Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1154 AM EDT Tue Jun 06 2023

Day 1
Valid 16Z Tue Jun 06 2023 - 12Z Wed Jun 07 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE FRONT
RANGE OF THE ROCKIES THROUGH THE SOUTHERN HIGH PLAINS AND IN
PORTIONS OF MONTANA...

...Southern High Plains through the Northern Rockies...
Pinched flow beneath a broad trough will continue to drive warm
and moist air across much of the High Plains and Intermountain
West today, as the synoptic pattern slowly begins to evolve today.
Above this trough, an axis of upper level deformation will
continue to be the axis upon which scattered to widespread aftn
convection develops as this ascent works across an environment
with PWs of +2 to +3 standard deviations according to NAEFS and
MUCAPE surging to 1000-2000 J/kg. The high-res simulated
reflectivity is again depicting widespread convection blossoming
this aftn, but struggling to depict any consensus as to where the
most organized convection will occur. It is this setup that drive
the nearly ever-present broad MRGL risk from west TX northward
through MT as rainfall rates of 1+"/hr within slow-moving storms
move across soils saturated from 7-day rainfall that is more than
300% of normal in many places leading to low FFG.

Despite the broad signal, there are two inherited SLGT risks that
have been adjusted only cosmetically to account for new guidance.
The first is in a large stripe from the Sacramento Mountains
northward through the Front Range and east onto the Southern High
Plains. This area consists of more vulnerable soils noted by many
USGS streamflow gauges between the 75th-90th percentile, as well
as numerous sensitive burn scars. This is also an area that will
likely have a lee trough providing at least a subtle focus for
convection, aided by weak impulses moving overhead and at least
the distant LFQ of a subtropical jet streak, and modest 0-6km bulk
shear of 25 kts to encourage modest storm organization. The HREF
probabilities across this area, for both 3-hr FFG exceedance and
3"/24hrs are greater than the surrounding region, indicating this
SLGT remains warranted for a higher potential for runoff and flash
flooding today.

The other SLGT is a targeted smaller area focused near Billings,
MT where the guidance features a better consensus for a pocket of
heavier rain. The HREF neighborhood and EAS probabilities are both
more aggressive in this small region, and there appears to be a
significant overlap between the highest probabilities and the
7-day AHPS rainfall exceeding 600% of normal. This is additionally
reflected by HREF FFG exceedance probabilities reaching 60-70%, so
the SLGT risk is definitely needed and was adjusted slightly for
updated guidance.


...Upper Midwest and Northern Plains...
A ribbon of MUCAPE above 1000 J/kg and PWs of around 1.25" will
provide a favorable environment for thunderstorms today as a
stationary boundary wavers across the Upper Midwest. This front
will produce low-level convergence, which into the favorable
environment should result in scattered to widespread diurnal
convection. The coverage of this is still uncertain as guidance
has backed off a bit based on the high-res simulated reflectivity.
However, 0-6km mean winds of just 5-10 kts parallel to the front
will support some short-term training and slow movement of any
cells that develop. This could produce more than 3 inches of rain
in some areas as reflected by 24-hr HREF neighborhood
probabilities as high as 30%, but confidence as to where those
higher rain totals will occur is low. Despite generally dry
antecedent conditions, USGS streamflow anomalies are still running
high, especially in eastern ND and down towards the Buffalo Ridge,
so the inherited MRGL was maintained for the isolated flash flood
risk today.


...Central California through the Northern Great Basin...
A strong closed low with 500mb heights below -3 sigma lifting onto
the CA coast this morning will move little while maintaining
intensity. This will drive pronounced deep layer ascent through
downstream divergence and upper diffluence across northern CA and
into the Northern Great Basin region. Low-to-mid level onshore
flow south of this system will rotate around it, increasing PWs to
0.75-1" as far north as central OR, while coincidentally MUCAPE
climbs towards 1000 J/kg. This overlap of impressive ascent and
thermodynamics will force diurnal convection once again, with the
HREF probabilities indicating a chance for short-duration rain
rates nearing 1"/hr. The high-res appears to have backed off a bit
on coverage today, especially for central CA, which may be due to
more enhanced cloud cover noted on visible imagery. However, there
is still a strong signal for the 2-yr ARI exceedance from the HREF
where antecedent soils are more saturated using the 7-day AHPS
rainfall and USGS streamflow anomalies as proxy, so only a few
cosmetic adjustments were made to the inherited MRGL risk.


...Central Gulf Coast...
A generally weakly forced but thermodynamically favorable
environment will again support slow moving thunderstorms this
aftn/eve. PWs on the morning U/A soundings at KLCH and KLIX were
both just around the median of 1.4-1.5 inches according to the SPC
sounding climatology, but with high WBZ heights and freezing
levels. This indicates that as the column moistens further this
aftn, convection should be capable of producing efficient warm
rain rates above 2"/hr as show by HREF probabilities. 0-6km mean
winds on the morning soundings were around 5 kts, and this is
expected to continue today, indicating slow movement of these
storms, with some more widespread development possible as a weak
shortwave traverses just south of the coast. This area has been
dry the past 7 days, but scattered convection did produce locally
as much as 3 inches on Monday according to MRMS, increasing at
least top-level soil moisture, and another day of these scattered
thunderstorms could produce an isolated flash flood instance.

