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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2026Z Feb 26, 2024)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
326 PM EST Mon Feb 26 2024

Day 1
Valid 16Z Mon Feb 26 2024 - 12Z Tue Feb 27 2024

...16Z Update...

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

Wegman


Day 2
Valid 12Z Tue Feb 27 2024 - 12Z Wed Feb 28 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO AND MID-MISSISSIPPI VALLEYS AND THE CENTRAL
APPALACHIANS...

...2030Z Update...

Very few changes were made to the inherited Marginal Risk area.
The pattern remains one quite favorable for severe weather, but
nowhere near as much so for flash flooding. Despite PWAT values
2-3 sigma above normal, a 60 kt LLJ, and 500-1000 J/kg of
instability noted in the latest guidance (with locally higher
maxima) these ingredients favor wind-related impacts more than
flash flooding impacts. The primary factor working against flash
flooding is the incredibly fast motion of any storms that develop
in this region. This will be supported by the 60 kt LLJ and a
supporting 150 kt upper level jet. Much of the CAMs guidance that
covers the Tuesday afternoon and overnight period suggests
anywhere from 1 to 3 rounds of storms...separated by a few
hours...will move across the Ohio Valley. These storms will be
capable of heavy rainfall rates, with embedded hail with the
stronger storms. Hail in this area will cut down on overall
precipitation totals. The very rapid movement of the storms
suggests any one area may see an hour or less of rainfall with any
one storm, so it will likely take 2 or 3 rounds of rain for flash
flooding to develop. The storms are likely to grow upscale into
MCS's and lines rather quickly, so that will favor a bit more
widespread rain, though not everyone will see heavy rain due to
more widely scattered storms and hail.

Secondly, atmospheric moisture. As noted, PWAT values are 2-3
sigma above normal. However as this is the dry time of year, that
only equates to PWATS of 1 to 1.25 inches. This will therefore
limit overall precipitation totals as well as more moisture goes
into maintaining the storms with less resulting in rainfall.

Finally we have very dry soil conditions, with NASA Sport
suggesting soil moisture over the Ohio Valley near 50% saturation,
which in some areas is less than 10% of normal. The empty river
basins will be easily able to handle any and all precipitation.
Flash flooding would most likely be in any local creeks, streams,
and poor-drainage urban areas.

Taking all of this together, think it will take multiple rounds of
heavy rain (or mostly rain) moving over a poor drainage area to
result in flash flooding. Thus, instances of flooding should be
very isolated. The Marginal was considered for cancellation, but
the prevalence of urban areas, somewhat low FFGs for Ohio and
points east (1-1.5 inches per hour FFG values), and then terrain
considerations in the east may be enough to support an isolated
occurrence or two, as well as keeping with consistency. For
western areas (IL/IN) where FFGs are a little higher, the storms
are likely to be a bit stronger as they will move through closer
to peak heating Tuesday afternoon and evening.

Wegman

...Previous Discussion...

A robust mid-level shortwave will propagate southeast out of the
Pacific Northwest on Tuesday with increasing difluent flow ahead
of the mean trough that will help initiate a round of convection
within the confines of the Mid-Mississippi and Ohio Valley's.
Focused meridional flow ahead of the mean longwave trough will
advect deeper moisture and associated theta-E's poleward with a
tongue of elevated instability building within the areal theta-E
advection regime. Getting more into the hi-res time frame, we are
seeing some forecast MUCAPE values approaching 500-1000 J/kg with
locally higher lying along a projected warm front situated to the
east of a developing surface reflection in the central Midwest.
PWAT indices soar to 2-3 standard deviations above normal within a
large zone spanning from the Mid-Mississippi Valley up into the
Great Lakes by Tuesday afternoon as the enhanced low and mid-level
flow usher Gulf moisture into the outlined area. By late-Tuesday
afternoon, convective development is likely thanks to the approach
a mid-level vorticity maxima moving out ahead of the shortwave
trough as it migrates eastward. Models are keying on a band of
heavy rainfall within the proximity of a warm front, denoted
fairly well within the theta-E fields on guidance. The question
that still needs to be solved is where that boundary will lie.
Current consensus is north of I-70 with some guidance as far north
as southern MI which leads this to be the focal point of expected
convective development. Recent HREF probability fields for at
least 1"/hr have risen to 25-30% within the confines of the
IN/MI/OH border which is where most of the CAMs currently have the
warm front situated when examining the surface moisture/temp
fields. Further south will have the best deep layer moisture and
elevated PWATs, but the lack of a focused boundary like the warm
front in question will lead to more isolated flash flooding
concerns, mainly within urbanized areas, and terrain as you head
further east.

Any convective development will continue into the overnight hours
as we approach Wednesday with upscale growth of convection likely
after 06z as the shortwave trough pivots east of the Mississippi
and begins to take on a more neutral tilt by the end of the
period. Large scale forcing will be at its maximum by the end of
the forecast cycle with a line of heavy rain likely extending from
eastern OH, south into the Tennessee Valley. This will continue to
advance eastward and the focus shifts into the Appalachian front
and terrain-centric areas in the Mid-Atlantic and Ohio Valley as
we roll into D3. The Marginal Risk from the previous forecast was
maintained, but adjusted to reflect the latest QPF trends within
the convective-allowing environment being forecast. A Slight Risk
upgrade is non-zero, but current forecasted rates within any
convection keep this capped for the time-being.

Kleebauer 

Day 3
Valid 12Z Wed Feb 28 2024 - 12Z Thu Feb 29 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE UPPER OHIO VALLEY THROUGH NEW ENGLAND, AS WELL AS THE COASTAL
PACIFIC NORTHWEST...

