
trough and moves northeast.
51.
Eta forecast 500 mb HT/VOR/T from 12:00 UTC 12 December 2003. Note the
intense shortwave trough that begins in southern AZ, and moves eastward over
the next 24-hr. Cooling at 500 mb associated with the upper low will result in
column cooling. For example, trace the -25 C isotherm as it advects across
Oklahoma by late in the period.
52.
HPC surface analysis from 12:00 UTC 12 December 2003 showing the bigger
picture. Large region of very cold high pressure dominates the Midwest. We
now see that the surface low pressure is centered over extreme southwest TX.
Warm front analyzed near DFW is forecast to move northward towards central
Oklahoma.
53.
Eta forecast MSLP (green)/QPF (shaded)/T (red)/Winds from 12:00 UTC 12
December 2003. Cold air in place over OK at 12:00 UTC, surface low
somewhere in TX. Precip moves up from the south over the next 6-hr (probably
WAA since low is to the southwest). At 9-hr a southwest-northeast warm front
becomes well defined from the low stretching into northeast OK with above
freezing temps moving in from the southeast. The Tulsa CWA seems to be right
on the border between snow and rain throughout most of the period. Two distinct
regions of precip for central OK, the first round associated with the warm
advection then moves east, the next wave associated with wrap-around the low.
Lingering precip into the next day in northeastern OK.
54.
Soundings taken at Norman, OK and Springfield, MO plotted on Skew T/Log P
diagrams for 12Z 12 December, 2003. At Norman, the temperature profile below
850 mb is mostly warmer than zero suggesting rain while at Springfield the
boundary layer is at or below zero. This suggests cold rain in Norman – at least at
this time and snow at Springfield with the rain/snow line in between. At Tulsa,
there is a good chance that the cold precipitation will cool the boundary layer and
rain will change to snow by 03:00 UTC. However remember that there was warm
air advection at 850 mb along the eastern Oklahoma border. We need to monitor
the temperatures closely since Tulsa seems to be right on the line.
55.
GOES-12 water vapor imagery from 16:01 UTC – 22:45 UTC December 12. As
the large shortwave trough moves into OK we see a WCB (with convection)
developing over eastern TX. The WCB expands to cover eastern KS, southern
MO, and eastern OK. Cold air cyclogenesis is taking placing, however note the
convection develop in eastern Texas (developing a secondary WCB).
56.
Eta 850 mb theta-e and GOES-12 IR imagery from 11:45 – 20:45 UTC December
12, 2003. Theta-e ridge advecting from eastern Texas northward into Oklahoma.
Convection forms along this ridge indicative of the unstable air. The theta-e ridge
moving towards the Tulsa region will advect the rain/snow line further north and
east (recall Eta 850mb forecast thermal ridge into this area). The development of
this secondary WCB means that the precipitation will probably begin as rain as it
acts to move the warm front north towards Tulsa. The warm front lines up with
the low level clouds that stretch from the Red River to southern Arkansas at 14:00
UTC. After that we see the low level clouds delineating the warm front move
quickly northward in central to eastern Oklahoma while it remains quasistationary in Arkansas. Notice that the line of convection is drifting east with
time towards more stable air.
57.
GOES-12 10.7 um IR imagery from 22:31 UTC, 12 Dec – 07:15 UTC 13 Dec
2003 with metars overlaid. We see the warm advection precip early in the loop
move off to the northeast followed by some drier air then followed by the wraparound precipitation. Precip type early on at CSM (Clinton, OK) is sleet but then
changes over to snow at 02:00 UTC when the wrap-around precipitation band
moves across. OKC changes from rain to snow at 01:00 UTC. Even though there
are cold cloud tops at 22:00 UTC there is no precip reported until the 00:00 UTC
observation and this is rain. Cold cloud tops exit the region while sfc temperature
goes down then snow is reported at 02:00 UTC. Switches back to rain at 03:00
UTC then at 07:00 UTC goes back to snow as the wrap-around precip band comes
in. Between the 06:00 UTC and 07:00 UTC observation, the temp remained 34 C
but it changes over to snow. The higher clouds wrap around into western OK by
03:00 UTC.
58.
GOES-12 water vapor imagery from 23:15 UTC December 12 - 11:45 UTC
December 13, 2003. Recall that the rain/snow line was near Tulsa and moving
north with time. Meanwhile the convection associated with the WCB is
propagating eastward into the more stable air. Water vapor imagery shows this
convection diminishing. The convection never completely fills in between the
WCB and the upper low, therefore this is not an instant occlusion type of
cyclogenesis. Notice the wraparound cusp associated with the upper low which
led to a changeover to snow.
59.
Radar mosaic with METARs (toggle) 22:30 UTC December 12 – 10:30 UTC
December 13, 2003. Early on we see the precip associated with secondary WCB
(theta-e ridge) is primarily rain. Note the temperatures are warmer than forecast.
As time goes on the WCB convection moves off to the northeast then some drier
air moves in behind it. Later, the wrap-around precipitation with the upper low
can be observed. Remember that the wrap-around precip is snow (500 mb
temperatures around -25°C and sufficiently cold at the surface), and the surface
temperatures drop several degrees to below freezing.
60.
Observed gridded precip.
61.
Observed snowfall map for the TUL CWA. Note the effects of the theta-e ridge
on the snowfall distribution. Heaviest snowfall in north-central Oklahoma,
northwest of the theta-e ridge.
62.
Case summary
63.
Session summary