Weiss


Day 2
Valid 12Z Wed Jun 07 2023 - 12Z Thu Jun 08 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE FRONT
RANGE OF THE ROCKIES THROUGH THE SOUTHERN HIGH PLAINS AND FOR
PARTS OF MONTANA...

...Southern High Plains through the Northern Rockies...
No significant changes again expected to the large scale flow
pattern across the mid section of the nation.  Blocking features
remain in place characterized by a slow moving closed low inching
north from south central CA into the southern Great Basin and an
upper high across the Northern Plains.  Similar to the day 1
period, there will be a broad region of favorable upper difluence
stretching from across the High Plains and Rockies between these
two blocking features.  With PW values again forecast to remain
well above average, 1-2+ standard deviations above the mean, there
will be a widespread region of convection and potential for
locally heavy totals.  No changes made to the primary slight risk
area from central CO, southeastward into northeast NM and
northwest TX where additional runoff issues are likely given the
recent heavy rains and resultant high soil moisture and anomalous
stream flows.  There were no big changes to the CSU first guess
fields for the day 2 period compared to the day 3 period across
this region, with CSU risk capped at marginal. 

...Northern Sierra/Northern California into portions of the Great
Basin...
While the strong closed low across south central CA day 1 may
weaken slightly during day 2 as it creeps northeast into the
Southern Great Basin, the overall large scale set up will remain
favorable for additional widespread scattered convection across
the Sierra, into northern CA and portions of the Great Basin. 
Well defined upper difluence to the north and northeast of the
slow moving closed low in an axis of persistent anomalous PW
values will support another day of potential locally heavy rains
and isolated flash flooding over regions of high stream flows or
recent burn scars.

...Coastal Southeast Florida...
Weak mean mid to upper level troffing over the central Gulf of
Mexico will keep west southwest mid to upper level flow from the
eastern Gulf and across South FL day 2.  While not showing great
agreement, there continues to be some model signal for heavy
southeast coast of FL heavy rains day 2 in the 1.75-2.00 PW axis
forecast across South FL.  Concerns continue for isolated urban
flash flooding from Miami north to approximately West Palm Beach.

Oravec

Day 3
Valid 12Z Thu Jun 08 2023 - 12Z Fri Jun 09 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE NORTHERN ROCKIES INTO THE NORTHERN HIGH PLAINS...


...Northern Rockies into the Northern High Plains...
The strong closed low creeping northward from south central CA
into the Great Basin on day 2 will begin to accelerate to the
north northeast day 3.  This will be increasing the heavy rainfall
threat for the Northern Rockies into the Northern High Plains
where strengthening easterly low level flow increases PW values to
2 to 3 standard deviations above the mean.  An overall very
favorable pattern for heavy rains will exist day 3 with strong
upper difluence and anomalous low level easterly moisture flux
impacts this region.  There is fairly good model consensus for
heavy rainfall potential from the MT/ID border region, eastward
into central MT, with many of these areas receiving much above
normal precip values over the past few weeks, 200-400% of normal.
Subsequently, we maintained a slight risk area across this region,
expanding it slightly from the old day 4 outlook on the western
end across the ID/MT border region.

..Sierra into the Great Basin...
PW values will remain much above average day 3, 1.5-2+ standard
deviations above the mean, ahead of the accelerating and weakening
Great Basin closed low.  Precip will begin to focus farther to
then north than on day 2, with widespread activity likely from the
central to northern Sierra, northern California and into the
northern Great Basin.  No changes to the previous broad marginal
risk area across this region where locally heavy rainfall across
area of high stream flow or burn scars from recent years may
produce isolated flash flooding.

...Southern High Plains...
There is a lessening excessive rainfall threat day 3 into the
Southern High Plains.  After days of persistently anomalous PW
values, veering low level winds to a west southwesterly direction
will bring in much lower PW values.  Model consensus is for little
precip potential day 3 over the Southern High Plains.  The
southern end of the marginal risk area was trimmed over the
Southern to Central High Plains given this from the old day 4
outlook.  There will be continued scattered convective potential
for north central to northeast CO into WY where a marginal risk is
denoted.

...Coastal Southeast FL...
With the mid to upper level trof forecast to persist across the
central Gulf of Mexico day 3, with west southwest mid to upper
level flow moving across South Florida, a small urban centered
marginal risk area was added from Miami north to approximately
West Palm Beach.  Similar to day 2, scattered convection in the
tropical PW axis expected across South FL, may result in isolated
urban runoff issues.  

Oravec



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