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Excessive Rainfall Discussion
 
(Latest Discussion - Issued 0048Z May 27, 2022)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
847 PM EDT Thu May 26 2022

Day 1
Valid 01Z Fri May 27 2022 - 12Z Fri May 27 2022

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
SOUTHEAST U.S. AND OVER PARTS WISCONSIN AND ILLINOIS...

...01Z Update...
A persistent stream of moisture from the Gulf of Mexico will
continue to be drawn northward over parts of the southern
Appalachians into parts of the Carolinas..fostering an environment
capable of supporting showers and thunderstorms with locally heavy
rainfall.  Given the progression of the system eastward along the
Gulf coast, was able to the Moderate Risk area and a sizable
portion of the Slight Risk area.  The latest runs of the HRRR
still suggest the potential for another round of rainfall later
tonight, with the probability of rainfall exceeding 2 inches per
hour in the 12 hour period ending at 12Z Friday inching back up
over 20 percent again over the Florida panhandle from Apalachicola
eastward...so maintained the Slight Risk there.  The periphery of
the Slight Risk was modified just a bit eastward based on trends
in short-term radar imagery.  Trimmed part of the Slight Risk area
farther north in portions of Wisconsin and Illinois to clear areas
where the heaviest rainfall has cleared.  Felt it was a bit too
premature to remove the Slight Risk entirely over northern IL
based on the approach of convection from the south-southeast...but
feel the risk should be tapering off quickly during the late
evening hours. Some convection elsewhere in the Marginal Risk area
still has the potential for some brief heavy rainfall rates, but
the flood guidance tended to be high enough that only isolated
problems from run-off are possible.

Bann



...19Z Update...
Introduced a Slight Risk area over portions of northern Illinois
extending into southern Wisconsin.  Convection was building over
western Illinois and will be tracking over areas that have become
more hydrologically sensitive due to several days of rainfall. 
Concern is that the environment will become somewhat more
supportive of higher rainfall rates as shown by some of the recent
runs of the higher resolution CAM guidance.  See WPC Mesoscale
Precipitation Discussion for additional details.

Bann


16Z Update: Began to trim away some of the western extent of the
Moderate and Slight Risk areas in the central Gulf Coast region
were the heavy rainfall threat has translated farther east and
additional redevelopment is not expected.  But overall, did not
introduce any large-scale changes to the previous risk areas that
extend from the Gulf Coast northeastward into the southern
Appalachians. 

Pereira

...Gulf Coast...
Another round of convection appears likely this morning/afternoon
across portions of the central Gulf Coast. The front remains well
to the west, and we should see better moisture/instability advect
back northward into the region this morning. As the better
synoptic ascent ahead of the trough and in the right entrance of
the upper jet moves over these areas convection should regenerate.
Expect that we will see a well organized area of convection
traverse the region from west to east...with some
training/backbuilding along coastal areas. An additional 3"+ of
rainfall is possible, with localized 5"+ amounts. Recent rainfall
over much of this area has saturated soils and elevated
streamflows...thus this additional rainfall will likely result in
scattered to numerous areas of flash flooding over portions of
southern MS and the FL Panhandle. We will maintain the Moderate
risk in the ERO, with some expansion off to the east to cover
areas hardest hit with rain over the past 24 hours.

...Ohio Valley and Southern Appalachians... 
A good signal for heavy rainfall over portions of the southern
Appalachians today and tonight. The expectation is that we will
see multiple rounds of convection over the period, with each round
capable of dropping a quick 1-1.5" in an hour or less. The
multiple convective rounds combined with upslope enhancement to
the rainfall should result in a swath of 2-3" of rain, with
localized totals of 4-6" possible. Hydrologic conditions over this
area are more sensitive than normal...with above average soil
saturation and streamflows noted. Given this, we gave some
consideration to a MDT risk upgrade over portions of far northeast
GA into the western Carolinas. In the end opted to stick with a
Slight for now, as there is some lingering uncertainty on
convective details, and there very well might be enough of a break
in between convective rounds to keep the flooding
coverage/magnitudes in check. Nonetheless, scattered flash
flooding still appears probable, and would characterize this as a
higher end Slight risk at the moment.

Portions of the TN and OH Valley will have an isolated flash flood
risk today as convection moves across. A bit more progression to
the convection and more neutral hydrologic conditions should keep
the risk more isolated in nature...warranting just the Marginal
risk that is in place.

...Upper Midwest...
We did go ahead and expand the Marginal to the northwest to
include portions of eastern IA, northern IL, southern WI and
southern/central MI. This is north of the mid/upper level low and
within an area of weaker mean flow. Current forecasts indicate
pretty good instability along this corridor, which should should
support convective development, which combined with the slow cell
motions and stationary convergent axis...should support some cell
mergers and localized heavy rainfall totals today.

Chenard


Day 2
Valid 12Z Fri May 27 2022 - 12Z Sat May 28 2022

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN THE EASTERN
UNITED STATES...

21Z Update: Extended the previous Slight Risk a little farther
south through southeastern NC into the Pee Dee region of South
Carolina.  Consensus of the 12Z hi-res guidance shows ongoing
convection late in the Day 1 period, continuing into early Day 2
across the region, with a good signal for additional development
farther to the south training across the same area during the
afternoon.  The extended Slight was drawn to include areas where
the 12Z HREF is showing high neighborhood probabilities for
accumulations of 2 inches or more.

Across the Northeast, extended the Marginal Risk farther to the
northeast to include portions of northern New York and New
England.  Pooling moisture and unidirectional flow along a
slow-moving boundary may support training storms -- raising the
threat for locally heavy amounts.  The 12Z HREF is showing a
significant signal for locally heavy amounts, especially across
northern Maine, with high neighborhood probabilities for
accumulations of 2 inches or more.

Pereira

...Eastern U.S....
Ahead of the slow moving closed mid/upper low and a surface
boundary advancing eastward during the day, warm/moist air will
quickly surge northward along the East Coast bringing precipitable
water values of 1.5-1.75", which is near 2 standard deviations
above normal for portions of the Mid-Atlantic. Peak heating will
allow for modest destabilization and MLCAPEs of 1500-2000 J/kg.
Soundings show unidirectional flow and surging low level flow at
850 mb. Convection should be somewhat progressive off to the east,
however there should be widespread/numerous showers/thunderstorms
with pockets of stronger clusters, and the mainly unidirectional
flow does support some south brief south/north training within the
eastward moving convective clusters. There is also the potential
for more than one round of storms as the better forcing looks to
lag the initial convective round. Potential exists for hourly
totals of 1-2" at times during the afternoon hours and these
storms are likely to fall along the urban corridor from central NC
to eastern PA/western NJ. Hydrologic conditions have started to
recover back closer to normal after a wet period...but nonetheless
neutral to slightly above average soil saturation/streamflows
should still exist over portions of the area on Friday morning. We
will maintain the Slight risk, with an isolated to scattered flash
flood risk possible.

...Great Lakes...
Added a small Marginal over southern MI this period. Slow moving
convection north of the closed mid/upper level low could pose a
localized heavy rainfall threat during the day Friday.

Chenard/Taylor

Day 3
Valid 12Z Sat May 28 2022 - 12Z Sun May 29 2022

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

The environmental setup appears favorable for one or more
convective complexes over the Northern Plains Saturday and
Saturday night. We may eventually need a risk area somewhere over
this region...however uncertainty in location and the potential
progressive nature of any convective complex precludes any risk
area at the moment. We will continue to monitor.

Chenard


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt