Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
419 PM EDT Wed Jun 07 2023

Day 1
Valid 16Z Wed Jun 07 2023 - 12Z Thu Jun 08 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE SOUTHERN
HIGH PLAINS NORTHWEST INTO THE LEE OF THE CENTRAL ROCKIES AND
ACROSS NORTHWEST WYOMING INTO SOUTH CENTRAL MONTANA...

...Southern Plains through the Northern Rockies...
Another day of scattered to widespread diurnal convection is
likely across much of the High Plains and Intermountain West as
the blocking synoptic pattern remains entrenched across the area.
An expansive ridge centered over the Northern Plains and an
anomalous closed low over the Great Basin will sandwich this area,
driving persistent moist advection from the Gulf of Mexico to push
PW anomalies to +2 to +3 standard deviations according to the
NAEFS ensemble tables. The corridor of highest PWs will be
collocated with MUCAPE of 500-1000 J/kg, and lay beneath a
persistent upper deformation axis, to favorably produce aftn
thunderstorms today. Weak impulses embedded within the flow will
likely yield areas of greater coverage, but with minimal overall
bulk shear, storms should generally remain disorganized today,
which is echoed by the simulated reflectivity among the various
high-res members.

This suggests that the broad MRGL risk inherited from overnight
remains warranted as any of these storms could have rain rates
above 1"/hr atop soils that are saturated from 7-day rainfall of
300-600% resulting in anomalous streamflows. The only significant
adjustment to the MRGL risk area was to expand it east into the
western Hill Country and RGV of TX where guidance indicates an MCS
may develop tonight with heavy rain rates falling atop locally
more sensitive soils.

The inherited SLGT risks were adjusted only cosmetically as they
still highlight the two areas of greatest concern. The first, from
the Front Range of the Rockies south into the TX Panhandle is
driven primarily by the most saturated antecedent soils from
repeated days of heavy rainfall. This has lowered FFG to
1-1.5"/3hrs in many areas, which has a 20-30% of being exceeded
according to the HREF. While coverage/organization of convection
may not be overly impressive here, the high risk of runoff leading
to flash flooding persists due to the more compromised soils.
Additionally, for the smaller SLGT risk across WY/MT, this area is
more firmly embedded within the greatest PW anomalies, and FFG is
reduced even more to as low as 0.5"/3hrs due to recent heavy
rains. Any storms that move across this area could quickly produce
runoff to renew flash flooding.


...Northern Sierra/Northern California into portions of the Great
Basin...
The anomalous closed upper low over CA this morning will begin to
drift northward while slowly filling today. The total latitude and
height gain will be pretty minimal today however, so significant
forcing for showers and isolated thunderstorms is still expected
across northern CA and into the Great Basin. The most significant
adjustment today should be a reduction in coverage over central CA
which did experience heavy rain on Tuesday, so the MRGL risk has
been trimmed just slightly from the south. However, the overlap of
impressive PWs noted by NAEFS standardized anomalies reaching
above +2 sigma and MUCAPE as high as 1000 J/kg will still support
efficient rain rates reaching 1"/hr in the scattered slow moving
convection. Across the Northern Sierra and northern coastal
ranges, 24-hr rainfall has been more than 2 inches in some areas
according to MRMS, compounding already wet soils noted by high
USGS streamflow anomalies and elevated CPC soil moisture. The HREF
has a more aggressive signal today for more than 3 inches across
the Klamath Mountain as well, which, while still only 10-20%,
suggests a higher-end MRGL risk for runoff and isolated flash
flooding than the past few days.


...South Florida...
Increasingly confluent mid-level flow across Florida will push a
modest shortwave northeast today, combining with the tail of a
70-90kt upper jet streak to produce deep layer ascent. This lift
will act upon an environment with PWs rising above 1.75 inches and
MUCAPE above 2000 J/kg to support scattered thunderstorms with
rainfall rates exceeding 2"/hr. The important environmental
difference today compared to earlier in the week is forecast 0-6km
bulk shear reaching 30-35 kts, sufficient for storm organization.
This combined with Corfidi vectors aligned to the mean wind to
imply training indicates that rainfall duration may be lengthened
in some areas today, and the HREF neighborhood probabilities
suggests the potential for 3-5 inches of rain in some areas.
Although FFG is high, 7-day rainfall has been 150-300% of normal,
suggesting the soils have at least a subtly higher threat for
runoff resulting in flash flooding. This will be most likely if
any training or organized cells can move across urban areas,
especially on the Gold Coast where the pinned sea breeze may
enhance rates and duration today.


