Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
347 PM EDT Tue Sep 17 2024

Day 1
Valid 16Z Tue Sep 17 2024 - 12Z Wed Sep 18 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER SOUTHEAST
VIRGINIA...

16Z Update...
Biggest change made to the Day 1 ERO was to expand the MRGL from
the FL Panhandle to include much of the central Gulf Coast. A
stationary front in the vicinity and PWATs of 1.8-2.0" will support
slow-moving storms and instantaneous rainfall rates around 3"/hr
between southeast Louisiana and the FL Panhandle. A weak mid-level
inflection stretching across the Lower Mississippi Valley rounding
a deeper stacked low over the southern Appalachians will also aid
in slower storm motions. 12z HREF neighborhood probabilities
remains elevated for at least 3" of rain in 6-hours through this
evening. This region remains somewhat susceptible for flash
flooding concerns with 0-40 cm soil moisture above the 90th
percentile per NASA SP0RT- LIS. Even with the saturated soils,
urban areas are most at risk to localized flash flooding.

Only minor changes were made elsewhere to incorporate 12z CAMs,
with the MRGL massaged in the northern High Plains and northern
Rockies to eliminate eastern sections where storms motions should
be fast.

Snell

Previous Discussion...
...Mid Atlantic...

The disturbance that pushed ashore yesterday will continue to churn
over the Southern Mid Atlantic with a funneling of moisture off the
Atlantic aimed across far Northeastern NC and the adjacent VA
Tidewater through the morning and afternoon hours. Modest SBCAPE
located across the above areas will justify enough instability to
trigger heavier convective cores capable of producing heavy
rainfall in an area that has a historically lower FFG due to the
urbanization factors over places like Norfolk, Virginia Beach, up
to Williamsburg. The pattern has produced a relatively robust precip
footprint as it is with models struggling to handle the overall
magnitude of the rainfall on the eastern flank of the low. 00z HREF
probability fields are still pretty exuberant for >3" (50-70%)
across the Tidewater with an extension of 30-50% further inland
towards Richmond. PWAT anomalies are sufficiently around +2
standard deviations with the 00z sounding out of MHX producing an
observed PWAT of 2.27", a similar airmass that will be advecting
north with time. This pinpoints the Southeastern VA area as the
most notable area for potential flash flood capabilities for the
period when you couple the moisture anomalies and anticipated areal
instability presence for tomorrow. This allowed for a general
maintenance of the SLGT risk from the previous forecast with only
an minor adjustment necessary on the western and northern flank of
the risk area.

Across the rest of Western NC and Western VA, the general
circulation of the remnant low will drift into the Carolina
Piedmont and become fairly stationary over the course of the
forecast. The weakening of what's left over from the disturbance
will mitigate the precip field enough to limit the flash flood
concerns further inland, although still within the low end of the
MRGL risk. In fact, some of the 24 hr totals within the means are
pretty high across Western NC and VA, but a lot of the rainfall
comes from a persistent, steady light to moderate rain with
embedded heavier echoes during the afternoon and evening hours with
marginal diurnal destabilization. Forecast 1"/hr rainfall
probabilities are relatively tame with basically no calls for 2"/hr
within the prob fields. The signal was sufficient to maintain the
MRGL for the forecast with most of the rain accumulating over a
long period of time and not so much a classic flash flood prospect.
Those areas along the Blue Ridge into the Shenandoah will have the
best opportunity for flooding in this setup, but with higher FFGs
across the region, the prospects are lower than normal for a
heightened flash flood concern.

...Florida Panhandle...

Lingering low-level moisture flux tied a quasi-stationary front and
subsequent weak surface reflection along the Gulf Coast will allow
for one more period of isolated to widely scattered thunderstorms
within the Western FL Panhandle, bisecting an area that is
currently under more sensitive FFG indices compared to
climatological averages. The most prone location resides near
Apalachicola where recent 5-day rainfall totals are upwards of
8-15" leading to a fairly water-logged position along the immediate
Gulf coast. Despite the signal not being as impressive as previous
periods, the instability gradient right along the coast could
provide the necessary element to maintain a risk for locally heavy
rainfall, especially any cells tied to the boundary running
parallel to the coast. 00z HREF neighborhood prob output signals a
stripe of 25-50% in any given location across the I-10 corridor
down towards Apalachicola for >3" potential with a 10-15% prob for
>5" over the same area. The convective coverage is more scattered
in nature, so the previous MRGL has merit given the probabilities
and anticipated rates hovering between 1-2"/hr.

...Southern Rockies and High Plains...

