Utilizing satellite imagery to help analyze / forecast Winter Storms Part 1
1.
Title. A prerequisite for this training session will be the VISIT teletraining
session “Cyclogenesis: Analysis utilizing Geostationary Satellite Imagery”. An
audio playback version is available for download to go through on your own time
at the student guide web-page for that session:
http://www.cira.colostate.edu/ramm/visit/cyclo.html
2.
Objectives
3.
Topics that will be covered.
4.
Winter weather factors
5.
Structure of winter storms
6.
Conceptual model of conveyor belts. Note the cyclonically curved branch of the
warm conveyor belt (WCB) which is referred to as the TROWAL by some
meteorologists. Others refer to this feature as the secondary WCB (as referenced
in VISIT Cyclogenesis training). The subtle difference is the timing, when
occlusion begins it is generally referred to as the TROWAL.
7.
Model Divergence. We are referring to different solutions between different
models.
8.
Let’s compare the differences between the Eta (green) and GFS (salmon) MSLP
field through the forecast time. We can see that the Eta has the low further south.
In terms of forecast implications the Eta would give northern Colorado a greater
threat of precipitation with the low further south.
9.
The GOES-10 IR loop shows that the low is following the more southerly track,
meaning that the Eta should be the solution that would have a higher probability
of verifying than the GFS.
10.
Observed position of the low at 12:00 UTC 10 November based on HPC surface
analysis. Notice that the position of the low at this time corresponds with the Eta.
This increases confidence in choosing the Eta over the GFS during the forecast
period.
11.
Average snow to liquid water ratios between 1971-2000 by Marty Baxter of St.
Louis University. Map shows that snow is generally drier in the west and in the
lake-effect snow belts. These differences need to be considered when using
automated snow algorithms (Does 10:1 usually work in your CWA?). The website (http://mnw.eas.slu.edu/CIPS/Research/snowliquidrat.html) provides the
option of clicking on individual CWA’s (toggle) on the interactive map link. The
example shown is the Boulder CWA. Note the variation across the small region,
and the large standard deviation in the sample.
12.
Introduction to satellite shear zone. Generally look for these ahead of the main
trough. The stationary type of shear zone doesn’t move very much relative to the
trough. The rotating type of shear zone rotates around the trough axis.
13.
GOES-10 IR imagery from 01:00 UTC – 23:45 UTC 29 October, 2003. This is
an example of the stationary type of shear zone. 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. Sometimes we may see just one vort max but in this
case we see multiple. Max PVA moving in a storm relative-sense just ahead of
southwestern most vort max.
14.
Denver/Boulder CWA for the March 17 – 20, 2003 blizzard. We will be
forecasting for the 12-36 hour time period with a start time of the morning of 17
March 2003.
We will be presenting only enough model output to bring the class up to speed on
the forecast problem for each of the cases that we present. The primary purpose
will be to show how satellite data can be utilized to give value added to the
forecast / nowcast situation. We simply don’t have the time to cover all aspects of
each case you would normally go through.
15.
Objectives for this case.
16.
Eta 300 mb Heights and Isotachs from 12:00 UTC 17 March, 2003. Shows
approaching trough. The trough becomes negatively tilted by 00:00 UTC 18
March and becomes cutoff 6 hours later. Note the jet max that develops around
06:00 UTC 18 March in northeast CO.
17.
Eta 500 mb hgt and vorticity (shaded) from 12:00 UTC 17 March 2003 showing
12, 18, 24, 30 and 36 hour forecasts. 12 hr forecast shows vorticity lobe near
Dalhart, TX. This vorticity center is forecast to move slowly northward over the
forecast period. Note that the 500 mb low if forecast to consolidate and deepen as
it moves into southeastern Colorado over the next 24 hours.
18.
MesoEta surface MSLP, precipitation, temperature and winds from 12:00 UTC
17 March 2003 going out 33 hours. Sfc low in southeast Colorado is quasistationary and remains deep. The deepening low in this region extends through
nearly the entire depth of the troposphere. Northeast Colorado is forecast to
experience very deep easterly (upslope) flow. The output shows precipitation
throughout north-central CO for the entire period. The forecast temperatures
suggest that the freezing line remains over the mountains until the next morning
while the temperatures are below freezing throughout most of southeast
Wyoming. Thickness analysis (not shown) suggests rain/snow line approximately
6000 feet throughout the period. If snow does occur it will be heavier than the
climatological 15:1 ratio. Heavier precipitation is forecast to develop over
OK/KS by 00:00 UTC and expand with time through 12:00 UTC.
19.
GOES water vapor loop 04:30 UTC 17 March 2003 – 00:00 UTC 18 March 2003
– A lot of the value of satellite imagery is in supplying context on the larger scales
to help the forecaster understand the NWP output. In this loop, notice the
developing warm conveyor belt (WCB) that stretches from Oklahoma into
Kansas. It is marked by deep convection associated with a jet max over KS
moving into NE. This precipitation seems to be developing in the same location
as that forecast by the Eta.
Satellite imagery should not be restricted to supplying synoptic overviews, but
should also be used to identify sub-synoptic components embedded in the larger
scale. In addition to the jet max in Kansas, there is a small shear zone rotating up
from north-central New Mexico into central Colorado. Finally, a jet max
becomes apparent as it rotates around the base of the trough from southern New
Mexico / northern Mexico into west Texas at the very end of the loop.
20.
MesoEta 310K potential temperature surface with pressure (mb), winds and
relative humidity (shaded) from 12:00 UTC 17 March, 2003. The Eta shows
weak isentropic lift at 00:00 UTC on the 18th. By 06:00 UTC the isentropic lift
intensifies over north central CO but by 12:00 UTC it weakens again.
(Remember that isentropic analysis is used to represent the thermal advection
contribution to QG omega. Thus, even though we’re utilizing isentropic analyis in
this case, both thermal and vorticity advection can contribute to vertical motion.
Also, remember that just because we’re using isentropic coordinates instead
pressure coordinates, QG assumptions still apply. Finally, remember that
isentropic surfaces are not static in time and space, and should not be thought of
as ramps that are frozen in time and space along which parcels ascend and
descend).
21.
Eta (yellow) and NGM (cyan) forecast MSLP 12 hour forecast valid 00:00 UTC
18 March, 2003. The Eta forecast the low to be in southeast Colorado while the
NGM. The NGM is chosen here just to make the point of comparing diverging
models with satellite imagery. The NGM has the low further north. In order to
assess which model is closer let’s go to the visible imagery in the next slide.
22.
GOES-10 visible imagery shows the low in extreme southeast Colorado meaning
that the Eta was more accurate in its MSLP forecast.
23.
Colorado topography. Air moving in from the east will undergo forced lift of
roughly 3,500 ft as it moves from the Colorado border into the Front Range
Range. Also, as the low-level air encounters the mountains it will also be
partially blocked. This effect causes the boundary layer right along the foothills
to deepen and the flow to accelerate (towards the south). This is the phenomenon
referred to as the barrier jet earlier.
Remember that the flow through the whole mid- to lower-troposphere was
easterly as well, implying that the upslope flow was deeper than the mountain
barriers. In a normal upslope situation, the upslope flow is contained by the
mountain barriers and snowfall totals west of the Front Range are light. The deep
upslope in this case improves chances for significant snowfall in these normally
shadowed areas.
24.
Eta MSLP and Precipitation forecast from 12:00 UTC 17 March, 2003 out to
18:00 UTC 18 March. Notice the precipitation band stretching from Kansas
across southwest Nebraska and wrapping back around into north-central
Colorado. Note also the forecast position of the surface low. We can compare
the position of the low with surface observations and the precipitation forecast
with IR satellite data to get an idea of how well the model is doing.
25.
Surface observations from 00:00 UTC 18 March, 2003 – 05:00 UTC 18 March,
2003 - The surface analysis for late afternoon shows pretty good agreement with
the MesoEta winds but the temperature forecast was a little high over northcentral / northeast CO as well as southeast WY. Note the strong northerly winds
in north central CO. This is a result of a barrier jet developing along the Front
Range of the Rocky Mountains. The barrier jet causes the boundary layer to
deepen and the winds to accelerate as they are deflected southward.
