Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 240844
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
344 AM CDT Sat Sep 24 2016

.SHORT TERM...(Today and tonight)
Issued at 344 AM CDT Sat Sep 24 2016

Main forecast concerns are on the potential for flash flooding
tonight into early Sunday morning with heavy rainfall possible.

Some light rain showers or drizzle is expected this morning mainly
along and east of the Mississippi River. Then focus quickly turns
to the potential for another round of thunderstorms tonight into
Sunday morning along a cold front. Precipitable water values
remain high across the region ahead of the front with values of
1.5 to 1.8. Looking at Standardized anomalies this is around plus
2 standard deviations above the mean. Warm cloud depths are also
3.5 to 4.0 KM. These parameters are all favorable for heavy rain.
However, the storms looks to be progressive and this may help
limit the flash flood potential. Also, forecast model QPF isn`t
overly impressive and generally produces widespread 1/4 to 1/2
inch amounts. Given the convective nature of the the rainfall and
high precipitation efficiency, this is likely underdone. We more
likely will see pockets of 3/4 to 1 inch amounts under the more
concentrated areas of storms. We are very vulnerable to flash
flooding with the recent heavy rains and ongoing flooding, so a
flash flood watch may be needed later today. Held off on issuing
one this morning given the latest CAMS showing the heaviest axis
of rain could fall closer to the Interstate 35 corridor. If this
axis of heavier rain takes a shift further east, a flash flood
watch will be needed. Monitor later forecasts and statements
closely today. We may also see a few stronger storms across
southeast Minnesota and northeast Iowa with the potential for some
gusty winds and maybe small hail. CAPE will be diminishing during
the evening hours and shear is lagging behind the front so the
overall severe weather threat, outside of heavy rain, looks low
at this point.

.LONG TERM...(Sunday through Friday)
Issued at 344 AM CDT Sat Sep 24 2016

Upper level trough digs into the upper midwest late Sunday into
Monday as low pressure deepens over southern Ontario. It will be
rather breezy late Sunday into Sunday night as the low deepens.
Strong winds are possible Sunday night as surface to 1 km lapse
rates remain fairly steep and a trop fold dives through the region
with PV 1.5 Pressure values lowering to around 650 mb. We may see
some wind gusts to around 35 to 40 mph at times. The NAM is the
most aggressive with this idea. Also, a few rain showers are
possible mainly across central into northern Wisconsin. Monday
looks to be a windy day across the region with northwest winds of
15 to 20 mph with gusts of 35 to 40 mph possible. We may even see
some stronger gusts Monday afternoon across the open areas of
southeast Minnesota and northeast Iowa.

The upper low meanders across the Great Lakes Region from late
Tuesday into Thursday as high pressure gradually builds into the
local area. Once the high settles in we may see some morning of
dense valley fog Thursday into Friday. Otherwise, mainly quiet
weather is expected Wednesday into the the start of the weekend.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1122 PM CDT Fri Sep 23 2016

MVFR to IFR ceilings in place across the area north of the
stationary front over Iowa. Expecting that both airports will end
up with IFR ceilings overnight that will persist into Saturday
morning. These should then improve to MVFR with some heating. The
24.00Z NAM forecast sounding would suggest these stay in place
into Saturday evening while some other aviation guidance indicates
they breakup late in the afternoon. For now, will stay with
pessimistic forecast and keep the MVFR going. Not expecting any
fog to form overnight as both the surface and winds just above the
surface should be too strong to allow it to form. A cold front
will approach Saturday evening with a broken line of showers and
storms along. Most models hold the rain off until after 25.06Z
except for the 23.12Z hi-res ARW which would bring some weakening
activity into KRST. Honored this solution with a VCSH for late
Saturday evening.


&&

.HYDROLOGY...Through Sunday
Issued at 344 AM CDT Sat Sep 24 2016

River flooding continues across much of the area today. Another
round of heavy rainfall is possible tonight into early Sunday
morning. At this time rainfall amounts of 3/4 to locally 1 inch
are possible. Given the recent heavy rains/saturated grounds,
flash flooding may become a concern again later tonight. It
appears the heaviest rain will fall closer to the Interstate 35
corridor across Iowa into southern Minnesota. If this heavier axis
of rain shifts any further east, a flash flood watch may be
needed. Also, any areas that would see repeated rounds of
thunderstorms tonight may experience flash flooding. There are
signals in the forecast models that thunderstorms will become more
scattered in nature through the overnight hours, so confidence was
not high enough to hoist a flash flood watch at this time.

&&

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

$$

SHORT TERM...WETENKAMP
LONG TERM...WETENKAMP
AVIATION...04
HYDROLOGY...WETENKAMP


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