Quantitative Precipitation Forecast
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FOUS30 KWBC 221920

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
319 PM EDT Mon Apr 22 2019

Day 1
Valid 16Z Mon Apr 22 2019 - 12Z Tue Apr 23 2019


...Southern Rockies to the Southern Plains...
A slow-moving cold front pushing south down across the southern
Plains today will become the focus for increasing coverage of
showers and thunderstorms initially this afternoon from far
south-central Colorado down through much of central and eastern
New Mexico. Then closer to 00Z and going into the evening hours,
the convection across the southern Rockies and immediate High
Plains will develop eastward in vicinity of the aforementioned
front and spread over western and northwest Texas and adjacent
areas of southwest to central Oklahoma. The amplifying closed low
over the Southwest will be ejecting gradually off to the east and
this will foster a broadly upper difluent pattern over the
Southern Plains and will help strengthen the moist low-level flow
into the front with a more focused area of stronger surface
convergence. Areas farther west back across the higher terrain of
the southern Rockies and especially the Sangre De Cristo mountains
will see a favorably moist easterly low level flow regime set up
that will favor a more efficient upslope flow contribution to the
heavy rainfall threat. PW values farther east along the front are
expected to rise to 1.5-2+ standard deviations above normal in a
region of increasing instability, with MUCAPE values forecast to
rise to as high as high as 1000 to 1500+ j/kg from west Texas
northeast to central Oklahoma. This will support widespread
convective rainfall this evening as the upstream energy/height
falls encroach on the region. The 12Z hires guidance favors some
of the heaviest rainfall initially over the southern Rockies and
especially along the eastern slopes of Sangre De Cristo mountains
where as much as 2 to 3+ inches of rain may fall through this
evening. Heavier rainfall rates of as much as 1 inch/hr and
snowmelt, coupled with localized burn scar sensitivities will
enhance the threat for flash flooding. Therefore, a Slight Risk of
excessive rainfall has been introduced across generally the higher
terrain of the southern Rockies from far south-central Colorado
down across central and eastern New Mexico. Farther east across
areas of northwest Texas and into central Oklahoma, the 12Z hires
guidance has trended somewhat wetter and is now favoring as much
as 2 to 4+ inches of rain this evening with a rather fair degree
of convective organization expected along the front. These totals
are likely to result in at least some isolated instances of flash
flooding, and thus a Slight Risk of excessive rainfall has been
introduced for this region as well.

...Upper Mississippi Valley into the Upper Great Lakes...
The stationary front stretching east-northeast from the Upper
Mississippi Valley into the Upper Great Lakes will continue to be
a focus for elevated convection to its north through the afternoon
and early evening hours. Persistent overrunning of this front in
an axis of PW values that are 2+ standard deviations above normal
will continue the heavy to locally excessive rainfall threat.
Initially the threat will focus from north-central Minnesota to
northwest Wisconsin and across the U.P. of Michigan where some
training of heavy rainfall is possible. Later this afternoon and
through the evening, some scattered convection is expected to
develop farther to the south ahead of a well defined surface wave
pushing east across Iowa into southern Wisconsin in a region of
greater instability. There is good model agreement that this would
be from northern Iowa into central to southern Wisconsin, and from
the northern portions of the L.P. of Michigan into the eastern
portion of the U.P. of Michigan. Thus the Marginal Risk area has
been maintained for these areas.


Day 2
Valid 12Z Tue Apr 23 2019 - 12Z Wed Apr 24 2019


A closed, mid/upper level low will slowly move east with strong
diffluence aloft transitioning across the Southern Plains.
Meanwhile, a surface cold front will advance south and east across
Texas and Oklahoma. Prior to its arrival, a steady plume of
moisture rich air will be advecting northward from the Gulf of
Mexico aided by a 30 to 40 kts southerly flow.  Precipitable water
values will be on the order of 1.25 to 1.50 inches, equating to
roughly 1.5 to nearly 2 standard deviations above the seasonal
average. Instability noted by surface based CAPE values nearing
2000 J/kg and modest shear coupled with a potent jet streak will
lead to strong forcing for ascent and organized convection over a
portion of the Southern High Plains. The area with the best
potential for high rainfall rates along or just south of I-20 from
Midland, TX to north of Fort Worth.

Based on the ingredients discussed and high resolution model
output, hourly rain rates of over 1.5 inches/hour seems plausible
with some slow moving/training cells exacerbating the flood
potential. With a bit of uncertainty with respect to coverage and
intensity of convection and after close coordination with impacted
WFO`s. kept this area under a Marginal Risk.  Though FFG is higher
across northern TX and west/central Oklahoma, there could be high
rain rates (1.5+ inch/hour) associated with the convection that
could result in localized flooding concerns. In addition,
precipitation from ongoing convection late today across far
north-central Texas and south-central Oklahoma may produce more
sensitive soils.  We will continue to monitor the progress of the
convection and resultant precipitation observed to determine if a
portion of central Texas will be upgraded to a Slight Risk.


Day 3
Valid 12Z Wed Apr 24 2019 - 12Z Thu Apr 25 2019


The frontal progression, as discussed in the Day 2 ERD, will
continue to trek across central Texas and Oklahoma. Precipitable
water values of 1.5 to 1.75 will advect into the Southern Plains,
which is on the order of 1.5 to nearly 2 standard deviations above
the mean, aided by 25 to 35 southerly low level flow. Strong
surface convergence, instability (noted by surface based CAPE
>2000 J/kg) and enhanced lift in the mid/upper atmosphere will
help sustain widespread convection over this region. The latest
model guidance continues to show high confidence of QPF of 3 to 5+
inches across central/eastern Texas into far southwest Oklahoma.
High resolution guidance depicts multiple hours where rainfall
rates are 1 to 1.5 inches/per hour associated with convective
activity. Current 1 hour FFG for much of Texas is 2 to 3 hours,
with 3-hourly values of 2 to 4 inches. Based on the expected
setup, anticipate QPF and high rainfall rates will likely be
problematic across central Texas. After careful coordination, a
Slight Risk was maintained and refined for central TX into far
southern Oklahoma. Confidence is fairly high that some locations
will exceed FFG, however, the exact placement of the front and
thus the heaviest QPF axis is still unknown as of this issuance.
Depending on model trends and changing soil saturation from rains
expected late today and on Day 2, portions of central Texas will
likely need to be upgrade to a Moderate Risk as the threat for
flash flooding rises.


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


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