Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
839 PM EDT Sun Aug 09 2020
Day 1
Valid 01Z Mon Aug 10 2020 - 12Z Mon Aug 10 2020
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE UPPER MISSISSIPPI VALLEY AND UPPER GREAT LAKES...
0900 UTC discussion
...Eastern Missouri through southern Illinois and Adjacent
States...
A fairly small scale but persistent threat of heavy rain is
expected to affect the middle Mississippi Valley near the
confluence with the Ohio River today. At 08Z a small MCS was
located near Saint Louis. During the formative stages, merging
thunderstorms had produced widespread 3-inch rainfall, with
radar-based estimates of over 6 inches northwest of Saint Louis.
The cold pool was pushing out so that totals will likely not be so
large later this morning, but the slow-moving MCV heading into
southern Illinois will be a focus for continued heavy rain, and
warm advection into eastern Missouri, aided further by the
presence of the cold pool, will continue to produce loosely
organized thunderstorms. And given the seasonably rich moisture in
place, with high wet-bulb zero levels, the storms will be
efficient rain producers.
Following perhaps a respite in the afternoon, the activity could
flare again Sunday night, a bit farther east, as low level flow
decouples and again accelerates, producing more pronounced warm
advection into a region of mid level vorticity riding the edge of
the southern Plains ridge / hot dome. We tended to favor the 00Z
WRF-ARW2 solution which is more generous in carrying spotty heavy
rains south and east, including into western Tennessee. It is a
difficult forecast with subtle forcing, however, so expect the
risk areas to be refined in each subsequent outlook.
...Eastern Minnesota into Northern Wisconsin and Western/Central
Upper Michigan...
0100 UTC Update -- Have trimmed the back edges of the Marginal
Risk across western MN and northwest IA. Otherwise, no other
changes were made since the last day 1 ERO update. The Slight Risk
area continues to encompass areas with higher surface moisture and
streamflow anomalies (and thus lower FFGs), ahead of the next
round of convection ahead of the approaching upper shortwave
trough/surface cold font.
0900 UTC Discussion...
The regime across the central to northern Plains and Upper Midwest
was marked by particularly large CAPE values in the vicinity of a
frontal boundary. Very warm mid levels across Nebraska and Iowa
were forcing convection to lift up into SD/MN this morning to find
parcels that would more readily reach their level of free
convection and support propagation northeastward. This weakening
MCS is forecast to deliver a half inch to inch of rain this
morning across southern MN into Wisconsin. Meanwhile, height falls
will continue throughout the day, as an upper trough over
Saskatchewan sinks into the northern United States. Sufficient
forcing and instability exists to yield additional rounds of
convection over parts of MN/WI/Upper MI, and eventually dropping
down into Iowa as well. The 00Z hi-res model suite generally
depicted a swath of maximum rainfall from central Minnesota into
western Wisconsin today, with blended mean techniques suggesting 1
to 3 inches of rain on average, and individual runs producing
4-plus. This appears like a strong enough signal for an upgrade to
Slight Risk, especially as areas from Duluth south and
southwestward had received pockets of heavy rain the past day or
two, thus lowering flash flood guidance. Marginal Risk extends up
through northwest Minnesota and eastern North Dakota where a
stronger, reinforcing front will arrive this afternoon.
...Iowa and Western Illinois...
This area will see rising instability today, but will remain in
between forcing mechanisms for the most part, with one notable
shortwave drifting through southern Illinois, and a larger scale
trough and frontal system affecting the northern Plains to Upper
Midwest. Very warm mid level temperatures may also hamper storm
development over Iowa, but eventually, and to varying degrees, the
models forecast convection either to break out in the warm
advection pattern over IA/IL, or to arrive from the northwest in
the form of MCS activity set off by the northern Plains weather
system. In any event, once things go and convection develops here,
the strong instability combined with northwesterly steering
currents could lead to quick forward motion / propagation, which
would limit the flash flood threat. Recent weather has also been
dry, and mature crops continue to take up soil moisture. We will
place a Marginal Risk here, but primarily for any early-developing
cells before they accelerate, and would expect problems only in
urban areas or sensitive basins.
Hurley/Burke
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
357 AM EDT Mon Aug 10 2020
Day 1
Valid 12Z Mon Aug 10 2020 - 12Z Tue Aug 11 2020
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE MIDWEST...
...Illinois / Indiana / Kentucky / Tennessee...
In the slowly evolving pattern over the Central U.S., upglide on
the 305K surface had provided forcing for rounds of thunderstorms
the past few mornings, along the edge of the instability gradient
and warm mid level temperature gradient east of the Great Plains.
A deep trough migrating across southern Canada and the northern
tier of states had dragged the gradients eastward, or migrated the
entire pattern eastward, and this morning thunderstorms were
beginning to break out across Indiana. Hi-res guidance varies as
to where the activity will focus once a cold pool becomes
established to intercept the westerly inflow. The WRF-ARW and some
HRRR runs place the axis of heavy rainfall across central Indiana,
whereas the WRF-ARW2 and 00Z NAM Nest were focusing on eastern
Illinois. Some of the more recent HRRR runs, from 04Z onward, do
not even indicate much of an event. The 06Z NAM Nest even reversed
course and trended toward no event. So there is a good deal of
uncertainty. Judging by the ingredients in place, however, and
fairly well defined height falls through 18Z, we would expect at
least some pockets of organized convection to occur over eastern
Illinois and into western Indiana, eventually driving or
developing southward into Kentucky and perhaps northern Tennessee.
These parts of the Ohio and Tennessee Valleys have been fairly dry
recently, but with regional soundings indicating steep lapse rates
and seasonably rich moisture, it will not take too many cell
mergers to push observed hourly rainfall above the 2.0 inch per
hour flash flood guidance values. Thus, we expanded Slight Risk
eastward from the Missouri frontal zone to encompass the warm
advection activity taking place mainly during the daylight hours.
...Kansas and Missouri...
With a well defined trough and lower heights pushing toward the
Great Lakes, a cold front will drop into Kansas and northern
Missouri by afternoon. The models are unanimous in developing
widespread rainfall - some of it heavy - but they vary greatly in
the details of location and timing. One concern is that increased
westerly flow will bring a mid level warm bubble eastward, seen in
some of the global guidance as 700mb temps 12C in the afternoon
over southern Iowa and northern Missouri. Convection could
struggle in the environment along this portion of the cold front.
Such thinking would appear to be supported by the NAM Nest, which
limits afternoon convection mainly to Kansas, but many other
models develop robust storms over Missouri during the same time
frame. Regardless, however, it seems that the pool of very strong
instability, MUCAPES 3000-5000 J/kg will eventually be consumed
and overturned via outflows arriving from the west and/or east, as
well as the deepening frontal lift. Flash flood guidance values
have been lowered by recent rain / wet soils across many parts of
eastern Kansas and northern Missouri. The HREF probability of
3-hour, 2-inch rainfall peaks above 50 percent this evening.
Therefore we maintained a Slight Risk area and expanded it
eastward to encompass the warm advection regime over Illinois and
adjacent states. Marginal Risk extends back along the front
through southwest Kansas where slow moving storms may pose a flash
flood threat before becoming outflow dominated.
...Michigan...
There are not many occasions to draw convective risk areas over
Lower Michigan. This afternoon the arrival of the upstream trough
is well-timed, coinciding with enhanced southwesterly low level
flow, drawing upon air marked by 1.60"-plus PW which is 2.0
standard deviations above climatology. Only a minority of hi-res
models generate much heavy precipitation, but those that do (the
WRF-NMM and FV3 CAM) indicate fairly good coverage. The NMM shows
potential for spotty 2-inch totals, and much of that would likely
occur in 1 to 2 hours at any given location. Flash flood guidance
is on the high side, so the area can handle some water, but the
signal is good enough for a Marginal Risk of excessive rainfall.
Burke
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
433 AM EDT Mon Aug 10 2020
Day 2
Valid 12Z Tue Aug 11 2020 - 12Z Wed Aug 12 2020
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE
LOWER/MIDDLE MISSISSIPPI VALLEY INTO THE OH VALLEY...
A low/mid-level trough will be passing through the Central Plains
into the Mid-Mississippi Valley, gradually weakening as it drifts
southeast. There are some timing issues among the guidance with
this feature - the NAM holds onto a stronger closed low while the
21Z SREF Mean deamplifies the wave faster than the GFS and NAM.
The spread decreases forecast certainty so a general blend of the
guidance was utilized for this issuance.
Ongoing convection from the day 1 period will likely track from
eastern Oklahoma into northwest Arkansas along a frontal boundary
draped across the region. Vorticity maxes traversing through this
trough, along with modest instability, height falls, and moist
airmass (PW values of 1.5 to 2 inches) should help convection
redevelop during the afternoon and evening hours. And given the
weak low-level winds, storm motion will likely be slow. Parts of
eastern Oklahoma, Missouri, Illinois, Indiana and Kentucky may be
sensitive to additional heavy rainfall
Areal average precipitation is around 1-1.5 inches with locally
higher amounts expected. And with the potential for high rain
rates, it may be enough to result in isolated flash flooding
across this region. The Marginal Risk area was expanded further
into southwest Missouri and eastern Kansas to reflect the latest
WPC QPF.
Campbell
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
407 AM EDT Mon Aug 10 2020
Day 3
Valid 12Z Wed Aug 12 2020 - 12Z Thu Aug 13 2020
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
Campbell
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
