Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
409 PM EDT Mon Jun 05 2023

Day 1
Valid 16Z Mon Jun 05 2023 - 12Z Tue Jun 06 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN HIGH PLAINS AND INTO THE FRONT RANGE OF THE ROCKIES...

...Southern Plains through the Northern Rockies...
The blocked pattern which has changed very little for the past
week will modify only slightly today as a ridge over the Northern
Plains and intensifying closed low over CA help to squeeze the
diffuse trough across the Intermountain West/Southern Plains.
Another day of persistent moist advection within this axis will
keep PWs above to well above normal, upon which weak impulses and
upper level deformation will drive ascent. The challenge today
primarily involves instability which has been forecast to be
somewhat modest today, and current cloud cover is quite extensive
across NM and into southern CO. Despite that, the guidance is more
adamant that instability will surge above 750 J/kg and cloud cover
will wane, so it is more likely that another day of scattered to
widespread convection will develop, but with again a struggle for
any specific focused area.

Despite a general vague signal from the ensembles as to where
convection may be more prevalent, there are two areas of most
concern today. The first is around the inherited SLGT risk from
the TX Panhandle into northeast NM. This region has been saturated
from 72-hr rainfall according to MRMS which is as much as 5" in
some locations, and 7-day rainfall nearing 600% of normal. This
has severely compromised FFG to just 0.5-1.5"/3hrs, which the HREF
indicates has a 20-30% chance of being exceeded by slow moving
storms today. While the total coverage may be tempered by morning
cloud cover, the antecedent soils are more than sufficient for the
current SLGT risk. However, after coordination with WFO BOU, an
extension of this SLGT was made northward into the CO Front Range
and foothills, and into extreme southern WY, where recent rain has
also been significant and slow moving storms with rain rates
exceeding 1"/hr are again possible today. Anywhere these slow
moving storms occur could result in runoff issues, but flash
flooding may be most likely should a storm drift atop a burn scar
or atop the most sensitive soils.

The other region of maybe greater risk is along the Gulf Coast of
Texas and Louisiana. While this region has been much drier
recently, there is an enhanced signal in the ensembles for more
than 3 inches of rain today, with potentially more than 5 inches
in isolated locations. This will be driven by a very favorable
atmosphere with 2000-3000 J/kg of MUCAPE and PWs around 1.75".
This will fuel thunderstorms with rain rates exceeding 2"/hr, and
storms are likely to be very slow movers on 0-6km mean winds of
just around 5 kts with anti-parallel and collapsed Corfidi
vectors. Considered a small SLGT risk for this area, but the high
FFG and low exceedance probabilities suggest more of an isolated
threat today, primarily across urban areas.


...Central California coast range into the Sierra Nevada...
Anomalous closed low with 500mb heights falling below -2 standard
deviations according to NAEFS will lift onshore the central CA
coast today. This feature will spread ascent into CA and the Great
Basin as upper diffluence, mid-level divergence, and subtle height
falls overlap. At the same time, this will help spread moisture
onshore with PW anomalies surging to around 1 inch, and
instability being pulled westward noted by MUCAPE exceeding 500
J/kg. This will result in scattered to widespread showers and even
isolated thunderstorms with rain rates 0.5"-1" per hour. Despite
antecedent dry conditions, streamflows are still quite high, and
this could also impact some recent burn scars. This continues to
support the expanded MRGL risk from overnight for isolated runoff
issues, and only some cosmetic adjustments were made to the
inherited area.


...Upper Midwest...
A cold front is still progged to drop out of Canada today and move
into the Upper Midwest, providing a focus for afternoon
thunderstorms. This cold front will drive convergent ascent into
an environment with around 1000 J/kg of MUCAPE and PWs of
1.25-1.5", more than 2 standard deviations above the climo mean
according to NAEFS, and approaching a daily record according to
the SPC sounding climatology. Although the high-res appears to
have backed off just slightly in coverage of thunderstorms today,
weak storm motions of just 5-10 kts parallel to the front suggest
the possibility of some training, with rain rates likely reaching
1"/hr or more as progged by the HREF probabilities. Despite
antecedent dry conditions with generally around 50-75% of rainfall
the past 2 weeks, USGS streamflow anomalies are still high so FFG
is modest. Where any of these slow moving storms train, locally 3"
of rainfall could occur, which could result in isolated instances
of flash flooding. The inherited MRGL is warranted for this
potential, and was adjusted cosmetically for new guidance.


Weiss


Day 2
Valid 12Z 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...