A strong, broad upper low over the Great Basin will continue to
slowly migrate to the northeast with increasing large scale forcing
downstream of the mean trough. Model consensus has grown over the
past succession of runs for a broader ascent pattern to develop
across the Southern Rockies with some aid of modest surface based
instability and terrain to generate a period of convection
beginning later this afternoon and early evening. Further east into
the NM and TX High Plains, a region of relatively favorable SBCAPE
between 500-1500 J/kg will allow for convection to develop
downstream of the mountains with cells becoming rooted at the
surface and helped along by shortwave energy ejecting out of the
adjacent terrain. 00z CAMs are more robust with the convective
pattern than even 24 hours ago, matching well with the recent ML
output from the ECMWF AIFS and Graphcast with some precip focus
over those areas. The HREF probs for >1" were much higher than
previous forecast as a result with some lower end probs for >2" and
>3" located across parts of the Southern Rockies and High Plains.
The focal points for flash flooding potential will continue to be
the complex terrain and burn scars situated within the Sangre de
Cristos along with the Sacramento Mountains, further south. The
High Plains area is low-end within the MRGL risk threshold, but the
favorable pattern lends credence to the chance at an isolated cell
or two over-performing and causing some flood concerns. This also
matches the current UFVS First Guess Field forecast for the period
allowing for the two previous MRGL's in the region to be bridged
together and expanded east to account for the threat.

...Northern Rockies and High Plains...

Approaching closed upper low over the Great Basin will pivot
northeastward while tilting negative as it progresses into the
Interior Mountain West, eventually situating over MT by the end of
the forecast cycle. Large scale ascent pattern will be maximized
downstream of the mean trough with ample mid-level moisture
advection being carried poleward into Big Sky Country, pushing PWAT
anomalies into the +2 to +3 standard deviation range. This is a
prominent signal when coupled with a strong upper forcing regime
and is co-located with an increase in boundary layer instability
when assessing the area MUCAPE field(s). The latest 00z
deterministic and relevant ensembles, including the HREF provide a
strong consensus on the potential for locally heavy rainfall
extending from Western MT, where the signal is the strongest, over
into the High Plains of Eastern MT and points south near the Bad
Lands of SD. Ensemble QPF output is still within that 1-2" range
for an areal average, but pockets of higher QPF are dispersed all
across the above area thanks to the approach of the potent mid-
latitude cyclone. Considering the overall setup and growing
convective support of the CAMs downstream of the previous MRGL
risk, the risk area was expanded to include much of the rest of MT
down into Northeast WY and Western SD, areas that have more
favorability for flooding potential due to lower FFGs and higher
run off capabilities given the soil type and topographic makeup in
those locations.

A SLGT risk was mulled over for Western MT, but with coordination
from the local WFO in Great Falls, decided against the upgrade with
retaining focus for the higher impacts anticipated the next period
(Wednesday).

Kleebauer


Day 2
Valid 12Z Wed Sep 18 2024 - 12Z Thu Sep 19 2024

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

Negatively tilted mean trough and closed low over the Northern
Rockies will become vertically stacked over Big Sky Country leading
to a prolific axis of deformation developing in central MT on
Wednesday. A TROWAL wrapping around the western side of the
deepening surface low will promote increase lift within a region of
anomalously high IVT (above the maximum climatological percentiles
for all hours per the 12z GEFS). PWATs are also forecast to reach
above the 95th percentile as actual values peak in the 1.00-1.25"
range. Ensemble mean QPF is quite impressive given the synoptic
regime has very little surface based instability to work with and
low MUCAPE (less than 200 J/kg). Precip totals off the latest
deterministic are gaudy with a wide swath of 2-4" with locally as
high as 5-8" located within the terrain to the east of Great Falls.
There has been some adjustments within the ensemble means for the
heaviest axis of QPF with a small displacement to the east. When
assessing the 500mb height anomalies between ensemble runs and the
current deterministic, there is a subtle, but noticeable difference
in the closed upper reflection being a bit more into East-Central MT
leading to a heavier QPF distribution being a bit further east than
previous iterations. This also follows with the ML outputs recently
with the trend having the axis of deformation a bit further east of
Great Falls, a general marker for where the heaviest precip will
focus. Ensemble bias corrected QPF is also a touch east with totals
relatively similar in the 2-4" range with a max of just over 5".
This will be something to monitor going forth, but the signal is
still very much present for a locally impactful rainfall thanks to a
strong mid- latitude cyclone entering the interior Northern U.S. A
SLGT risk was maintained with an additional small extension
eastward to reflect the recent trends in guidance.

Kleebauer/Snell


Day 3
Valid 12Z Thu Sep 19 2024 - 12Z Fri Sep 20 2024

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

Snell

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