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QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
252 AM EST Tue Dec 24 2024

Day 1
Valid 12Z Tue Dec 24 2024 - 12Z Wed Dec 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE NORTHERN
AND CENTRAL SIERRA NEVADA FOOTHILLS AND ACROSS EAST TEXAS INTO THE
LOWER MISSISSIPPI VALLEY...

..Sierra Nevada Foothills..

Current atmospheric river will progress inland during the overnight
and morning hours with the primary shortwave trough axis moving
through CA by late-morning through at least the early evening
before precip potential wanes. The recent forecasts for a skinny
axis of MRGL risk potential remains steadfast with the best forcing
likely to cause some low-end flash flood prospects in-of the
Foothills of the northern Sierra down through the adjacent hills of
the central Sierra and central Valley. Latest HREF blended mean QPF
has come up a bit across the central CA areas, likely attributed to
a more favorable mid-level ascent pattern as the shortwave trough
pivots inland and the height falls allow for a period of elevated
instability across the areas downstream of the greatest forcing.
PWAT anomalies running up to +2 deviations also coincide with a
better prospect for locally heavy rainfall as this matches the
general expectation for places already in the previous MRGL further
north. Neighborhood probabilities for >2" of rainfall are very high
(70-90+%) for areas along and north of US50 that are below the
expected snow level of <5500ft MSL. These same probabilities are
not as high further south, however they are now upwards of 25% for
the same >2" threshold and are well above 70% for >1", a precip
amount more conducive for flooding in the central Valley when
assessing the current FFG intervals. Rainfall rates will likely max
out at 0.5-0.75"/hr at peak, but that is right on the cusp of many
locations within that hourly FFG index.=20

Considering the current forecast probability fields and the trends
within the upper level evolution, have decided to expand the
previous MRGL risk further south along the central Sierra Foothills
with some adjustments on the northern periphery of the MRGL to
encompass an area that may still hang on to heavy rainfall in first
few hours of the period.=20

..East Texas and Lower Mississippi Valley..

The recent trend for heavy rainfall across east TX through portions
of the Lower Mississippi Valley have grown on the current=20
succession of runs with the 00z hi-res ensemble and noted=20
ensemble bias- correction output really enhancing the areal average
QPF for the D1 time frame. A lot of this is likely due to the short
range guidance catching on to the anticipated positive mid-level
shortwave evolution entering the plains with better theta_E
advection out ahead of the increasingly favorable ascent as the
return flow pattern aids in fueling a more readily available
environment conducive for scattered to widespread convection.
Latest HREF average for MUCAPE across east TX through the Arklatex
has shot up over 1500 J/kg with the upper quartile for CAPE now
hovering closer to 2000 J/kg, a testament to a much more favorable
instability pattern that correlates with heavier convective
premise. As a result of the expected surface evolution with an
advancing cold front east and southeast through the primed
environment ahead of the advancing trough, increased boundary layer
convergence along with mid-level ascent will ultimately generate a
period of more organized multi-cell convective clusters with cold
pool mergers likely forming towards sunset and beyond as we
initiate a modest LLJ development.=20

HREF neighborhood probabilities are now as high as 60% for >3"=20
across a large area extending from northern Houston up towards the=20
I-20 corridor near Tyler with lower probabilities (20-25%) for=20
amounts that could exceed 5" in the same general area. The saving=20
grace for this setup is the convective development and advancement=20
will be on the progressive side, so the threat for extensive=20
training is very likely. This is still within reason for isolated=20
flash flood concerns, especially considering the heaviest precip=20
will likely only last a few hours at best, and the totals being=20
reflected are very much signaling rates >2"/hr in the stronger=20
cores.=20

The previous MRGL risk was relatively unchanged, however it was
extended a bit further south and west given the latest CAMs output
indicating some stronger cells developing upstream under the=20
advancing shortwave, as well as on the southern flank of the cold=20
front as it moves to the southeast. Storms will likely hold=20
together as they approach the TX Gulf coast, so the risk area does=20
cover for the main urban areas downstream of the frontal=20
progression, including the greater Houston area and surrounding=20
suburbs.=20

Kleebauer

Day 2
Valid 12Z Wed Dec 25 2024 - 12Z Thu Dec 26 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL
WASHINGTON AND OREGON...

There has been little change in the forecasted evolution of the
next atmospheric river (AR) surging off the Pacific and into the=20
PAC NW by Wednesday afternoon. Model guidance is keen on a surge of
moisture represented by fairly elevated IVT signatures on the=20
order of 600-800 kg/ms within the current global ensemble blend and
12z NAEFS. A steady stream of moisture will advect onshore of the=20
Pacific Northwest with the current progression indicating a wave of
heavier precipitation entering coastal WA/OR by Wednesday=20
afternoon and continuing through Thursday morning. The latest NBM=20
mean QPF is pretty substantial for the areas between the central OR
coast up into the Olympic Peninsula with the latter being the=20
beneficiary of the expected QPF max thanks to added orographic=20
enhancement from the Olympic terrain just inland of the coastal=20
plain. Totals of 2-4" along the coast will be common over the span=20
of 12-18 hrs with 3-5" likely occurring across the Olympic=20
Mountains and adjacent Olympic National Forest where soil moisture=20
anomalies are hovering ~80% leading into the event. The forecasted=20
precip is causing some forecast rises in rivers within the local=20
watersheds of the areas anticipating the heaviest rainfall,=20
especially within the Skokomish basin south of the Olympics. The=20
elevated stream flows and heavy rain will enhance local flash flood
prospects for anywhere in proxy to those watersheds, so the threat
is a bit higher than normal compared to what is usually forecast=20
for an AR of this strength.=20

The coastal OR area will be more prone to urban flooding,
especially for places like Astoria that are within the secondary
maxima of the ensemble QPF for the incoming event. The previous
forecast was generally maintained, but did trim a small area across
the Olympics that will likely maintain wintry precipitation during
the entire period leading to a mitigated flash flood threat and
more of a snow load/heavy snow concern.=20

Kleebauer


Day 3
Valid 12Z Thu Dec 26 2024 - 12Z Fri Dec 27 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE PACIFIC NORTHWEST AND ACROSS EAST TEXAS THROUGH THE LOWER=20
MISSISSIPPI VALLEY...

..Pacific Northwest..

Secondary IVT pulse will occur on Thursday during the prolonged AR
event creating a flash flood potential extending into the D3 time
frame. The multi-day precipitation forecast is upwards of 3-6"
with locally higher for many areas extending from the Olympics down
through coastal OR with the highest centered over northwest WA and
some of the coastal ranges in OR. The second pulse will actually
protrude inland further creating a risk for flash flooding inland
compared to the previous period, so the MRGL risk is more
pronounced in areal coverage compared to D2. The AR will eventually
slide southward with more emphasis placed on southwest OR into
northern CA leading to more heavy precip over likely saturated
soils. This lead to the MRGL risk extending further south,
encompassing areas that will be at risk for back-to-back events
within the last 72 hours.=20

..East Texas and Lower Mississippi Valley..

A repeat of the D1 setup is progged for east TX extending into the
Lower Mississippi Valley as a shortwave disturbance will make
headway into the Southern Plains with favorable ascent aloft
coupled with surface cyclogenesis and an increasingly=20
destabilizing environment ahead of the trough. This setup does have
a bit more vigor within the 500mb evolution as the ensemble output
has a better defined closed height field with neutral to negative=20
tilt within the mean trough. This will likely lead to another=20
period of convection with heavy precipitation cores in-of a tongue
of enhanced theta_E's that will position over eastern TX into the
Arklatex. As of this time, the QPF output via ensembles is
generally within the 1-1.5" range, but is lacking the data input
from regional hi-res guidance and relevant CAMs. If the current
500mb evolution being forecast goes as planned, expectation is for
a general crescendo of QPF in future model output.=20

Considering the threat expected in the D1, it is very plausible=20
that there could be a higher risk area (SLGT) embedded over a=20
broader MRGL risk forecast. As of now, the forecast inherited was=20
only changed to capture the areal QPF average >0.75" which extended
the western flank of the risk a bit further west compared to=20
previous issuance.

Kleebauer


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!-4Nfgux9JS5Al0WAyc3aOO-0Qjq2KC9-H9GuOWMS3Ys2=
KuWLjO59Vdr8l8XHcWECxte17IN7Ps-RFNvlLw09rmYius0$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!-4Nfgux9JS5Al0WAyc3aOO-0Qjq2KC9-H9GuOWMS3Ys2=
KuWLjO59Vdr8l8XHcWECxte17IN7Ps-RFNvlLw09PZYl46E$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!-4Nfgux9JS5Al0WAyc3aOO-0Qjq2KC9-H9GuOWMS3Ys2=
KuWLjO59Vdr8l8XHcWECxte17IN7Ps-RFNvlLw09zhoeoZM$=20

=3D =3D =3D
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