Throughout the evening temperatures remain in the low 40s and high 30s along
the northern Colorado Front Range, while southeast Wyoming observations
showed low 30s and snow most of the afternoon. Note the intensifying northerly
winds and changeover to snow at both Laramie and Cheyenne, WY by 01:00
UTC. Notice that the low pressure and circulation center indicate that the low is
in reasonable agreement with the Eta. The changeover from rain to snow took
place in Front Range cities that were 5000 feet or greater elevation around 05:00
UTC. Note this is the time of the arrival of the shortwave seen on IR imagery.
26.
GOES-12 10.7 um IR loop 20:15 UTC 17 March 2003 – 06:00 UTC 18 March
2003. Early in the loop we can see north-south bands of colder cloud tops in
north-central CO. These tops were associated with north-south oriented squall
lines (radar loop soon). From 02:30 – 05:30 UTC the short wave moving into
Colorado from the southeast causes the areal coverage of the colder cloud tops to
expand due to upward motion. Notice that the cold cloud tops associated with the
WCB now extend from southwest Nebraska through northeast CO. During this
time, the precip type along the Front Range was rain or rain mixed with snow.
Recall the Eta had the warm conveyor belt (WCB) further north and east, but
satellite shows the WCB is a little further south than the forecast. As the
shortwave moves into the Front Range the precip area broadens and precip rates
increase. The short wave observed in west Texas continues to expand in areal
coverage as it moves north.
27.
Platteville, CO profiler data for 19:00 UTC 17 March – 07:00 UTC 18 March
2003. A reflection of the changing dynamics can be seen in the vertical wind
structure as the shortwave moves into the area. This corresponds with the WCB
enhancement noted on the IR. The winds at all levels increase substantially
between 02:00 and 04:00 UTC. The areal coverage of the precipitation increases
substantially during this time, as well. Between 05:00 – 05:30 UTC observers
from Fort Collins through the Denver area report a changeover from rain/snow to
snow (coincident with the arrival of this disturbance). The changeover probably
occurred as a result of the continuous and deeper precipitating layer. With precip
falling from higher levels, there would be greater cooling of the lower levels.
28.
KFTG 0.5 degree tilt reflectivity for the period 00:47-10:27 UTC 18 March 2003.
During the period 00:04 through about 02:30 UTC the precipitation takes the form
of discrete, north-south lines of intense convection during which severe weather
was observed on the Plains. The convective lines were associated with periods of
relatively heavy rain along the Front Range. Beginning shortly after 02:30 UTC
precipitation becomes widespread (associated with the arrival of the shortwave
and enhanced WCB) and transitions to snow between 05:00 – 05:30 UTC along
the Front Range. Notice the small circulation moving in from the southeast. This
was the small wave that moved up from New Mexico (referred to in the water
vapor imagery discussion earlier).
29.
GOES water vapor loop 00:30 UTC 18 March 2003 – 12:00 UTC 18 March 2003.
Between 11:00-12:00 UTC we notice the thick clouds associated with the first
period of heavy snow move off to the northwest and weaken. Fort Collins had on
average 8” of snow reported by 14:00 UTC. However, note the intense
shortwave/jet max in southwest TX that moves northeastward into western
Kansas by 12:00 UTC. This feature was not evident on the model output from the
morning of the 17th. New convection can be seen forming as the shortwave
reaches the WCB. What is the forecast implication of this? (It is key to holding
the low in place, allowing heavy snow to begin agan, and continue for nearly 24
more hours).
30.
GOES IR loop 10:15 UTC 18 March – 01:15 19 March 2003. The new
convection seen late in the previous loop is indeed associated with a second
shortwave, and an invigorated WCB. The intensified WCB moves into northern
Colorado during this period, leading to a long duration of heavy snowfall starting
around 19:30 UTC (18 March) and continuing through the morning of the 19th.
31.
GOES-10 IR loop for the entire event. This loop reviews the entire event from
00:00 UTC 18 March to 23:45 UTC 19 March, 2003. Remember that the imagery
tracks the development and movement of WCB’s and provides nowcasting
guidance for the beginning or ending of the various phases of this event. The Eta
represents the first and second shortwave/WCB’s discussed, however in both
instances the model has the max precip amount associated with the features too
far north. The role of satellite data here is critical as you can see how the Eta
forecast precipitation fields are evolving and adjust your forecast positioning and
consequent QPF amounts utilizing both in tandem.
32.
Snowfall totals. The satellite data in this case helped confirm/deny certain aspects
of the model outputs. The snowfall pattern clearly shows that the terrain played
the major role in this event.
