Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1209 PM EDT Sun Jun 13 2021
Day 1
Valid 16Z 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. No significant changes were
noted in the 12Z hi-res guidance over the Marginal Risk area
compared to the previous cycle.
...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. A minor southward shift of the
Marginal across the Florida peninsula was drawn for the 16Z update
based on the latest hi-res guidance.
...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 that
weak shortwave energy over Colorado may play a role 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. No significant changes were noted in the
12Z hi-res guidance over the Marginal Risk area, but the newer
guidance does not support much in the way of rainfall over Kansas,
presumably due to a warm layer near 700 mb as noted on the 12Z LBF
sounding, which likely extends into western Kansas.
...Southern Plains...
A well defined MCV was noted over southwestern Oklahoma this
morning with a small area of ongoing convection on its southern
side. This MCV will likely remain convectively active as it slowly
drops southeastward into north central TX. Confidence is low on
the movement of the MCV however...and current CAMS continue to
struggle with current radar trends. It remains likely that
convection continues to flare up with daytime heating, 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 holding onto the
Marginal risk is warranted.
...Oregon Cascades...
An out-of-season atmospheric river has reached the Pacific
Northwest with highly anomalous moisture noted in morning
observations. The 12Z sounding from SLE showed a PW of 1.42
inches, which is not a record for June, but ties for the top 20th
highest recorded for the month. With surface dewpoints in the low
60s for the Willamette Valley and mid 50s for the northern Oregon
Cascades, forecasts of CAPE by about 21Z peak in the 500 to 1000
J/kg. This could support 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 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 PM EDT Sun Jun 13 2021
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.
The surface evolution is complex but there will be two main
boundaries of interest. First, a stationary boundary should extend
westward into the Mid-Atlantic region Monday afternoon, joining up
with an eventual low near WV/MD/PA and a diffuse front extending
southwestward from the low. Near and north of the Atlantic
stationary front will be where more noteworthy PWs of 1 to 1.5
inches should be in place. Second, a reinforcing cold front will
approach from the Great Lakes region, tracking south and east
during the daytime. Moisture values are only forecast to be about
1 to 1.3 inches across Upstate New York into New England ahead of
the secondary cold front, near seasonable for this time of year.
Confidence remains a bit below average on how everything evolves
on Monday, but an initial round of showers and thunderstorms
should be ongoing 12Z Monday across central New York into New
England. Additional shower and thunderstorm activity should
develop from late morning into the afternoon from portions of New
York into New England with low level warm advection and out ahead
of the advancing secondary cold front from New York into
Pennsylvania. 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. Most of the storms should be
forward propagating so we are not expecting anything too
significant for rainfall totals at the moment, but enough of a
risk with time to warrant the Marginal risk inclusion.
There will be somewhat better support for flash flooding over
portions of the northern Mid-Atlantic given higher values of
moisture and instability, combined with wetter antecedent
conditions over the past week. The ENE to WSW orientation of the
instability/moisture axis could favor some repeating rounds of
heavy rain with potential for 2 inches or greater in a 3 hour
period, especially from northern Virginia/Maryland into eastern
Pennsylvania and New Jersey.
...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. The western edge
of Atlantic low-level ridging should support southwesterly flow
into southern Florida while westerly flow is forecast over central
Florida, supporting confluence in the low levels and potential for
training where enhanced 850 mb flow sets up. While the upper level
flow is not very strong, there could be some weak divergence aloft
within the entrance region of strengthening 250 mb flow just east
of the Peninsula. 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 are
seeing heavy rain today (Sunday) as well. Therefore, the potential
for 2-day totals across northern Florida and localized potential
for 5 inch totals over southern Florida supports a Marginal risk.
...Central to Southern Plains...
There is broad agreement that the mid-level ridge over the western
U.S. translates northward from New Mexico into Colorado for Monday
into Tuesday. Low level ridging over the eastern Great Plains
should support some moisture transport to the west and north
across the High Plains supporting at least weak PW anomalies, but
uncertainty remains with afternoon/evening convection, especially
from the Texas Panhandle, northward. However, one area of greater
confidence is with a remnant MCV that was over southwestern
Oklahoma early Sunday afternoon which is expected to continue
tracking southwestward into western TX, reaching southern portions
of the Permian Basin near 00Z Tuesday. Some increase in moisture
is expected into southwestern portions of Texas based on moisture
advection from the east around the low level ridge to the north
and from the forecast MCV itself. Slow movement of storms within
the marginally anomalous moisture could support locally heavy rain
with 2-3 in/hr rates and totals of 3-5 inches across portions of
west-central Texas.
Chenard/Otto
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
