FOUS30 KWBC 230758
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
258 AM EST Mon Dec 23 2024

Day 1   Valid 12Z Mon Dec 23 2024 - 12Z Tue Dec 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA AND SOUTHWEST OREGON...

The next atmospheric river will make headway into southwest OR and
northwest CA coastal plain with a protrusion of elevated IVT inland
as we work the back end of the period. Consistency among all major
deterministic, both global and hi-res indicate a widespread area of
2-4" with locally as high as 5.5" in-of the impacted areas, mainly
north of Santa Rosa up into southwest OR. Latest HREF probs for >3"
are very high (80+%) across areas like the King and Siskiyou Ranges
along with the Foothills of Mount Shasta. Despite the elevation, a
strong warm nose with this event will send snow levels spiking
upwards with the base pushing close to 8000ft MSL for the
rain/snow delineation point. This will create a better heavy
rain threat even away from the coast with areas inland also
maintaining a threat for localized flash flood concerns, especially
as we move into early Tuesday morning. The heaviest rainfall will
likely be within those coastal ranges which are some of the harder
areas to flood, so that will help limit the extensive flash flood
prospects we see with some events. The progressive nature of the AR
regime will also aid in the anticipated impacts, however the threat
is still within the low to medium end of the MRGL risk threshold
leading to a continuance of the MRGL from the previous forecast
issuance. The area(s) with the greatest potential are those that
are still dealing with burn scar aftermath with very sensitive
runoff capabilities. Those are included within the MRGL risk,
especially across northern CA. 

Kleebauer


Day 2   Valid 12Z Tue Dec 24 2024 - 12Z Wed Dec 25 2024

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

..Sierra Foothills..

Atmospheric river from previous period will bleed into the D2 time
frame with the primary shortwave trough quickly propagating inland
with increased forcing upon arrival into the Northern and Central 
Sierra, less so for areas back into the coast due to negative 
vorticity advection. Locally enhanced rainfall will be plausible 
the initial portion of the period with the heaviest likely aligned 
along those Foothill regions of the Northern Sierra, mainly within 
elevations below 8000ft, although snow levels will fall below 
7000ft towards the end of the more significant precipitation time 
frame late Tuesday morning. Additional totals of 1-2" are possible
within a 6-10 hour window prior to the precip ending leading to a 
low-end potential for flash flood concerns just outside the Valley 
that extends from Redding down to Sacramento. Higher runoff 
capabilities due to terrain orientation and soil moisture anomalies
running closer to normal will present some potential for flood 
concerns despite this being an event that doesn't maintain a more 
prolonged precipitation signature. 

The previously inherited MRGL risk was generally maintained,
however some of the risk area was cut out due to provide a gradient
between the Sierra locations that will trend to more winter
precipitation and the areas that will remain liquid through much
of, if not the entire duration of the event. 

..Arklatex and Lower Mississippi Valley..

Surface ridge over the eastern CONUS will slowly drift further to 
the northeast allowing for a more broad return flow regime to 
affect areas across the Western Gulf into the Lower Mississippi 
Valley. A steady flux of higher theta_E's will begin moving
northward out of the Western Gulf, carrying as far north as the Red
River before stabilizing as we reach into OK. Aloft, a digging 
mid- level shortwave will exit the Central Rockies with sights on 
the Southern Plains leading to enhanced left exit region dynamics 
and surface cyclogenesis in-of the Red River Valley with a cold 
front extending from the base into TX and a small warm front 
lifting northward around the Arklatex. The classic mid-latitude 
cyclogenesis will create a sector of modest destabilization with 
the primary axis aligned from southwest to northeast across east TX
up through the Arklatex, eventually extending northward into AR 
and southern MO. The current indications are the best organized 
convective schemes will be situated around the small warm sector 
along and ahead of the approaching cold front with relative 
buoyancy and increasing upstream shear allowing for scattered 
convection to develop late Tuesday afternoon through the evening. 

As of this time, the deep layer moisture pattern is still
relatively meager compared to some of the more impactful events
that occur in this area of the country, however there is enough
instability and PWAT anomalies creeping between 1-1.5 deviations
above normal to constitute some isolated flash flood concerns,
mainly within those stronger cores. SPC D2 risk includes a targeted
Marginal Risk for severe weather, overlapping the inherited MRGL
risk ERO for the period. Areal rainfall averages will be between
0.5-1" across east TX and 0.75-1.5" across AR, but there is a
growing consensus among the CAMs to have scattered instances of a
quick 2-3" of rainfall within the best convective environment
during the pattern evolution with a 5-10% risk of >3" within the
tail end of the 00z HREF neighborhood probs. This threat is likely
still within the lower end of the MRGL risk threshold, but the
convective premise was enough to maintain general continuity from
the previous issuance. 

Kleebauer


Day 3   Valid 12Z Wed Dec 25 2024 - 12Z Thu Dec 26 2024

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

Another atmospheric river surging off the Pacific will make an
appearance into the Pacific Northwest by Wednesday afternoon
through the end of the D3 cycle. Model guidance is keen on a surge
of moisture represented by fairly elevated IVT signatures on the
order of 600-800 kg/ms within the global ensemble blend. There is
some discrepancy between one of the main deterministic (GFS) and
the other global members carrying more of a 25th percentile outcome
in total precip due to a less robust IVT pulse comparatively. When
assessing the ensemble means from the GEFS compared to the
deterministic, the ensemble sways more in favor the scenario of
greater magnitudes into the IVT channel leading to a more 
pronounced atmospheric river regime. This trend leaned more into 
maintaining continuity in the inherited MRGL risk across the PAC 
NW, although a few changes were made in the proposal. 

The first change was to scale back on the eastern extension of the
MRGL risk due to considerably less deep layer moisture advecting
inland at this juncture for the D3 time frame. FFG indices are
still pretty high all the way towards the windward side of the
Cascades, so the prospects for FFG exceedance were pretty low and
generally fall below the 5% threshold criteria. The second change
was to cut out the higher elevations in the Olympic Peninsula due
to the primary ptype trending towards snow with more emphasis on
rainfall closer to the coast and below 4000ft MSL. The risk area
still encompasses the lower elevations surrounding the Olympics and
channels into the Olympic National Forest on the southern flank of
the Peninsula. Anticipate totals of 2-4" with locally as high as 5"
in the risk area extending from coastal southwest OR up through the
Olympic coast of WA. 

Kleebauer
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