33.
Summary of Colorado Blizzard case.
34.
Introduction for 16-18 February, 2003 Presidents Day snowstorm. We will be
forecasting for the 12-36 hour time period beginning on the morning of 16
February 2003.
35.
Case objectives
36.
Eta 300 mb Height and Isotachs from 12:00 UTC 16 February 2003 through
06:00 UTC 18 February 2003. At 00:00 UTC 17 February the right rear quadrant
of the jet and confluence zone helps keep the surface high in place. The persistent
ridging over New England and confluent flow will help keep the surface high in
place which does 2 things 1) Keep the cold air in place and 2) Delay the arrival of
the coastal low. By 12:00 UTC ridging develops over the NYC region along with
weaker winds. Also note the coupled jets over the PA/NY region. The smaller jet
max over the southeast is forecast to arrive in the NYC region by 18:00 UTC.
37.
Eta 500 mb Height and Vorticity from 12:00 UTC 16 February 2003 through
06:00 UTC 18 February 2003. A large amplitude trough over the central US is
forecast to move east. The trough orientation transitions from positive to negative
through the course of the loop.
38.
Northeast topography. The rain snow line often runs from Washington to NYC to
Boston following the terrain at some elevation. This terrain helps keep the cold
air locked in during cold air damming situations (coastal wedge). For New York
City the low-level ageostrophic winds should be from about less than 50 degrees
for snow. Winds from about 70 degrees or greater typically mean rain. Cold air
often advects down the Connecticut and Hudson river valleys on a north wind.
This will bring the coldest air into NYC.
For the Boston CWA, the rain-snow line is commonly found in one of 3 locations.
Either very close to the coast (near Buzzards Bay), or up against the Berkshires or
all the way to the Connecticut river valley.
39.
Eta12 MSLP, winds, and precipitation from 12:00 UTC 16 February 2003
through 06:00 UTC 18 February 2003. Note the wedge of high pressure on the
east side of the Appalachins, firmly in place before the storm begins. 1040 mb
high pressure by 00:00 UTC over Quebec. The low pressure center takes shape
off the Carolina coast and deepens to 1006 mb by 00:00 UTC on the 18th - not
particularly deep but in combination with the 1032 mb high to the north, the
resulting gradient results in strong onshore winds.
Note: Coastal lows during the cold season tend to be deeper and closer to the
coast than forecast, because neither the Gulf stream nor coastal frontogenesis are
handled adequately by the models. The consequence of the low being too far east
is that the maximum frontogenesis is also too far east and thus the gradient going
from light snow to very heavy snow is often positioned incorrectly. (We will use
satellite data later to monitor the position of the low).
The Eta shows that the precip over NJ/NYC begining around 06Z on the 17th.
The trend between 12 and 18Z is for a decrease in the areal coverage / max
amounts of precipitation. It also keeps a weak low to the west of the primary low
through most of this time period.
40.
GOES-East Water Vapor imagery from 07:45 UTC 15 February – 23:45 UTC 16
February, 2003. Convection along Texas Gulf coast indicates cyclogenesis.
Baroclinic leaf can clearly be seen by 16:00 UTC 15 February. Convection over
southern portions of the Mississippi river by around 19:15 UTC indicate Warm
Conveyor Belt (WCB) development with a tropical connection.
Note the jet max coming out of northeast Mexico moving north/northeast as well
as the large shortwave crossing north Texas. The jet max propagates
northeastward and later colder cloud tops can be seen over the central
Appalachians. Between 00:00 UTC 16 February and 08:00 UTC 16 February the
original WCB weakens while a new WCB forms to its east. By around 15:00
UTC 16 February the new WCB can now be seen well into the southeast portions
of the Gulf, indicating that unstable air from the tropics is being advected into the
system.
By mid-morning on the 16th, a secondary WCB (TROWAL) begins to extend
westward from the primary cyclone, possibly in response to the proximate
shortwave to the west. (Is it possible that some unstable air from the secondary
WCB helps intensify the cold air cyclone?) We can also see, beginning around
17:00 UTC (in southern IL) an apparent CCB (warmer cloud tops) stretching back
in a soutwestward direction. The regions where the CCB and TROWAL are
developing will eventually form a large deformation zone situated northwest (and
perhaps north) of the primary cyclone. This deformation will become one of the
ingredients that brings about mesoscale snow banding later in this event.
