Area Forecast Discussion
Issued by NWS La Crosse, WI

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671
FXUS63 KARX 172336
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
536 PM CST Tue Jan 17 2017

.SHORT TERM...(This evening through Friday)
Issued at 219 PM CST Tue Jan 17 2017

The main surface low was moving through the southern Great Lakes
into Ontario with a trough of low pressure extending back over
Wisconsin. Local and regional radars still showing some returns
along this trough of low pressure. Observations suggest this is
mainly drizzle/freezing drizzle but some rain with the higher
returns along with a bit of sleet and snow. The short range meso
scale models all suggest this precipitation will move slowly
southeast through the afternoon, in association with the secondary
upper level low moving across, and may not exit the area until
early this evening. These trends have been worked into the
forecast and expect the area to be dry by 03Z.

The next chance for precipitation will start to come in Thursday
night and then continue into Friday. The upper level low currently
over southern Arizona and New Mexico will lift northeast into the
Midwest through Thursday before transitioning to an open wave and
weakening Thursday night and Friday. The 17.12Z GFS is slower with
the weakening process than the 17.12Z NAM and brings some moderate
pv advection in the 500-300 mb layer across the area. The GFS qg
convergence generally looks to be on the weak side but it rather
deep extending from 1000 mb up through 500 mb. The low level warm
air advection ahead of this wave doesn`t look to be very strong
which results in a rather broken up glide signal of 1 up to 3
ubar/s on the 290K surface. Unlike the system that just went
through, there is little to no moisture transport into the area.
So while this system looks to be bring good moisture north with
it, precipitable waters should be around three-quarters of inch,
concerned the forcing may be on the weak side. For now, will keep
the precipitation chances in the 40 to 50 percent range but could
see these going up if the forcing looks to be stronger.

Precipitation type will again be a concern with this system.
Current indications are that there may be a lack of ice in the
clouds at times and even if there is ice, the temperatures aloft
will be warm enough for melting anyway. With these problems, the
main precipitation type will be rain or freezing rain depending on
surface temperatures Thursday night transitioning over to mainly
rain Friday as temperatures warm. Some sleet and a little bit of
snow could also occur for a while late Thursday night into Friday
morning where some ice could be present and the warm layer may
only produce partial melting. If the forcing starts to look
better, this could be another icing event for a while Thursday
night and Friday.

.LONG TERM...(Friday night through Tuesday)
Issued at 219 PM CST Tue Jan 17 2017

As the first wave lifts north of the area Friday night, the next
wave will be quickly coming in behind it. The 17.12Z ECMWF and GFS
now both show this second wave intensifying as it comes into the
Upper Midwest Friday night with and upper level low forming
Saturday. Considerable differences between the two models where
this low will be but is should continue to lift north through the
weekend as yet a third system comes across the Rockies but moves
east across the southern portion of the country. Some additional
light precipitation will be possible with the second system
through Saturday, but then the moisture feed from the south gets
cut off by the third system. This could then limit the
precipitation with the second system to the north side of it,
which would be past the local area by then. Then there is the
question of whether the northern edge of the precipitation shield
from the third system will affect the area or not. So for now, all
this will result in some on and off low precipitation chances for
most of the period and will keep it simple for now with rain or
snow based on low level temperatures.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 536 PM CST Tue Jan 17 2017

Moved both KLSE/KRST into a more pessimistic forecast for the
18.00Z TAFs. Short-range forecast guidance now suggests low IFR/
LIFR stratus will be slow to move out this evening and overnight.
In addition, some guidance hints at the development of widespread
fog with visibility dropping below 1/2 to 1SM, especially at
KRST. While confidence is higher for the low clouds to hang tough
through much of the night, much lower confidence concerning the
fog and potential visibility reductions. For now, will carry 1
1/2SM in mist at KRST until 18.08Z and 4SM in mist at KLSE later
this evening and into the overnight.

Will need to closely monitor observational trends for possible
adjustments as low level winds pick up overnight and clearing
across southwest MN/northwest IA approaches TAF airfields.
Improving conditions back to VFR are expected by sometime Wednesday
morning. Light northwest wind early this evening will gradually
shift to the southwest through the period.

&&

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

$$

SHORT TERM...04
LONG TERM...04
AVIATION...Rogers



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