Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 230738

Area Forecast Discussion
National Weather Service La Crosse WI
238 AM CDT Sat Sep 23 2017

.SHORT TERM...(Today through Monday)
Issued at 238 AM CDT Sat Sep 23 2017

Not much change has occurred in the overall upper air pattern this
morning. Maria continues to move slowly north over the southern
Atlantic keeping the ridging in place across the eastern conus
with troughing remaining over the Rockies. At the surface, an area
of low pressure was over northwest Minnesota with a trailing cold
front back across far western Minnesota into Nebraska.

Through at least Sunday, the upper air pattern will only show a
very slow overall change. Maria will continue to work north with
the upper level ridge axis expected to gradually shift around to
the north side of the hurricane but maintaining the ridge axis
over the lower Great Lakes into the region. This will keep the
troughing and the surface cold front well to the west of the area
to maintain the dry conditions. The warm air aloft will be
maintained over the region as well but with some slight cooling
expected each day. 925mb temperatures Friday topped out around 27C
and there should be about 1-2C of cooling today with about the
same expected for Sunday. Forecast soundings indicate we will be
able to mix deep enough both days to tap into this warm air. Based
on the temperatures that were achieved Friday, bumped up highs for
today with upper 80s to lower 90s for much of the area. Expecting
the river valleys to again be the warmest spots with middle 90s
possible there. With just a touch of cooling expected for Sunday,
highs that day should primarily be in the upper 80s with some
lower 90s possible in the river valleys.

By Monday, there is the possibility that the trough may edge far
enough east to allow some short wave troughs moving north to
possibly work across the region. The cold front will attempt to
work east with a model consensus of this extending from northern
or northwest Wisconsin into Iowa. This should be enough to support
at least some scattered showers and storms, but not expecting a
lot of coverage at this point as the overall forcing does not look
that strong. All the deep moisture to support any rain looks to be
confined to areas along and behind the front. Any short wave
troughs that come across only look to be able to produce some very
weak pv advection in the 500-300 mb layer. The front will be
weakening as it moves east with only some weak frontogenesis in
the 1000-850 mb layer expected. Plan to have the highest rain
chances Monday afternoon and keep this primarily in the 30 to 50
percent range.

.LONG TERM...(Monday night through Friday)
Issued at 238 AM CDT Sat Sep 23 2017

The expectation in this period is that the upper level ridge will
continue to get flattened north of Maria early in the period and
eventually shift to the northeast side of the storm. A piece of
energy in the base of the long wave trough will split off to form
a cutoff upper level low over the base of the Rockies and allow
the northern portion of the trough to move east across the Great
Lakes and southern Canada. A short wave trough in the northern
stream is then expected to drop into the back side of the trough
and dig down across the Great Lakes toward the middle of next
week. At the surface, the cold front should still be over the
region into Tuesday or Tuesday night before moving off to the
east. This will maintain the rain chances with this feature
through Tuesday night with a dry period Wednesday before some more
rain chances for Thursday with the northern stream short wave
trough. Temperatures will be cooling down through the period and
are expected to return to more seasonable values for parts of the
area Tuesday and then for the entire area starting Wednesday.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1146 PM CDT Fri Sep 22 2017

Multiple clusters of thunderstorms have developed this evening
across eastern SD into central and northern MN, but all convective
activity will remain north and west of TAF airfields through the
period. For KRST/KLSE, low-level wind shear will be a concern
through 23.09Z to 23.10Z with southwest winds in the 1500 to 2000
ft agl layer between 35 and 40 kts. Thereafter, winds aloft
weaken. Surface winds will remain from the south through the
period, generally under 15 kts, but some gusts to 20 kts are
possible at KRST Saturday afternoon. For the most part, skies will
be mostly clear/sunny, although some scattered cumulus are
possible Saturday afternoon in the 4000 to 5000 ft agl range.




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