Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 181137

Area Forecast Discussion
National Weather Service La Crosse WI
637 AM CDT Wed Oct 18 2017

.SHORT TERM...(Today and tonight)
Issued at 335 AM CDT Wed Oct 18 2017

At 3 AM, a 1029 mb surface high was located over West Virginia and
a 975 mb surface low was located over east-central Saskatchewan.
This resulted in a moderate pressure gradient across our area. The
18.00z models are in general agreement that this will continue to
be located across the area through mid afternoon and then
gradually weaken as we head into the evening. Sustained southwest
winds will be in the 15 to 25 mph range and the winds will gust to
around 30 mph. The highest winds will be found in the higher
terrain areas of southeast Minnesota and northeast Iowa.

A Canadian cold front will move through the region late this
afternoon and evening. With the air mass very dry across the
region, only expecting a scattered to broken cirrostratus deck
and a wind shift associated with its passage. 925 mb temperatures
will be about 1.5C warmer than they were on Tuesday afternoon.
However, the clouds will temper this warming some, so only bumped
the high temperatures for this afternoon by 1 to 2F.

Moderate cold air advection in the wake of this front will bring
in much cooler temperatures into the area for tonight. Under clear
skies and light winds, temperatures will fall into the mid and
upper 30s north of Interstate 90 and into the lower and mid 40s
for the remainder of the area.

.LONG TERM...(Thursday through Tuesday)
Issued at 335 AM CDT Wed Oct 18 2017

On Thursday and Thursday night, the 500 mb ridge will build
quickly across the Upper Mississippi River Valley. As this occurs,
moderate warm air advection will develop across the region. This
will result in the 925 mb temperatures climbing 4.5C during the
day. As a result, this will temper the drop in the high
temperatures for Thursday, so we are only looking at the
temperatures being 5F cooler than they will be this afternoon.

On Friday, there will be a tightening surface gradient ahead of a
long wave trough and the departing high. This will result in a
sustained south winds in the 15 to 30 mph range and wind gusts of
30 to 40 mph. With the NAM not mixing quite as deep as the GFS,
its winds are not quite as strong as the GFS. If the GFS would
happen to verify, we might end up with a Wind Advisory west of the
Mississippi River. Definitely something to watch over the next 2

With 925 mb temperatures warming into the 16.5 to 18C on Friday
afternoon, we are looking at temperatures ranging from the mid
70s to lower 80s. These temperatures will be warm enough that they
could potentially approach record high temperatures in a few
locations. However, at this time, no record highs are anticipated.

On Saturday afternoon and night, a cold front will move through
the region. Like yesterday, the 0-3 km wind shear climbs into the
25 to 35 knot range, but the most unstable CAPES are a bit lower
(only up to 500 J/kg). As a result, not anticipating any severe
storms to develop ahead of this front.

On Monday, there continues to be considerable difference in the
timing in the passage of a Canadian cold front through the region.
Both the ECMWF and GEM moves this front through the region during
the morning. Meanwhile, the GFS holds off until the late afternoon
and evening. Due to this, the high temperatures for this day are
highly uncertain.

On Tuesday night, there is a mention of rain and snow showers in
the grids. However from looking at the latest deterministic
guidance, this is starting to look very unrealistic. If this trend
continues, they will be likely removed.


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 637 AM CDT Wed Oct 18 2017

VFR conditions are expected through the TAF period. A cold front
will move through the area this afternoon. South to southwest
winds will increase to 12 to 17 kts with gusts to around 27 kts
possible ahead of the front. Winds will then shift to the west
and northwest this evening in the wake of the front and decrease




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