Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
852 PM EDT Tue Jun 30 2026
Day 2
Valid 12Z Wed Jul 01 2026 - 12Z Thu Jul 02 2026

..THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE UPPER
MIDWEST...

Upper Midwest and Upper Great Lakes Regions...

20Z Update: The previous forecast remains on track with the overall
synoptic overview still very much in-line in run-to-run continuity.
Hi-res models have been initiating heavier convection across
southeastern MN with a focus on the area south of MSP towards
Rochester and neighboring areas. This is likely coincident with the
alignment of a sharp theta_E gradient oriented along a cold front
that will bisect the region from west-southwest to east-northeast
across southern MN into WI. Multiple shortwaves will ride across
the northern periphery of a ridge expanding across the eastern
CONUS leading to aided mid and upper forcing to couple with a
primed environment positioned over the Upper Midwest. Heavy
rainfall is likely with any convective core that develops leading
to localized rates of 1-2"/hr as noted via the modest hourly rate
probabilities from the 12z HREF. Neighborhood probabilities for >2"
are running between 60-90% across all of southeastern MN into
Central WI with >3" probs still a respectable 30-60% with a
bullseye located just east of Rochester, MN. This was enough of a
signal to expand the SLGT risk further southwest into the region
and generally maintain the previous forecast over WI with some
minor trimming on the northern edge of the risk as noted via trends
in a slight southward shift of the frontal alignment.

Kleebauer

..Previous Discussion..

The large scale flow changes little during the Day 2 period with
deep low pressure moving slowly from Manitoba into Ontario while a
strong and sprawling upper high remains quasi-stationary over the
eastern US (except for portions of New England). That leaves a
broad region of enhanced moisture-transport from the central and
southern portion of the Rockies front-range and nearby High Plains
to the Upper Mississippi Valley and the Upper Great Lakes region.

There was a growing overlap in the models between shortwave energy
rotating around the Canadian upper low and the deep-layered
moisture in the channel of the moist southwesterly flow across the
Upper Mississippi Valley with a corresponding increase in model
QPF. The UKMET and NAM NEST were most aggressive with their
QPF...perhaps from convective feedback...but even other models
tended to place their QPF maximum values somewhere over or near
Wisconsin. This region had precipitable water values in excess of
2.5 standardized anomalies above climatology on Wednesday
afternoon/evening when daytime ML CAPE values are forecast to be in
the 1000-2500 J/KG range. This environment should remain
sufficient to produce a few more robust downpours with the threat
for some isolated flash flooding so introduced a Slight risk area
there. A broad...unfocused Marginal extended from the Upper Midwest
southwestward to the Southern Rockies foothills. Weak and
difficult-to-time shortwaves embedded within the broader flow can
easily trigger storms capable of downpours almost anywhere within
the channel.

Bann

Northeast US...

20Z Update: No changes were necessary across the Northeast U.S. as
the forecast remains steadfast from the previous update. Pockets of
heavier rain are plausible across Central NY and points north with
a general maxima between 2-3" in the hardest hit locations. Best
chance for those heavier totals are forecast over Northern New
England with prob fields indicating modest potential for such
totals over northern VT down through the Green Mountain corridor.
The MRGL risk remains in effect.

Kleebauer

..Previous Discussion..

Additional showers and thunderstorms are expected over parts of New
England in the northwest flow to the east of the ridge axis of the
upper high. Seasonably high precipitable water value should support
at least some isolated heavier downpours. Antecedent conditions may
also be more sensitive depending on the evolution of more
widespread/potentially organized convection on Tuesday. No major
changes were needed to the Marginal Risk area added on Monday.

Bann

Southeast Florida...

20Z Update: MRGL risk was maintained with the current update, but
did trim back some on the western and northern edge of the previous
update. Hi-res trends were pretty aggressive across Miami proper
which correlated to elevated probs for >3" locally and even some
relevant probs (30-50%) for >5" within the latest HREF output.
This allowed for maintaining the MRGL risk with a chance at a
targeted upgrade pending further trends. Convergence pattern across
the southeast FL coast is pretty well-documented as this point in
the CAMs, so there will likely be an area that receives appreciable
rainfall in the region. Whether that settles over an urban zone or
not will dictate the potential for flash flooding as FFG's remain
high across the area.

Kleebauer

..Previous Discussion..

The front which is expected to help focus showers and
thunderstorms on Tuesday will continue to make its way into the
southern Florida peninsula on Wednesday. Despite the front
becoming weaker and more diffuse with time...the expectation is
that the boundary will still act to help focus showers and
thunderstorms in the unstable airmass...aided by convergence along
the sea breeze along with potential for sea breeze collisions.

Bann

...Louisiana...

Inverted trough will maneuver around the southern periphery of a
strengthening ridge positioned to the north leading to an
increasing convergence pattern across the southern half of
Louisiana as we move into Wednesday. Recent trends have been more
aggressive with the signature over the I-10 corridor extending
from the Lower Sabine through the Lake Charles to Lafayette, LA
areas with the latest HREF blended mean QPF output pushing closer
to 2-3" across the area. Probabilities for >3" locally are up to
40-70% within that zone along the interstate with some lower end
probs for even >5" showing up between Lake Charles and Lafayette
proper in the latest update. One positive in this setup is the area
FFG's for all 1/3/6hr thresholds are very high given the drier
antecedent soils remaining over the area. This should curb a more
enhanced flash flood scenario, but still offer a modest prospect
given the heavy totals in the deterministic outputs. In
coordination with the local LCH and LIX WFO's, a MRGL risk was
added in this forecast update to encompass the aforementioned area.

Kleebauer


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