Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
821 PM EDT Sat Aug 18 2018
Day 1
Valid 01Z Sun Aug 19 2018 - 12Z Sun Aug 19 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL AND SOUTHERN PLAINS...
...Eastern Colorado...western Kansas...TX/OK panhandle into
central Oklahoma...
An increasingly difluent upper level pattern will be spreading
eastward across much of the High Plains southeast of an upper
level trough. Convection has developed along a low-level cyclonic
shear axis, which is currently congealing near the KS/CO border.
This activity should be progressive in nature given the eastward
progression of the closed mid/upper low. This quick forward speed
should overall limit the extent of the flash flood risk. The
general consensus amongst the 12z HREF members and recent HRRR
runs is for good coverage of 1-2" amounts...which would not likely
cause widespread flash flood issues. However this guidance does
suggest at least localized 3"+ amounts. Given this...and the
expectation of organized convection within a strongly forced,
unstable and anomalous PW environment...suggests that some
clusters of flash flooding are certainly possible. Thus will
maintain the broad Slight Risk across portions of the central and
southern Plains.
May end up with two separate swaths of higher totals...one in the
existing thunderstorm cluster near the KS/CO border and another
across the TX Panhandle into southwest OK, where, depending on the
orientation of the approaching convective line, increasing
southerly moisture transport may be a bit more supportive of
backbuilding and training later tonight.
...Northeast Wyoming...southeast Montana into central South
Dakota...
Model consensus is for a northern precipitation max closer to the
primary mid to upper level center associated with the amplifying
area of height falls moving out of the Northern to Central Rockies
and into the High Plains, where a Marginal Risk across the
Northern Plains has been depicted. Convection has been widely
scattered so far and will be quite progressive off to the
east...and any comma head rainfall overnight is expected feature
rates too low to be considered much of a flash flood threat. The
12z HREF members and HRRR generally depict 1-2" amounts, which is
not expected to pose an organized flash flood risk...although
isolated concerns could still develop.
...Northeast TX/northern LA...
Thunderstorms are trying to build westward along an old outflow
boundary across northeast LA due to earlier convection in
south-central MS while a sea breeze boundary approaches from
central LA. Inflow at 850 hPa is convergent in this area though
light -- random heavy rain issues are possible. Complicating the
forecast is an upper disturbance across LA, which is expected to
weaken/split in the next several hours and become a non-factor.
Other convection across northeast TX may attempt to move eastward
into northwest LA with time. CIN should take hold and continue
the general downward trend in heavy rain coverage, but since the
thunderstorms have not completely faded as of yet, maintain a
marginal risk in this area.
...Southeast...
Ongoing convection is expected to last a few more hours. There
was a chance for convection to organize across NC as a disturbance
aloft moved into the area, but has not come to pass, and chances
for such are fading. However, thunderstorms are trying to move
into the area from northern GA and SC, so there is a chance for
cell mergers before convection fades for good .
...NJ/Southeast NY...
While afternoon convection has moved stage right into the
northwest Atlantic, the guidance suggests that there is a chance
for nocturnal convection within the TROWAL of a surface low that
is expected to move south of the region. MU CAPE values are
expected to fall just below 500 J/kg per RAP forecasts but enough
effective bulk shear exists in the area to support organized
convection in the area. The 12z ECMWF and some of the mesoscale
guidance were showing this signal, with the ECMWF showing local
maxima in the 4"+ range. One learns not to underestimate the
TROWAL in this area, so have maintained a marginal risk across
portions of northern NJ and southeast NY as a precaution.
Roth/Chenard
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Sun Aug 19 2018
Day 1
Valid 12Z Sun Aug 19 2018 - 12Z Mon Aug 20 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS EASTERN
PORTIONS OF THE NORTHERN...CENTRAL AND SOUTHERN PLAINS...EASTWARD
INTO THE LOWER ARKANSAS RIVER VALLEY AND MID TO LOWER MISSOURI
RIVER VALLEY...
...Eastern portions of the Central to Northern Plains into the mid
to lower Missouri Valley...
A broad elongated slight risk area was maintained for the upcoming
day 1 period to the east of the amplifying upper trof across the
Plains into the lower Arkansas and middle to lower Missouri River
Valleys. Overall there is good agreement this period with the
mass field evolution of this elongated trof---with the closing off
of the mid to upper level center over eastern portions of the
Central Plains. With this closing off of the mid to upper level
center and the associated deepening of the surface low...an
increasingly well defined comma head/deformation precipitation
band likely to form over eastern Kansas...eastern
Nebraska...northwest Missouri...western Iowa...far eastern South
Dakota into far southwest Minnesota. While instability will be
modest across these areas...mu-cape values in the 500-1000
j/kg...the slow movement/pivoting of the comma head will support
widespread heavy precip totals. The slight risk area was aligned
along the model consensus heavy precipitation axis in this comma
head/deformation region.
