Hi Everyone,
Prompted by the ongoing
flooding on the Mississippi River that in spots is rivaling and
exceeding all-time records set in 1927 and 1937, I would like to present
a further synoptic-scale perspective
on the antecedent flow conditions that enabled the flooding to occur
(the presence of substantial snow melt from a deep snow cover over upper
Midwest certainly contributed significantly to the flooding problem as
well). This post should be viewed as complementary
to a previous posts by a number of you to the map listserv. I will
concentrate on the period from 11-29 April 2011 when most of the heavy
rains and related severe weather occurred. Most of the attached images
were derived from the NOAA/ESRL/PSD interactive
web link and are based on the coarse resolution 2.5 degree gridded
operational analyses. A few additional images were derived from
NCEP/CPC.
The attached anomalous
precipitation rate (caveat: may be as much model fiction as reality)
shows a core positive anomaly of 4-12 mm/day that runs from extreme
northeastern Texas to Ohio and western
Pennsylvania A secondary precipitation rate maximum extends from Lake
Huron to Quebec. These two features are the only significant positive
rainfall rate anomalies over the U.S. and Canada during this period.
Attached time series of accumulated precipitation
(source: NCEP/CPC) for the 90-day period ending 6 May 2011 for New
Orleans (NEW) and Lake Charles (LCH), Louisiana, show that virtually no
rain fell at NEW in April while at LCH the only significant April rain
fell on 25-26 Apr when almost 90 mm was measured.
Farther north at Cincinnati (CVG), Ohio, rainfall was more frequent and
substantial, and accumulated to ~300 mm (see attached image) between
11-29 April. These rainfall accumulation snapshots from NEW, LCH, and
CVG are consistent with the model-derived precipitation
rate anomaly map in showing that the bulk of the rainfall occurred north
of the Gulf coast from the Mississippi Valley to the Ohio Valley.
The aforementioned rainfall
analysis serves as background information to a discussion of mean and
anomaly maps (attached) of 925 hPa equivalent potential temperature, 850
hPa temperature, 850 hPa geopotential
heights, 850 hPa vector winds, 850 hPa moisture flux, 700 hPa moisture
flux, 850 and 700 hPa lapse rates (centered on these levels and
expressed in deg C per km), 250 hPa geopotential heights, and 250 hPa
vector winds. A comparison of these additional figures
enables the following points to be made about the antecedent structure
of the large-scale flow pattern over North America prior to the onset of
major Mississippi River flooding.
1) A corridor of positive 4-8 K equivalent potential temperature
(theta-e) anomalies at 925 hPa extends northward from eastern Mexico
across Texas and then northaestward across the Ohio Valley. This
anomalous 925 hPa theta-e corridor coincides with the axis
of positive anomalous precipitation rate. The anomalous theta-e gradient
which arises from negative theta-e anomalies to the northwest and
positive theta-e anomalies to the southeast when compared with the mean
and anomalous 850 hPa temperatures suggests that
the anomalously heavy precipitation rate is concentrated along the
southern edge of an anomalously strong lower tropospheric baroclinic
zone.
2) At 850 hPa, a southwesterly low-level jet (LLJ) between 11-13 m/s is
present from extreme northeastern Texas to Kentucky. This LLJ, manifest
by positive wind speed anomalies of 4-7 m/s. coincides nicely with the
corridor of positive anomalous theta-e values
at 925 hPa and the warm boundary of the primary 850 hPa theta-e and
temperature gradients. The mean and anomalous 850 hPa geopotential
heights are consistent with the observed LLJ and show the importance of
an anomalous trough over the high Plains and a strong
Bermuda anticyclone in driving an anomalously strong LLJ along the
observed rainfall corridor.
3) The moisture flux (mixing ratio times vector wind) at 850 hPa extends
from the western Caribbean, northwestward across the western Gulf of
Mexico, and then northward and northeastward from Texas to the Ohio
Valley and is a maximum in the latter region. The
850 hPa moisture flux positive anomaly maximum coincides very well with
the corridor of maximum moisture flux. At 700 hPa, the moisture flux
positive anomaly maximum is farther to the northeast and extends from
the Ohio Valley to New England, suggestive of
moisture transport along an upward-sloping path consistent with
large-scale ascent on the warm side of an existing baroclinic zone and
consistent with point 1) above. The near-absence of rainfall at NEW (and
the modest rain in a single storm at LCH) during
this period suggest that moisture from the western Caribbean and Gulf of
Mexico crosses the Gulf coast but is only realized as precipitation
farther north when it is forced to ascend where the 850 hPa LLJ
intersects the aforementioned 850 hPa baroclinic zone.
