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Excessive Rainfall Discussion
(Caution: Version displayed is not the latest version. - Issued 0818Z Feb 26, 2024)
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
318 AM EST Mon Feb 26 2024

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

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


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


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.


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


...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).


Day 1 threat area:
Day 2 threat area:
Day 3 threat area: