Flash Flooding and Flooding in Wisconsin, September 3 to 10, 2018

Area Forecast Discussion
National Weather Service La Crosse WI
552 AM CDT Mon Sep 3 2018

..SHORT TERM...(Today and tonight)
Issued at 314 AM CDT Mon Sep 3 2018

Main short-term fcst concerns are fog this morning then SHRA/TSRA
chances and heavy rain threat this afternoon and tonight.

Data analysis at 06z had a rather weak frontal boundary from NE WI
to western IA. Pressure gradients on either side of the boundary
were quite weak, with light winds at the sfc and aloft to near
700mb. Very moist BL either side of the front across WI, IA,
southern MN. All this a very favorable set up for BR/FG formation
early this morning, even under some cirrus debris clouds from
convection in SW IA to eastern KS.

03.00z models initialized without major errors. Initializations
could have been 'cleaner' with the pieces of shortwave energy
rotating thru the weaker troughing flow over the southwest CONUS.
Models in reasonable agreement today/tonight as the next stronger
wave in the flow lifts NE across the Upper Midwest today/tonight.
However, detail differences with the shortwaves continue to impact
SHRA/TSRA timing/chances into mid-week. Short-term fcst confidence
is average to good this cycle.

Abundant BL moisture to remain trapped in/under an shallow inversion
this morning, with rather light winds in the sfc-700mb layer and
cirrus clouds aloft. Fog is looking to persist at some locations
through mid morning, before slowly lifting/dissipating with eventual
diurnal warming later in the morning.

The rest of the short-term fcst continues to hinge on timing/
strength of shortwave energy thru the broad SW flow over the
region. Each wave to bring with it a round of increased lower
level thermo- dynamic forcing and moisture transport of a 1.75 to
2 inch PW airmass into/across the area. Loose model consensus for
the next of these waves in the train to lift into/across the area
this afternoon/evening with its round of deeper layered forcing
and moisture transport. Detail differences with timing/placement
of convection across the area but overall consensus is for a round
of SHRA/TSRA to lift north across the area this afternoon/evening.
Precip chances increasing to 60-70% along/south of I-90 by later
this afternoon spreading northward tonight looking well trended.
Deep layered shear is on the weak side thru this evening, with
main threat from TSRA more heavy rains. Given the very moist
airmass (PWs of 2+ std deviations above normal per NAEFS) and warm
cloud depths around 4K meters, TSRA will continue to be efficient
rain makers. Local rain amounts of an inch or more are possible
from stronger TSRA this afternoon/tonight. Even with TSRA looking
to be progressive today/tonight, attention will have to stay
focused on the rainfall and potential for runoff as much of the
landscape has been saturated by recent rains. Blend of guidance
highs in the 70s today, held down by plenty of clouds and
SHRA/TSRA spreading north this afternoon, look good.

.LONG TERM...(Tuesday through Sunday)
Issued at 314 AM CDT Mon Sep 3 2018

For Tuesday thru Wednesday night: main concerns this period are
heavy rain/flooding threat later Tuesday into Wed morning.

Model runs of 03.00z in reasonable agreement for portion of the
western CONUS troughing to move into the plains Tue, with this
trough and stronger pieces of shortwave energy lifting NE across the
region Tue night into Wed morning. Quieter westerly flow to build
across the region behind the trough later Wed/Wed night. Fcst
confidence this period is average to good with the larger scale
picture but average with the details.

Convection from later tonight lifts north of the fcst area with the
shortwave Tue morning for a lull in rain chances. Little change of
the airmass through Tue/Tue night though, with PW values progged in
the 1.75 to 2.25 inch range and warm cloud depths of 4K+ meters.
Lower level thermo-dynamic forcing and divergence aloft increase
again Tue afternoon/night with approach of the next shortwave energy
and trough axis. Lower level trough/front accompanies the mid level
features into/across the area from later Tue afternoon into Wed
morning. Strong F-gen forcing signal with the lower level trough
axis for linear forcing along the front/trough axis. Lower thru mid
level flow progged to be parallel to the forcing, with strong signal
for training/repeating SHRA/TSRA and a band of heavy/excessive rains
centered on Tue night. Models progging potential of 2 to 5 inches in
the band but Model differences impacting the location of the band.
Consensus has it impacting north-central IA to NE WI, which passes
across roughly the NW half of the fcst area. Model consensus may
still shift so will keep threat for heavy rains Tue night raised
across the entire fcst area. Consensus 70-90% SHRA/TSRA chances
translating across the fcst area Tue afternoon thru Wed morning well
trended for now. A few strong TSRA possible Tue afternoon/evening
but main threat is the heavy rains. See SWODY2 for more info.

Once the sfc thru mid level trough axis pass Wed morning, cooler and
much drier Can high pressure to slide into the Upper Midwest for Wed
afternoon/night for a much needed break from the rains. Any
lingering rain chances by Wed night limited to the south end of the
fcst area.

For Thursday through Sunday (days 4-7: main fcst concerns this
period are cooler, more September like temperatures and some
SHRA/TSRA chances mainly on Sat.

