Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
849 PM EDT Mon Jul 27 2026
Day 2
Valid 12Z Tue Jul 28 2026 - 12Z Wed Jul 29 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE TRI-STATE
AREA OF NEW JERSEY, NEW YORK, AND CONNECTICUT...

...Southern Appalachians through New England...
An anomalously strong upper low will produce impressive dynamics to
create the potential for significant flash flooding across the
Tri-State area on Tuesday.

A shortwave moving across the Great Lakes today will deepen into a
closed low over Upstate NY Tuesday evening, and then amplify as it
curls southeast into New England. NAEFS 500-850mb heights fall to
below the 1st percentile (or even below the minimum climatology)
according to NAEFS by 00Z Wednesday. This intense system will help
drive increasing divergence downstream, which when combined with
the accompanying height falls will result in broad deep layer
ascent from the Mid-Atlantic into New England. At the same time,
the deepening system will force acceleration of the 850mb inflow,
reflected by NAEFS percentiles in the 850mb wind to above the 99th
percentile, forcing robust moisture convergence into the Mid-
Atlantic and Northeast.

As is typical for late July, PWs and SBCAPE will be near
climatological maximums (reflected via the SPC sounding
climatology), and this event will be no different as PWs are
progged to be around 1.75 inches overlapping MUCAPE of 2000-3000
J/kg. Strong ascent in the vicinity of this anomalous closed low
and near the accompanying occluded surface low into these strong
thermodynamics sets a situation favorable for heavy, training
rainfall, and flash flooding.

Both HREF and REFS probabilities for 2"/hr rainfall reach 15-25%,
and Corfidi vectors aligned anti- parallel to the mean wind which
will be aligned to the approaching front, suggests a strong
backbuilding and training potential. While there is still quite a
bit of timing spread among the various CAMs as to when the
strongest convection will pivot through, the exceedance
probabilities for both 5"/24hrs and 8"/24hrs are impressive (40-70%
and 10-30%, respectively) despite some uncertainty in position.
This combined with the 100-yr RI exceedances exceeding 20% indicate
increasing confidence for widespread flash flooding with locally
significant events possible. After coordination with the affected
WFOs, a MDT risk has been introduced, focused in the Tri-State
region where FFG is already more vulnerable due to recent rain and
the general urban corridor.

Despite the MDT risk in place for the Tri-State area, it should be
noted that the REFS probabilities are a bit displaced south from
the HREF, and focus the heaviest rainfall closer to Philadelphia.
The surrounding SLGT risk has been expanded to account for this
potential, and there is potential for additional adjustments to the
higher-end risk areas in place for Tuesday/Tuesday night.

...Four Corners States across the Central Plains...
The expansive monsoon ridge will become additionally squeezed Tuesday
as shortwaves rotate anti-cyclonically around it both to the north
and south. This will again support at least scattered monsoon
convection as PWs above 1 inch rise northward into the Four Corners
overlapped with sufficient instability for afternoon development.
Once again the risk will likely initiate in the higher terrain
during the onset of peak heating, and then there appears to be
sufficient shear (20-30 kts) for some organization off the terrain
and into the lower elevations, primarily in NM and CO.

Storm motions will again be weak as reflected by 0-6km mean winds
of just 5-10 kts, and locally chaotic motion is likely where
storm clusters interact with other boundaries or
mergers/collisions. The CAMs, as well as HREF/REFS PMM suggest a
local enhancement to QPF from far southeast Arizona through western
New Mexico and into eastern Colorado, which is where multiple
rounds of storms are more likely. After coordination with the
affected WFOs, a SLGT risk was introduced for this axis where a
greater risk for flash flooding exists, especially across
vulnerable terrain features.

...Western Florida Peninsula and the Keys...
A shortwave diving along the Florida Peninsula/Western Gulf will
interact with broad diffluence aloft to maintain deep layer ascent
for convection across the eastern Gulf and into parts of South
Florida and the keys. Mean 0-6km winds of just 5-10 kts will be
aligned SW to NW, anti- parallel to collapsed Corfidi vectors, and
parallel to the Keys. This suggests that convection will repeatedly
build across the Gulf where PWs will be pooled at 2-2.25 inches
within impressive 2000-3000 J/kg MUCAPE. This will support
widespread activity which is reflected by high-res CAM simulated
reflectivity. Within these impressive thermodynamics, rainfall
rates of 2-3"/hr are likely at times, leading to 2-4" of rainfall
with locally higher amounts. Some of these areas will be sensitive
from heavy rain on D1, so a MRGL risk was added for the potential
for isolated impacts on Tuesday.

...Missouri Valley...
A weakening but notable shortwave and accompanying vorticity
impulse will crest the monsoon ridge to the rest and then race
steadily southeast over the Central Plains and into the Missouri
Valley by Tuesday night. This feature will help push a cold front
southward towards the Ozarks, but become aligned parallel to the
mid-level flow causing it to stall in a NW to SE oriented fashion.
As the shortwave moves along this front Tuesday night it will help
accelerate the already intensifying LLJ to draw robust
thermodynamics northward. This will support organizing clusters of
convection or an MCS aligning to the front as both 0-6km mean winds
and Corfidi vectors become parallel. With rainfall rates likely
reaching 1-2"/hr at times, any training has the potential to
produce 2-4" of rain (HREF and REFS probabilities for 3+" 20-25%)
leading to instances of flash flooding.

Weiss


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