FOUS30 KWBC 170804
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 AM EDT Thu Oct 17 2024

Day 1
Valid 12Z Thu Oct 17 2024 - 12Z Fri Oct 18 2024

Relative progressive nature of precipitation across Northern NM 
will alleviate much of the flash flood concerns this afternoon, 
however the very low FFG's in place over each existing burn scar 
within the Sangre de Cristos will offer an opportunity for 
localized flooding within each impacted zone. Current HREF probs 
for >1" of precip is around 50-60% within the neighborhood derived 
analysis. This is still very lacking within the EAS prob field 
leading to a pattern of less widespread heavier precipitation 
totals and more relegated to very localized maxima likely over the 
tops of the mountain chain. The best depiction is outlined over the
San Juan Mountains where a majority of the precipitation will fall
as snow or a rain/snow mix mitigating the threat for flash flooding
during the period. This has allowed for a continuation of the nil
ERO across the CONUS, but still a non-zero presence within the burn
scars of Northern NM.

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

Kleebauer


Day 2
Valid 12Z Fri Oct 18 2024 - 12Z Sat Oct 19 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE FOUR CORNERS REGION THROUGH NORTHERN NEW MEXICO...

Longwave pattern will continue evolving into a fairly prolific 
upper trough/closed-low over the Great Basin, slowly migrating 
eastward with sights on the Four Corners. Large scale ascent will 
become maximized downstream of the mean trough with a pronounced 
diffluent pattern in-of the Southern Rockies, including the Sangre 
de Cristo and San Juan mountain ranges where heavier precip is 
forecast. A cold front will slowly propagate eastward from the 
primary disturbance with a return flow setup pressing westward 
across the Eastern NM plains on the western flank of a sprawling 
surface high east of the Mississippi. A secondary cold front to the
north will plunge southward thanks to a strong ridge of high 
pressure developing upstream in wake of the primary western low, 
along with a lee side trough developing over the Front Range. The 
tandem of boundaries and backed flow will converge over Northern NM
with a strong isentropic ascent pattern initiating later Friday 
evening through Saturday AM and beyond.

There has been very little deviation in the guidance to amass much
of a change compared to the previous forecast, so a general
continuity was maintained from the inherited MRGL risk across the
Four Corners into Northern NM. 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 Friday evening into Saturday
morning and beyond. Each areal maxima is similar with regards to
precip magnitude with a general neighborhood 1-2" max with a small
chance at upwards of 2.5" across the western maxima within the best
upper ascent within the nose of the 100kt 250mb jet streak. The
secondary maxima is generally within the last 12-hr window
encompassing 00-12z Saturday with the heaviest QPF footprint
actually highest in D3 (More on that in the D3 period below). In
any case, the very dry antecedent conditions, 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 NM. This allowed for a maintenance
of the previous MRGL risk. 

Kleebauer


Day 3
Valid 12Z Sat Oct 19 2024 - 12Z Sun Oct 20 2024

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

Closed upper circulation will slowly migrate eastward with a
persistent diffluent channel focused in-of the Eastern NM High
Plains up into the Colorado Front Range for Saturday. A uni-
directional flow located within the lowest 600mb will persistent
through much, if not all the forecast period allowing for moist
unstable air to be pulled poleward into the confines of the Caprock
and points north. ECENS PWAT anomalies are forecast to run upwards
of 3-3.5 deviations above normal with an M-Climate Percentile
(Forecasts relative to the last 20 years during time of year) that
is peeking into the seasonal max, a testament to the anomalous
nature of the moisture advection regime. With the poleward
advancement of the airmass, regional theta-E's spike with a theta-E
ridge positioning located right over the NM/TX border running clear
into Front Range of CO and nearby KS. This signal is consistent
with a persistent band of rainfall likely to be situated within the
theta-E ridge mentioned above and just out ahead of the slow-moving
quasi-stationary front trailing the very slow moving to perhaps stationary
low pressure across Southern CO. 

Precip totals of 2-4" with locally higher amounts are increasingly
likely within that corridor between Clovis up through Southeastern
CO with the max likely driven by repeating convection as small 
mid-level perturbations rotate around the general upper circulation
and move overhead of the areas above. The only saving grace for a 
more significant flash flood potential is the antecedent conditions
leading in are very dry with the 0-10cm relative soil moisture 
percentiles running between 5-20% across the area anticipated to 
see the most rainfall. Initial rains will be beneficial, but will 
add through the course of the period with waves of heavier rain 
likely exacerbating areas by later in the period into early Sunday.
The previous SLGT risk was generally maintained with a MRGL 
surrounding. Both risks were expanded somewhat on the southern and
northern peripheries to account for a trend in an expansion of
precip latitudinally in either direction. The heaviest will likely
encompass Eastern NM into Southeast CO with these areas likely
trending towards the higher end of SLGT risk if the signal remains
and/or amplifies further. 

Kleebauer
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