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FOUS30 KWBC 270043
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
842 PM EDT Fri Jul 26 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 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 PORTIONS OF COLORADO AND NEW
MEXICO...

...Northern Plains...
Moisture and instability increase ahead of a shortwave trough
crossing the northern Plains Saturday night. Convective capable of
producing excessive rainfall are expected. There was good model
agreement with the evolution of the synoptic scale systems and QPF
among the 12Z guidance, which is a little farther south toward the
instability axis than the 00Z guidance. Therefore, the WPC QPF and
Excessive Rainfall Outlook were based on a multi model blend of
12Z guidance.

Shortwave energy ejects from a long wave trough over western
Canada crosses the Northern Rockies Friday night, with an
associated surface low developing over the central High Plains
Saturday. Ahead of the front and short wave energy, a 20 to 30
knot low level southwest flow transports 1.75 to 2 inch
precipitable water air (which is between two and three standard
deviations above the mean) across northeast WY and western SD by
peak heating Saturday. Expect instability (1500/2000 J/Kg MLCAPE)
and the to feed organized convective development over northeast WY
Saturday afternoon, which tracks into western SD that evening.

Initially, the convection is expected to be mainly outflow and
hail dominated over northeast WY, but there is a window for short
term training just ahead of the short wave axis. Hourly rainfall
totals could exceed 1.25 inches here, where three hour flash flood
guidance values are as low as 1.50 inches. As the convection
crosses western SD, there is a window for short term training and
possible cell mergers before the front moves over the region.
There is a multi model signal for local 2.00/2.50 inch rainfall
amounts across southwest SD into northern NE. Three hour flash
guidance values are as low as 1.00/1.50 inches (north of the NE
Sandhills), as 7 day rainfall amounts across western SD has been
300+ percent of normal.

The Slight Risk was shifted a bit south over northeast WY across
much of western and central SD to the NE border. The convection
moves east overnight with the threat of flash flood continuing.
Given timing differences, the Marginal Risk still covers eastern
SD.


...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 12Z guidance.

A low level east to southeast continues to transports 1.00/1.25
inch precipitable water air 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. Under the mid level ridge,
instability reaches 1000/1500 J/KG along the terrain, and model
soundings show the cap breaking in the afternoon. After the
convection develops, it shifts 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.

Hayes/Jackson

Day 3
Valid 12Z Sun Jul 28 2019 - 12Z Mon Jul 29 2019

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

...Upper Mississippi Valley into the Upper Great Lakes...
Deepening moisture and strong instability ahead of short wave and
cold front crossing the Northern Plains into the Upper MS Valley
and Upper Great Lakes feeds convection capable of producing
excessive rainfall Sunday. There was generally good model
agreement concerning the overall synoptic setup, so the WPC QPF
and Excessive Rainfall Outlook were based on a multi model blend
of 12Z guidance which is a bit farther south than 00Z.

Convection is expected to be ongoing (and weakening) as it tracks
from SD into MN Sunday morning. Debris cloudiness from this
activity could delay heating across MN into WI. However, ahead of
the front and mid- level trough approach from Manitoba/MT,
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, peaking Sunday evening.

The convection develops in 2.00+ inch precipitable water air
(which is about two standard deviations above the mean) on a 30/35
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 to the western UP. This airmass could
support hourly rainfall rates near 1.50 inches, 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 shifted south
a bit, but still covers far eastern SD, much of southern and
central MN, northwestern WI and the UP from the Keweenaw Peninsula
and west. Given this narrow axis of instability, the Slight risk
area is expected to continue to waiver a bit with subsequent
forecasts.


...New Mexico...
While the mid-level ridge eases to the west, and the monsoon flow
becomes suppressed temporarily, deeper moisture and instability
across eastern NM could support convection with heavy to locally
excessive rainfall Sunday. Storms developing in an axis of 1000
J/KG of MLCAPE extending from KTCC to KSRR 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
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 be realized where storms
merge or where short term training occurs. Therefore, the Marginal
Risk was maintained over eastern slopes of the mountains of
central NM.

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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