Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
405 AM EDT Mon Aug 17 2026
Day 1
Valid 12Z Mon Aug 17 2026 - 12Z Tue Aug 18 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE OHIO VALLEY AND THE CENTRAL APPALACHIANS...
...OH/TN valley into the Appalachians...
Cold front currently analyzed across the western edge of the Ohio
Valley will continue pushing east-southeast through the overnight
hours tonight and over the course of the D1 time frame. Multiple
small mid-level perturbations will zip through the mean flow aloft
at the same juncture leading to periods of localized ascent within
the proximity of these little disturbances. A small cluster of
convection currently situated across southern IL will translate to
the east, a relevant depiction of at least one of the shortwaves
migrating across the Ohio Valley with sights downstream on the
eastern half of the Ohio Valley into the Central Appalachians.
This particular zone extending down the Central Appalachians from
WV down through eastern KY and neighboring areas of southwest VA
and northwest NC are the most noteworthy for potential impacts due
to the antecedent moist soils remaining from what was a tumultuous
period of heavy rainfall events that greatly impacted Appalachia
with widespread flash flooding and heightened riverine responses.
Conditions remain favorable for flood prospects, noted via an
abnormally low FFG threshold for all 1/3/6 hour periods. Latest
00z HREF 3hr FFG exceedance probabilities for later this afternoon
across eastern OH into WV are >70% for a large area, mainly along
and west of I-79 within that area that has seen 4+ inches of
rainfall in the past 5-6 days. Flash flooding occurred once again
this past evening from another round of organized convection over
the area, so the hydrologic state located within that region of the
CONUS remains extremely sensitive and will not take much to breach
those low FFG thresholds currently in place.
Considering the environmental priming anticipated by the afternoon
with ample destabilization and sufficient low to mid-level forcing
thanks to the incoming cold front and shortwave advection, yet
another round of thunderstorms capable of rates between 1-2"/hr
will impact the aforementioned region leading to another instance
of scattered to perhaps widespread flash flood prospects. Timing
is likely to be cell initiation by the early afternoon hours,
lingering through sunset before conditions finally improve with the
frontal passage and drier air advecting into the area, at last. A
SLGT risk was maintained in the zones referenced above with the
highest threat likely over southeast OH into WV, and the terrain
located across eastern KY into southwest VA and northwestern NC.
...Southwest...
Upper ridging centered over the Southern Plains and Lower
Mississippi Valley will lead to the Southwestern U.S. positioned on
the western flank of the anti-cyclonic flow for yet another day
leading to another round of scattered thunderstorm activity over
the sensitive terrain within eastern AZ through much of NM. Signals
within the CAMs are consistent in the scattered convective coverage
with focal points likely over the terrain extending from the NM
Bootheel up through the Southern Rockies, including areas of the
Sangre de Cristos and Sacramento Mtns. Locally heavy rainfall
capable of a quick 1-2" will be the theme of any higher impact
scenarios with the convective pattern, so the threat will remain
more within the MRGL risk category when assessing the magnitude
and expected coverage of the convection. As a result, the previous
MRGL risk was maintained with little to no change as models remain
consistent in their interpretations of the setup for today.
...Central Plains to Upper Midwest...
Troughing across Canada into the Northern tier of the CONUS will
promote the advancement of the next disturbance over the Plains and
Upper Midwest with a trailing cold front likely to shove southeast
over the next several days. The advancement of the first front in
question will allow for an initiation of thunderstorm activity over
the Northern High Plains thanks to the aforementioned front, as
well as a quick-moving mid-level shortwave ejecting out of the
Rockies by later today. Models have been all over the place with
regards to the final convective output, but CAMs consensus has
grown for a more consolidated convective cluster maturing over the
NE Sandhills, migrating southeast into the eastern half of NE by
the overnight period with some enhancement provided by a modest LLJ
structure situated over the Plains. Current probs and general
blended mean output of QPF from the latest HREF aren't exactly
spectacular, but some deterministic depictions are much more
prolific, enough to warrant some consideration for a flash flood
threat outside the Sandhills. The threat extends into southern MN
when assessing the mean QPF alignment, but the best ascent seems
fixed just a bit south. Still, some threat of heavier convection
developing within that area between northeastern NE into southern
MN when looking at the ensemble mean. Enough mid-level vorticity
will be advecting through the area with the trough, so its
plausible heavier rain sets up a bit north of the current maxima.
Previous MRGL risk was generally maintained for the threat as
forward propagation speeds of any convection should limit more
substantial impacts resulting in more isolated to widely scattered
flash flood concerns for the D1.
Kleebauer
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
405 AM EDT Mon Aug 17 2026
Day 2
Valid 12Z Tue Aug 18 2026 - 12Z Wed Aug 19 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MIDWEST AND MISSOURI VALLEY...
Longwave pattern across the Northern CONUS into Canada will lead to
continued trough amplification down into the northern tier with
multiple shortwaves migrating under the trough base. Tuesday will
see one of the more prominent waves eject out of the Northern
Rockies with an anticipated downstream amplification as it motions
into the Northern Plains. Mid and upper level height falls with an
attendant vigorous shortwave will translate eastward on Tuesday
with sights on the eastern Northern Plains with heavy convective
prospects extending from eastern NE down into the Missouri Valley
and adjacent Corn Belt (Iowa). A secondary cold front will
accompany the disturbance a strong surface ridge follows in wake of
the trough swinging through the area. A combination of diffluent
flow aloft and ample mid-level vorticity advection into a well-
positioned theta_E ridge will lead to convective initiation across
the Sandhills, once again but with a greater focus downstream given
the vigor of the shortwave and sharper cold front. Ample
instability and PWAT anomalies between +1 and +1.5 standard
deviations above normal will compliment the upper forcing necessary
to promote a period of heavy thunderstorm activity where
convergence is maximized near the surface and diffluent flow aloft
works in tandem. Add in a modest LLJ developing across the Central
Plains with a nose into the Missouri River Valley and we'll see a
period of enhanced convective impacts that will materialize Tuesday
evening, carrying through the overnight period across the
intersection of MO/IA/KS/NE.
ML outputs have been consistently favoring this area over the past
several model iterations which historically provides a great enough
consensus to warrant more confidence in the forecast. Local totals
between 2-4" are forecast in the more heavily impacted areas when
assessing the latest ensemble probs and deterministic forecasts
that extend out through the D2 period. The CAMs window ends 00z
Wednesday, a bit before the more appreciable impacts occur, so the
next forecast update for the threat should provide more detail on
the threat itself, including more up to date probabilities from the
hi-res suite. In any case, there was enough confidence to maintain
the previous SLGT risk forecast with some modifications on the
southwest flank of the risk to account for the latest trends in
available guidance, as well as matching some of the consensus
within the ML fields.
Further downstream into the western Great Lakes, additional mid-
level vorticity will stream into the region with some of the global
deterministic more emphatic in the potential for heavy rainfall
over portions of southern WI into northern IL, including both the
Chicago and Milwaukee metros. Current CAMs are a mix with regards
to the potential with some looking more aggressive than others at
the end of their runs. In any case, this will be an area to monitor
for possibly an expansion of the SLGT to encompass those areas of
southern WI and northern IL, especially in those spots that are
still recovering from the onslaught that occurred this past week.
For now, a MRGL risk is sufficient to cover for the threat, but
bears watching the next few forecast cycles.
Kleebauer
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
405 AM EDT Mon Aug 17 2026
Day 3
Valid 12Z Wed Aug 19 2026 - 12Z Thu Aug 20 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID-MISSISSIPPI AND OHIO VALLEY'S...
...Mid-Mississippi and Ohio Valley...
Shortwave amplification from the previous period will continue its
approach downstream into the Mid-Mississippi Valley with an
increasing threat of organized heavy rainfall in-of the confluence
area of MO/KY/IL/TN. Both numerical and ML guidance have been
upping forecast totals over the area over the past few days as
consensus grows for an upscale growth of convection along and ahead
of a cold front continuing its approach from the northwest.
Increasing low-level convergence along with a strong mid-level
shortwave will enhance ascent within that area of the country
leading to prospects for locally heavy rainfall capable of
scattered flash flood signals as we move into Wednesday. Some of
the guidance is more keen on enhancing the storm threat further
southeast into TN as outflows propel the convective cluster further
into the backed flow orienting into the Tennessee Valley. In any
case, QPF maxima over the confluence areas of the rivers is the
current consensus with both the numerical ensembles and ML guidance
outputs leading to greater confidence in a SLGT risk addition for
the D3 time frame. Will continue to monitor the threat closely as
small shifts could spell more significant impacts to some major
population centers located within that area. Until then, the SLGT
remains sufficient for the current outlook.
...Southwest into the Front Range...
Persistent upper ridge positioned over the Southern Plains will
allow for another burst of convective potential across the
Southwestern U.S, extending northeastward into the Front Range and
western High Plains. Multiple shortwaves embedded in the overall
flow across the deserts up through the Southern Rockies into the
Front Range will provide scattered thunderstorm activity and
potential for localized flash flooding scenarios. The most notable
shift from previous periods is the expansion of the threat back
into the Mogollon Rim with some concern for outflow driven
convection to bring back flash flooding threats into the adjacent
deserts adjacent to the terrain. Mixed signals so far in terms of
magnitude, but the threat is certainly within the bounds for at
least a MRGL risk which agrees with the current QPF outputs and
isolated to widely scattered nature of the convection forecast.
The previous MRGL risk was relatively unchanged in this update,
however some small modifications we made to extend a touch further
west into the desert areas of southwestern AZ.
Kleebauer
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
405 AM EDT Mon Aug 17 2026
Day 4 and Day 5
Valid 12Z Thu Aug 20 2026 - 12Z Sat Aug 22 2026
...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST, OHIO VALLEY, AND MID-ATLANTIC...
...Ohio Valley into Mid-Atlantic...
Large scale mid to upper level troughing moving into the eastern
U.S. will drive an active weather pattern across portions of the
OH Valley and Mid-Atlantic Thursday into Friday. On Thursday (day
4), an embedded shortwave should track across the OH Valley into
the Mid-Atlantic, initiating convection from portions of AR and MO
northeastward into WV. While some recovery is expected by this
time, recent heavy rainfall has left soils saturated over portions
of this corridor, and thus hydrologic sensitivity will remain
heightened and an isolated flash flood threat should exist.
Further east, a stronger model QPF signal exists across the
eastern Mid-Atlantic, spanning from VA into NJ. In addition to a
surface frontal boundary acting as a low level focus, upper level
jet dynamics within the favorable right entrance region will
enhance large scale lift. PW values exceeding 2" overlapping
strong instability will support heavy rainfall rates Thu into Thu
night. An embedded Slight risk may eventually be required somewhere
from eastern VA into southeast NJ...however lingering uncertainty
regarding mesoscale positioning and the climatologically higher FFG
across the coastal plain favor keeping the risk at Marginal for
now.
By Friday (day 5), the mid/upper trough will deepen over the East,
resulting in a stalling surface front. With anomalously high PWs
and moderate to strong instability remaining in place, the
environmental setup will remain conducive for additional heavy
rainfall and possible flash flooding. Model spread increases
regarding the timing and strength of individual shortwaves
traversing the deepening trough which will impact where the better
ingredient overlap will be. The GFS and ECMWF are in relatively
good agreement bringing a heavy rainfall threat from the Carolinas
into MD, whereas the AIFS and AIGFS shunt more of the deeper
moisture offshore, keeping the heaviest rain either offshore or
along coastal areas. While a Slight risk could eventually be
needed, will hold at Marginal until model guidance better converges
on the exact spatial overlap of forcing and moisture.
...Southwest...
A Marginal risk remains in place across portions of the Southwest
for both Thursday and Friday, with the risk expanding slightly
northwestward on Friday relative to Thursday. A strong upper level
ridge will remain anchored over the region, trapping embedded
shortwave energy within the light flow. Plentiful monsoonal
moisture and instability will remain in play, making localized
flash flooding likely both days. Because convective forcing will be
primarily terrain induced, overall storm coverage should stay
below Slight risk thresholds. However trends will continue to be
monitored, as any increase in convective coverage/organization
within the favorable CAPE/PW footprint could support a more
concentrated flash flood risk.
Chenard
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
405 AM EDT Mon Aug 17 2026
Day 4 and Day 5
Valid 12Z Thu Aug 20 2026 - 12Z Sat Aug 22 2026
...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST, OHIO VALLEY, AND MID-ATLANTIC...
...Ohio Valley into Mid-Atlantic...
Large scale mid to upper level troughing moving into the eastern
U.S. will drive an active weather pattern across portions of the
OH Valley and Mid-Atlantic Thursday into Friday. On Thursday (day
4), an embedded shortwave should track across the OH Valley into
the Mid-Atlantic, initiating convection from portions of AR and MO
northeastward into WV. While some recovery is expected by this
time, recent heavy rainfall has left soils saturated over portions
of this corridor, and thus hydrologic sensitivity will remain
heightened and an isolated flash flood threat should exist.
Further east, a stronger model QPF signal exists across the
eastern Mid-Atlantic, spanning from VA into NJ. In addition to a
surface frontal boundary acting as a low level focus, upper level
jet dynamics within the favorable right entrance region will
enhance large scale lift. PW values exceeding 2" overlapping
strong instability will support heavy rainfall rates Thu into Thu
night. An embedded Slight risk may eventually be required somewhere
from eastern VA into southeast NJ...however lingering uncertainty
regarding mesoscale positioning and the climatologically higher FFG
across the coastal plain favor keeping the risk at Marginal for
now.
By Friday (day 5), the mid/upper trough will deepen over the East,
resulting in a stalling surface front. With anomalously high PWs
and moderate to strong instability remaining in place, the
environmental setup will remain conducive for additional heavy
rainfall and possible flash flooding. Model spread increases
regarding the timing and strength of individual shortwaves
traversing the deepening trough which will impact where the better
ingredient overlap will be. The GFS and ECMWF are in relatively
good agreement bringing a heavy rainfall threat from the Carolinas
into MD, whereas the AIFS and AIGFS shunt more of the deeper
moisture offshore, keeping the heaviest rain either offshore or
along coastal areas. While a Slight risk could eventually be
needed, will hold at Marginal until model guidance better converges
on the exact spatial overlap of forcing and moisture.
...Southwest...
A Marginal risk remains in place across portions of the Southwest
for both Thursday and Friday, with the risk expanding slightly
northwestward on Friday relative to Thursday. A strong upper level
ridge will remain anchored over the region, trapping embedded
shortwave energy within the light flow. Plentiful monsoonal
moisture and instability will remain in play, making localized
flash flooding likely both days. Because convective forcing will be
primarily terrain induced, overall storm coverage should stay
below Slight risk thresholds. However trends will continue to be
monitored, as any increase in convective coverage/organization
within the favorable CAPE/PW footprint could support a more
concentrated flash flood risk.
Chenard
