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FOUS30 KWBC 260803
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
402 AM EDT Fri Jul 26 2019

Day 1
Valid 12Z Fri Jul 26 2019 - 12Z Sat Jul 27 2019


...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE
SOUTHERN ROCKIES...

...Central and Southern Rockies and Great Basin...

Afternoon convective development will occur over the CO/NM Rockies
following destabilization and upslope flow to generate lift.  With
monsoonal flow continuing, expect precipitable water values to
climb to 1.0 to 1.25 inches. This
combined with moderate instability will lead to locally heavy
rain.  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.
Locally heavy rain is possible where cell mergers occur.  NAM
forecast soundings show low directional shear today, promoting
localized training.

Other cells are forecast to develop on the central high Plains in
the vicinity of a wavy front in western NE, where dewpoints are as
high as the mid 60s.  Cells are expected to develop in the peak of
the diurnal cycle, with slow motions possibly leading to locally
heavy amounts, with several high res models showing a couple of
inches of rain.  Higher flash flood guidance values precluded a
more significant
threat.

...Upper Peninsula of MI...

The models have increased amounts this cycle over the western UP
of MI as a 700 mb shortwave rotating under the upper trough in
southern Canada induces a combination of well defined 700 mb
convergence and theta-e advection crossing from northeast MN
through the UP of MI Fri evening.  Convection generates in
northeast MN during the peak diurnal period and continues east
southeast across the UP of MI.  The high res WRF ARW/ARW2, NAM
Conus Nest, and NSSL WRF all forecast a could of inches of rain,
near flash flood guidance.  Runoff problems could stem from
localized training in the area Fri night.


Petersen



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 become focused along and ahead of a
frontal boundary crossing northeast WY into SD, which supports
convection capable of producing excessive rainfall. There was
generally good model agreement concerning the evolution of the
synoptic scale systems (though the 00z GFS may be too weak with
the front over SD), so the WPC QPF and Excessive Rainfall Outlook
were based on a multi model blend.

Short wave energy ejected from a long wave trough over western
Canada crosses the Northern Rockies during Day 2, pushing a
frontal boundary over the Northern Rockies into the Northern
Plains, mainly during the second half of Day 2. Ahead of the front
and short wave energy, a 20/30 knot low level southwest flow
transports 1.75 inch precipitable water air (which is between two
and three standard deviations above the mean) across northeast WY
and western SD, generally between 27/18z and 27/00z. Model
soundings showed 1500/2000 J/KG of MLCAPE over this area, and the
combination of moisture and instability are expected to feed
convection over northeast WY before 28/00z, which tracks into
western SD between 28/00z and 28/06z.

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. Hourly rainfall rates
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 western into central SD. Three hour flash guidance
values are as low as 1.00/1.50 inches, as 7 day rainfall amounts
across western SD has been 300+ percent of normal.

Based on the above, a Slight Risk was placed over northeast WY
across much of western and central SD. As the convection moves
east late in the period, the threat of flash flood continues, but
the storms could be more progressive over eastern SD before
28/12z.


...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 during Day 2. 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.

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 27/18z to 28/00z time
frame. After the convection develops, it trundles 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 rates in excess of an inch (per the 00z
NAM CONUS Nest). 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, a Slight Risk was placed over portions of
eastern NM into central NM for Day 2.

Hayes

Day 3

The Day 3 outlook will be updated by 0830Z.


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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