Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
320 AM EDT Sat Jun 10 2023

Day 1
Valid 12Z Sat Jun 10 2023 - 12Z Sun Jun 11 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE PLAINS, ARKLATEX, AND THE MIDWEST...

Northern High Plains...  
Anomalous moisture continues early on before dropping off and
southward as an upper level shortwave potentially splits, with
some energy moving north into Canada and some undercutting the
ridge and dropping southward.  Instability lingers early on, so
the existing Slight Risk still makes sense, but it was refined
using the latest guidance.  
  
  
Midwest...  
A shortwave drops south around a forming closed low across
southeast Ontario, with the overall system on a strengthening
trend.  A low- to mid-level low drops southeast and lures PW
values up to 1.5-1.75" into portions of KS, MO, NE, and IA. 
Inflow at 850 hPa and effective bulk shear should be increasing,
particularly late in the period when the low-level jet tends to
ramp up Saturday night into Sunday morning which portends
organized convection.  CAPE values upstream are forecast as high
as 4000 J/kg on the 21z SREF guidance during daytime heating,
falling towards 1000 J/kg overnight.  The guidance signal is more
scattered than on previous nights in and near the complex border
junction of NE/MO/KS/IA, near where mid-level heights/thickness
pattern are diffluent (eastern NE, western IA, and western MO),
with the guidance (particularly the 00z NAM and 00z Canadian
Regional) still showing a signal for local 3-5" amounts, with the
00z HREF guidance showing a non-zero chance for 5"+ amounts. 
Flash flood guidance values, particularly outside MO, are modest,
but the Kansas City urban area would naturally be susceptible to
heavy rainfall.  The ingredients available could support hourly
rain totals to 2.5", which would be a problem in urban areas.  The
Slight Risk area was re-expanded due to the above, but care was
taken not to go too far into IA per their lesser sensitivity
mentioned in prior coordination.  A broad Marginal Risk area
continues to surround the Slight due to some lingering spread as
the guidance is no longer coherent if a convective complex is
going to form, even if all the ingredients appear to be there for
the formation of such a complex.  The re-expansion was coordinated
with the DMX/Des Moines IA and OAX/Valley NE forecast offices.
  
  
Portions of CA/NV...  
An anomalously strong closed low drops into CA Saturday into
Sunday.  Precipitable water values remain elevated, with moisture
increasing in the Central Valley as the cold low develops and
moves in.  Instability shouldn't be a problem as temperatures
aloft decrease.  Believe a Marginal Risk of excessive 
rainfall/flash flood should continue Saturday into Sunday.  Hourly
rain totals up to 1.5" should be possible where cells backbuild,
train, and/or merge.  
  
  
ArkLaTex to the Central Gulf Coast...  
As a southern stream shortwave phases with a deepening positively
tilted upper trough, PWs rise towards 2" along a section of the
polar front making some progress eastward.  Low-level
inflow/effective bulk shear appear to be sufficient for convection
with some level of organization, which would support hourly rain
totals to 2.5" somewhere between eastern TX, LA, and MS.  The
NAM/GFS still advertises local 4-6" amounts in eastern TX
especially, so the Slight Risk area remains in place in that
region.  
  
Roth


Day 2
Valid 12Z Sun Jun 11 2023 - 12Z Mon Jun 12 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
COLORADO & THE TENNESSEE/OHIO VALLEYS... 
 
Great Lakes/Midwest/Ohio Valley/Tennessee Valley... 
A broad area of locally heavy rainfall is expected in the eastern
portion of the country -- much of it welcome because of recent
dryness -- as a deepening closed cyclone drops into the Great
Lakes and phases with a shortwave moving eastward from the
southern Plains.  The best signal for heavy rainfall is in the TN
Valley, where strengthening low-level inflow, CAPE of 2000+ J/kg,
and PWs of 1.5-2" are forecast.  There appears to be enough 
effective bulk shear for convective organization.  These
ingredients allow for hourly rain totals to 2.5".  The 00z UKMET
and 00z NAM guidance indicate local 3-4" amounts, while the 00z
Canadian Regional advertises local 4-6" amounts.  Outside of
northernmost GA, the region has seen much below rainfall over the
past week, if not two months.  However, with the rain rates
expected, particularly in urban  areas, widely scattered to
scattered instances of flash flooding are expected despite the
recent dryness, so raised the TN Valley remains in a Slight Risk. 

To the north, a front moves in while 1.5"+ PWs stay over portions
of OH for 12+ hours.  Some instability is expected to sneak in to
southwest OH from KY with time, but the combination of lowish
instability, increasing 850 hPa frontogenesis, and the low-level
inflow exceeding the mean 850-400 hPa wind are expected to lead to
better than average rainfall efficiency.  Within this region
that's been dry, some areas for quite a while, there's a concern
that soils will be hard/brick-like initially, with rainfall all
running off until soils can soften and allow infiltration.  With
hourly rain totals maxing out in the 1.5" range at the southwest
portion of the new Slight Risk, and closer to 0.75-1" in the
northeast portion of the new Slight Risk, the three hourly flash
flood guidance should be able to be exceeded on a scattered basis,
hence the new risk area.  Coordination with JKL/Jackson KY,
LMK/Louisville KY, and ILN/Wilmington OH forecast offices led to
the current Slight Risk configuration. 
 
