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FOUS30 KWBC 270802
QPFERD

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

Day 1
Valid 01Z Sat Jul 27 2019 - 12Z Sat Jul 27 2019


...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL NEAR NEW MEXICO &
THE WASATCH MOUNTAINS OF UTAH...

...Central and Southern Rockies and Great Basin...
Convection over the Colorado/New Mexico Rockies has been able to
fire in an area where Precipitable Water values were in the 1.0 to
1.25 inch range owing to the elevated heat source of the terrain
and low level upslope/convergent flow.  This, combined with
moderate instability, will continue to lead to locally heavy
rainfall with rates in excess of an inch per hour.  will lead to
locally heavy rain, perhaps as high as 2" an hour.  Areas with
prior showers/storms have reduced 3-6 hr flash flood guidance,
between 1-1.5 inches on 3 hours, thus the need for a slight risk
area in the terrain of southeast CO across NM, including the
Sangre de Cristo and Manzano Mountains.

A shortwave trough moves east from California into Intermountain
West today.  As a result, the deep high centered over the four
corners region will shift south and west in time. This will send
further mid-level impulses around the ridge and help promote
widespread shower/thunderstorm development from over NV/UT due to
insolation within the disturbance's dry slot, currently shifting
northward across central NV and southern UT. Locally heavy rain is
possible where cell mergers occur, up to 2" an hour.  NAM forecast
soundings show low directional shear today, promoting localized
training.  An upgrade to a slight risk was coordinated this
morning with the SLC/Salt Lake City forecast office.


...Central Plains...
Convection flared up again from the Western High Plains to the
Central Plains along and near a wavy and poorly defined boundary.
Until the storms dissipate shortly after sunset, storms will be in
an environment where CAPE values early this evening were in excess
of 1500 J per kg but forecast to drop to 500 - 750 J per kg by
06Z.  Southerly flow at 850 mb will be feeding moisture into the
area, allowing for a few cells to produce intense rainfall and
isolated flash flooding problems.  Once cells begin to move, they
should be generally be moving southeasterly paralleling the low
level thermal field.  The 12C isotherm still seemed to be working
as a guide for its movement.  Maintained the Marginal Risk
area...thinking that the better risk of heavy rainfall was over
the Sandhills...and rainfall rates should be tapering off as the
cells move out the that area.


...Western Great Lakes to Upper Midwest...
Expanded the Marginal Risk area southward to encompass a second
area of active convection that was clipping the northern portion
of the Twin Cities and areas downstream into Wisconsin.  Several
rounds of convection have been moving into an area with an
unstable atmosphere despite fairly modest moisture.  The 00Z
sounding from KMPX showed over 4000 J per kg with under 1 inch of
Precipitable Water...with the storms being fed moisture on
southwesterly flow at the lowest levels.  Farther north...little
change made to the on-going Marginal Risk area over parts of the
Minnesota Arrowhead into parts of the western Great Lakes.  A 700
mb shortwave rotating under the upper trough in southern Canada
combined with well defined 700 mb convergence and theta-e
advection has resulted in convection over parts of the Arrowhead
which has produced locally heavy rainfall.  That convection will
continue eastward southeast across the UP of MI.  The high res WRF
ARW/ARW2, NAM Conus Nest, and NSSL WRF all forecast local amounts
of 1-2" of rain which was near flash flood guidance, though only
in a couple random locations. While the storms should be
progressive given the deep layer flow around the southern Canadian
trough, some brief back-building of storms can't be ruled out
which leads to localized flash flooding.

Bann


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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