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

Day 1
Valid 01Z Wed Sep 18 2024 - 12Z Wed Sep 18 2024

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

...Mid Atlantic...

Occluded low wrapped up over South Carolina will continue to
funnel moisture off the Atlantic on easterly flow into the Mid-
Atlantic. Sizable area of >4" rainfall has already fallen over
northeastern NC but has largely moved northward to the VA/NC border
as of 0030Z. Still, with a saturated column and PW values >2" the
convergent flow could still yield some heavier rain totals through
the overnight hours, so the Slight Risk outline remains (but focus
a bit more toward and east of I-95. Broader Marginal Risk area
covers the region westward to the Appalachians/Blue Ridge where
additional rainfall is likely, though not as heavy as near the
coast.


...Florida Panhandle...

With the loss of daytime heating and diminishing activity, removed
this Marginal Risk area for the overnight hours.

...Southern Rockies and High Plains...

Trimmed the Marginal Risk to just northeastern NM into the TX/OK
Panhandle with a small expansion to the northeast, given recent
trends. With the lead line of rainfall already through the region,
upstream echoes could creep toward now lowered 3-h FFG values in
the SW flow. Most sensitive areas will still be around terrain
areas of northern NM but also into far SW KS this evening/overnight
should any convection flare up enough again.

...Northern Rockies and High Plains...

Robust closed upper low over the northeastern Great Basin will
pivot into MT overnight and continue to deepen, favoring
strengthening of the system. Some rainfall has fallen over much of
the region, but more vigorous dynamics should support some higher
rates and an expanse of the precip shield overnight, and especially
into D2 (Wednesday). Will keep the Marginal Risk for the region
with a slight eastward shift into the warm sector to cover any
northward-moving and potentially training cells this evening.

Fracasso


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