...Eastern portions of the Southern Plains into the Lower Arkansas
River valley...
More progressive areas of convection expected to push eastward on
the southern side of the developing Central Plains closed low
across eastern portions of the Southern Plains into the Lower
Arkansas River Valley. An initial area expected to push eastward
early day 1...a continuation of early morning organized convection
moving into the Southern Plains. This will be followed by
potential for a second round of pre-frontal/frontal convection
late Sunday afternoon into Sunday night from eastern OK/northeast
TX into western AR. With pw values remaining 1 to 2 standard
deviations above the mean along this front and greater
instability...1000+ j/kg mu-cape...heavy rains possible with each
convective episode. The slight risk area was maintained from the
previous issuance to reflect the heavy rain potential with each
convective episode.
...Southern Mid-Atlantic...Central to Southern Appalachians into
the middle Gulf Coastal region...
An axis of above average pw values will persist this period along
and to the south of the frontal boundary stretching from the
Southern Mid-Atlantic...across the Southern Appalachians and into
the Tennessee Valley region. Widespread scattered convection
likely in the moist unstable airmass along and south of this
front. This will support locally heavy rainfall amounts and
potential for isolated runoff issues, No significant changes made
to the previous broad marginal risk area across these regions.
Oravec
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Sun Aug 19 2018
Day 2
Valid 12Z Mon Aug 20 2018 - 12Z Tue Aug 21 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE MID
MISSISSIPPI VALLEY TO THE SOUTHERN GREAT LAKES AND WESTERN
OHIO/TENNESSEE VALLEY...
...Mid-Mississippi Valley arcing to the Western OH/TN Valleys...
Strong and deep moisture transport ahead of a system advancing
across the Central/Southern Plains on Day 1 will bring
Precipitable Water values of 1.75 to 2.00 inches through much of
the Midwest. The model agreement has gotten better on the
development of a notable deformation zone over portions of Iowa,
Illinois and Wisconsin as the system wraps up. A warm conveyor
belt should result in organized afternoon thunderstorms acrpss the
area. Many of the adjustments made to the Day 3 Slight Risk area
on Saturday were still valid and only a few minor adjustments were
made as the Day 3 outlook becomes the Day 2 outlook.
...Northern Rockies...
A positively tilted upper trough persists over the northwest CONUS
Monday and Monday night. PW is one standardized anomaly above
normal and some instability could lead to localized heavy rain
across western MT which has low FFG. A Marginal Risk was
maintained.
Bann
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Sun Aug 19 2018
Day 3
Valid 12Z Tue Aug 21 2018 - 12Z Wed Aug 22 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE
EASTERN GREAT LAKES REGION TO THE CENTRAL APPALACHIANS...
...Great Plains region into the Southern Rockies...
A shortwave trough rotating rough the base of a larger-scale
trough in the Great Lakes should be moving towards the
east/northeast on Tuesday before it gets drawn northward by the
larger scale dynamics. Southerly flow, deep moisture and the
formation of a mid/upper level jet streak will aid set the stage
for another round of afternoon and night time storms capable of
producing heavy rainfall. A Slight Risk was placed over portions
of the Southern and Eastern Great Lakes given the Precipitable
Water Values of 1.75 to 2.00 inches expected to be in-place.
Those Precipitable Water values run some 2 to 2.5 standardized
anomalies above climatology. With Flash Flood Guidance lowered
significantly from heavy rainfall in the past 3 to 5 days, placed
a broad Slight from the Eastern Great Lakes southward into the
Central Appalachians. The Marginal Risk was extended into portions
of southeast Virginia where Precipitable Water values are forecast
to be in the range of 2.00 to 2.25 inches, setting the stage for
local heavy rainfall there.
...SOUTHWEST U.S....
A plume of deep moisture begins to move back into the Southwest
U.S. on the western periphery of a mid- and upper-level high, with
models generally in agreement that Precipitable Water values reach
or exceed 1.75 inches over far southwest Arizona during by Tuesday
evening. Precipitable Water values of an inch or more should
start to nose as far north as the southern Wasatch by late Tuesday
afternoon. This should moisture to support widely scattered late
day and evening convection that will be capable of producing
locally heavy rainfall. Given enough flow around the periphery of
the high, there should be enough steering flow to keep cells
propagating to prevent more than localized run off or ponding
problems.
Bann
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
