← Previous June 2, 2023 3:05 PM

FOUS30 KWBC 022005

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
405 PM EDT Fri Jun 02 2023

Day 1
Valid 2000Z Fri Jun 02 2023 - 12Z Sat Jun 03 2023


...Southern Plains...
Another active day expected for the Southern Plains as a shortwave
ejecting out of New Mexico overlaps with the LFQ of a subtropical
jet streak and an advancing dry line to drive impressive ascent.
This lift will occur within an extremely favorable thermodynamic
environment characterized by PWs of +1 to +2 standard deviations
above the climo mean according to the NAEFS ensemble, and a plume
of 2000-3000 J/kg MUCAPE being drawn northward from the RGV. The
result of this evolution is likely to be rapidly increasing
convection which should intensify and expand across the High
Plains of NM and organize into an MCS over the TX Panhandle. As
convection expands, Corfidi vectors are progged to collapse to
just around 5 kts, indicating the potential for storms to
backbuilding into the greater instability and then train to the
E/NE. Eventually, the complex is progged by the high-res simulated
reflectivity to surge eastward this evening, but until that time
rain rates of 2+"/hr training across areas could result in locally
more than 3" of rain as shown by a good consensus among the
HREF/ECENS/GEFS/SREF ensembles, with locally more than 5"
possible. This rain will occur atop primed soils from recent heavy
rains which have been 300-600% of normal the last 14 days leading
to already high USGS streamflows. This has compromised FFG to as
low as 0.25-1"/3hrs, for which the HREF indicates has a better
than 60% chance of being exceeded.

Although the ensemble probabilities have shifted just a bit east,
the inherited MDT risk was adjusted only slightly/cosmetically as
the antecedent conditions over the Panhandle are more susceptible
than areas farther east. This did necessitate some expansion of
the surrounding SLGT risk, but the area of greatest concern for
more numerous flash floods remains over the TX Panhandle through
this evening.

...Northern/Central Rockies and Northern High Plains...
A fairly blocked mid to upper level pattern will persist today
from the Northern Rockies, eastward through the Northern Plains
into the Upper Mississippi Valley and Great Lakes. Very slow
moving mid to upper level troughing will persist on the western
side of the blocking upper high forecast to remain in place from
the Upper Mississippi Valley into the Great Lakes. Within this
trough, a wave of low pressure drifting along a nearly stationary
boundary across central/eastern MT today will allow increased
moisture to rotate into the area. 850mb inflow with a source
region of the Gulf of Mexico will increase to 20-30 kts over the
Dakotas and then turn eastward into MT, lifting isentropically
atop the front, converging into weaker flow to the west, and also
resulting in enhanced ascent through upslope. Additionally, an
axis of deformation is still progged to pivot over the central
part of the state, resulting in prolonged ascent to wring out
moisture which will be nearing +3 standard deviations according to
the NAEFS ensemble tables. AS MUCAPE rises to more than 1000 J/kg
to the east, some of this will resupply into central MT, resulting
in thermodynamics that are favorable for rainfall rates that have
a greater than 50% chance for exceeding 1"/hr according to the
HREF probabilities.

The inherited rare (for this part of the country) MDT risk still
appears warranted as it overlaps where all the global ensembles
indicate at least a low-end risk for more than 3 inches of rain
today, with the HREF also indicating a 10-20% chance for more than
5 inches. This will fall atop saturated soils from 14-day rainfall
that is generally 150-300% of normal, including pockets of 2-3" of
rainfall yesterday according to MRMS. The MDT risk was pulled east
just a bit from inherited however to better align the overlap of
heavy rates and antecedent soils, but there will likely be
numerous instances of flash flooding today.

...Central and Southern Florida...
Although Tropical Depression Two will remain well west of Florida,
the associated mid-level trough and tropical moisture plume (PWs
to 1.75 inches) will continue to plague the peninsula today. A
weak surface trough will be a focus for low-level convergence,
along which scattered to widespread showers and thunderstorms are
once again expected. With favorable thermodynamics in place to
support efficient 2"+/hr rain rates, storm motions that are
expected to be just around 5 kts based on the 0-6km mean wind will
support local rainfall amounts of 2-4 inches, locally higher where
any mergers or collisions can occur.  This could result in
isolated instances of flash flooding, especially if any cells
drift across the more urban southeast or southwest coasts.

...Northern New York State and New England...
The backdoor cold front that will drop across New England and
Upstate New York today is still progged to move through by this
evening, with associated showers and thunderstorms developing
along it. Overall the area has been quite dry based on 14-day
rainfall departures, which has left FFG around 2-3"/3hrs. This
suggests that despite a favorable overlap of instability and
moisture to support rainfall rates of 1-2"/hr, these dry
antecedent conditions will somewhat cap the excessive rain threat.
This is also aided by storm motions that should be generally to
the south at 15-20kts. However, in some places multiple rounds of
heavy rain producing convection could occur, and there are also
some more sensitive terrain features across this region. This
could result in some isolated runoff or flash flooding instances,
and the MRGL risk was broadened to account for Mesoscale
Precipitation Discussion #410.  See that discussion for more


Day 2
Valid 12Z Sat Jun 03 2023 - 12Z Sun Jun 04 2023


...Southern and Central Plains
Another day of scattered to widespread convection is likely across
the Southern/Central Plains on Saturday, but with somewhat less
focus and intensity than the previous few. The mid-level pattern
becomes more elongated with a trough across the Southern Plains as
an expanding ridge builds to the north. This will begin to
suppress the intense meridional moisture transport which has
plagued the area recently, but modest 850mb inflow from the Gulf
of Mexico should still produce PWs around +1 standard deviations
according to the NAEFS ensemble tables, coincident with MUCAPE
surging to 2000 J/kg during peak heating from the RGV into the TX

Weak impulses aloft and some modest upper diffluence will help
drive large scale ascent across the area, while residual
boundaries left over from Friday's convection will also serve as a
trigger for thunderstorms during the day. The guidance has little
consensus in developing any organized areas of convection, which
is also reflected by scattered HREF probabilities for 1"/hr
rainfall rates. This area has been worked over by heavy rainfall
multiple times in the past two weeks though, so any additional
heavy rain could result in runoff and instances of flash flooding.
The most sensitive area is again across the TX Panhandle, so the
inherited SLGT risk was maintained since 3-hr FFG i still just
0.25-1 inch, and may lower from heavy rain anticipated on Friday.

Some more organized convection is possible as an MCS which some
high-res tries to develop in the evening near the Big Bend of
Texas and then dropping southeast into the better moisture along
the RGV and into South Texas or the western Hill Country.
Considered a small SLGT risk for this area based on HREF EAS
probabilities suggesting a local maxima in rainfall, but this area
is not as sensitive as points north, and confidence is marginal as
to how this will evolve. A SLGT risk may be needed for this region
with later updates, but for now the MRGL risk was maintained.

...Central to Northern Rockies...
Blocked mid-level pattern will cause a slow retrograde of features
on Saturday as an amplifying ridge over the Northern Plains pushes
shortwaves embedded within a longwave trough to the northwest.
Several of these impulses are progged to lift northwest from the
Central Plains through the Northern Rockies on Saturday, producing
periods of enhanced deep layer ascent. This lift will occur within
favorable thermodynamics noted by PWs of 0.75-1.25 inches, above
the 90th percentile according to the SPC sounding climatology, and
pockets of MUCAPE of 500-1000+ J/kg. This will support another day
of scattered to widespread convection, especially from the Front
Range of CO northwest through much of WY, ID, and the western half
of MT. Rain rates within deeper convection could reach 1+"/hr as
noted by HREF probabilities as high as 30%, while Corfidi vectors
aligned to the mean wind will permit some short term training to
offset generally progressive storms to the northwest.

The large inherited MRGL risk was adjusted to account for new
guidance. Considered a small SLGT risk for the Northern Rockies
where HREF neighborhood and EAS probabilities feature a greater
potential for an additional 2+ inches of rain as instability gets
resupplied from the east into a slowly weakening deformation
boundary atop pre-conditioned soils and low FFG. However, after
coordination with TFX, opted to maintain the MRGL risk since rates
may not be that impressive and additional rain D2 may not overlap
efficiently with heavy rain from the prior few days.

Tropical Storm Arlene should be a TD on Saturday over the eastern
Gulf of Mexico as it begins to decay, with the remnant vorticity
lobe becoming absorbed into the larger scale trough extended from
the Southeast Atlantic Coast. This will allow the accompanying
shortwave to shift towards the southern FL Peninsula by Sunday
morning, leading to at least subtly enhanced ascent in a still
tropical environment characterized by PWs of 1.5 to 1.75 inches.
With a subtle convergence axis/surface trough still likely
positioned across South Florida, coincident with the LFQ of a weak
jet streak aloft, this additional forcing should result in another
day of scattered to widespread showers and thunderstorms. The
favorable thermodynamics should support rain rates of 2"/hr at
times, with cells again moving slowly on 0-6km mean winds of just
5-10 kts. Additionally, weak bulk shear to support generally pulse
convection could result in storm mergers and outflow interactions,
to enhance rain rates and cause chaotic storm motions, and
rainfall of more than 3" is likely in some areas as reflected by
the HREF probabilities. The southern FL peninsula has received
significant rain recently, with more forecast on D1. Any of these
slow moving storms could produce isolated flash flooding,
especially if they occur atop any more urban or sensitive areas.


Day 3
Valid 12Z Sun Jun 04 2023 - 12Z Mon Jun 05 2023


...Southern Plains through the Northern Rockies...
More of the same for this part of the country on Sunday, although
the overall coverage and intensity of convection may continue to
wane. The driver of this pattern will remain the expansive
mid-level ridge over the Northern Plains, resulting in more
diffuse troughing arced from the Northern Rockies through the
Southeast. With still above normal PWs more than +1 standard
deviation above the climo mean according to NAEFS, and elevated
afternoon instability, and diurnal convection across the region
could produce excessive rain rates. However, there is little
signal at this range for any organized convection atop saturated
soils from prior rain events during the past week. If any signals
develop during the next few model cycles, it would not require a
significant upward trend to drive targeted SLGT risks due to the
antecedent soil conditions, but otherwise the runoff/flash flood
threat appears just isolated on Sunday.


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