Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1131 AM EDT Mon Sep 24 2018
Valid 15Z Mon Sep 24 2018 - 12Z Tue Sep 25 2018
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL KENTUCKY...FAR SOUTHWEST OHIO AND FAR SOUTHWEST WEST
...Lower MS Valley...TN Valley...OH Valley..Central Appalachians
1500 UTC Update...Made minor modifications to the day 1 ERO based
on the latest observational (radar/satellite) and mesoanalysis
trends, along with some of the latest (12Z) high-res CAM output.
Highest 3-6 hour QPF>FFG exceedance probabilities based on the
latest (12Z) HREF neighborhood probabilities are aoa 60% within
the Moderate Risk area, particularly between 2100-0100 UTC.
Previous discussion below...
A well defined area of upper difluence expected to push
northeastward this period in association with shortwave energy
moving from the lower TN/lower MS Valley...through the OH Valley
and into Southern to Central Appalachians. With pw values
expected to remain 2.0 to 2.5+ standard deviations above the mean
in this area of enhanced upper difluence...widespread heavy
precipitation likely to spread through much of the TN Valley...OH
Valley...Central Appalachians into the eastern Great Lakes and
Northern Mid-Atlantic this period. A broad risk area was
maintained from portions of the lower MS Valley...across large
portions of the TN Valley...OH Valley and portions of the Central
Appalachians. Changes to the previous outlook for this period
were to upgrade to moderate risk over portions of central KY into
far southwest OH and far southwest WV where ffg values are the
lowest. Neighborhood exceedance probabilities from the arw and
nmmb and nam conest show probabilities of over 50% for 1 and 3
hour ffg values in the moderate risk area. The nam conest is also
showing exceedance probabilities over 50% into far northeast AR
and western to central TN for 1 and 3 hour ffg values. After
collaboration with WFOs across this region...the risk level was
kept at slight. The slight risk area was...however...extended
farther to the southwest into northeast AR...far northern MS and
far northwestern AL from the previous issuance given this and the
lower ffg values from recent heavy rains.
..Northeast MN into far northwest WI...
No significant changes made to the marginal risk area previously
depicted this period over northeast MN into far NW WI. Models
agree on the potential for a line of heavy frontal/pre-frontal
rains Monday afternoon into Monday night/early Tuesday. The
expected progressive nature of any organized precip should keep
any runoff risk at marginal with isolated .50-1"+ amounts.
Valid 12Z Tue Sep 25 2018 - 12Z Wed Sep 26 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO AND TENNESSEE VALLEYS AND THE NORTHEAST...
...Ohio southwest to MS/AL...
After a wet Monday, the OH/TN valleys remain in the warm sector of
a long wave trough shifting east across the north-central CONUS
Tuesday. Prefrontal activity is expected to develop/enhance as the
right entrance of a strengthening jet streak over the Great Lakes
Tuesday night. Mean flow is oriented with the cold front, so the
activity may not be all that progressive. Given notable
preconditioning, a localized Moderate Risk may become necessary.
As of now, similar coverage of the Slight Risk was maintained from
AL to the OH/PA border with areal average rainfall up to 1.5
inches which was a non-CMC 00Z general model blend.
...Northern Mid-Atlantic/Southern New England...
The retreating cold-air damming wedge lifts across the
northeastern CONUS Tuesday. Rising PW (1.75 inches which is 2 to
2.5 standard deviations above normal) allows for a heavy rain axis
to develop. 00Z consensus is for the heaviest rain over southern
New England with up to two inches on areal average for CT/RI.
Maintained the Slight Risk over this area with a consistent target
of heavy rain.
...Northern Great Lakes...
Surface low pressure develops along a cold front shifting east
over the Great Lakes Tuesday. Development of this low should allow
longer residence of heavy rain with PW of 1.5 inches (about 1.5
standardized anomaly) along the WI/MI border. Lower FFG in this
area warrants a Marginal Risk.
Low pressure approaches eastern NC Tuesday. Two inch PW looks to
converge over the Outer Banks which would result in locally heavy
rainfall. However, 00Z consensus is for this to either remain
offshore or just clip Cape Hatteras. Therefore, so excessive
rainfall is expected as of this time.
Valid 12Z Wed Sep 26 2018 - 12Z Thu Sep 27 2018
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A PORTION
OF SOUTH TEXAS...
A cold front stalls over south Texas Wednesday with enhanced
easterly flow from a 1026 surface high shifting east over the
southern plains. Two inch PW (which is a 1.5 standardized anomaly)
pumps off the Gulf. A period of low level convergence and
increasing upper level dynamics warrants a Marginal Risk for
excessive rainfall over south-central TX.
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