FOUS30 KWBC 180833
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
433 AM EDT Fri Oct 18 2024

Day 1
Valid 12Z Fri Oct 18 2024 - 12Z Sat Oct 19 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN INTERMOUNTAIN REGION AND ADJACENT SOUTHERN HIGH PLAINS...

Overall...the synoptic pattern continued to maintain good
continuity with previous runs...showing ab upper trough/closed- 
low over the Great Basin that eases eastward with time and the
strongest large scale ascent maximized downstream of the mean 
trough with a pronounced diffluent pattern. This has allowed the 
the Day 2 outlook from Thursday to propagate into the Day 1 outlook
without significant change. Ensemble guidance is in 
relative agreement on the placement of two maxima during the 
forecast period; one located Northeast AZ into Southeast UT with 
the initial diffluent signature ahead of the evolving upper low 
over the Great Basin, and the secondary maxima across Northeastern 
NM where the convergence pattern aided by increasing large scale 
ascent will generate a period of heavier precip this evening and
into the early morning hours of Saturday morning. Did make a minor
adjustment in moving the Marginal risk area around most of the 
Sangre de Cristo area to avoid too much overlap with the WPC winter
weather desk deterministic snowfall forecast. Slot canyons, and 
flashier river basins within the Four Corners area will offer a 
greater risk of flash flooding with a secondary threat over the 
remnant burn scars in northern New Mexico, This allowed for a 
maintenance of the previous MRGL risk.

Bann


Day 2
Valid 12Z Sat Oct 19 2024 - 12Z Sun Oct 20 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN HIGH PLAINS INTO SOUTHEAST COLORADO...

...Southern Plains...

Forecast rainfall continues to slowly increase across the southern
Plains in response to a quasi-stationary stationary upper low which
begins to tap deeper moisture will begin to tap into increasing 
Gulf moisture. By Day 2/Friday the moisture plume will be well 
established. Given the steering flow changes will be slow, the 
excellent forcing east of the upper level low and increasing 
instability will all favor the development of storms on Saturday capable
of producing heavy downpours that lead to flooding/flash flooding 
over an otherwise mostly parched/arid area. While individual storms
may be progressive given the low level wind speeds involved, the
placement of the strongest forcing moves little on
Saturday/Saturday night suggesting the threat of repeat convection
or training of cells...especially over northeast New Mexico.
Modified the previous Marginal risk area to avoid the highest
terrain of the Sangre de Cristo range given the elevation forcing
the predominant precipitation type as snow. 

...Northwestern Washington State...

As the Day 3 period begins at 12Z on Saturday...a well defined
atmospheric river should be settling into Washington from the 
north, resulting in local rainfall totals of 3-5 inches for the 
period. Integrated Vapor Transport values at the peak could exceed 
900 kg/m/s, which is a bit stronger than the typical A.R., but 
nothing major. When added to Friday's rain totals, this will likely
be enough rain to result in isolated flash flooding. Even if the 
highest elevations see snow, the large majority of the rainfall 
into the mountains will be in the form of rain. Saw little reason 
to make to many changes to the region of greatest excessive risk 
based on terrain- forced upslope/downslope regions at this point. 
Only the first 12 hours of the Day 2 period were covered by the 
18/00Z HREF probabilities...but the probabilities of 2 inch amount 
of liquid precipitation are roughly 50 percent in the terrain of 
the Olympics and the far northern Cascades.

Bann


Day 3
Valid 12Z Sun Oct 20 2024 - 12Z Mon Oct 21 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN AND CENTRAL HIGH PLAINS...

There threat of excessive rainfall is expected to linger into
Sunday from parts of northeast New Mexico and adjacent portions of
the Texas panhandle northeastward into southwest Kansas in a region
of upper level difluence east of the closed low/upper trough
back closer to the four-corners region. The additional rainfall in
this period does not look to be blockbuster in nature...perhaps 1 
to 1.5 inches...but it will be in addition to the amounts that fall
mainly in the Day 2 period. During the period... model guidance 
suggests a weakening of the upper level feature near the four- 
corners and surface high pressure building southward east of the 
front range will help lead to decreasing coverage and rainfall 
intensity with time.

Bann
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