...2030Z Update...

...Appalachians through New England...

A strengthening surface low starting the day near Lake Huron will
track east on the Canadian side of the border until its absorbed
into the cold front associated with a stronger low over northern
Quebec by Wednesday evening. Convection ongoing Wednesday morning
will reform in some areas and continue racing eastward into the
west-facing slopes of the Appalachians Wednesday morning. This
area is where there is the greatest confidence that isolated flash
flooding will occur, when added to any rainfall from Day 2/Tuesday
and any snowmelt contributions. For western portions of the
Marginal Risk, the cold air will follow in earnest, with
below-freezing temperatures ending both rainfall and flooding
threats as early as Wednesday evening.

East of the Appalachians into New England, this area will be in
the warm sector for all but the wee hours of Thursday morning.
However, despite the continued LLJ of 50+ kts, instability will be
largely lacking, keeping any rainfall largely showery with some
convective elements. Thus, for most of the I-95 corridor, rainfall
rates are unlikely to materialize enough to constitute a
significant flash flooding threat. As this period moves into the
high-resolution models time frame, new details may determine that
the Marginal Risk threat may need to come east, but for now all of
the guidance suggests there will not be enough rain overall to
result in even an isolated flash flooding threat.

The Marginal Risk area was expanded northeast to include the rest
of New York State and almost all of interior New England. The line
was largely drawn following the NOHRSC depicted areas of snow
cover. As temperatures rapidly warm well above freezing Tuesday
and especially Tuesday night, snowmelt will become an increasingly
significant factor driving the potential for flash flooding across
New England. Rainfall totals don't vary all that much across New
England except into extreme down-eastern Maine, where
strengthening flow off the Atlantic will locally enhance rainfall
totals. Thus, the flash flooding risk and the rationale for
expansion of the Marginal is largely driven by snowmelt rather
than rainfall, though up to an inch of rain on top of snow should
be enough to cause flash flooding. The real question is how
substantial the snowmelt component will be as temperatures remain
above freezing for as little as 24 hours in some areas.

...Pacific Northwest...

No changes were made to the inherited Marginal Risk area for
coastal WA and OR. Widespread 2 to 3 inches of rain totals are
forecast for this area. However, there are a couple points to note
working against flash flooding. First off is low snow levels and
cool air. This will be a cold rain for sure along the coast, as
the area spends much of the day in between 2 separate reinforcing
shots of cold air. Thus, a substantial portion of the
precipitation that falls in the coastal mountains and definitely
the Cascades will be snow, and therefore not contributing to
rising river levels. Limiting the area of potential flooding to
the coastal plain, which is largely able to handle large amounts
of rain due to flora and a wet climate, even these larger amounts
of rain will struggle to result in flash flooding. Thus,
confidence is low for this Marginal Risk area, but any
poor-drainage areas will be susceptible to flooding given the
large amounts of rain forecast.

Wegman

...Previous Discussion...

...Mid-Atlantic and Northeast...

Potential mid-level shortwave trough will pivot eastward, taking
on a neutral to eventually negative tilt as it progresses through
the Great Lakes and Ohio Valley on Wednesday afternoon. Large
scale ascent pattern will be maximized downstream of the mean
trough with a line of rainfall extending northeast to southwest
across the interior Northeastern U.S down into the Central
Appalachians. Locally heavy rainfall embedded within the main line
will allow for isolated flash flood concerns, especially within
the terrain focused areas of western PA down into WV and eastern
KY. A secondary area of interest will be across western NY state,
mainly to the east of Lake Ontario where snowpack with modest
snow-water equivalent (SWE), based on the latest NOHRSC data set
will couple with warming temperatures ahead of the cold front and
locally heavy rainfall on top of the snow causing rapid snow melt.
QPF totals exceeding 1" are forecast within the Tug Hill and
adjacent Adirondack area of Upstate NY where the highest SWE is
located. Considering rates between 0.55-0.75"/hr possible along
with the anticipated snow melt, this could lead to localized
flooding as rivers and streams would have to take in a lot of the
excess water and deep soil moisture is already running above 80%
for much of the aforementioned area. QPF forecast of 1-1.5" is
currently situated over the above area, as well as within Central
Appalachia, but there is potential for higher QPF pending the
evolution of the convection over the Ohio Valley as synoptic
pattern is favoring fairly robust low to mid-level forcing within
a moisture laden environment given the +2.5-3 deviation PWAT
anomaly across guidance. There is some agreement within the spread
on the heavy rainfall making it across the mountains and into the
Piedmont of NoVA and MD up through eastern PA. If there is more
consensus in future runs, would not be surprised to see the MRGL
risk expanded to encompass part of the population centers west of
I-95. For now, will maintain the Marginal Risk over areas west of
the Blue Ridge in the central Mid-Atlantic and west of the Capital
District in NY state.

...Pacific Northwest...

Another atmospheric river will indulge on the Pacific Northwest
coast with the primary concern focused over the coastal plain of
the Olympics in Washington and coastal Oregon. Current AR forecast
is for a minor impact event with IVT values between 550-650 kg/ms
anticipated to advect over the aforementioned areas, protruding
inland to the Cascades. This will aid in copious amounts of
rainfall over the span of 24-36 hrs, but rates will be lacking as
probability of exceeding anything over 0.5"/hr will be modest, at
best. Totals in-of the coast will be between 2-3" with up to 4"
possible over the northwestern OR coast near Astoria. Overall, a
low-end MRGL risk area was maintained from previous forecast due
to the rainfall totals anticipated, as well as the moist-leaning
soils based off the recent NASA SPoRT soil moisture analysis
(Mainly within coastal WA and northwest OR).

Kleebauer


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