Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
910 PM EDT Wed Sep 18 2024

Day 1
Valid 01Z Thu Sep 19 2024 - 12Z Thu Sep 19 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
CENTRAL MONTANA...

...Montana...

...0100 UTC Update...
Based on the observational trends, was able to trim the western
edges of the Slight and Marginal Risk areas a bit. Deep layer
instability within the TROWAL is rather weak/shallow, thus the
short term rainfall rates have been lacking somewhat (Isolated 1"
amounts within 3 hours). Given the persistent coverage however,
along with the low FFG values (many areas 0.50" or less in 3
hours), will maintain the Slight Risk over north-central MT.

Hurley

...Previous Discussion...
Current WV satellite imagery is a sight this evening with a broad
upper circulation engulfing much of the Inter Mountain West through
Big Sky Country in MT. Height falls have been impressive with this
disturbance as it pivots northeastward with an intensifying dual
surface low reflection located across Central MT with pressures now
down below 1000mb. The expectation is for the cyclone to continue
to intensify through the evening with an occlusion phase occurring
later this morning allowing for a strong, well- defined deformation
axis to center itself over the Northern Rockies, east of Great
Falls. PWAT anomalies approaching the upper end of climatological
peak for not just this time of year, but all years according the
latest NAEFS and ECENS moisture return intervals. PWAT anomaly
forecast is between +3 to +5 deviations, a solid 97th to 99th
percentile outcome given climatology which is impressive to say the
least. The strong large scale forcing focused on the western side
of the occluded cyclone will induce a focused TROWAL across the
aforementioned area above, in good agreement amongst all guidance,
including the CAMs and global deterministic. Regardless of which
model output you assess, the prospects for 3-5" of rainfall with
locally higher are being forecast in-of the terrain east of Great
Falls and the I-15 corridor in MT.

Latest HREF probabilities are pretty stout within both the
neighborhood and EAS fields across that general area with the
neighborhood probs running between 50-80% for >5" and even some
15-30% probs for the >8" totals being expressed. EAS probabilities
are >80% for at least 2", a testament to both the expected rainfall
and the consistency among the CAMs. This is no different from the
totals expressed within the global deterministic, and even the
ensemble blends are signaling over 4" in places. The bottom line
is; this will be a pretty formidable storm for the Northern Rockies
and adjacent Great Plains of MT with scattered instances of
flooding increasingly likely as we move through the daytime,
Wednesday. The storm will drift north and begin losing steam the
back end of the forecast leading to the organized TROWAL to fade
with lighter rates before tapering off later in the evening. The
previous SLGT risk was maintained given the strong continuity from
guidance. The only limitation to this being a higher risk is the
lack of sufficient surface instability with most of the positive
buoyancy relegated above the boundary layer. Still, rates of
1-1.5"/hr are still possible in the stronger cores, enough to
solidify the SLGT risk forecast.


...North-Central Sierra and Western NV...

...0100 UTC Update...
Expanded the Marginal Risk area a bit farther north-northwest to
include more of the northern Sierra Nevada, along with some of the
foothills in the eastern Sacramento Valley. This given the latest
radar and satellite trends, recent HRRRs, and 18Z HREF
probabilities of >0.50"/hr rainfall rates. The expansion also
includes the Park Fire north of Chico.

...Previous Discussion...Another broad closed upper low will come
ashore over Northern CA from the Pacific leading to increased
moisture and favorable upper forcing across the terrain of CA/NV
beginning later this afternoon. Despite cooler surface temps, steep
low and mid- level lapse rates with a decent outline of positive
buoyancy within the forecast soundings out of KREV signal the
potential for some heavier convective cell initiations across the
North-Central Sierra over into Western NV. There has been an uptick
in the ensemble QPF across this area the past succession of runs
leading to a bit more potential than what was thought prior.
Historically, including with the past system, we've seen these
strong upper lows tend to overachieve at times with the magnitude
and spatial coverage of the convection downstream. In this case,
it's favoring the prospects for locally heavy rainfall in the
complex terrain and over an area of lower FFGs like Reno which
carry the urbanization factor. The latest UFVS First Guess Field
has also introduced a recommended MRGL risk for the first time in
its runs as of late with the uptick in the signal. A MRGL risk was
added to account for the threat with the threshold still on the
lower half of the risk spectrum.

Kleebauer

Day 2
Valid 12Z Thu Sep 19 2024 - 12Z Fri Sep 20 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE TRANSVERSE RANGES OF CALIFORNIA AS WELL AS FOR THE URBAN
CORRIDOR OF SOUTHEAST FLORIDA...

...California...

A strong slow-moving cutoff low will drift southeastward down the
coast of California on Thursday. Limited, but still notable amounts
of Pacific moisture will advect into southern California on
southwesterly flow ahead of the low, where the forcing will be
maximized. Rainfall into central California today will drift
southward towards the Santa Barbara region by Thursday. Burn scars
in this area will greatly enhance the flood risk locally should
persistent upslope flow result in long-duration rainfall. For this
reason, the Marginal Risk was hoisted for the Transverse Ranges
north of Oxnard. Resultant flash flooding should be isolated, due
to limited moisture and burn scar support, but the threat will be
high enough that local impacts from any flash flooding are
probable.

...Florida...

A stalled out front over Florida will interact with moisture with a
tropical wave on Thursday. This will result in more widespread and
heavier storm coverage when the usual afternoon storms form. South
Florida has seen several afternoons of storms this week, so a more
active day on Thursday could result in isolated flash flooding. The
threat will be greatest along the urban I-95 corridor from West
Palm Beach south to the Keys. In addition to timing of storms and
urban concerns, flooding will be most likely at the time of high
tide, when drainage from the adjacent beaches and urban areas will
be hindered due to the higher sea levels. In coordination with
MFL/Miami, FL forecast office, a Marginal Risk has been added.

Wegman

...Previous Discussion...

A non-zero flash flood threat does exist across portions of the
Upper Midwest, however the antecedent dry conditions will allow for
much of this rain to be beneficial over the area of interest
(Northern IA, Southeastern MN, Western WI, and the Arrowhead of
MN). Urban corridors will be the most favorable spots for flash
flood prospects. Rates generally capped around 1-2"/hr, falling shy
of the current FFG forecast in the area. If the signal becomes more
impressive in the next 24 hours, a targeted MRGL is possible.

Kleebauer

Day 3
Valid 12Z Fri Sep 20 2024 - 12Z Sat Sep 21 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE URBAN
CORRIDOR OF SOUTHEAST FLORIDA...

The stalled out front over south Florida will remain in place again
on Friday. Once again the convection which will be capable of flash
flooding will be generally during the afternoon and evening, over
the urban I-95 corridor from West Palm Beach south, and at high
tide. Otherwise, the weather pattern remains status quo with very
little changing day to day.

Wegman

Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt