Area Forecast Discussion
Issued by NWS La Crosse, WI

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406
FXUS63 KARX 222304
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
604 PM CDT Thu Sep 22 2016

.SHORT TERM...(Tonight and Friday)
Issued at 318 PM CDT Thu Sep 22 2016

Rainfall records continue to be set and rivers continue to rise.
We have reached a vulnerable point where forecast rainfall is of
utmost importance. Good news is there may be some relief in sight.

We continue to watch progression of large upper low moving through
western U.S. Unlike past few days, minimal signs of an upstream
short wave to focus low level response into boundary that divides
very moist airmass to the south and drier, cooler airmass north.
Combined with backing flow as upper trough digs in west, seeing
signs in short term model data that main moisture transport and
focus will be shifting more west for tonight.

Nonetheless, primed airmass remains along southern tier with higher
precipitable water values and some convergence. Convection has
slowly been on the increase closer to Highway 20 area in Iowa,
closer to CAPE gradient region. As the evening goes on, would expect
this focus to gradually shift north unless convection becomes
extensive enough to hold boundary where it is at. Still concerned if
convection does spread north, we could see another 1-2" of rain so
will keep Flash Flood Watch going at least into Friday to cover any
threat. Given how saturated things are, anything over a half inch
could pose a problem.

On Friday, as upper flow continues to back and becomes more
southerly, may see remaining area of elevated convection gradually
shift north and east as it dissipates. Banking on coverage being on
the lighter side with loss of forcing. Also hoping this leads us
into a period of dry weather and flood headlines can be cleaned up a
bit.

.LONG TERM...(Friday night through Thursday)
Issued at 318 PM CDT Thu Sep 22 2016

As previously mentioned upper low lifts northeast over the weekend,
could see lingering dry spell Friday night into Saturday ahead of
approaching cold front. Models in fair agreement bringing elongated
north-south band of rain with cold front passage Saturday night into
Sunday. Some model differences on timing with 22.12z NAM the fastest
to move rain through, and the GFS/ECMWF more alike holding it back
more, but still not as bad as previous model runs had. Convection
may not be as intense as earlier this week, but prolonged (?) which
could bring more concern for areas hard hit by flooding. Will
certainly have to watch how this plays out and future flood watches
may be needed.

Now for the really good news. Drier flow regime looks in the offing
for much of next week, although some differences in medium range
guidance.

22.12z ECMWF is far more progressive sweeping upper low by and
northwest flow sets up with eventual upper ridge building in. GFS is
a bit slower with this evolution and swings upper low into Great
Lakes. This might not generate as much rain but may hold back
temperatures far more than ECMWF. Blends of operational models
suggests a small chance of rain into mid week but we could see a lot
of dry time in there.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 604 PM CDT Thu Sep 22 2016

A weak boundary was across southern Minnesota into northeast
Wisconsin late this afternoon with the main surface boundary
across central Iowa into southeast Wisconsin. The convection so
far as been just north of the southern boundary and well south of
both airports. A mixed bag of ceilings along and north of the
northern boundary with VFR going down to MVFR. Plan to start both
TAFs with VFR conditions and then go down to MVFR ceilings by the
middle of the evening. Some potential, all be it a small one, that
some showers could impact both airports overnight once the low
level jet starts to interact with the southern front. Think most
of this activity should stay to the south though and have just
shown a VCSH overnight. More likely to occur is the ceilings going
down to IFR at KRST overnight with some MVFR fog forming at both
airports. The winds aloft off the 22.18Z NAM forecast soundings
look too strong to allow much of visibility restriction. The
forecast soundings do not show much potential for the ceilings to
improve Friday so will only show some minor improvements through
the day.

&&

.HYDROLOGY...
Issued at 318 PM CDT Thu Sep 22 2016

Many areas saw 4 to nearly 12 inches of rain last few days.
Combined with recent wet periods most area rivers into flood.
Potential for another 1 or 2 inches tonight could generate more
problems but confidence in that is low given weaker forcing tonight
and boundary further south. Concerns are shifting from immediate
flash flood threat to high water or areal concerns, along with river
flooding.

Also look for continued rises on the mainstem of the Mississippi
River going into next week, which could lead to an increase in flood
warnings there. Water levels will be fall slower than normal as well
given how saturated everything is.

&&

.CLIMATE...
Issued at 318 PM CDT Thu Sep 22 2016

La Crosse WI...

Top September rainfalls...
1. 10.87 inches in 1881
2. 10.52 inches in 1965
3. 10.33 inches in 2016
4. 10.01 inches in 1884
5. 9.84 inches in 1901

Rochester MN...

Top September rainfalls...
1. 10.50 inches in 1986
2. 9.95 inches in 2010
3. 8.96 inches in 2016
4. 8.08 inches in 1978
5. 7.95 inches in 1938

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Flash Flood Watch through Friday afternoon for WIZ041>044-
     053>055-061.

MN...Flash Flood Watch through Friday afternoon for MNZ086>088-
     094>096.

IA...Flash Flood Watch through Friday afternoon for IAZ008>011-018-
     019-029-030.

&&

$$

SHORT TERM...SHEA
LONG TERM...SHEA
AVIATION...04
HYDROLOGY...SHEA
CLIMATE...SHEA



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