Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 220736

Area Forecast Discussion
National Weather Service La Crosse WI
236 AM CDT Fri Sep 22 2017

.SHORT TERM...(Today through Sunday)
Issued at 236 AM CDT Fri Sep 22 2017

Water vapor satellite this morning shows ridging aloft extended
from northern Mexico/Texas northeast into Quebec. Troughing
extended from the Rockies toward Hudsons Bay. Very little eastward
movement in this pattern is expected through the weekend as the
tropical systems over the Atlantic hold the ridge in place which
in turn keeps the troughing over the Rockies. Very little movement
is expected in the low levels as well with ridging remaining over
the eastern conus. The cold front that is over the Dakotas this
morning will not move much through tonight. From Saturday into
Sunday, the northern portion of the front will attempt to edge
east as the area of low pressure over Colorado and Nebraska moves
northeast along the front and then east over the top of the
surface ridge pulling the front with it.

With this pattern, the strong low level south/southwest flow will
remain in place. This will continue to pump warm air into the
region with 925 mb temperatures today expected to be around 26 to
27C. Forecast soundings indicate the mixing will be deep enough to
tap into this warm air and with some compressional warming, highs
today for much of the area will be in the lower 90s with some mid
90s possible. For Saturday, there is the suggestion that
temperatures aloft will cool slightly, maybe 1-2C, and the mixing
will still be deep enough to tap into this warm air. Current
forecast highs for Saturday are in the upper 80s which seems like
too much cooling compared to today`s forecast with very similar
temperatures aloft. If the lower to mid 90s are realized today,
then Saturday will definitely need to be warmer. Little change
for Sunday with highs again in the middle to upper 80s.

Little chance for any rain through the weekend with the front
remaining well to the west of the area. Maybe some activity that
develops along the front Sunday afternoon over Minnesota could
possibly edge far enough east to graze the far northwestern
sections of the forecast area, but this looks to be a very long
shot and will only have some very small rain chances for this

.LONG TERM...(Sunday night through Thursday)
Issued at 236 AM CDT Fri Sep 22 2017

How the pattern will evolve next week all revolves around what
happens with the tropical systems over the Atlantic. Right now,
Jose is still meandering off the northeast coast. By early next
week, the models are all showing that Jose will have weakened to
the point to no longer act as a block to the ridging which is then
expected to move to the north and then northeast side of Maria is
it comes north off the east coast. With this eastward progression
of the ridge, the western trough is then finally allowed to work
east and move past the Upper Midwest Wednesday. With this
evolution in the upper air pattern, the surface front is then
finally allowed to move east as well. Current timing would have it
approach the area Monday, moving across the area Tuesday and
essentially be to the east for Wednesday and Thursday. The 22.00Z
CMC is slower to bring the trough and cold front east and would
suggest the need for some rain chances through Wednesday night.
With overall confidence in any one model solution low at this
point, will keep the rain chances generally in the 30 to 50
percent range as the front comes through and then linger some low
chances to honor the CMC.


.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1145 PM CDT Thu Sep 21 2017

Scattered thunderstorms have developed across central/northern MN
in the past couple hours, but are expected to remain north of TAF
airfields. Bigger aviation concern will be low-level wind shear
overnight given 40 to 45 kts in the 1500 to 2000 ft agl layer.
After sunrise Friday morning, wind shear will decrease as the
boundary layer deepens and surface winds become breezy from the
south. Expect frequent gusts into the low-mid 20 kt range
throughout the late morning/afternoon. Some VFR cloud cover is
possible overnight, but a strong thermal inversion will develop on
Friday, resulting in mostly sunny/clear skies through the rest of
the period.




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