Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1148 AM EDT Wed Sep 18 2024

Day 1
Valid 16Z Wed Sep 18 2024 - 12Z Thu Sep 19 2024

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

...16Z Update...

The only change of note with this update was the removal of both
Marginal risk areas in the Mid-Atlantic. No changes were made to
the Slight in Montana nor the Marginal in Nevada.

...Mid-Atlantic...

Across the Mid-Atlantic thus far much of the heaviest rainfall has
remained well offshore of the Jersey coast, headed for the Cape of
Massachusetts, where FFGs are very high. For the DMV and
Pennsylvania, all of the rain has been in the form of slow-moving
showers, which have largely moved into the Appalachians, with
little left around any urban centers. Through this afternoon,
additional showers and maybe a few storms may form with daytime
heating, but meager instability, lack of convection so far, and
general agreement among the 12Z CAMs suggests any cores of heaviest
rainfall will be brief enough to not cause too many problems.
Further, much of the area has been in a drought, and the light
rainfall that has been seen and expected has been largely
beneficial.

While an isolated instance of flash flooding can't be completely
ruled out given the slow-moving nature of the convection, given the
meager showing both up to this point and in the guidance going
forward, expect the threat for flash flooding is less than 5
percent.

...Montana and Nevada...

No changes were made to the ERO risk areas in each of these
regions. For Montana, ongoing light to moderate rain over areas
with low FFGs will continue to pose a flooding threat into tonight
in this otherwise very dry part of the country. Meanwhile in Nevada
the event is just now approaching the Marginal Risk area in the
Lake Tahoe region, so no change was needed given little change in
the guidance.

Wegman

...Previous Discussion...

...Montana...

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

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

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

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

Kleebauer

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