03.00z medium range model runs in decent agreement for quieter
westerly flow over the region Thu as troughing digs into the central
plains. Trough to continue digging Fri, then advance east across the
mid/upper MS valley Sat. Westerly flow to return Sunday. Day 4-7
fcst confidence is average this cycle.

Thu/Fri continue to trend as much needed drier days with below
normal temps as the Can high settles over the region. Some small
lingering precip chances across the far south end of the fcst area
but the overall trend for later in the week is dry. Plenty of lower
level detail differences with the passing mid level trough on Sat
but some SHRA/TSRA chances across mainly the south half of the fcst
area OK for now. Westerly flow Sunday brings with it some warming a
return to near normal temps along with a small SHRA/TSRA chance.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 552 AM CDT Mon Sep 3 2018

The same complicated convection pattern as we've had the past few
days continues across the taf sites today, and looks to continue
thru Wed morning before drier Canadian high pressure builds into the
region. BR/FG problematic across the area this morning. Surprising
that KRST remains VFR while some nearby sites are IFR/LIFR in
cigs/FG. Will continue trend for some MVFR/IFR BR at KRST for a time
this morning. BR/FG around KLSE is more on the ridge tops this
morning, with an IFR cig at KLSE. Will continue a mention of some
lower vsbys at KLSE thru mid morning. BR/FG lifts but mainly IFR
cigs looking to be stubborn thru the morning, only lifting to MVFR
for the afternoon. Meanwhile, the next round of SHRA/TSRA to lift
north across the area thru the afternoon/evening hours. Limited
mention of TSRA to VCTS/CB as confidence when a TSRA would move
across one of the airfields is not great and overall duration of TS
at one of the sites looks limited. As with the past few days, if a
stronger SHRA or TSRA would move across one of the airfields, a
short period of MVFR/IFR cigs/vsbys can be expected in the precip
core. Plentiful low level moisture and some weaker lift to remain
over the area later tonight for persistent MVFR cigs into Tue
morning.

&&

.HYDROLOGY...
Issued at 314 AM CDT Mon Sep 3 2018

The highest potential for additional heavy rains and possible flash
flooding, remains focused on the later Tuesday afternoon and Tuesday
night period. The next cold front will be advancing into the area
with strong moisture transport focused into the front. An axis of
high precipitable waters, at and above 2 inches, will reside ahead
of the front. Strong linear forcing signal with the approaching
front, with the sfc-mid level flow parallel to the front/forcing.
The concern is where will the strongest forcing/lift/training set up
to produce a band of heavy/excessive rains. Consensus at this time
is from north-central IA to NE WI, cutting across the NW half of the
fcst area. However, plenty of variability exists among the 03.00z
model solutions, with potential for the axis to still shift. Much of
the landscape is saturated from rains of the past week. Any
additional heavy rains have the potential to quickly producing
runoff and rises/flooding on area rivers and streams. A flash flood
watch is likely to be needed for some portion of the area Tue night,
but uncertainty remains on the location.

Flooding continues along parts of the Kickapoo River and for now,
levels continue to go down. High water is also occurring along
portions of the Turkey River. See the latest flood statements for
more details.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&

SHORT TERM...RRS
LONG TERM....RRS
AVIATION.....Lawrence
HYDROLOGY....RRS

Hazardous Weather Outlook
National Weather Service La Crosse WI
331 AM CDT Mon Sep 3 2018

IAZ008>011-018-019-029-030-MNZ079-086>088-094>096-WIZ017-029-
032>034-041>044-053>055-061-041000-
Mitchell-Howard-Winneshiek-Allamakee-Floyd-Chickasaw-Fayette-
Clayton-Wabasha-Dodge-Olmsted-Winona-Mower-Fillmore-Houston-
Taylor-Clark-Buffalo-Trempealeau-Jackson-La Crosse-Monroe-Juneau-
Adams-Vernon-Crawford-Richland-Grant-
331 AM CDT Mon Sep 3 2018

This Hazardous Weather Outlook is for portions of southeast
Minnesota...northeast Iowa and southwest into central Wisconsin.

.DAY ONE...Today and Tonight

Showers and storms are likely this afternoon and tonight. Heavy
rain is possible with most thunderstorms, with the potential for
localized flooding.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

More showers and storms are expected mainly Tuesday afternoon
into Wednesday morning. This round of storms could bring more
widespread heavy rains to the area, with several inches of
accumulation possible. With wet soils unable to hold much more
rain, heavy rainfall would likely result in localized to
widespread flooding. A few stronger storms with hail and gusty
winds are also possible Tuesday afternoon and evening.

After Wednesday, smaller shower and storm chances are generally
confined to sections of northeast Iowa and southwest Wisconsin.

.SPOTTER INFORMATION STATEMENT...

Rainfall reports are appreciated.

.EMERGENCY MANAGEMENT COORDINATION...

No webinar is scheduled.

RRS

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE LA CROSSE WI
657 AM CDT MON SEP 3 2018

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0651 AM     HEAVY RAIN       2 NNW FAYETTE           42.87N 91.82W
09/03/2018  M1.00 INCH       FAYETTE            IA   AMATEUR RADIO

&&

JSB