The observed 850 hPa moisture flux in this case is also broadly similar
to the vertically integrated moisture flux for June and July 1993 during
a warm season episode of severe flooding along the Mississippi River
(Bell and Janowiak 1995; see their Fig. 15).
4) The mean and anomaly lapse rates centered on 850 hPa (usual caveats
apply about lapse rates in the rocks over the western U.S.) suggest that
the boundary layer is moderately and anomalously stable along the Gulf
coast beneath the moisture inflow region and
poleward of the baroclinic zone from the Great Lakes to eastern Canada
while boundary layer lapse rates are steeper, but still anomalously
stable, in the dry air over western Texas and eastern New Mexico. The
mean and anomaly lapse rates centered at 700 hPa
tell a different story. Steeper and anomalously less stable lapse rates
prevail from eastern Mexico to eastern Texas and eastward across much of
the Southeast, consistent with the presence of anomalously frequent
and/or stronger elevated mixed layers (EMLs)
that are surface-based over the elevated terrain of northern Mexico in
the region of frequent severe weather outbreaks.
5) The 250 hPa geopotential height patterns feature a strong zonally
oriented flow across the U.S. with anomalous ridges centered over the
Southwest and, especially, east of the Middle Atlantic and New England
regions. These twin positive 250 hPa height anomaly
centers sandwich a weak trough over the southern Plains. Likewise, these
twin 250 hPa positive geopotential height anomaly centers when taken
together with the large negative geopotential height anomaly center over
much of central and northern Canada are indicative
of an anomalous strong 250 hPa jet across the U.S. More specifically,
the mean and anomalous 250 hPa vector wind maps show that an anomalously
strong (17-23 m/s) westerly jet enters the U.S. over northern
California and southern Oregon and then dives southeastward
toward the Texas panhandle. A prominent jet-exit region is situated over
the Red River Valley of the South. An even stronger anomalous 250 hPa
jet (20-25 m/s) is situated over eastern Canada with a prominent
jet-entrance region located over the middle Mississippi
Valley. A coupled 250 hPa jet structure is clearly indicated in the
time-mean flow for 11-29 April over the lower Mississippi Valley. The
entire pattern is also consistent with time-mean anticyclonic wave
breaking.
The bottom line here is that
the "big wet" from the lower Mississippi Valley to the Ohio Valley and
the record-breaking severe weather outbreaks that is observed along the
southern flank of the "big
wet" can be linked to strong dynamical forcing in the form of key
players that include a coupled 250 hPa jet pattern, a well-defined
downstream equatorward jet-entrance region at 250 hPa, and an
anomalously strong 850 hPa LLJ. An anomalously strong poleward
moisture flux along the LLJ corridor, an anomalously strong baroclinic
zone that favors warm-air advection and implied ascent along the
southern edge of this baroclinic zone, an anomalously high theta-e
corridor at 850 hPa that coincides with a relatively
stable boundary layer beneath a relatively unstable lower tropopshere
associated with an EML (cap) rounds out the key players in this flooding
event on the thermodynamic side of the ledger. The resulting favorable
thermodynamic environment is supportive of
organized severe weather-producing deep convection that is triggered by
cap-breaking dynamically forced ascent associated with transient
disturbances (not shown) that are embedded in the aforementioned jets.
Not addressed here is to what
extent low-frequency tropical heating anomalies helped to condition the
anomalous middle latitude flow patterns discussed above. A thorough
investigation of the problem
would need to consider the tropical connection to the observed highly
anomalous jet stream circulation that prevailed over North America
during 11-29 April 2011.
Reference:
Bell, Gerald D., John E. Janowiak, 1995: Atmospheric Circulation
Associated with the Midwest Floods of 1993. Bull. Amer. Meteor. Soc.,
76, 681–695.
Lance
