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FOUS30 KWBC 251558
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Thu Jul 25 2019

Day 1
Valid 16Z Thu Jul 25 2019 - 12Z Fri Jul 26 2019


...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NEVADA, COLORADO, & NEW MEXICO...

...Southwest...
Mid/low-level monsoonal flow will persist across the southwest
CONUS around a high moving southwest from the Four Corners.
Warming at 500 hPa should allow for some downtown across AZ today
when compared to previous days.  Weak impulses will provide
enhanced lift beyond the usual diurnal destabilization.  Light
mean flow will persist which will continue to lead to slow storm
motions and increased risk for excessive rain.  RAP guidance shows
a 700 hPa convergence zone across central NV which, along with
portions of the Sierra Nevada, could act as foci for
convergence/lift.  Given the moisture anomaly of 2 to 3 standard
deviations over central Nevada with most of the state forecast to
have precipitable water values < 1" and an existing flash flood,
the Slight Risk area was maintained.

Another slight risk area is over the Sangre De Cristo Mountains of
northeast NM and southeast 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 within the region of light northwest
flow.  Low-mid level lapse rates average near 7-8 deg C/KM, so
lift in terrain should result in formation of showers/storms this
afternoon/evening.  Isolated pockets of heavier rain are possible,
with flooding concerns as a result, in burn scar areas and areas
with low flash flood guidance in the Front Range.


...Northern Plains/Upper Midwest...
A front and outflow boundary could act as foci for convection
today.  The mesoscale guidance is scatter shot, with a general
signal for heavy rain in northern MN and a less obvious signal
near the IA/SD border.  Temperatures at 700 hPa do not appear to
be prohibitive, generally in the 6-9C range.  Low-level
convergence is forecast to broaden and increase with time, with
the low level boundaries moving least in IA/SD.  Precipitable
water values are sufficient for thunderstorms with potential heavy
rainfall, with 1.25-1.75" forecast.  A shortwave moving across
central SD is allowing for an area of 25 kts effective bulk shear
which could organize storms.  Hourly rain totals to 2" should be
possible.  Due to the uncertainty in the guidance presently and
the two week precipitation anomalies in the area being generally
below average, yet the favorable ingredients, thought it prudent
to introduce a marginal risk of excessive rainfall for the region.


...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
which maximize in the afternoon hours.  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, an abnormal situation for July.  Therefore,
anticipate convection to move across the northern and central FL
peninsula and storms with higher rain rates focusing near the East
Coast where sea breeze boundaries/outflow boundaries collide and
near the front itself.  Once activity initiates across west
central FL in the morning, low to mid level westerly flow promotes
movement to the eastern peninsula.  A slight risk was not issued
due to their high flash flood guidance -- rainfall during the past
couple weeks has been a bit below normal in the eastern peninsula.

Roth/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 Bear tooth 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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