Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 270243
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
943 PM CDT FRI AUG 26 2016

.UPDATE...
Issued at 943 PM CDT Fri Aug 26 2016

Million dollar questions for tonight: just how far west will it
rain and how much rain will fall? Clues seem to be emerging per
radar trends regarding where the band of better rains will set up.
Said trends strongly support our ongoing forecast (nice work day
crew!) with the best coverage of showers and perhaps embedded
storms later working mainly east of a line from New Hampton IA to
La Crosse to Wisconsin Rapids. That lines up nicely with where
recent near term guidance suggests the better 850-700mb
frontogenetic circulation setting up in advance of an approaching
850-700mb shortwave trough axis, and per radar extrapolation of an
expanding rain shield north of Des Moines.

Still have a hard time buying into recent NAM solutions which
really amplify the low level and surface circulation centers, thus
creating an axis of intense FGEN forcing which puts down 1-3" of
QPF over much of the area. Moisture increase through the night is
still pretty robust, with PWAT values progged to climb to around
1.5-1.75 inches with warm cloud depth around 3.5km. However,
majority of other near term guidance as alluded to above line up
more with current trends and RAOB analysis of a weaker low level
circulation. Still, do foresee the potential for up to an inch of
rain to fall across the southeast third of the CWA or so,
especially considering the likelihood of at least some convective
enhancement after 06-08Z with increasing moisture transport and
better inflow above 850mb supporting maybe a couple hundred J/kg
elevated CAPE...just enough in a warm cloud/saturated environment
to support localized higher rain rates. So the upshot of it all?
Will really have to watch to see where the heavier rain band in
fact ends up, but current radar analysis would suggest the
heaviest rains lining up just south and east of the worst-hit
areas from Tuesday night.

As for how far west precip will end up, confidence there just
isn`t all that high. Lack of more organized forcing and being on
the downward side of the frontogenetic circulation won`t help
matters, but can`t rule out at least a few showers with the
approach of the upper trough.

&&

.SHORT TERM...(This evening through Monday)
Issued at 318 PM CDT Fri Aug 26 2016

The focus of the forecast is on the rain chances tonight and the
associated rainfall amounts.

Water vapor satellite this afternoon shows a short wave trough
over the western sections of Kansas. This wave will lift northeast
through the night with differences between the 26.12Z models on
how strong this wave becomes. The NAM is by far and away the
strongest with this short wave trough which results in a stronger
and farther northwest surface low moving across the area. With
little to no support from the other model guidance have pretty
much discounted this solution. The GFS and ECMWF keep this wave
much flatter allowing it to come through much quicker. With this
solution, the wave should produce a quick shot of moderate pv
advection in the 500-300 mb layer overnight. There is some
suggestion this wave could get a little additional lift from the
upper level jet with the right entrance region coming over the
region. Most of the forcing will be in the low levels with the GFS
indicating a band of weak to moderate frontogenesis setting up in
the 1000-500 mb layer from northeast Iowa into central Wisconsin.
This will be lined up well with the best area of isentropic up
glide which the GFS shows to be on the order of 3 to 6 ubar/s on
the 300K surface. The moisture transport does not look to be all
that strong but should be focused right into the region of
frontogenesis and isentropic up glide. Expecting to see the the
area of developing rain across western Iowa become oriented into a
northeast to southwest band that moves into the area this evening
and should then move out of the area early Saturday morning. Have
aligned the rain chances to capture this scenario with the highest
chances of 70 to 80 percent across parts of northeast Iowa into
southwest and central Wisconsin. Still some uncertainty on just
how much thunder will occur with this event. Very little is in
place over the area now and only about 500 J/Kg of MU CAPE is
expected to come in across the southern sections tonight. Have cut
the mention of thunder back to chance/scattered.

Once this system moves out of the area Saturday morning, expecting
the rest of the day to be dry. The next short wave trough is
expected to rotate across the Dakotas and northern Minnesota
which should keep the rain associated with it to the northwest of
the area. Some suggestions that a weak short wave trough on the
trailing end of the Saturday afternoon system could come across
the northern parts of the area Sunday afternoon. The GFS is the
strongest with this wave and shows the most potential rainfall
amounts so only plan to carry a small 20 to 30 percent chance with
this system. Another weak short wave trough should come across the
region Monday and push a cold front into the area for additional
rain chances Sunday night into Monday.

.LONG TERM...(Monday night through Friday)
Issued at 318 PM CDT Fri Aug 26 2016

Scattered showers and thunderstorms will be possible Monday night as
a surface cold front approaches the region from the northwest.
Beginning Tuesday, the 26.12z GFS/ECMWF build an upper level ridge
over the Northern Plains, placing our area under northwest flow
until the ridge axis slowly shifts eastward late in the week. Weak
disturbances are progged to move over the ridge, keeping low chances
for showers and storms in the forecast through much of the week.
Temperatures should remain slightly above normal, especially later
in the week as the ridge axis shifts farther to the east and low
level warm air advection increases.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 643 PM CDT Fri Aug 26 2016

High pressure holds firm at the moment but weak low pressure is
expected to lift north toward the area late tonight into Saturday
morning. With the arrival of that feature and increased pooling of
moisture north of a warm front, still looking for ceilings to be
on the decline very late tonight into Saturday, with MVFR to even
IFR conditions expected, lowest at RST. With that said, confidence
remains rather low with regard to how far west any rain will
expand with the approaching low. Current trends suggest the more
organized rainfall will likely skirt south of both RST and LSE,
but with LSE standing the best risk for at least a period of some
light rain/showers. We will also be fighting entrenched dry air
through much of the night along and north of I-90, so it is
possible that RST ends up staying mainly dry the next 24 hours.
Winds through the TAF period will remain under 10 knots.


&&

.HYDROLOGY...
Issued at 318 PM CDT Fri Aug 26 2016

Still some ongoing river flooding across parts of northeast Iowa.
All the rivers have either crested and are on their way down or
will be doing so very soon. See the latest flood statements for
specific river information

Of concern for tonight is whether the incoming rain will cause any
problems. Currently expecting rainfall amounts to be in the 0.5 to
1 inch range but there is the potential for locally higher
amounts. Precipitable waters are forecast to reach the 1.75 to 2
inch range overnight with warm cloud depths in the 3.5 to 4 km
range. With the main forcing being the frontogenesis and
isentropic up glide, expecting this could end up being similar to
an overrunning event with light to moderate rains and then some
higher amounts were a stronger shower or thunderstorm tracks.
Given that parts of northeast Iowa are already saturated and still
have ongoing flooding, there has been some thought given to
issuing a flash flood watch. However, currently believe the
expected rains will not be heavy enough or have a high enough rate
for a long enough period to generate new flash flooding. It may
be enough to slow down the current falling river levels or cause
some minor increases. Will let the evening shift monitor the
development of the rain and determine if a short fused watch will
be necessary.

&&

.ARX Watches/Warnings/Advisories...
WI...None.
MN...None.
IA...None.
&&

$$

UPDATE...Lawrence
SHORT TERM...04
LONG TERM...MH
AVIATION...Lawrence
HYDROLOGY...04



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