 
In and near Colorado... 
Upslope flow behind a frontal zone imports moisture from the
southern and central Plains into eastern Colorado, with PWs
exceeding 1" in easternmost CO in a col point in the mid-level
flow.  CAPE is expected to rise towards 1000 J/kg.  With the
mid-level flow opposing the low-level flow, enough effective bulk
shear should exist for organized convection, which could allow
hourly rain totals to rise to 2" where cells train and/or merge. 
Once cold pools becoming dominant, organized convection should
plow east to southeast into the low-level flow, increasing storm
scale inflow.  The guidance has a good signal for local amounts of
2".  Flash flood guidance is rather low, so a scattered risk of
flash flooding is anticipated as both hourly and three hourly
flash flood guidance values are exceeded.  The Slight Risk area
remains similar to continuity which matches the overnight model
guidance. 
 
 
Remainder of the West... 
An anomalously deep closed low pivoting through southern CA into
southern NV will help cause pockets of 1000+ J/kg of CAPE and PWs
of 0.75-1.25".  This is expected to cause isolated heavy rain
concerns particularly at elevation across portions of the West and
northern Rockies.  Where cells train and/or merge, hourly totals
of 1.5" are possible.  Considering recent wetness, the existing
Marginal Risk area looks reasonable at this time as cell coverage
should not be dense enough to cause scattered flash flood issues. 
Some expansion was made to the Marginal Risk area to account for
the 00z HREF guidance.
 
Roth


Day 3
Valid 12Z Mon Jun 12 2023 - 12Z Tue Jun 13 2023

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR
COLORADO & THE ARKLATEX...

In and near eastern CO...
An upper low moves from the Desert Southwest into the Great Basin
with time.  Moisture in the High Plains of CO and western KS
remains potentially problematic for the region, with PWs of
0.75-1".  Daytime heating is expected to lead to 1000-1500 J/kg of
CAPE.  While there is modest low-level upslope flow, the flow
aloft is westerly -- enough effective bulk shear should be in
place to promote organized convection capable of heavy rainfall,
up to 2" an hour. Activity should tend to move eastward and
southeastward per the forecast 1000-500 hPa thickness lines and
the forward propagating convective motion vectors which would
enhance storm-relative inflow.  Local amounts of 2-4" are forecast
by the guidance, which is expected so long as convection remains
progressive.  Heavy rainfall should be aided by storm mergers and
short periods of training.  The Slight Risk area from continuity
has been generally maintained.
 

Near and west of the ArkLaTex...
Inflow into a front drifting across the region near the apex of a
mid-level ridge is expected to maintain a pool of 1.5-2" PWs
downwind of 2000-4000 J/kg of CAPE.  This is a traditional
location for a convective complex, even if it would normally be a
bit north of here in mid-June.  Outside of the 00z NAM, there is a
coherent and fairly agreeable signal for local amounts in the 3"
range near the eastern border of OK and TX.  Hourly rain totals to
2.5" are possible, and local amounts to 4" are considered
possible.  Rainfall on Saturday/Saturday night is expected to
allow for some saturation across the area.  Combined with the
guidance QPF and available ingredients, a Slight Risk is raised
for the area.


Intermountain West...
Under the upper level low migrating across the region, enough
moisture (particularly at elevation) and instability (up to 1500
J/kg of CAPE) should be available to aid convective development
due to daytime heating in the afternoon and evening hours.  Hourly
rain totals are possible should cells merge and/or train.  The
ceiling for hourly rain totals should be 1.5", but cell coverage
is not expected to be dense enough for more than isolated to
widely scattered heavy rain-related issues.  The existing Marginal
Risk area from continuity still looks good.


Northeast/Great Lakes...
With instability expected to be minimal across the region,
frontogenesis in the low- to mid-levels would be doing the heavy
lifting from a heavy rainfall perspective.  Across the Northeast,
the expected rainfall/moisture plume is expected to be progressive
enough to be more helpful than harmful (considering recent
dryness) despite the wet 00z Canadian Regional and 00z ECMWF QPF
solutions showing 2-4" totals over portions of NY, so the Marginal
Risk area was dropped, a conversation started by the CTP/State
College PA forecast office.  The situation is trickier across MI,
as frontogenesis will move minimally across the Upper Peninsula of
MI.  The center of cold lows sometimes surprise if enough daytime
heating is around.  Still, hourly rain totals are expected to max
out in the 0.5-0.75" range and aren't expected to pile up above
2".  Since it's been so dry, per coordination with APX/Gaylord MI
and MQT/Marquette MI forecast offices, went ahead and dropped the
inherited Marginal Risk area for MI.

Roth


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