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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0105Z Sep 20, 2019)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
905 PM EDT Thu Sep 19 2019

Day 1
Valid 01Z Fri Sep 20 2019 - 12Z Fri Sep 20 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS EASTERN
TEXAS, WESTERN LOUISIANA, SOUTHWESTERN ARKANSAS AND FAR
SOUTHEASTERN OKLAHOMA...

...Eastern Texas and Oklahoma/Lower Mississippi Valley...
Locally heavy rains continue fall along an east-west convergence
axis currently sinking south of I-10, east of San Antonio. 
However, convection across much of southeastern Texas continues to
become generally less organized and/or move into areas with drier
soil conditions/higher flash flood guidance values.  While
additional widespread flash flooding is not expected, the
potential for more scattered flash flooding is expected to
continue into the overnight from the Texas Hill Country, Upper
Texas and Southwest Louisiana coasts northward into southeastern
Oklahoma and southwestern Arkansas.  Recent runs of the RAP show
deep tropical moisture remaining in place (PWs 2-2.25 inches)
across the highlighted areas, with low level winds increasing as
the mid level remnants associated with Imelda lift north overnight
toward the ArkLaTex region.  Several 12Z hi-res guidance members
showed locally heavy amounts developing along near the track of
the mid level center, with recent runs of the HRRR also showing
some signal for locally heavy amounts along the axis of inflow
extending back to the southwestern Louisiana and Upper Texas
coasts.  Therefore, a Slight Risk area was maintained across these
areas.  Also extended the Marginal Risk to include areas farther
to the east across the lower Mississippi valley, where slow moving
convection along a diffuse cold front may continue to produce some
locally heavy amounts into the overnight hours.
  
...High Plains...
Convection continues to redevelop this evening from the TX
Panhandle into western KS/NE along the dry line. This activity may
stay tied to the low level boundary long enough to pose a
localized flash flood threat. Do note at least some mid/upper
level connection to the tropical Pacific in the layered PW fields,
which may help increase rainfall efficiency.

Pereira/Chenard


Day 2
Valid 12Z Fri Sep 20 2019 - 12Z Sat Sep 21 2019

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL NEAR THE ARKLATEX
AND IN THE NORTHERN PLAINS...

...Northeast Texas / Southeast Oklahoma / Southwest Arkansas...
In the 2030Z update we expanded the coverage of Slight Risk here,
The risk is conditional, betting that nocturnal convergence into a
remnant mid level circulation will be sufficient to spark a new
area of heavy rainfall. There are a few lesser developed solutions
from the mesoscale models, namely the 12Z NAM CONUS Nest and
WRF-NMMB. More often than not, however, mid level vorticity
centers that originated as part of a tropical cyclone are
efficient at generating these nocturnal rainfall maxima, more like
that seen in the 12Z WRF-ARW and ARW-2. By Thursday afternoon the
mid level circulation was evident over the Texas Piney Woods,
while convergence was supporting a band of deep convection closer
to the coast of southeast Texas. This daytime convection should
gradually loosen or propagate westward, leaving room for deep
layer convergence to increase into the mid level center late
tonight as it lifts into northeast Texas. Once initiated,
southerly and southwesterly inflow of 20-plus knots should
maintain a convective cluster or two, with a fairly well portioned
balance between northward directed mean layer winds and southward
directed propagation vectors - or in other words, slow movement.
This balance could yield isolated very heavy rainfall from late
Thursday night through the first half of the day on Friday. One or
more clusters could occur as the mid level center begins to shear
and continues to lift northward, and models have some heavy rain
signal extending into the Ozarks in northwest Arkansas, hence the
need to expand the Slight Risk. Fortunately, antecedent conditions
are dry here, but tropical rain rates may overwhelm creeks and
streams. Precipitable Water values around 2.00 inches for
mid-September are well above the 90th percentile, and will
approach daily records at LZK and SHV.

We also expanded Marginal Risk westward to capture a separate
convective environment where more traditional high plains storms
are likely to form in the increasingly difluent upper flow regime
ahead of a deep western U.S. trough. Rain rates 1 to 2 inches an
hour are possible with supercell and multicell modes from the
Texas panhandle into western Oklahoma. Flash flooding will be
dependent on local cell mergers / brief training, and antecedent
wetness is not especially pronounced.


...Northern Plains...
Here in the 2030Z update we maintained a broad Slight Risk area
affecting the central and eastern Dakotas and western Minnesota
Friday / Friday night. The odds that this event would need to see
an upgrade to Moderate Risk do not appear to have trended in any
direction. The PW anomalies are impressive, and there will be a
good deal of instability. Convective mode, and widespread
overturning / cold pool production, however, along with
displacement of the heaviest QPF from the lowest flash flood
guidance values, suggests Slight Risk best captures the storyline.
We did expand Marginal Risk a little farther west to catch an
extension of weak instability toward the evolving mid level low
center expected to come out of the mountains in Wyoming and
Montana early Saturday.

An anomalously deep upper trough and associated surface front will
pull toward the northern Plains Friday night. An increasingly
coupled upper jet structure is forecast as one deep layer
shortwave lifts into central Canada during the evenign, followed
quickly by a secondary jet max rotating through the base of the
trough. The initial wave is well timed to promote vigorous
thunderstorm growth during peak heating and beyond, into the low
level jet cycle Friday evening.

Flow from 925-500mb will become southerly, driving tremendous
moist advection northward, noted by large 850mb moisture transport
vectors and PWATs surging to 1.75+", more than 3 standard
deviations above the climatological mean. As the 850mb wind
reaches 40-50 kts immediately ahead of the cold front, the
accompanying WAA pushes MUCape to 3000 J/kg. Although storm
motions are forecast to be rapid to the north as evidenced by
0-6km mean wind vectors of 25-35kts, deep layer unidirectional
shear parallel to the length of the cold front suggests training
of echoes with heavy rainfall is likely, particularly across North
Dakota with the lead wave during the evening. Somewhat lesser
coverage is expected in South Dakota, but upscale growth is
possible throughout the region owing to favorable inflow and deep
layer ascent, resulting in the broad Slight Risk area.

Burke/Weiss

Day 3
Valid 12Z Sat Sep 21 2019 - 12Z Sun Sep 22 2019

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER NORTHEAST
KANSAS AND NORTHERN MISSOURI...

...Northern Kansas to Northern Illinois...
In the 2030Z update, and along with input from WFOs Topeka and
Pleasant Hill, we upgraded to a Moderate Risk of excessive
rainfall from northeast KS into northern MO for rainfall expected
Saturday night. No doubt, it is difficult to pinpoint a mesoscale
heavy rain area this far out in time, and the bulls-eye may move a
little bit between now and Saturday. The increased lead time was
deemed worth it, though, to get the message out well before the
weekend. There are a number of synoptically evident heavy rainfall
ingredients checking a lot of boxes over this area. In simplest
terms, a deep western U.S. trough will pull out into the Plains,
running into rich moisture that is enhanced by high level blow-off
from two tropical storms in the eastern Pacific. Model QPFs are
not overwhelming in magnitude, but they are relatively consistent
in placement, and both the UKMET and ECMWF have produced greater
than 5 inch spot totals on consecutive runs. Most likely, 5-inch
amounts or greater would focus along the CAPE gradient which the
NAM predicts over the Moderate Risk area. This matches exactly to
the QPF placement in the 00Z ECMWF and 12Z UKMET. Especially of
concern is the persistence of strong 850-mb moisture flux which
sets up Saturday night to the rear of a 110-knot upper jet streak.
Despite what should be rapid storm motions to the east on 30+ kts
cloud layer winds, 0-6km mean flow parallel to the frontal
boundary suggests that training is likely. Rainfall rates should
exceed 1"/hr, and may approach 2"/hr the latter half of Day 3 as
warm cloud depths surge above 13kft to increase rainfall
efficiency. Wherever outflow becomes established as a barrier to
the moist southwesterly low level flow, some local extreme totals
pushing closer to 10 inches are not out of the question, although
with no mesoscale guidance that goes that far out in time - this
is a little bit of conjecture based on experience.

Rainfall will also be quite persistent downstream through parts of
MO / IA / IL, covered by a Slight Risk. Rain rates and event
totals should decline steadily with eastward extent, as
instability becomes exhausted in favor of the ongoing activity
back to the west. Some pronounced training could occur over
northeast Missouri and adjacent IA/IL, however, especially early
in the event Saturday afternoon. Based on the latest large scale
guidance we would expect the back edge of heavier rain should
begin to shift eastward early in what would be the Day 4 period,
Sunday.

...North Dakota...
The NAM indicates continued generation of a few hundred J/kg
MUCAPE over western North Dakota Saturday morning within the
elevated frontal zone / wrap-around moisture associated with an
ejecting, tight vort max. Given 2-week precipitation running
6-times normal, and the anticipated widespread 0.50 to 1.50 inches
occurring through Friday night, this environment looks to warrant
a Marginal Risk that will linger into the Day 3 period, daytime on
Saturday. Rain rates briefly up to one inch with convective
enhancement - per the 12Z NAM CONUS Nest - could exacerbate any
ongoing flooding.

Burke/Weiss


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt