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FOUS30 KWBC 272029
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 PM EDT Sat Jul 27 2019

Day 1
Valid 16Z Sat Jul 27 2019 - 12Z Sun Jul 28 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE NORTHERN PLAINS AS WELL AS PARTS OF THE CENTRAL AND SOUTHERN
ROCKIES...

...Southern Louisiana and Far Southeast Texas...
Daytime heating, in addition to deepening moisture along a weak
northward moving surface boundary will likely support increasing
shower and thunderstorm coverage across the region this afternoon.
 Although flash flood guidance values are relatively high across
the region (greater than 3-inches for 1-hr), weak steering flow
through the low to mid levels is expected to support slow moving
cells, raising the potential for locally heavy amounts producing
runoff concerns.  Within the Marginal Risk area, HREF 40km
neighborhood probabilities for the 12-hr period ending at 06Z are
generally 50 percent or greater for amounts exceeding 2-inches.


...Northern Plains...
The upper ridge over the northern-central Rockies will flatten
somewhat today, in response to the shortwave trough skirting the
northwest U.S./southwest Canadian border along with a
convectively-aided vort lobe that will drop into the high plains
during the evening. Strengthening s-se low-level flow ahead of the
aforementioned vort and associated surface low will help draw a
more moist and unstable airmass farther northwest and into eastern
WY and southeast MT. MUCAPES climb between 1500-2500 j/kg during
peak daytime heating hours this afternoon, then are expected to
drop below 1000 j/kg overnight. PWs climbing to 1.25" across the
high plains and to 1.50+ farther east are modestly anomalous for
late July (around 2 standard deviations above normal). Relatively
high-based convection with DCAPEs ~500 j/kg or greater, along with
the degree of vertical wind shear, will promote fairly swift cell
progression. However, the high-res CAMs show the potential for
renewed convection along/ahead of the surface warm/stationary
front, as the compact area of difluent upper flow pushes rather
slowly eastward overnight. The Slight Risk area delineates areas
of lower FFG -- i.e. 1" or less within one hour and 1-1.5" within
3 hours. The latest HREF and experimental HREF both depict the
highest exceedance probabilities across a portion of southwest SD
between 21-00Z -- with 50-60% or greater probabilities that 3
hourly QPF will exceed the current FFG.


...Central and Southern Rockies/Intermountain West...
Moisture from the Central Plains and instability over much of the
terrain and plains of CO and NM combine to produce storms with
heavy to excessive rainfall Saturday. There was generally good
model agreement with respect to the synoptic setup and placement
of the highest rainfall amounts, so the WPC QPF and Excessive
Rainfall Outlook were based on a multi model blend of 00Z guidance.

A low-level east to southeast flow continues to transport
1.00-1.25 inch PWs across the plains of CO and NM into
the terrain from the Front Range in CO into the Sangre de Cristo
Mountains of CO into northern NM. Underneath a flattening
mid-level ridge, modest instability will be present, as the models
continue to advertise MUCAPES between 1000-1500 j/kg along the
terrain, and model soundings show the cap breaking in the
afternoon. After the convection develops, it will shift slowly
southward along the terrain into the plains, as the mid-level flow
is fairly weak under the ridge.

Slow moving storms along the terrain fosters an environment
favoring cell mergers and short term training, which in turn
results in hourly rainfall totals in excess of an inch. Seven day
rainfall totals over portions of eastern CO have been 300+ percent
of normal, with three hour flash flood guidance values as low as
1.00/1.50 inches. Given the moist antecedent conditions, and the
potential for high rainfall rates with slow moving cells, the
Slight Risk was maintained over portions of eastern NM into
central NM.

Pereira/Hurley/Jackson


Day 2
Valid 12Z Sun Jul 28 2019 - 12Z Mon Jul 29 2019

......THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM A PORTION
OF THE NORTHERN PLAINS TO LAKE SUPERIOR...

...Northern Plains to Upper Great Lakes...
Deepening moisture and increasing instability ahead of a short
wave trough moving from SD into MN before merging into a long wave
trough crossing the Canadian Prairies feeds convection capable of
producing excessive rainfall Sunday into Sunday night. There
remained good model agreement with the large scale pattern, though
time and position differences with the surface low made for
differences with the QPF. WPC QPF and the Excessive Rainfall
Outlook were based mainly on a compromise between the 12Z ECMWF
and 12Z NAM Nest. The 12Z GFS remained faster a weaker with the
surface low compared to consensus.

Convection is expected to be ongoing as it tracks from SD into MN
Sunday morning. This activity will be reinvigorated as the low
deepens through Sunday as it tracks toward Lake Superior. The
developing convection is fed 1.75 to 2 inch precipitable water air
(which is 2.5 standard deviations above the mean) on a 30 to 40
knot low level southwest flow. Lift ahead of the long wave trough
should allow the convection to expand in coverage, and become
better organized as it moves over southern and central MN into
northern and central WI. The amount of instability is uncertain
with at least 500 to 1000 J/Kg MUCAPE expected.

This airmass could support hourly rainfall totals near 1.5 inches
and there is a multi model signal for 2 to 2.5 inches of rainfall
over central MN to the Lake Superior shore. Three hour flash flood
guidance values are generally 1.5 to 2.5 inches over portions of
MN into WI (including the Minneapolis metro area), and these
numbers could be exceeded where convection trains ahead of the
front. Based on this, a Slight Risk was maintained over much of
southern and central MN into northern WI and the western UP along
with some more of eastern SD to cover the ongoing storms in the
morning.


