Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
435 AM EDT Sun Jun 13 2021

Day 1
Valid 12Z Sun Jun 13 2021 - 12Z Mon Jun 14 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID-ATLANTIC, APPALACHIANS, NORTHERN FLORIDA AND COASTAL
GEORGIA, AND THE SOUTHERN AND CENTRAL PLAINS...

...Mid Atlantic into the Northeast...
Convection along/ahead of an approaching cold front is expected to
result in an isolated flash flood risk from eastern KY, across
portions of the Mid Atlantic and into southern NY today. Good
model agreement that scattered convection will develop...and
enough instability/shear present to suggest some organization and
intensity to the activity. PWs are only modestly above average and
cells should be fairly progressive off to the east. Nonetheless,
setups such as this do favor some pre frontal development along
terrain and/or downstream troughing...and this activity could
merge with the later frontal convection. Thus some areas could end
up seeing multiple convective rounds and/or merging of cells.
Indications in the HREF and GEFS based machine learning output
indicate we'll likely see pockets of 2"+ rainfall with this event.
However the probabilities of 3"+ are considerably lower. Typically
this would likely put this event in the low end of a Marginal
risk. However given the wet antecedent conditions over portions of
this area...the risk of flash flooding is a bit higher than
typical for an event of this nature. With that said, there will
have been about 1-2 days of drying over most of this area...and
soil saturation fields suggest the recent rains have primarily
saturated the top soil layers...with deeper layers remaining
closer to average levels. Thus areas have likely seen some
recovery, and so FFG may be a tad too low. Thus, after
coordination with the local WFOs, we decided to stick with a
Marginal risk and not upgrade to a Slight risk. Still would expect
isolated flash flood issues over the area, but the hope is that
the progressive nature of cells and modest moisture will prevent
anything more organized or widespread.

...Northeast FL into eastern GA...
A surface frontal boundary and upper level trough with embedded
vorticity maxima should help focus scattered convective
development over this area on Sunday. PWs between 2.0 and 2.3"
combined with slow cell motions will support locally heavy rain.
Weak shear should not allow for much in the way of organization to
the convection, and activity will tend to be of the pulse variety.
However the high PWs and slow cell movements will still support
localized heavy rainfall. 00z HREF would support some swaths of
2"+...with localized max values between 3-5". The flash flood risk
is likely pretty limited given the high FFG across the
region...however a few localized issues over any more sensitive
urban/low lying areas are possible.

...Central Plains...
Some flash flood risk may exist over portions of the central High
Plains later Sunday into the overnight as convection develops on
the northeast side of a mid-level ridge centered over New Mexico.
There is plenty of instability forecast, and some indications a
shortwave may round the top of the mid level ridge but capping
aloft and coverage of storms is in question. Current thinking is
for convection to develop over southeastern Wyoming, northeastern
CO into western Nebraska and evolve into a forward propagating MCS
as a low level jet develops over eastern Colorado beyond 00Z
Monday. Current model QPF fields would probably not result in much
of a flash flood risk. However, CAMs tend to struggle over this
portion of the country, with locally higher QPF totals often
verifying. The only flood risk is probably in the initiation phase
of convection...when movement will be slower and cell mergers are
more likely. Thus an isolated flash flood risk could evolve here
and will carry a Marginal risk. Convection should become forward
propagating downstream of this Marginal risk area, suggesting an
even lower flood risk.

...Southern Plains...
Growing confidence that the convective complex moving across the
TX Panhandle will leave behind a well defined compact MCV today.
Also looks increasingly possible that this MCV will become
convectively active as it drops southeastward into north central
TX. Confidence is low on these details...and none of the CAMs are
doing a great job with the ongoing activity...so not confident
that any of them have a handle on the evolution the rest of the
day. Certainly possible that nothing too organized ends up
redeveloping. But beginning to lean towards the idea of convection
flaring up today, and possibly lingering into tonight near the MCV
center. Localized heavy rainfall could occur...and isolated flash
flooding can not be ruled out. Again, low confidence...but think
adding in a Marginal risk is warranted.

...Oregon Cascades...
An out-of-season atmospheric river will reach the Pacific
Northwest tonight into Sunday. There are some indications of
convective development along crest and the eastern slopes of the
OR Cascades, which could drop a quick 1" of rain. The concern for
localized flash flooding would be across burn scars from last
year...however it appears only the Lionshead burn scar is at
increased risk of this convective activity...with the other burn
scars generally on the western slopes where intense convection is
less likely. Given the small spatial scale of the risk, opted to
not add in any Marginal risk area.

Chenard/Otto


Day 2
Valid 12Z Mon Jun 14 2021 - 12Z Tue Jun 15 2021

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER THE MID
ATLANTIC AND NORTHEAST, AND ACROSS PORTIONS OF FLORIDA...

...Northern Mid-Atlantic into the Northeast...
Troughing and embedded shortwaves diving into the Northeast
supports convection along/ahead of an eastward progressing front.
Confidence remains a bit below average on how this all plays out,
but seeing enough ingredients in place to carry a Marginal risk
from the Mid Atlantic into the Northeast. The greater areal
averaged rainfall should end up from eastern NY into western New
England. The persistent upstream forcing and low level moisture
transport into this region seems to support the potential for
multiple rounds of continued scattered convection into the area
through Monday...and this will be in addition to a round of
locally heavy rain today (Sunday). So while antecedent conditions
are quite dry...areas may eventually begin to saturate with time.
Thus the multiple rounds on Monday may be enough to push some
areas into localized flash flooding. Not expecting anything too
numerous or significant at the moment, but enough of a risk with
time to warrant the Marginal risk inclusion.

The northern Mid Atlantic is a tricky forecast. A stronger
shortwave and mid level jet max should overrun the frontal
boundary over this region. Expect this to result in convective
development...and given the forecast shear/CAPE
overlap...potentially strong to severe convection. How much drying
we see behind today's convection and the exact timing of the
shortwave Monday will be key in determining exactly where stronger
convection forms. While it is possible that this results in
activity along just the coastal plain...there seems to be a
growing indication that robust development could occur as far west
as WV and southwest PA. Cells that do form should be quick moving
and mid level drying will at least have lowered PWs. Nonetheless,
given already wet ground conditions, and an additional round of
rain expected today...any robust convection could at least pose a
localized flash flood risk Monday.

...Central and Northern FL into far southeast GA...
A lingering surface frontal boundary and upper level trough with
embedded vorticity maxima should help focus continued scattered
convective development over this area on Monday. Convective
details are uncertain and partially dependent on convectively
induced MCVs embedded within the broader troughing. Nonetheless,
instability and PWs will remain conducive for areas of locally
heavy rain. Portions of this same area will likely see heavy rain
today (Sunday) as well. Thus it is the two day totals that pushes
us into the Marginal risk category...with localized flash flooding
a possibility.

...Central to Southern Plains...
There is broad agreement that the mid-level ridge over the western
U.S. translates northward a bit for Monday into Tuesday. Low level
ridging over the eastern Great Plains should support some moisture
transport to the north across the High Plains supporting at least
weak PW anomalies, but uncertainty with convectively generated
vorticity maxima from Sunday night and uncertain potential for
additional storms late Monday to be slow moving limits confidence
in depicting a Marginal Risk at this time. There is non-zero
potential anywhere from southwestern South Dakota into Texas, but
confidence in the details remains too low for a risk area at this
time.

Chenard/Otto

Day 3
Valid 12Z Tue Jun 15 2021 - 12Z Wed Jun 16 2021

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Chenard


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