Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
844 PM EDT Fri Jul 17 2026
Day 2
Valid 12Z Sat Jul 18 2026 - 12Z Sun Jul 19 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE FOUR CORNERS AND SOUTHWEST, AND ACROSS PARTS OF THE UPPER MID
ATLANTIC INTO SOUTHERN NEW YORK AND NEW ENGLAND...

2000 UTC Update: The most notable change to the Day 2 ERO was the
eastward and northward expansion of the Slight Risk area across
northeast AZ, southeast UT, central NM, and into southwest CO. The
expansion incorporates the Slot Canyons across southern UT, and was
supported by the high HREF and REFS probabilities (>50%) of hourly
rainfall rates of at least .50". Maximum FFG probabilities (40km
neighborhood probs) of the 1/3/6 hr FFG peak above 25% in these
areas per the 12Z HREF, while climb above 40% (over 70% in northern
AZ) per the 12Z REFS. Based on the 00Z guidance, and particularly
over UT, the ECMWF EFI does indicate spotty values over 0.7, with
shift- of- tails values < 1.0.

In addition, also based on the latest guidance/trends (particularly
the CAMs), have expanded the Slight Risk over the Northeast to
include more of CT, southern NY, eastern PA, much of NJ, and far
northern DE. Quite a few CAMs show spotty 3-5" totals ahead of the
mid/upper shortwave and associated surface cold front, with the 12Z
REFS showing 30-40% probabilities of >5.00" across CT-RI and
southern MA.

Hurley

Previous discussion..

...Western U.S.....
An expansive monsoonal convective pattern remains locked in place
across the Western CONUS, supporting the continuation of a broad
Slight Risk extending from West Texas and southern New Mexico
northwestward across central and northern Arizona into southern
Utah. An increase in low level moisture transport into the region,
coupled with the moisture already in place will support PW
anomalies 1.5-3 std dev above normal across much of this region.
This moisture will likely once again support widespread convective
development across the area.  With ample daytime heating and
sufficient instability, these storms will be capable of producing
efficient rainfall rates. Given the history of recent
precipitation, soils have become increasingly sensitive across
portions of the region, elevating the potential for rapid runoff
and flash flooding concerns.

...Ohio Valley/Appalachians to the Mid Atlantic and Northeast...
A progressive cold front dropping southeastward will interact with
an anomalously moist airmass pooling ahead of it. Atmospheric
profiles will be highly favorable for efficient warm-rain
processes, featuring PWs over 2 inches and deep warm-cloud layers.
Where daytime heating and mid-70s dewpoints break capping
inversions, prefrontal convection will tap into greater instability
and raise rainfall rates.

This environment along with the concentration of urbanized areas
susceptible to fast runoff provided enough confidence to extend
the Slight Risk farther east from parts of eastern Pennsylvania and
northern New Jersey across southern New York and New England.
Limiting factors to highlighting a more widespread flash flooding
threat include fast storm moisture and lingering model discrepancy
regarding the exact placement of the heaviest precipitation axes.

Farther west through the central Appalachians into the Ohio Valley,
a broader Marginal Risk remains in place to cover what is
anticipated to be more isolated concerns.

...Florida Gulf Coast into Southeast Alabama and Southwest Georgia...
Deep tropical moisture and a slow-moving mid-to-upper level low
over the eastern Gulf will maintain a heavy rainfall threat, which
translates into the Day 3 ERO. Model QPFs have come up in this
area, now within the CAM windows, with high (>60%) probabilities of
24hr QPF exceeding 3" per both the 12Z HREF and REFS. Both are also
fairly bullish with the >5" probabilities west of the FL Big Bend
(30-40% per the HREF, 40-50% per the REFS).

Hurley/Pereira


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