...Central Plains...
A cold front trailing the shortwave trough crossing the northern
plains will push into IA/MO Sunday night. Southwesterly low level
flow ahead of the trough will bring PWs up to 1.75 to 2 inches
Sunday night from eastern KS to eastern IA. Scattered convection
developing late Sunday afternoon over eastern NE/western IA could
organize and track east while propagating south given westerly
mean flow and northerly propagation vectors. Despite 3hr FFG of
2.5 to 3 inches, localized repeating cells could raise flash flood
concerns and a Marginal Risk was raised that extends to across IA
to southwest MO.


...New Mexico...
The mid-level ridge eases to the west Sunday and the monsoon flow
becomes suppressed temporarily. However, deeper moisture and
instability across eastern NM could support convection with heavy
to locally excessive rainfall, especially in the afternoon and
evening hours. Storms developing in an axis of 1000 J/KG of MLCAPE
are expected to tap 1.25 inch precipitable water air (which is
about two standard deviations above the mean) in place over
eastern NM to support convection capable of producing hourly
rainfall near an inch along and just west of the terrain (as the
storms moving slowly to the west with the mid-level flow).

Three hour flash flood guidance is generally near 1.50 inches and
it is possible that these amounts could occur where storms merge
or where short term training occurs. Since there is some spread in
coverage of storms during Day 2 (as instability could be a
limiting factor for a more organized flash flood threat), a
Marginal Risk was maintained over the eastern slopes of central NM
mountains.


...Northeast...
Moisture and instability south of a frontal boundary extending
from southern Ontario into northern ME could feed convection
capable of producing heavy rainfall Sunday afternoon and evening.
Weak short wave energy in the mid level flow south of the front is
expected to interact with 1.25/1.50 inch precipitable water air
and 1000/2000 J/KG of MLCAPE stretching from western NY/western PA
across NY state into northern and central New England during peak
heating.

The weakly difluent flow ahead of the short wave could allow small
clusters of storms to move across this area, generally following
the best instability axis, but much of the activity is expected to
be fairly short lived in nature. For the most part, the mid-level
flow should be sufficient to keep the convection progressive,
limiting an organized flash flood threat. In addition, much of the
area has been fairly dry over the past seven days, with three hour
flash flood guidance values generally over 2.50 inches. While
there could be some locally heavy rainfall, the overall threat of
organized flash flood threat seems to be low. No excessive area
was assigned again, but given some ingredients, a Marginal Risk
could be needed here eventually.

Hayes/Jackson



Day 3
Valid 12Z Mon Jul 29 2019 - 12Z Tue Jul 30 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN ROCKIES AND SOUTHWEST AS WELL AS THE LOWER
MISSISSIPPI VALLEY...

...Southern Rockies/Southwest...
As the mid-level ridge reasserts itself over the Southwest states,
the monsoonal flow starts to become reestablished over the
Southwest and Southern Rockies Monday. While the best thrust of
moisture is expected across southern AZ (where precipitable water
values rebound to near 1.75 inches Monday night), the best
instability is expected across the Southern Rockies into the
Sangre de Cristo Mountains. An axis of 500/1000 J/KG of MLCAPE is
expected to develop from the Front Range of the Rockies in CO into
northern and southwest NM, peaking Monday evening.

The developing storms will tap 1.00 inch precipitable water air on
a 10/15 knot low level southeast flow to produce local 1.00 inch
rainfall amounts over the higher terrain, where the storms will be
almost stationary against the upslope flow. Once maximum heating
occurs, the storms will begin to move slowly west northwest with
the weak mid-level flow, retaining the potential to produce hourly
rainfall rates between 0.50/1.00 inches away from the terrain.
Three hour flash flood guidance values are as low as 1.50 inches
in these areas, so a low end flash flood threat is expected. Based
on the above, a Marginal risk was placed over the higher terrain
and adjacent locations.

Further west across southeast AZ, the moisture plume from northern
Mexico possesses more moisture than
CO/NM, but instability could end up being the limiting factor for
a more organized flash flood threat. Scattered storms could
produce local 1.00 inch rainfall amounts over the higher terrain
before moving out into lower terrain, but the general lack of
forcing would suggest that the storms should be short lived. Slow
cell motions under the ridge position could allow for cell mergers
and short training. Based on the above, the Marginal Risk was
maintained from the San Juans to much of the higher terrain in NM
to the southeast corner of AZ.


...Lower Mississippi Valley...
Moisture and instability extending ahead of a slowing cold front
are expected to feed convection that could produce locally heavy
rainfall from the Lower MS Valley into the OH Valley Monday into
Monday night. The best moisture feed ahead of the front is
expected across MO into AR/OK and LA during and just after peak
heating. Model soundings showed an axis of 1500/2500 J/KG of
MLCAPE coincident with the deepest moisture, and developing storms
are expected to drop southeast with the mid-level flow.

There is a model signal that a cold pool could form with storms
over AR, resulting in the storms dropping south into northern LA
as they weaken. Three hour flash flood guidance values are
generally above 2.50 inches from MO into northeast OK, and above
3.50 inches over much of the Lower MS Valley. Given the organized
thunderstorm threat, raised a Marginal Risk for southern MO/much
of AR and the lower MS Valley with an understanding that this
threat area should become more focused in the future.

Hayes/Jackson


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


$$




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