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Excessive Rainfall Discussion
(Caution: Version displayed is not the latest version. - Issued 0802Z Feb 27, 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
302 AM EST Tue Feb 27 2024

Day 1
Valid 12Z Tue Feb 27 2024 - 12Z Wed Feb 28 2024


Two main features to note via WV satellite; digging shortwave
trough over the northern Rockies will continue to propagate to the
southeast, eventually shifting east, then northeast by the end of
the forecast cycle. Out ahead of the mean trough out west, a fast
moving vort max will motion out of the mid-Mississippi Valley
overnight and push through the Mid-Atlantic and Northeast U.S by
morning, exiting out of the picture by the afternoon hours. These
two disturbances will be the primary rain makers for the eastern
half of the CONUS Tuesday into Wednesday morning. The bigger
impact will be from the former of the two disturbances as the
shortwave progression out of the Rockies into the plains will
begin to ramp up the convective pattern east of the Mississippi.
Broad difluent flow ahead of the shortwave trough will allow for
greater large scale forcing to take shape within a priming
environment thanks to boundary layer flow out of the south
ushering warm, moist air poleward from the Gulf. Increasing
instability axis in-of the Tennessee and Ohio Valleys will be
setup for the incoming shortwave trough as theta-E fields signal a
strong meridional push of low-level moisture with sights on areas
as far north as the Great Lakes by later this afternoon. A surface
low will spawn within the central Midwest with a triple-point
depiction fairly consistent on deterministic guidance, especially
as it matures heading into northern IL. By late afternoon,
scattered showers and storms will fire within the warm sector and
quickly move to the northeast as the steering flow is fairly
progressive through the period. Locally heavy rain can be found
within any of these convective cells, but the forward motion of
the showers/storms will likely mitigate the flash flood threat to
a low-end probability when coupled with the expected rates
(0.5-1"/hr based off latest HREF neighborhood probability fields).

As we step forward into Tuesday evening through the overnight time
frame, more showers and storms will organize into a tight linear
segment just out ahead of a swift-moving cold front plowing
eastward out of the Mississippi Valley with greater coverage
downstream over the central Appalachians up through western PA
thanks to increasing difluent pattern pivoting eastward with the
progression of the mean trough. QPF forecast shows local maximums
within the confines of central and southern OH into the previously
stated zones, mainly due to greater convective clustering as cells
merge over time after their initiation over places further west.
Some hi-res deterministic are being fairly liberal with their QPF
footprint indicating some local 1.5-2" totals within the confines
of the Alleghanies in western PA, back into southern OH thanks to
the organized convective pattern. Elevated PWAT indices
approaching 2-3 deviations above normal would constitute an
environment favorable for locally heavy rain, and with the
topographic elements coming into play within those areas, that is
where the best opportunity for localized flooding concerns is
anticipated. The saving grace is the progressive propagation of
the system as a whole which will limit a larger scale event and
keep this closer to a Marginal Risk which has been the forecast
for the past several cycles, and will be maintained in this
forecast package. The main adjustments were related to the latest
QPF footprint of the CAMs and alignment with the favorable
instability fields which will drive the rates and convective
regime necessary for flooding to occur.


Day 2
Valid 12Z Wed Feb 28 2024 - 12Z Thu Feb 29 2024


...Mid-Atlantic and Northeast...

Progressive, but potent shortwave trough will eject east-northeast
while evolving from a neutral to negative tilt as it crosses east
of the Mississippi. A strong cold front will press east through
the Ohio Valley into the Mid-Atlantic and Northeast by the second
half of the forecast period. A line of organized showers and
embedded thunderstorms will rumble eastward with locally heavy
rainfall situated within the confines of the linear segment thanks
to focused ascent along the cold front. Meridional flow ahead of
the mean trough will pump deeper moisture poleward allowing for
2-3 deviations above normal PWATs to be positioned across the
Mid-Atlantic and Northeast with as much as 4 standard deviations
forecast for portions of Northern New England. There are two main
areas of concern for flash flooding potential, both of which are
within the terrain and also have the added effect of snow melt
with NOHRSC snow-water equivalent (SWE) estimates pinpointing some
modest SWE available to couple with the heavier rainfall. The
first area is within the central Appalachians across the high
country of WV into Western MD where organized convection out of OH
will likely propagate eastward with forcing maximized within the
terrain encompassing Tucker/Grant/Randolph counties in WV, and
Garrett county in MD. There's a very good signal for at least 1"
of rainfall during Wednesday morning with locally as high as 2"
within some hi-res deterministic. Given the current SWE available
and expected heavy rainfall, some flood concerns will arise as
regional streams and rivers could flood their banks in a few
locations, especially within the rugged terrain situated in those
aforementioned counties. There will be a transition back to snow
after the cold front moves through the region and upslope flow
takes over, so the threat is mainly in the first half of the
forecast cycle, but the threat is certainly warranted.

The second area of interest is across Upstate NY into New England
where the same issues will be found as locally heavy rains mixed
with the rapid snow melt could induce some flooding prospects
where SWE is readily available and rainfall rates reaching up to
0.5"/hr prior to the cold front passage. The the main concerns
will be during the morning and early afternoon hours Wednesday
before the cold front pushes through and the falling height
pattern and 1000-500mb thicknesses crash rapidly post front with
snow taking over shortly after. There's a bit less enthusiasm for
the rainfall totals reaching above 1-1.25" on guidance currently,
but the environment is ripe for some totals to reach within that
1.5-2" range which would spell a better risk for flood potential
given the components mentioned above. The Marginal Risk in place
from the previous forecast was changed very little as the overall
consensus of the pattern evolution and expected rainfall is fairly
lock steady outside some shifts in the QPF maximums due to
convective variability.

...Pacific Northwest...

The next atmospheric river will take shape across the PAC
Northwest with a minor to bordering moderate AR forecast expected
as IVT values between 500-700 kg/ms are expected to advect into
WA/OR by Wednesday morning, onward. This event will be
characterized by longevity of moderate rainfall and less of
short-term magnitude of rainfall as probabilistic fields are all
fairly tame for the prospects of any hourly rates reaching the 1"
mark. Most of the heaviest rain will settle between 0.5-0.75"/hr
with the heaviest rainfall positioned over northwest OR where both
the GEFS/ECENS means both pinpoint the heaviest precip totals
within the period. Expect 2-4" to be fairly common within the
coastal plain extending from the Olympic Peninsula down through
southwest OR as we move into Thursday morning. When you add that
on top of what is expected during the prior forecast period (12z
Tue - 12z Wed), local totals within that above corridor will reach
3-6" over the course of 48 hrs. While this is not a textbook flash
flood concern, any poor drainage areas and urbanized locales will
be at the highest risk for flooding during the period. The
atmospheric river will continue to propagate southward down the
Pacific coast with a continued threat heading into the D3 period.
The previous MRGL risk was largely unchanged as the synoptic
pattern and associated rainfall forecast remains on track.


Day 3
Valid 12Z Thu Feb 29 2024 - 12Z Fri Mar 01 2024


Atmospheric river will continue to push southward with heights
falling rapidly across the PAC NW as the mean trough begins
pivoting towards the coast. A steady stream of moisture will be
positioned from southwest to northeast with flow generally banked
against the terrain over northern CA where heavy precip will
likely develop Thursday morning and continue through much of the
afternoon before waning. Totals of generally 2-4" will be common
across the northwest CA coast, inland into the foothills of Mount
Shasta down into the northern Sierra. This area has been involved
in multiple events over the past month with top soils generally
moist with NASA SPoRT soil moisture readings running into the 80th
percentile. Will be monitoring the snow levels as the system moves
closer to the coast as height falls and surfae cold front will
induce a rapid drop of the forecast snow levels as colder air is
brought down from the upper levels, shifting the forecast dynamic
from rain to snow accumulation. As of now, the best chance for
flooding will be along the coastal plain and areas below 6000ft
MSL leading to a MRGL risk to encompass the lower elevations from
coastal OR down into northern CA.


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