Detailed Analysis
Detailed charts from the surface to 100 mb from the storm's very beginning to the very end would
be great, but I don't have unlimited storage space, so I must be selective. Even so, the 17
charts below require 1.86 MB. If some don't load, you can copy their location using your
right mouse button, load them separately into your cache, then return to the article. Save them
on your hard drive if you desire ! You may ask why the archived upper air charts are
insufficient. They aren't per se - they do illustrate basic features; though after seeing the
detailed analyses below, perhaps you can tell me. Another purpose is so that you can see
additional examples of how these charts can be analyzed if you are unfamiliar with this.
The following conventions are used on the charts :
500 mb
Geopotential height is contoured black every 60 m, labeled as decameters.
Temperatures are contoured red, labeled every 3 °C.
Black script L's are geopotential height Lows, red capital K's are cold centers.
850 mb
Same as for 500 mb, but purple is used instead of black, and geopotential height contours are
every 30 m.
Surface chart
Only winds, present weather symbols, temperatures, and altimeter settings shown.
Altimeter setting are contoured dark green, every .10 inches of Hg.
Isotherms are contoured red, every 4 °F.
Cold front is blue, trofs are dashed blue, and ridge is swiggly blue line.
Precipitation charts
Weather symbols from METAR & SYNOP reports are green.
0 & -3 °C 850 mb isotherms are red.
1270 & 1300 gpm (geopotential meters) 100-850 mb thickness contours are dark blue.
2800 & 2840 gpm 1000-700 mb thickness contours are light blue.
States and provinces are discussed, so knowledge of their locations is helpful. I first
describe the upper air charts then the precipitation charts.
2 January 1999, 00 UTC
The storm was already formed @ this time, though not yet a strong system. The 500 mb chart shows
a closed Low over E ND embedded in a broad trof over the northern Plains
(A) and a sharp, well-developed trof over the southern Plains
(B) :
The Lows and trofs correspond with cold regions. As previously discussed, temperature determines
geopotential heights - between mean sea level (MSL) and 500 mb for these. Cold conditions at 500
mb likely mean cold conditions directly below. Heights are particularly low at locations where
cold air is at both 500 & 850 mb. Thus, cold air advection (CAA) behind or
below a trof strengthens it (C), and warm air advection
(WAA) weakens it (D). Thus, seeing only this chart,
a reasonable assumption is that the trof over northern Texas (where little CAA was occurring)
would move eastward (where CAA was occurring). Likewise, that over the Northern Plains should
develop SSE (E).
The 850 mb chart reveals a cyclone over N OK, with cold and warm fronts and a prefrontal trof
extending from it :
The Low center is just ahead of the strong trof aloft (at 500 mb), with a weaker area of low
pressure extending NNW to the Low aloft. Though better defined at the surface, fronts retain
identity well into the troposphere. I analyze 2 warm fronts because 2 separate poleward surges
of warm air were evident. The top one is the leading edge of warm air with an associated trof
(which the data supports), the bottom one accompanied with a wind shift separating strong S
winds and further warming from lighter SSE winds. Typically, you'll see none of this analyzed
on an upper air chart. Rather you'll see the smoothed height contours of an objective analysis.
My analysis for this map can improve, though the height contours generally agree with gradient
balance. That's definitely not a requirement for 850 mb - especially over mountainous regions.
No winds or temperatures were reported for Lander, WY & Denver, CO because the surface
pressure was less than 850 mb - heights were extrapolated underground ! A height is always
reported for every mandatory level - above or below ground.
Strong CAA & WAA are behind the cold & warm fronts, respectively. Thus, you may expect that
the 500 mb ridge over the eastern U.S. should strengthen as should the trof over TX. Such a
pattern is considered as amplifying - meaning that if such a pattern were a wave, its
amplitude (up & down distance) would be increasing. Further eastward movement of the low-level
cold front (850 mb) would also suggest that the 500 mb trof may progress eastward. Such a trof
aloft is called progressive - moving to the same zonal direction of the flow. This can
make a person believe that the upper air heights are advected - though that's not often a bad
approximation, it's not what really happens. The wave characteristics are real though.
2 January 1999, 12 UTC
Comparing the 500 mb chart :
with its predecessor, consolidation of features is noticeable. The separate trofs became one,
with a single closed Low (closed height contours; or more properly a closed circulation, because
contour spacing is arbitrary) over the northern Plains. This Low is in the vicinity of the
coldest air. The pattern is much more amplified with a strong trof extending southward to
southern Mexico (blue dashed), and a sharp ridge extending NNW
over the Great Lakes. These correspond with the CAA & WAA advection locations below 500
mb previously mentioned. I suppose a front can even be drawn extending from SE SD thru TX and
to near the Mexican Gulf coast, corresponding with the trof line and sharply turning winds from
NW to SW.
The 850 mb low was over E MO,
much larger and a bit stronger than previously; its location still ahead of the trof aloft. Why
not much stronger ? One reason is that though the trof aloft is very well unified, it is not
very much stronger than previously. Why much larger ? Because the trof aloft now extends uniformly
from the northern Plains to southern Mexico. For reasons I shall hopefully explain soon, the
area ahead of a trof aloft (sketched green) is a preferred region for
cyclone development below - especially just equatorward of an associated Low.
On this 850 mb chart, I analyze 2 cold fronts but only 1 warm front. A pronounced wind shift
was associated with the leading cold front with associated thundershowers, some severe. This was
actually a squall line at the surface, causing storms but not a large temperature decrease. The
temperature decreases much more behind the second front I drew. The wind shift across the warm
front is quite noticeable. This is the southerly one from the previous map, with the strong S
winds behind it. The northerly one probably became diffuse and generally vanished. A strong trof
extended NW from the Low toward the coldest location on the map. This is actually behind the
trof aloft, though is at a region of sharp wind shift.
3 January 1999, 00 UTC
Maps for this time should look familiar because they were shown in the previous feature. As cold
air continued moving toward the base of the 500 mb trof and closed Low, it continued deepening
(height became lower) - a very strong closed Low over S IA :
A trof extended from it to the Gulf coast, across which winds sharply turned from a westerly
to southerly direction. When a Low becomes closed like this, development begins decreasing and
occurs more toward the center of the Low aloft. The occlusion process often begins :
Though an open frontal structure is evident at 850 mb,
where a warm sector clearly exists, the warm sector is much less defined at the surface,
where only a small area of (relatively) warm air was SE of the cyclone center over the E KY
vicinity. The eastward push of warm air to the NE of the 850 mb Low corresponds well with both
the warm front and trof analyzed. Heaviest snow was occurring over S MI, E WI (where flow off
Lake Michigan enhanced it) and N IL, where the WAA advection at 850 mb was very strong north
& east of the surface front. You can think of this region (circled navy
blue on 850 mb chart) as one where warm air was quickly rising over potentially colder
surface air, though the entire lower air mass has this rising characteristic. Note the sharp
surface ridge associated with cold air damming E of the Appalachians.
Here altimeter settings instead of isobars are plotted. Some aspects of each are
(dis)advantageous, though every station reports altimeter settings, and they are very
consistent on this chart.
3 January 1999, 12 UTC
The 500 mb Low continued strengthening while both progressing eastward and building northward
toward the coldest air :
The 850 mb Low attained near maximum intensity while moving slowly northward - a consequence of
development of the Low aloft :
Fronts are again analyzed. Though the one extending SE from the Low center appears like a warm
front, it is clearly associated with CAA behind - thus a cold front wrapping around the storm's
vigorous circulation. This can also occur with warm fronts - particularly during winter. The
bulge of isotherms at F in the strong easterly winds indicates
warm air similarly wrapping around the system. Each of these fronts would eventually lose
definition as they continued to uncharacteristic environments, though even at the surface,
a poleward-moving cold front or equatorward-moving warm front can sometimes occur.
Weather features also circulate around the system. Snow shown below on the precipitation chart
for this time was quite light at locations where it previously was heavy, and heavier N & E of
those locations (though weakening).
4 January 1999, 00 UTC
A huge closed 500 mb Low over the northern Great Lakes was evident @ this time :
which continued deepening. This did not imply continued deepening of the surface or 850 mb
cyclone though :
Though cyclone development requires more than a trof aloft, not much of a trof existed then in
the rather circular flow. Considering the area of interest, a hint of a trof is at
G - the general direction the remnants of the Low was heading.
Precipitation was generally diminishing then, though quite heavy for affected areas still. The
850 mb cyclone was accelerating NNE and occlusion was even evident at that level then (as the
closing warm sector suggests).
I drew a broken front over the E-SE U.S. on the 850 mb chart. The associated shortwave
trof/ridge couplet was also on the previous map, so I think it was a real feature.
For all of these charts, you can think of the low-level cyclone as moving with the flow aloft.
Though (again) that's not really what's happening, a good estimate for surface cyclone movement
is the 600 mb wind direction and slightly more than half its speed - lower/faster for shallow
systems and higher/slower for deeper ones.
Precipitation Charts
An interesting question is how observed precipitation types corresponded with the cyclone
characteristics and thickness thresholds I previously mentioned. Below are such a precipitation
charts for 2 January 1999 :
The storm was already beginning @ 00 UTC. You can see the snow areas to the north of
the warm front and cyclone (remember the weather symbols ?) with freezing rain further S and
rain still further S. The boundary between precipitation falling as snow and falling as rain
(including freezing rain) corresponded approximately with 1000-850 mb thickness of 1280 gpm
(though extending northward of the 1270 gpm curve in NE KS), 1000-700 mb thickness of 2825 gpm
(though similarly varying), and 850 mb temperature of -2 °C (though extending both northward
of the -3 °C and southward of the 0 °C curves). The 12 UTC chart indicates warm air
progressing poleward ahead of the cyclone and equatorward & eastward behind. Maximum precipitation
intensity was off the SW shore of Lake Michigan. Below are charts for 3 January 1999 :
Warm air continued progressing northward and eastward with progression of the cyclone.
Maximum precipitation intensity was over SE to central MI to the SW shore of Lake Michigan and
inland over WI. Thresholds for snow/rain dividing curve were similar as previously. Below is
the chart for 4 January 1999 :
All precipitation in the region was snow, after CAA occurred behind the storm system.
The thresholds I previously presented were a slight overestimate for this storm, for which
the best were generally 1000-850 mb thickness of 1285 gpm, 1000-700 mb thickness of 2830 gpm,
and 850 mb temperature of -2 °C. This occurs most often when strong WAA occurs aloft as ahead
of this storm system. For cold frontal types of precipitation, thresholds can be much greater
than that - when snow can melt thru a shallow, warm surface layer. Melting generally occurred
above 850 mb (but lower than 700 mb, where most temperatures were also subfreezing at similar
locations), and supercooled water exists.
Text and embedded images are copyright of Joseph Bartlo, though may be used with proper crediting.