...Northern Plains/Upper Midwest...
Stationary front positioned north-to-south across the Dakotas will
begin to weak today, but continue to provide an impetus for ascent
and scattered thunderstorms this aftn. The environment around the
front will be thermodynamically favorable for heavy rain noted by
PW anomalies around +2 sigma according to NAEFS ensemble tables
and a ribbon of MUCAPE reaching 2000 J/kg. This will fuel
convection with rain rates that could reach 2"/hr according to the
HREF probabilities. Storm motions will generally be around 10 kts,
but aligned to the front so some training is possible. Despite the
expected scattered nature of convection this aftn, training of
these rain rates could produce locally more than 3" of rain, which
will fall atop soils still primed noted by USGS streamflows that
are above the 90th percentile, especially in eastern ND.

The MRGL risk was extended into MN as well which, despite having
soils that can likely handle more rain, could also experience a
flash flood risk tonight. The high-res simulated reflectivity has
some consensus in developing an MCS type event late tonight along
a potent CAPE gradient and the elevated front to the east. This
could result in training 2"/hr rates, and the HREF has some higher
EAS probabilities for 2" tonight. Confidence is not extremely high
for this event, and it is possible some further adjustments to
this ERO area will be needed later today.


Weiss


Day 2
Valid 12Z Thu Jun 08 2023 - 12Z Fri Jun 09 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE
NORTHERN ROCKIES INTO THE INTERIOR PACIFIC NORTHWEST...


...Northern/Central Rockies into the Pacific Northwest...
Closed low over the Great Basin will open and fill Thursday and
then move slowly almost due north as its eastward progression gets
blocked by an amplified ridge over the Northern Plains. This
evolution will drive increasing height falls into the Northern
Rockies, with lobes of vorticity swinging through the trough
combining with intense mid-level divergence to drive robust
ascent. Moisture will increase significantly on Thursday as well
as persistent E/SE flow around this trough advects PWs as high as
1.25 inches, above the daily record according to SPC sounding
climatology and approaching +4 sigma according to NAEFS. This
exceptional PW will overlap with MUCAPE of 500-1000 J/kg to
support intense rainfall in widespread convection on Thursday,
with rates likely eclipsing 1"/hr in many areas. 0-6km bulk shear
reaching 25-35 kts and southeast upslope flow will contribute
additionally to storm organization and intensity, and with this
rain falling atop soils pre-conditioned from 7-day rainfall as
much as 600% of normal (highest in MT), the flash flood risk is
increasing.

The inherited SLGT risk was expanded as far west as Spokane, WA,
and cosmetically adjusted for the new ensemble guidance. Despite
eastern WA and the stovepipe of ID having been dry recently as
reflected by AHPS rainfall data and below normal USGS streamflow,
these intense thermodynamics and resultant rainfall rates should
still produce rapid runoff and scattered flash flooding. However,
the higher threat, which is reflected by the CSU first guess
fields indicating a greater than 25% risk, is across north-central
MT where antecedent soils are more saturated and HREF exceedance
probabilities eclipse 50%.


...Sierra/Northern CA/Great Basin...
As the closed low opens and drifts northward, the overlap of
intense synoptic ascent and impressive thermodynamics will persist
one more day. Forcing provided by height falls and downstream
divergence will continue to impact the environment with PWs of
0.75-1 inch and MUCAPE of 500-1000 J/kg. This will again support
scattered diurnal convection, and HREF rain rate probabilities
reach as high as 25% for 1"/hr. While the coverage of convection
may be a bit less/more scattered than anticipated on D1, it will
be occurring across the same areas, suggesting the soils may be
more saturated, resulting in a faster onset of runoff. Additional
rainfall could reach 1-3" in some areas, which, especially if
occurring over the same areas as on Tuesday or Wednesday, could
yield rapid runoff and isolated flash flooding noted by 3-hr
exceedance probabilities peaking at 30%.


...South Florida...
The tail of an upper level jet streak centered over the Atlantic
will remain over the Florida Peninsula on Thursday, providing
another day of favorable ascent for scattered showers and
thunderstorms. Mid-level flow will become increasingly confluent
on Thursday as well the anomalous New England trough digs farther
south, within which subtle impulses will rotate over the region.
This ascent into PWs above 1.75 inches collocated with a ribbon of
2000 J/kg MUCAPE will fuel 2"/hr rain rates within the convective
activity. Storms that do develop are progged to move progressively
to the northeast on 20-25 kts of mean 0-6km winds, but aligned
Corfidi vectors and 25 kts of bulk shear indicate that training or
some multi-cell clusters are likely, which could enhance both the
rain rates and rain duration. The HREF probabilities for more than
3 inches of rain reach 40%, with the highest risk occurring atop
more saturated soils from 7-day rainfall that has been more than
200% of normal according to AHPS. While the greatest risk for any
isolated flash flooding appears to be along the urban Gold Coast
where the sea breeze pinning may occur, any place across South
Florida that experiences training could see instances of flash
flooding.


...Northern Plains/Upper Midwest...
A weakening stationary front will dissipate during Thursday ahead
of a secondary cold front approaching from Canada late in the
period. This forcing moving across a region of elevated PWs of +1
to +2 standard deviations according to NAEFS collocated with a
ribbon of MUCAPE reaching as high as 2000 J/kg will support
intense rain rates which at times may reach above 1"/hr according
to HREF probabilities. Although Corfidi vectors reach 20 kts, they
are aligned to the mean wind suggesting some training potential,
while bulk shear of nearly 30 kts indicate the potential for some
multi-cell clusters. These together could result in pockets of
heavier rainfall as repeated rounds of storms occur, with locally
3" possible. Overall the flash flood risk looks isolated, with the
MRGL risk tailored to the highest antecedent streamflows
suggesting a greater risk for rapid runoff but any heavy rain.


Weiss

Day 3
Valid 12Z Fri Jun 09 2023 - 12Z Sat Jun 10 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE INTERIOR
PACIFIC NORTHWEST THROUGH THE NORTHERN HIGH PLAINS...

...Sierra through the Pacific Northwest and Northern Rockies...
Amplified but weakening mid-level trough, the remnants of the
closed low from CA Wednesday, will continue to lift
north/northwest as its eastward progression gets halted by a
strong ridge over the Northern High Plains. This feature will
continue to produce robust deep layer ascent through height falls
and divergence, with mid-level impulses rotating through the flow
contributing. Additionally, a modest jet streak on the NW side of
this trough will place favorable RRQ diffluence over the Northern
Rockies. This overlapped ascent will work upon an extremely moist
environment with PWs remaining as high as +4 standard deviations
near the Canadian border, while a plume of 1000+ MUCAPE advects
westward from the High Plains. This setup should result in
scattered to widespread heavy rainfall, which will occur on top of
soils that will be highly vulnerable to runoff after what is
expected to be even more expansive and intense rainfall during D2.
The inherited SLGT risk was expanded to the west to account for
the highest PW anomalies atop the forecast rainfall footprint from
D2. Although some uncertainty persists due to consideration of D2
rainfall, the SLGT risk expansion was coordinated with the local
WFOs and covers both the highest deterministic QPF and greatest,
albeit small, probabilities for an additional 3" of rain in the
ECENS/GEFS/SREF ensembles.

Farther to the south, another day of scattered slow moving showers
with isolated thunderstorms is forecast from the Sierra northward
through the northern Great Basin and into Oregon. Here, the
organization and duration of convection is expected to be less
than points northeast, but rainfall which the GFS indicates could
exceed 0.5" in 1 hr at times Friday will be occurring atop soils
that are likely saturated from rainfall earlier in the week, with
high streamflows still indicated by USGS. This could result in at
least isolated instances of runoff or flash flooding on Friday.


...Southern to Central High Plains...
Modest eight falls pushing across the Southern to Central Rockies
into the Southern to Central High Plains will occur Friday as a
shortwave traverses through the southern stream. These height
falls should strengthen the southerly flow across the region,
noted by increasing low level moisture transport through the
Southern Plains. While there is some uncertainty as to how this
will manifest as convection, there is at least some consensus that
a stripe of showers and thunderstorms will develop and push
eastward beneath this shortwave into the better moisture. Both the
GEFS and ECENS ensemble probabilities for more than 1" of rain are
spread broadly across the High Plains and into the Central Plains,
and while it is possible some heavier rain will occur the signal
is not sufficient at this time to highlight any specific region
above a MRGL risk. The inherited MRGL risk was trimmed from the
east to account for drier soils and lower streamflows, but
otherwise changes were negligible.


Weiss


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt