Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
835 PM EDT Wed Jul 18 2018
Valid 01Z Thu Jul 19 2018 - 12Z Thu Jul 19 2018
...A SLIGHT RISK OF EXCESSIVE RAINFALL CONTINUES TONIGHT ACROSS
PORTIONS OF THE MID MISSOURI VALLEY AND NORTHERN PLAINS...
...Northern and Central Plains/Mid Missouri Valley...
2100 UTC Update...Expanded the Slight risk area farther N-NW
through much of central-eastern SD and into southern ND, based on
the latest mesoanalysis and observational trends downwind of the
compact mid-level circulation. The uptick in the WPC deterministic
QPF reflected the trend in the guidance, especially high-res CAMs,
with pockets of modest deep-layer instability (MUCAPES 1000-1500
j/kg) present even across areas where the cloud cover remains
extensive. The expansion of the Slight risk area encompasses the
most recent HREF and SSEO elevated neighborhood probabilities of
QPF exceedance over FFG (i.e. pockets of 30-60+ percent probs of 6
hourly QPF > 3 inches).
Showers and thunderstorms are expected to develop ahead of an
organizing low associated with a mid level shortwave moving across
the Dakotas this afternoon. Increasing instability along with
deepening moisture (PWs increasing to around 1.75 inches) along an
axis of 30 kt southerly inflow will support the growing potential
for heavy rainfall rates. Hi-res guidance shows the greatest
potential for heavy accumulations centering from southeast South
Dakota and northeast Nebraska into southwest Minnesota and
northwest Iowa. Models show slow moving cells developing south
during the late afternoon and evening hours ahead of the low and
along the previously noted axis of southerly inflow. Within much
of the Slight Risk area, neighborhood probabilities from the 00
UTC HREF Mean are well above 50 percent for 24-hr amounts
...Lower Missouri Valley into Arkansas...
Weak mid level energy interacting with a deep moisture pool (PWs
1.75 to 2.25 inches) coinciding with a northwest-southeast
oriented low level boundary may continue to foster convective
development well into the morning hours today, with some of the 00
UTC deterministic guidance, as well as the HREF Mean, offering
some signal for locally heavy amounts early in the period.
Models continue to show a mid level ridge helping to keep areal
average precipitation amounts in check. However, with ample
moisture still in place, expect showers and thunderstorms to
develop once again this afternoon and evening, especially along
the high terrain from west-central New Mexico eastward along the
Mogollon Rim across the Arizona into southern Utah and southern
Deep moisture (PWs at or above 2 inches) along a slow moving
frontal boundary, in addition to weak flow aloft may support heavy
rainfall, resulting in localized runoff concerns along the
Southeast coast from North Carolina back into eastern Georgia.
...Central to the Eastern Gulf Coast...
Mid level energy embedded within weak flow aloft may once again
interact with the deep moisture pool (PWs at or above 2 inches)
that remains in place along the central and eastern Gulf coast to
produce locally heavy rainfall amounts. Any additional heavy
amounts may be problematic, especially across portions of southern
Mississippi, where recent heavy rains have resulted in low flash
flood guidance values.
Valid 12Z Thu Jul 19 2018 - 12Z Fri Jul 20 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE UPPER MISSISSIPPI VALLEY...
...Upper Mississippi Valley...
A compact upper low closing off over the Dakotas will drift
downstream on Thursday. Much of the instability available to this
system is forecast to remain tied to a developing surface warm
front which achieves roughly the shape of our Slight Risk area by
midday Thursday. Slow cell motions are expected near and north of
the small deep layer low center, over eastern North Dakota and
adjacent states. Elsewhere multiple rounds of thunderstorms
producing locally heavy rain are possible within the instability
axis and directly beneath the coldest air aloft and stronger upper
jet energy translating to the south and east of the low center.
