Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
900 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.

...Mid Atlantic...

The period can be characterized by uncertainty within the guidance
as a disheveled surface and mid-level reflection left over from
what was Potential Tropical Cyclone 8 (PTC8) continues to meander
over the Mid Atlantic with a broad PWAT anomaly positioned across
Northern NC through the Delmarva. The interesting aspect of the
forecast is multiple deterministic outputs have some prospects of
heavy rainfall across parts of the Mid Atlantic, but there is a
large disagreement on where exactly that could take place. The
issue is the lack of an organized surface reflection that can help
bring everything together, but still has a presence that can allow
for smaller zones of opportunity to crop up. Assessing the ensemble
QPF has helped curb some of the discontinuity within the
deterministic leading to two areas of focus within the next 24 hrs.

The first is across the coast of NJ where an axis of convective
potential seems reasonable as the weak surface low analyzed over
Eastern NC begins to drift northeast with some resemblance of
organization as it enters over the Gulf stream. A modest jet streak
is forecast to develop over the Western Atlantic with the surface
low likely to lie within the left exit region of the jet allowing
for some modest strengthening as it moves northeast off the coast
of DE/NJ. The synoptic evolution is moving towards the "classic"
axis of deformation prospects as the low begins to intensify and
the western side of the cyclone exhibits a band of heavier precip
close to the DE/NJ coastal plain. This was outlined within the
latest HREF ensemble blended mean QPF with the matched mean QPF
displaying a relatively significant heavy QPF signature within the
Southern and Central NJ coast impacting areas from just north of
Cape May up through Atlantic City to just to the east of Toms
River. These areas aren't as flood prone compared to some of the
focused urban corridors inland, but the combination of some heavy
surf with rainfall rates between 1-2"/hr could cause some localized
flooding within the coast. Considering some of the uncertainty, the
only risk to add was a MRGL as the probabilities for >3" and >5" are
still favorable (40-60% and 15-30% respectively), but with only
modest agreement amongst guidance, it would be hard to issue
anything higher.

The second area of interest lies within much of Northern and
Eastern VA where the closed 500mb low will continue to spin south
of the region prompting some weak ascent focused over a region of
relatively modest instability and deep layer moisture. This is
another scenario where the guidance has precip, but the focus is
anywhere from Southern PA down to Hampton, VA. HREF probabilities
signal a broad axis of 40-60% probs for >3" somewhere from the WV
Panhandle down to Virginia Beach with some low end >5" probs
lingering within the same corridor. Ensemble QPF footprint depicts
an areal average of 1-1.5" located across the above zone with a
maxima centered over Northern VA. The prospects for heavy rainfall
also include areas along the Potomac, including the DC metro which
has notoriously lower FFGs thanks to urban sprawl. There is too
much uncertainty again for anything more than a MRGL risk, however
the environment is favorable for somewhere to see a heavy cell or
two that could cause issues. For that, have opted to add a second
MRGL risk within the Mid Atlantic with points between the two risk
areas void from the threats due to a greater model consensus
pointing at a relative QPF min within that area of the Delmarva.

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

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