Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 PM EDT Mon Jun 30 2025
Day 2
Valid 12Z Tue Jul 01 2025 - 12Z Wed Jul 02 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER A PORTION OF
THE MID ATLANTIC AND NORTHEAST...

...2030Z Update...
The Slight Risk was expanded across the Mid-Atlantic and Northeast
ahead of the cold front. Thunderstorms across the region today are
expected to "prime" soils across the region, which are already well
above average according to NASA SPoRT 0-40 cm soil moisture
percentiles. By early tomorrow afternoon, slow moving cold front
and right entrance region ascent are forecast to drive widespread
thunderstorms within an airmass characterized by PWAT values in the
97-99th percentile per the NAEFS. While individual cell motions
will likely be progressive (15-30 kts), steering flow oriented
parallel to the forcing should favor periods of cell training and
repeating ahead of the cold front. Both the 12Z HREF and REFS
neighborhood probabilities highlight a high (60-90%) chance of 24
hour QPF exceeding three inches across the area, with embedded 30-40%
maxima of at least five inches noted. Considerable to locally
significant flash flooding is possible tomorrow within sensitive
urban areas along the I-95 corridor, and over complex terrain in
the Appalachians. 

Asherman

...PREVIOUS DISCUSSION...

The boundary that helps focus some of the threat for heavy to
potentially excessive rainfall on Day 1 will continue to shift
eastward and provide the focus for another round on Day 2. The
flow aloft becomes more supportive over the Northeast US as
divergence aloft increases in response to a digging trough in the
upper level while convergence is focused at the low-levels
convergence increases along the boundary. Portions of the Mid-
Atlantic have suppressed Flash Flood Guidance from a period of
above normal rainfall making that area a bit more susceptible to
excessive rainfall...while faster cell motions should generally
preclude more than isolated instances of flash flooding across the
Northeast US. The inherited Slight Risk was nudged a bit to the
west of its placement in the previous outlook towards a region of
better overlap between the deterministic QPF and the lower flash
flood guidance. The surrounding Marginal Risk area was changed
little from the previous outlook.

Farther south/west along the boundary...convection is expected to
be develop within a region of decent CAPE and precipitable water
values in excess of 2 standard deviations above climatology. With
weaker flow aloft...locally heavy rainfall totals could result in
isolated instances of excessive rainfall. A stronger push of
moisture is expected into the Four Corners by Tuesday in response
to tropical moisture pushing northwestward across Mexico and
lifting to the east of a closed low churning near the California
coast. This could lead to greater thunderstorm coverage across the
Southwest/Four Corners on Tuesday and a broader isolated flash
flooding threat.

Bann/Snell


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