41.
MSLP and surface obs overlaid on IR from 22:00 UTC 16 February through
21:00 UTC 17 February, 2003. Analyses show surface low developing and
slowly moving north up the Atlantic seaboard while the high in southeast Canada
slowly drifts east maintaining a strong pressure gradient along the mid-Atlantic
seaboard. The dewpoints from upstate NY northeastward are very low. Notice
that the IR imagery shows a broad band of cold cloud tops. However,
precipitation during the early period is sparse from NYC northward. Remember
that cold cloud tops don’t always mean precipitation, particularly in winter. The
transition from light to snow to very heavy snow can be abrupt in these situations.
There is good agreement with the Eta during this period. Between 00:00 UTC
and 04:00 UTC the best snow is over the tri-state area. Just after 04:00 UTC a lull
develops over NYC area, while heavier precip moves into southern NJ. A region
of heavier precip begins moving up from the south in association with a tighter
surface pressure gradient but the northward movement stalls for about 3 hours
before moving into NYC around 11:00 UTC. Remember this is the approximate
time (12:00 UTC 17 February) that the Eta 300 mb forecast showed coupled jets.
42.
GOES-East Water Vapor imagery from 00:15 UTC 17 February – 07:45 UTC 18
February, 2003. Early in the loop the cold cloud tops associated with the jet max
that had moved northeast from the Gulf have arrived in the eastern PA / NYC
area. There does not appear to be a correlation between these tops and the banded
precipitation that is occurring on Long Island at this time. Notice that the WCB
and the cold air cyclone further west have become separate. Beginning around
12:00 UTC the 17th we can see a new CCB emerging from beneath the WCB in
central NC, by 16:15 UTC a cusp can be seen developing over Chesapeake Bay.
We would expect to find a developing low pressure just east of the cusp.
Remember that the Eta forecast a surface low just east of the Delmarva peninsula
at 18:00 UTC on the 17th. The satellite imagery suggests a low in the same area.
Now let’s analyze the dry slot evident in the WV imagery. The dry slot is first
observed over Georgia around 08:00-09:00 UTC on the 17th where the nose of the
jet impinges on the WCB. It continues to move northeastward and expand in
time. Draw a line to extrapolate the position of the dry slot. The line should
show that the dry slot will move east of NYC (moving across eastern
Massachusetts around 21:00 UTC). Noticing the early signs of dry slot
development enables you to draw a line to extrapolate the position and timing of
the dry slot.
Note the CCB that develops over Ohio around 08:15 UTC. Does this play a role
in terms of a more favorable environment for deformation over the region of
interest?
43.
Radar reflectivity mosaic from 00:24 – 18:00 UTC 17 February 2003. The snow
in the NYC area came in different surges (i.e. periods of heavier snowfall rates).
The heaviest snowfall occurred in the mesoscale bands, especially those persistent
for long periods over the same location. Note the deformation zone in central NY
and the banding that sets up 21Z on the 17th through 02Z on the 18th. Compare
this loop to the water vapor imagery, note that not all cold cloud tops are
associated with precipitation.
44.
GOES-East IR imagery from 01:45 UTC 16 February – 02:45 UTC 18 February,
2003. Review the conveyor belts we analyzed, the deformation zone on the
west/northwest flank of the cyclone and the effects of the dry slot. The
deformation plays a key role on the northwest flank of the cyclone.
45.
Snowfall totals for the event. The arctic air mass played a key role in locking in
the heaviest snowfall along and near the coast.
46.
Summary
47.
Intro for 5-7 December 2003 case. We are forecasting for the 12-24 hour time
period (for NYC) based on data available at 00:00 UTC 5 December 2003.
48.
Case objectives
49.
Eta 300 mb heights and isotachs forecast from 00:00 UTC 5 December, 2003.
Early on we see a confluence zone over the northeast which acts to help keep the
surface high in place. A ridge sets up off the northeast coast around 18:00 UTC.
At the same time a jet over the north central plains begins moving southward.
Diffluence over New England around 00:00 UTC 6 December combined with the
warm SST’s suggests the potential for cyclogenesis. Transition from zonal to
meridional flow as the trough deepens and moves eastward over the 24-30 hour
forecast time period.
50.
Eta 500 mb heights and vorticity forecast from 00:00 UTC 5 December, 2003.
Vorticity center associated with northern plains jet mentioned above intensifies
and rotates around the deepening trough. As the closed 500mb low approaches
the Atlantic seaboard it is blocked by the offshore high. The height gradient
increases and the low becomes deeper and more focused. By 12:00 UTC on the
6th notice the vorticity maximum over the northern Great Lakes moving
southward into the trough acting to deepen it further.
51.
Water Vapor imagery from 21:45 UTC 3 December – 23:45 UTC 4 December,
2003. This loop bring us up to the 00 hour forecast time. Notice the high clouds
over the NC, VA, KY, TN region – radar (not shown) showed this was associated
with precipitation. Very strong jet across the southeast moving towards Georgia
and the Carolinas, seems to be more intense than the 300mb analysis implies.
Well defined shortwave moving into Missouri by the end of the loop. This shows
up on the 00 hour forecast. Note the confluence over the northeast.
52.
Eta MSLP/QPF and winds forecast from 00:00 UTC 5 December, 2003. Early
on, we see the precipitation mentioned previously. From 06:00 UTC the 5th
through 06:00 UTC the 6th we see two distinct regions of precipitation. One over
the Ohio valley in association with the shear zone, the other with the gradually
developing low off the Atlantic seaboard. Elongated east-west low stretched back
to the west. The low off the Atlantic seaboard as of this time is not that deep
(1006 mb), however the strong high to the north (1032 mb) makes for a strong
pressure gradient just north of the low. Just after 06:00 UTC the coastal low
deepens and becomes dominant. QPF values quite high in the wrap around region
of this low (which is slow to move).
53.
Eta 700 mb deformation (yellow dashed), 850-700 mb frontogenesis (shaded),
700 mb heights (black), and 700 mb isotherms (red dashed) forecast from 00:00
UTC 5 December, 2003. Signature for banding potential (max in deformation
juxtaposed with warm side of frontogenesis maximum) begins to show up
between 18:00 UTC 5 December and 00:00 UTC 6 December. Potential for
banding looks good through 00:00 UTC 7 December. Mesoscale bands orient
themselves parallel to the isotherms (for the level in question). Notice how the
isotherms (and thus forecast band orientation) changes from east-west (18:00
UTC Dec 5) to north-south (18:00 UTC Dec 6).
54.
HPC surface analysis with fronts from 00:00 UTC December 5 through 21:00
UTC December 5 in 3 hour increments. We see 2 distinct weak lows early on
with a high pressure in place in the northeast providing for sufficiently cold air for
snow (except perhaps on the coast). Weak low pressure trough oriented from
Kentucky southward into Alabama. Overrunning precipitation north of the warm
front – mixed precipitation locally. Notice how strong the wind observations are
north of the low in the strong pressure gradient. Also notice the slow movement
of the low. Precip type will be a forecast issue near/along the coastline.
55.
GOES 10.7 um satellite 21:15 UTC 3 December – 21:15 UTC 5 December, 2003.
Your attention is immediately drawn to the diffluent flow (deformation) over and
east of PA/NJ, lifting north with time. This matches the forecast position shown
for the eta 700mb deformation forecast. Notice the low-level clouds off the coast
of Maine extending southward, indicating cold air advection providing for cold
enough air for snow before the main system is forecast to move into this region.
Jet max moves through Georgia and the Carolinas (early in the loop) which leads
to convection developing above the Gulf stream. Associated shear zone sets up
north of this jet, where snowfall is occurring across Indiana/Ohio (notice vort max
on the southwest side of this). By late in the loop the precip extends from the
northeast coast westward to Indiana. A jet max is moving rapidly across Virginia
moving towards the ocean, we will investigate this in more detail later.
56.
GOES-12 IR satellite image at 05:45 UTC 5 December, 2003 with Eta 6 hour
forecast boundary layer theta-e and winds. Note the tongue of high theta-e air
where the convection formed and the front developing along the coast. Recall in
the previous loop that a jet max is approaching this region.
57.
GOES Visible imagery for the daytime hours of 5 December, 2003. *loop fast
and rock* Vorticity maximum evident just west of Cincinnati. Deformation
zone stretching from the vort max northeastward (snowfall in this region of the
shear zone). Surface low off the coast is clearly visible with convection
developing along the WCB. Fast moving cirrus traveling across Virginia will be
addressed later in more detail. Note the convective appearance of the cloudiness
over the Gulf stream.
58.
Radar reflectivity mosaic 00:06 UTC 5 December – 08:00 UTC 6 December,
2003. Updated forecast simulation time of 23:48 UTC. Early in the loop, we see
the largest areal coverage of precipitation near the coast, reflectivity’s are quite
high – recall the rain/snow line was some distance inland so some of this was
rain/ice. By 08:00 UTC on the 5th we can see snowfall associated with the shear
zone extending westward over western PA, OH, and northern VA. The low
further west can be observed moving into western PA around 18:00 UTC 5
December then move eastward and lose its identity around 02:00 UTC on the 6th
as it merges with the coastal low. The area of higher reflectivity mentioned above
progresses north/northeast through eastern VA, MD, eastern PA, the Atlantic City
NJ vicinity. Note that during the later period of the loop we observe banding
signatures, more on this later.
59.
GOES-12 Water Vapor imagery from 00:15 UTC 5 December – 07:15 UTC 6
December, 2003. System digs and becomes meridional. The coastal low
continues its northeastward track along the eastern seaboard. The western low is
associated with a rotating type of shear zone (associated with sfc low just
observed).
The jet max noted earlier on the 10.7 um imagery can be picked out by the
enhanced cloudiness on its leading edge over eastern TN and western VA, by late
morning. This feature moves rapidly northeast and reaches the WCB by early
afternoon. Note the enhancement of the WCB as this jet merges with the system.
Also note the CCB over eastern NY to western/central PA.
The enhanced cloud tops move out fairly quickly, but as the western low (shear
zone) moves from Indiana eastward and merges with the coastal low (between
00:00 UTC and 03:00 UTC December 6) another intensification occurs. Note the
intensification of the coastal low as this new system moves offshore and merges
with it. Usually the shear zone doesn’t play as significant a role in these major
snowstorms because a vertically stacked low generally doesn’t have a shear zone
or it happened earlier in the evolution. However, in this case we see an elongated
deformation zone develop as the narrow jet goes by. Observing this on satellite
was important not only for the snowfall forecast in the Ohio valley but also for the
northeast coast as the shear zone interacts with the coastal low to intensify the
system.
60.
Visible imagery for the daytime of December 6, 2003. Eye of the storm clearly
visible at the surface low location. Numerous convective tops visible in the
WCB. Deformation zone and banded structure to clouds over the mid-Atlantic /
New England region.
61.
Radar reflectivity mosaic for the period 12:00 UTC 5 December 2003 – 12:00
UTC 7 December, 2003. Snowfall bands are oriented east-west early in the loop
(as the low approaches) then switched to north-south on the back side of the
circulation. Note the persistent reflectivity maximum in eastern Massachusetts,
keep in mind that the rain/snow line was in this vicinity. Around 17:00 – 19:00
UTC on the 6th we see a fast moving feature offshore (long narrow N-S line) Note
the intense band in the Hudson River Valley later in the loop (north-south
oriented), this created a strong snowfall gradient in this region. As the band
dissipated the single band transitions to multiple east-west bands.
62.
GOES IR loop for the duration of the event. Note how the small cirrus clouds we
earlier noted moving across Virginia to offshore coincides with the change in
orientation of the system (and thus the bands). Also note the secondary WCB
(TROWAL) signature that appears soon after the Ohio Valley low merges with
the coastal low. We observed various jet streaks that helped intensify the coastal
low as well as the shear zone which played a significant role in intensification.
63.
Gridded output of precipitation observations for a) 12:00 UTC December 5 –
12:00 UTC December 6. This seems to agree well with the HPC QPF slide
shown earlier. Note data are missing over Ohio, and b) 12:00 UTC December 6 –
12:00 UTC December 7. Note the two maxima that we observed on satellite, first
is the north-south in the Hudson river valley and the large area of warm advection
near the coast northwest of the low.
64.
Maximum snowfall amounts for each county for the event. Note the maximum
associated with the banding, the minimum very near the coast associated with a
period of rain and heavy amounts in New England as the low intensified while
this area was northwest of the low.
65.
Summary