...Southern High Plains through the Northern Rockies...
Once again, the High Plains will be sandwiched between a ridge of
high pressure to the northeast, and a strengthening area of low
pressure across the Pacific Coast. This will continue to draw
moisture and instability northward beneath a diffuse mid-level
trough, with the favorable thermodynamics for heavy rain
continuing across a large area. PWs are forecast to reach +1 to +2
standard deviations above the climo mean from west TX through MT,
overlapping MUCAPE of 500-1500 J/kg, locally higher. With weak
impulses lifting northward within the flow, any place could
experience diurnal convection once again Tuesday, with storm
motions generally around 10-15 kts in the overall weakly forced
environment. Since much of this region has experienced 7-day
rainfall that is 200-400% of normal, FFG is low (less than
1.5"/3hrs in many areas) and streamflows are already high
according to USGS. This suggest that any slow moving cell with
rain rates of 1+"/hr could produce runoff and flash flooding.

While the signal for organized activity is modest on D2, there
does appear to be a need for an expansion of the SLGT risk
inherited for northeast NM and the TX Panhandle. From CO southward
through west TX, there is a good model consensus for higher 0-6km
bulk shear reaching 20-30 kts to help organize storms, resulting
in potentially higher rain rates of longer duration. This is
supported by an axis of modest 24-hr probabilities for more than
3" of rain from southern CO through the El Paso area of TX.
Additionally, some of this same area will likely experience heavy
rain on Monday, making the soils even more susceptible to flash
flood concerns. After coordination with the area WFOs, the SLGT
risk was expanded northward along the Front Range of the Rockies
and southward to the Sacramento Mountains to account for this
greater potential, enhanced even moreso the sensitive burn scars
across the area.

Note as well the CSU first-guess field indicates the potential
need for a small SLGT risk across southern MT where the HREF
probabilities also suggest a narrow corridor of higher 3"/24hr and
3-hr FFG exceedance probabilities. The simulated reflectivity for
that area does develop a more organized cluster of storms, and
heavy rain rates are likely. However, this is a pretty small
geographic area that would likely need to fall directly on top of
the 72-hr MRMS footprint from heavy rain to more than an isolated
threat. For that reason have opted to maintain just a MRGL D2, but
note this could require a future increase to SLGT if the signal
persists or becomes more impressive.


...Upper Midwest and Northern Plains...
The cold front from Monday is expected to stall and become nearly
stationary draped north to south across ND/MN as it encounters an
expansive ridge to the west to block its progression. This front
will remain in a region of favorable thermodynamics for heavy
rainfall, with MUCAPE forecast to reach 1000-2000 J/kg coincident
with PWs exceeding 1.5" in a narrow corridor, 2-3 standard
deviations above the climo mean according to NAEFS. This will
likely manifest as scattered storms as reflected by high-res
simulated reflectivity, tracking slowly along the front to support
short term training. With rain rates possibly exceeding 1"/hr at
times, any training could produce locally 2-3" of rainfall as
reflected by the HREF neighborhood and EAS probabilities. This
rain falling atop some more saturated soils from additional rain
on D1 could result in isolated flash flooding.


...Central California through the Northern Great Basin...
The anomalously strong closed low moving onshore CA D1 will move
little during 2, basically spinning in place through the period.
This will allow for persistent synoptic ascent downstream, with
upper diffluence, mid-level divergence, and some upslope flow
driving ascent. At the same time, instability and moisture will
remain favorable with PWs at +2 standard deviations, and MUCAPE
above the 90th percentile according to the SPC sounding
climatology. The overlap of this forcing and thermodynamics is
likely to drive another day of scattered to widespread showers and
even isolated thunderstorms, with the HREF indicating a better
chance of 0.75-1"/hr rates than what occurs on Monday.
Additionally, with rainfall expected on D1, this will further
increase the pre-conditioning of the soils so even where recent
rainfall has been modest, the chance for runoff will begin to
increase. With a signal from the ensembles for 1-3" of rainfall
expanding into OR/ID where recent rainfall has been more
substantial, the MRGL risk was expanded just slightly north and
east from inherited where any heavy rain rates could quickly
result in runoff, with isolated flash flooding also possible,
especially over recent sensitive burn scars.


...Gulf Coast...
Continued extreme thermodynamics with MUCAPE above 2000 J/kg and
PWs above 1.75 inches in an otherwise weakly forced environment
will support another day of slow moving, heavy rain producing,
convection. Synoptic ascent will be generally modest with a weak
jet streak positioned to the south, but subtle mid-level impulses,
possibly residual vorticity from D1 convection, could re-ignite
thunderstorms during the aftn. This will likely be aided by sea
breeze convergence and storm interactions as storm motions fall to
just around 5 kts in the pulse environment. Although the signal
for heavy rainfall is a little less robust here than on D1,
rainfall rates of 2"/hr in these slow moving storms could still
locally produce more than 3" of rain as shown by HREF neighborhood
probabilities. Where these slow moving storms are most intense,
especially if they fall atop an urban area or overlap with the
heaviest rain footprint from Monday, isolated flash flooding could
occur.

Weiss

Day 3

The Day 3 outlook will be updated by 2030Z.


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