Moisture is seasonably rich (PW values greater than 1.50 inches
along the warm front). This is not particularly anomalous, a
relatively wet summer in this region / somewhat compromised FFG
values, along with the potential for slow moving and/or repeat
convection, leads us to maintain a Slight Risk of excessive
rainfall from ND/MN to eastern Iowa.
...Middle Mississippi Valley...
Marginal Risk extends southward along the developing warm front,
and including portions of th warm sector of the northern stream
cyclone (although a remnant polar boundary remains placed even
farther south). Here from eastern Kansas to Illinois / Indiana,
flash flooding will be more dependent on periods of west to east
training that may occur on a local scale, while synoptic forcing
is greater to the north, and convection will be more cold-pool
driven across the mid MS Valley. Confluent low level flow setting
up - especially during the Thursday nighttime hours - may support
these local training episodes.
...Northern Florida Peninsula and the Southern Atlantic Coast...
Persistent mid-upper trough and surface front will act as a focus
for convection and locally heavy rainfall. PW values of 2-2.25"
and weak mid-level flow suggests some heavy rainfall producing
storms that could lead to flash flooding. A Marginal Risk is shown
along/south of the front that coincides with the highest PW values.
Moisture values will slowly ease in the Southwest but convection
will still fire off along the favored terrain, with modest upslope
flow and favorable lapse rates near 7 deg C/km. Localized flash
flooding could still be a potential so maintained a Marginal Risk
over mainly the higher terrain, and extending as far north as the
forecast extent of the 1.00 inch precipitable water values.
Valid 12Z Fri Jul 20 2018 - 12Z Sat Jul 21 2018
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE CENTRAL
OHIO RIVER VALLEY...
...Lower Great Lakes / Ohio Valley / Tennessee Valley...
WPC introduced a Slight Risk of excessive rainfall over the Ohio
Valley, including portions of IN/OH/KY. Marginal Risk surrounds
this area from lower Michigan to Tennessee and far northern AL/GA.
This region will become wedged into a narrow warm sector ahead of
the closed and strengthening upper low over the Midwest. Per
trends in the GFS/ECMWF, the greatest overlap of instability,
inflow, and unidirectional mid level flow will occur straddling
the Ohio River on Friday. The global models are fairly well
clustered here, producing a heavy rain signal that is currently
centered around Louisville and extending north and south. While
the placement may evolve some more, we felt there were enough
ingredients (strong instability per the NAM and seasonably rich
moisture) met with the developing model QPF signal to introduce
Slight Risk probabilities.
Farther south the height falls will be more subtle, but a difluent
upper jet and sufficient instability should yield deep convection
over the Tennessee Valley. This convection will be more forward
propagating judging by wind profiles, but cell mergers or brief
episodes of training could lead to flash flooding.
As a trough digs through the eastern United States, difluent
500-250 mb flow will begin to impinge upon a plume or rich
tropical moisture crossing northern Florida and extending along
the southern Atlantic coast to North Carolina. Heavy rain may tend
to focus offshore at night, but heating inland during the day
should give rise to the opportunity for deep convection to form
inland as the synoptic coastal front slowly organizes. The model
QPF signal is trending upwards, and also trending more inland. For
now we will maintain Marginal Risk, but could see this area
upgraded in time, especially if 925mb-850mb southerly inflow ends
up backing up any farther west.
Fairly standard monsoonal thunderstorms are again expected on
Friday. That is not to understate the threat, as any one could
produce flash flooding depending on the watershed. Deep layer
moisture of 1 to 2 standard deviations above climatology will
expand northward as southerly flow deepens around the west side of
the ridge center. We therefore expected Marginal Risk
probabilities northward into Utah. Southerly deep layer flow often
does present an opportunity for embedded vorticity maxima or
productive outflow boundaries that may anchor a better organized
threat of flash flooding. We are too far out to determine those
details in this case, but the Marginal Risk is fairly generous in
size to allow for later upgrade to Slight should it become
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