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FOUS30 KWBC 270826
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
426 AM EDT Sat Jul 27 2019

Day 1
Valid 12Z 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...


...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.

Hurley/Jackson


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

......THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE UPPER
MISSISSIPPI VALLEY INTO THE UPPER GREAT LAKES...

...Northern Plains across the Upper Mississippi Valley into the
Upper Great Lakes...
Deepening moisture and increasing instability ahead of a long wave
trough and associated cold front crossing the Northern Plains into
the Upper MS Valley and Upper Great Lakes feeds convection capable
of producing excessive rainfall during Day 2. There was generally
good model agreement concerning the overall synoptic setup
(outside of the 00z GFS, whose solution continues to be weaker
with the frontal passage), so the WPC QPF and Excessive Rainfall
Outlook were based on a multi model blend.

Convection is expected to be ongoing (and weakening) as it tracks
from SD into MN during the first part of Day 2. Cloudiness
associated with weakening activity could delay heating across MN
into WI during the morning hours. After the clouds break,
instability climbs to 2500/3550 J/KG of MLCAPE stretching from
eastern NE/IN across southern MN into WI and the northern portion
of the LP of MI ahead of the front and long wave trough, peaking
between 29/00z and 29/06z.

The developing convection is fed 2.00+ inch precipitable water air
(which is about two standard deviations above the mean) on a 25/35
knot low level southwest flow during this time frame. 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. Simulated radar
imagery from the 00z NAM CONUS Nest indicated that rounds of
progressive convection cross MN and WI, which could result in
areas with repeating cells.

This airmass could support hourly rainfall rates near 1.50 inches
(as shown by the 00z NAM CONUS Nest), and there is a multi model
signal for 2.00/2.50 inches of rainfall over central MN into
northern WI, closest to the best instability gradient. Three hour
flash flood guidance values are as low as 1.50 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 placed over much of
southern and central MN into northern WI.


...New Mexico...
The mid level ridge eases to the west during Day 2, 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 rates near an inch (in agreement with
the 00z NAM CONUS Nest) 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 placed over a portion of east central NM.


...Northeast...
Moisture and instability south of a frontal boundary extending
from southern Ontario into northern ME could feed convection
capable of producing heavy rainfall during the afternoon and
evening hours of Day 2. 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, mainly in the 28/18z to 29/03z
time frame.

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. With this in mind,
no excessive area was assigned, but if later model solutions
indicate a better threat, a Marginal Risk could be needed here in
later forecast.

Hayes



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...

...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 during Day 3. While the best thrust
of moisture is expected across southern AZ (where precipitable
water values rebound to near 1.75 inches before 30/12z), 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 near 30/00z.

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
extended across southeast AZ for Day 3.


...Lower Mississippi Valley into the Ohio 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 during Day 3.
The best moisture feed ahead of the front is expected across MO
into AR/OK and LA, generally between 29/18z and 30/03z. 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. The 00z NAM is the most bullish with the
convection, showing 2.50 inch rainfall amounts from MO into
northeast OK, while the remainder guidance is further south with
the QPF (and not quite as high with the amounts). At this point,
it is not clear just how extensive the areal coverage of
convection will be, and it could be that the main threat is
limited to a small location.

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. Based on this, and the spread in the
placement of the highest rainfall amounts with the storms, no
excessive area was assigned here for Day 3. However, if later
model runs show better agreement with the placement of the highest
rainfall amounts ahead of the front, a Marginal Risk could be
needed here in later forecasts.

Hayes


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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