Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
446 AM EDT Mon Sep 21 2026
Day 2
Valid 12Z Tue Sep 22 2026 - 12Z Wed Sep 23 2026

..THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTHERN NEW MEXICO...

...New Mexico and West Texas...

A potent, multi-day heavy rainfall event is set to begin across NM
and far southwest TX Tuesday. An upper level trough approaching
from the west will help strengthen an upper level jet,
persistently placing the region in the favorable right entrance
region. Deep southerly flow will transport tropical moisture
northward, while embedded mid level shortwave energy advances north
out of Mexico and into NM by Tuesday night. Low level easterly
upslope flow will also enhance lift in this setup.

PW values are forecast to approach mid to late Sep. record levels.
With sufficient instability expected, this deep moisture profile
will support heavy rainfall rates. Scattered flash flooding is
expected across much of NM and far southwest TX, with a more
concentrated area of slow moving convection possible over southern
NM.

Guidance is in pretty good agreement regarding high QPF potential
over southern NM. The 00z RRFS and 3km NAM depict swaths of 3"
rainfall totals, with the GFS/AIGFS/Gem reg depicting higher QPF
across a similar axis. With a day 3 Moderate risk already in
effect, and the flash flood threat expected to ramp up by Tuesday
night, we opted to upgrade the day 2 to a Moderate as well.
Scattered to numerous instances of flash flooding are anticipated,
some of which could be locally significant. This threat includes
the sensitive burn scar areas within the Sacramento Mountains.

...Ohio Valley and Mid-Atlantic...

An approaching mid-level trough and favorable positioning within
the right entrance region of an upper level jet will support
widespread rainfall across portions of the OH Valley and Mid-
Atlantic. At the surface, a backdoor cold front with strong
easterly flow will enhance low level convergence, further amplified
by a developing inverted surface trough.

Well to the north of the frontal boundary, limited instability
will keep rainfall primarily stratiform, with rain rates remaining
below flash flood thresholds. Considerable model spread remains
regarding where the strongest convergence and sufficient
instability will overlap, complicated by dry air advecting in from
the northeast. While the 12z EC and 00z GFS were previously in
agreement with a heavy QPF axis along the coastal plain from
eastern VA into the Delmarva, the 00z EC has trended further
inland. Meanwhile, the 00z RRFS and REFS favor a further inland
and southward axis stretching from south central VA into central
NC.

Ultimately, the forecast hinges on far south cold air damming
forces the instability axis and also how far inland the surface
trough gets nudged. A localized sweet spot may emerge where
persistent convergence intersects sufficient instability to drive
a heavy rainfall and flash flood threat. Where these ingredients
overlap, the environment will be conducive for some low topped
convective cores and efficient warm rain processes. However given
the current uncertainty, we will keep the risk level at Marginal
for this update, and we will reevaluate later today when we have
the full suite of high res guidance available.

...South Florida...

A Marginal Risk has been maintained for portions of South Florida.
Locally heavy downpours will remain possible with anomalously high
moisture in place, which could lead to some isolated
flooding/ponding in urban areas.

Chenard


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