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FOUS30 KWBC 250812
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
411 AM EDT Thu Jul 25 2019

Day 1
Valid 12Z Thu Jul 25 2019 - 12Z Fri Jul 26 2019


...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NEVADA AND SOUTHEAST CO TO NORTHEAST NM...

...Southwest...
Mid/low-level monsoonal flow will persist across the southwest
CONUS around a high centered on the Four Corners through the end
of the work week.  Weak impulses will provide enhanced lift beyond
the diurnal destabilization.
For example, THE 00Z nam shows 300 mb divergence maxima crossing
from near the CA/NV border across central NV. The NAM Conus Nest
shows localy heavy downpours this afternoon and evening.  Light
mean flow will persist which will continue to lead to slow storm
motions and increased risk for excessive rain.  Given the moisture
anomaly of 2 to 3 standard deviations over central Nevada and a
flash flood watch issued, an upgrade to a Slight Risk was made.

Another area where an upgrade to a slight risk was made was over
the Sangre DeCristo Mountains of northeast NM and southweast CO.
Recent diurnal showers and storms have occurred in the mountains
and both the NAM/GFS focus 700 mb vertical motions underneath 300
mb divergence maxima in this region centered on 00z Fri. low-mid
level lapse rates average near 7-8 deg C/KM, so lift in terrain
should result in formation fo showers/storms thias
afternoon/evening.
Isolated pockets of heavier rain are possible, with flooding
concerns as a result, particularly in burn scar areas.


...FL peninsula to far southeast GA...
A stationary boundary remains draped across northern Florida which
will be the focus for periodic shower/thunderstorm activity
through Thursday night. An area of 2+ inch precipitable water will
exist across this region in addition to pockets of instability.
With the upper level jet streak just to the north, the right
entrance region will assist with large scale forcing along with
mid-level impulses moving atop the surface boundary.  Therefore,
anticipate convection to move across the northern and central FL
peninsula.  Once activity initiaties across west central FL in the
morning, low to mid level westerly flow promotes movement to the
eastern peninsula, so areas from Orlando to Melbourne and West
Palm Beach should be active with showers/storms this afternoon.
Additional showers/embedded storms could develop tonight near the
front over northern FL while the east central to southeast coast
should go into a diurnal minima of activity.  A slight risk was
not issued following high flash flood guidance of 4-5 inches of
rain in six hours.

Petersen



Day 2
Valid 12Z Fri Jul 26 2019 - 12Z Sat Jul 27 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE
CENTRAL AND SOUTHERN ROCKIES AND THE CENTRAL GREAT BASIN...

...Four Corners Region...

Ridging across the region will become undercut by an approaching
shortwave moving from California into Intermountain West. As a
result, the mid-level high once situated across the four corners
region will start to shift south and west in time. This will allow
mid-level impulses rounding the ridge axis to help promote
convection along the central/southern Rockies during the
afternoon.  In addition, the aforementioned shortwave combined
with divergence aloft will enhance lift resulting in more
widespread shower/thunderstorm activity across NV/UT.  With
monsoonal moisture still available, expect precipitable water
values to climb to around 1.0-1.25 inches, which is 1.5-2.5
standard deviations above the mean. This combined with pockets of
2000+ J/kg will lead to periods of heavy rain associated with
convection during the afternoon/evening hours. Rain rates look to
approach an inch/hour in some locations. Anticipate convection
will initiate on the windward side of the higher terrain, slow to
move off the mountains until later in the afternoon.  Feel the
best training potential and slower storm motions will be across
the central/southern Rockies where the flow is a bit weaker
overall and mean flow is well aligned with the corfidi vector in
both direction and magnitude. Thus, the QPF across this region is
higher overall.  If models continue to illustrate higher QPF
amounts and Day 1 rainfall leads to more sensitive soils in this
region, a Slight Risk may be warranted. Regardless, at this time,
isolated to localized flash flooding is expected given the
terrain/soil vulnerability in this region and residual burn scars
that currently exist.

Pagano

Day 3
Valid 12Z Sat Jul 27 2019 - 12Z Sun Jul 28 2019

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL THAT EXTENDS
FROM THE SOUTHWEST INTO THE NORTHERN PLAINS...

...Southwest/Rockies/Northern Plains...

Mid-level ridging holds strong across the southwest CONUS.
Meanwhile, a weak shortwave will undercut the ridge as it moves
across the northern/central Rockies into the Northern Plains
forcing the mid-level high to shift farther to the southwest
Saturday into Saturday night.  As a result, impulses riding the
ridge will interact with the higher terrain and surface boundaries
draped across the central/southern Rockies.  In addition, the
aforementioned shortwave will move east from eastern WY into SD
with strong divergence aloft moving atop a surface front. This
will promote strong atmospheric lift leading to more organized
convection across this region.  As low level southerly flow
increases, it will allow precipitable water values to respond
accordingly with values climbing to around 1.25 inches, near 1.5
inches into SD along the front.  Modest instability is also noted
with pockets of 2000+ J/kg across the Rockies and >3000 J/kg along
the front draped across SD.  With plenty of lift, moisture and
instability, expect convection to develop during the afternoon
across the higher terrain in CO/NM with a more organized area of
thunderstorms developing in eastern WY into SD during the evening
hours. The latter convection will likely ride the instability
axis.  The models are struggling to depict the exact
front/instability gradient and overall movement of the shortwave.
Therefore, confidence is slightly below average with both the
placement and intensity of the precipitation.  However, if QPF
increases and model continuity improves, feel a Slight Risk will
likely be warranted across a portion of this area as soils will be
vulnerable to additional rainfall.  For now have placed this
region in a Marginal Risk to account for the isolated to
potentially localized flash flooding.  Added a Marginal Risk
across the Teton Range and Beartooth Mountains to account for the
vulnerable terrain and soil.

Pagano


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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