FOUS30 KWBC 150811
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
411 AM EDT Wed Jul 15 2026

Day 3  Valid 12Z Fri Jul 17 2026 - 12Z Sat Jul 18 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF WEST
TEXAS, SOUTHERN NEW MEXICO, ARIZONA, AND THE SOUTHERN GREAT BASIN...

...Texas into New Mexico...

We'll finally see a decay in the heavy rain setup across RGV and
Edwards Plateau as the upper pattern evolves to where the mid-level
trough over west TX finally caves and begins to dissolve through
daytime Friday. Despite that forecast, lingering elevated moisture
under the lower heights will allow for at least a scattered threat
of heavy convection further northwest into the Permian Basin and
Pecos River Valley out in west TX and southeastern NM. Precip
totals are considerably lower for the general forecast in the
region, however some areas that will have been hit prior will be
subject to more rainfall, so the prospects for flash flooding
remain elevated for many. Scattered thunderstorms will also
continue into the terrain over southwest TX with the greatest focus
over the Davis Mtns. Local totals over 2" are projected within the
latest ensemble bias corrected output, enough to warrant a
continued SLGT risk across the region.

Further west into the area of El Paso and southern NM, elevated
moisture and lowering heights under the westward progression of the
trough to the east will allow for a period of scattered convective
activity that could prompt heavy rain for areas that are more
susceptible to heavy rain threats. Areas like the Sacramento Mtns.
over into the Bootheel are likely to see thunderstorm genesis by
Friday afternoon, lingering into the evening before diminishing
with the loss of diurnal heating. Already have areas in the
Sacramento's still dealing with remnant burn scars from past wild
fires, so those area have the greatest threat in this setup to see
flash flooding. In coordination with the local El Paso WFO, have
expanded the SLGT to include the rest of far west TX into southern NM.

...Western U.S...

Despite a weakening presence of the ridge that's been dictating the
overall weather pattern for the Western U.S., another active day
of widespread convection and locally heavy rainfall will plague the
region from southern AZ up through the Great Basin with a northern
extension even into parts of western MT. PWAT anomalies out west
will remain between the 90-99th percentile according to the latest
NAEFS and ECENS climatology with PWATs even encroaching on 2" down
by the Mexican border in Pima county. Textbook pattern for
monsoonal convection and likely a scattered signal for flash flood
potential with the terrain and slot canyons across the interior
west being the main focus. Heaviest QPF signature remains over
southern UT, the Mogollon Rim and southern AZ with the threat. This
is a general repeat from the previous period with the SLGT risk
coverage relatively similar as the forecasted convective pattern
maintains a similar outlook.

...Ohio Valley through the Great Lakes...

Broad theta_E ridge centered over the Ohio and Tennessee Valley's
will act as focus for diurnally driven convection during the
afternoon Friday, lingering into the evening before finally
dissipating with the loss of daytime heating. Further north into
the Northern Great Lakes, our next trough will be sweeping across
Manitoba into western Ontario with the nose of a prominent jet max
associated with the disturbance nudging into the MN Arrowhead and
neighboring U.P. Friday night. Expectation is for a blossoming of
nocturnally driven thunderstorms to materialize within Arrowhead
and neighboring Canada and press southeast as the disturbance
migrates southeastward, north of the Great Lakes. Flow will remain
generally pretty fast, so the residence time of any convection will
be short-lived. However, PWAT anomalies will be pushing upwards of
+2 standard deviations according to both the NAEFS and ECENS
outputs meaning heavy rain potential is likely with rates between
1-2"/hr likely in the stronger cell cores. MRGL risk remains over
both the Great Lakes and Ohio River Valley given the two threats.

Kleebauer
 

Day 4 and Day 5  Valid 12Z Sat Jul 18 2026 - 12Z Mon Jul 20 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE WEST ON SATURDAY AND SUNDAY, AND OVER THE OHIO VALLEY INTO
SOUTHERN NEW ENGLAND ON SATURDAY...

...Western U.S...
A broad active monsoonal pattern will continue across much of the
western U.S. Saturday into Sunday. An anomalous PW axis stretching
from AZ northward into MT is forecast to remain above the
climatological 90th percentile, and locally higher in some spots.
This airmass will sustain a localized flash flood threat across an
expansive corridor spanning from portions of west TX, AZ, and NM
northward to MT. On Sunday troughing over ID/MT should suppress the
ridge enough to lower PWs there, and shift the moisture axis
southeastward into more of WY and central/western CO.

We are carrying Slight risk areas on day 2 and 3 over portions of
this region, and some chance embedded Slight risks will eventually
be needed on day 4 and/or 5 as well. There should be plenty of
moisture and CAPE across the risk area to support heavy rainfall
rates and a localized flash flood risk. However, the mesoscale
details which may end up determining the eventual flash flood
coverage can not really be pinned down at this lead time, and
ensemble QPF is a bit lower compared to day 2 and 3. Thus will
hold at Marginal and continue to monitor.

...Ohio Valley into Southern New England...
On Saturday the core of the high PW airmass shifts eastward, with
2"+ PWs forecast from portions of the Mid-Atlantic into Southern
New England. Troughing building into the Northeast, accompanied by
embedded shortwave energy and favorable right entrance jet
dynamics, should support a organized convective threat from OH into
the northern Mid-Atlantic and possibly portions of the Northeast.
The Marginal risk was expanded northward into more of NY and
Southern New England with this update. Spatial uncertainties exist
regarding how far north the surface front and axis of better
instability will get, which will ultimately dictate the northern
bound of any flash flood risk. Overall this should be a progressive
system, although the eventual evolution could support a couple
rounds of convection. Given the high PWs combined with strong
850mb moisture transport and synoptic forcing...at least a
localized rainfall rate driven flash flood threat exists. We may
eventually need a targeted Slight risk as the event nears and
confidence on convective evolution and rainfall axis increases.

Further south and west across the OH Valley into the southern Mid-
Atlantic large scale forcing may be weaker, but plentiful moisture
and instability may still drive localized areas of heavy rainfall.

...Florida...
Watching the potential for heavy rainfall across the west coast
coast of FL this weekend with multiple models showing a mid-level
weakness developing over the eastern Gulf in between two strong
ridges. This setup would support a heavy rainfall threat, and
possible low pressure development. However, run to run model
consistency has not been great, and any flash flood threat over FL
will depend on if and exactly where this low to mid level
circulation sets up (heavier rain could stay primarily offshore).
Thus we will hold off on introducing any Marginal risk area at this
time and continue to monitor trends.

Chenard
$$

--- Scorpio BBS
 * Origin: capitolcityonlinet.net * KY, USA (618:250/10)