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FOUS30 KWBC 180059

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
859 PM EDT Tue May 17 2022

Day 1
Valid 01Z Wed May 18 2022 - 12Z Wed May 18 2022


01Z Update...

Few changes needed with the Slight Risk area in portions of Kansas
and Missouri...although there was a slight expansion southward and
westward based on latest guidane from the CAMs and late-afternoon
runs of the WoFS.  They have been suggesting that convection over
central Kansas as of 0030Z will continue to build as a low level
jet feeds deeper moisture into the storms from a region of greater
instability.  With that becoming the primarly focus...and on-going
convection generating outflow boundaries...trimmed the northern
extent of the Marginal Risk area.  The southwestward expansion of
the Slight encompasses some of the heavier amounts shown by the


Previous Discussion...

...Central Plains to Mid-Mississippi Valley...

An area of low pressure moving from the Plains into the Midwest
will drape a warm front eastward into the Mid-Mississippi Valley
late Tuesday into Wednesday morning. Gulf moisture will be
advecting northward within the increasing low-level winds, will
encroach the frontal boundary, experience enhanced ascent with
increasing areas of upper-level diffluence that results in the
expansion of the precipitation coverage over the central U.S.
Additionally, there may be multiple impulses of energy that move
through the upper-level trough that result in multiple rounds of
heavy rain that progresses eastward along the frontal boundary.
Most of the guidance is depicting a QPF axis oriented NW to SE
from the Plains to far western reaches of the Ohio Valley, with
areal averages of 1 to 4 inches within it.

With PWs surging to +1 to +2 standard deviations above the climo
mean, warm cloud depths deepening to above 10,000 ft to support
warm rain processes, and Corfidi vectors slowing to less than 10
kts, slowing convection with rainfall rates reaching 2-2.5"/hr as
shown by the HREF probabilities will move across areas that have
FFG as low as 1.5"/3hrs. The guidance continues to show favorable
conditions for training of cells especially from eastern
Nebraska/Kansas to southern Illinois. The Slight Risk area was
expanded westward farther into southeast Nebraska and northeast
Kansas, as well as, eastward toward the St. Louis metropolitan

For the remainder of the Mid-Mississippi Valley, and into the
Upper Midwest and parts of the Central Plains, more disorganized
convection is likely as multiple shortwaves move eastward embedded
within increasingly confluent and zonal flow. While this activity
will also likely feature impressive rain rates of 1-2"/hr at
times, fast 0-6km flow and a lack of support for much in the way
of storm organization should limit the flash flood potential to
more of an isolated risk. The Marginal Risk that is already in
effect continues to reflect the threat for excessive rainfall and
local flooding concerns; therefore minimal adjustments were made.


Day 2
Valid 12Z Wed May 18 2022 - 12Z Thu May 19 2022


21Z Update...

A southward trend in models has led to some tweaks among the risk
areas in place. Our Slight Risk area has been shifted south and
extended to the west as much of the latest guidance depicts a
swath of heavier QPF through central KY into southern WV. HREF
probs for 3hr QPF > 1" sit near 40% with some > 2" bullseyes of
around 30% within this region. Left the Marginal Risk area up
through northern parts of WV since this area does have some very
low FFG.


Previous Discussion...

An area of low pressure along with its attendant warm front will
slowly move eastward through this period, in response to a potent
shortwave digging from the Midwest towards the Mid-Atlantic, and
will continue to provide a focus for showers and thunderstorms
across the Ohio Valley. Meanwhile southwesterly flow from the Gulf
of Mexico will transport moisture into the area with confluent
mid-level flow, and with mid-level energy passing through to boost
ascent, there will likely be periods with enhanced rainfall
intensity. The PW values will be nearing 1.5 inches at its peak,
which is +1 sigma for mid-May for this region. Rainfall rates of
greater than 1"/hr are likely, which through training could
support 2" of rain in some areas. The latest guidance is signaling
for the highest QPF to setup over parts of central/southern West
Virginia and northeast Kentucky, an area that historically is more
sensitive with these synoptic patterns. The FFG over this
sensitive terrain is as low as 1"/3hrs. While much of the area has
been drier of late, there are still locations that have been
upwards of 300% of normal in the past 2 weeks. Given the
sensitivity and the potential for 2+ inches a small Slight Risk
area was hoisted for this period.


Day 3
Valid 12Z Thu May 19 2022 - 12Z Fri May 20 2022


21Z Update...

Only minor updates were made to the Slight and Marginal Risk areas
in place for the Northern Plains and Upper Great Lakes region. Low
FFG combined with well above normal soil saturation of 300-600%+
over the past several weeks with added precipitation as a robust
low center pushes southward will only enhance flash flooding
concerns. There is still a bit of uncertainty, mostly with the
eastern flank of the Marginal Risk with models wavering a bit on
how much and where to place QPF, but for portions of northern ND
into MN, flash flooding is more of a concern.


Previous Discussion...

Another closed low/upper trough will be digging through the
Intermountain West and High Plains, sending a robust, low pressure
system through the Northern Plains and Upper Mississippi Valley. A
potent upper-level jet will be rounding the base of this trough
and will be providing enhanced forcing for ascent, especially
across the Dakotas and portions of Minnesota. Prior to the arrival
of the front, PW values near 0.50/0.75 will be pooled over the
region which is above the climatological average for mid-May. The
highest QPF is expected near/north of the surface low, which for
this period will likely be along the International border. The
latest models are suggesting 1 to 2 inches, locally higher, will
span from southern Saskatchewan to southern Wisconsin. Much of
northern North Dakota is still recovering from high stream flows
and multiple days of recent heavy rainfall. Given the forecast and
the degree of sensitivity, a Slight Risk for excessive rainfall
was hoisted from north-central North Dakota to north-central
Minnesota. A Marginal Risk spans from west-central North Dakota to
far western Upper Michigan to the Minnesota/Iowa border.


Day 1 threat area:
Day 2 threat area:
Day 3 threat area: