Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 191120

Area Forecast Discussion
National Weather Service La Crosse WI
620 AM CDT Sat May 19 2018

.SHORT TERM...(Today through Tuesday)
Issued at 130 AM CDT Sat May 19 2018

Fine line of showers/storms currently sparking along a sfc front
across northeast MN/northwest WI, with some aid from low level
moisture transport. Expectation is for the bkn line to continue to
lift northeast, diminishing as it does.

The front will slowly sag south/southeast during the day, roughly
extending from northeast WI into southern IA later this afternoon.
Models pool some instability around the front - with the NAM a bit
more rambunctious with upwards of 1500 J/kg of SBCAPE - due to its
overly moist sfc dewpoints (progged mid 60s while mid/upper 50s more
reasonable). That said, bufkit soundings still suggesting some long,
skinny CAPE would be available for the frontal boundary to work on.
Wind shear is weak. All said, wouldn`t surprise to see another fine
line of sct/bkn showers/storms develop late afternoon around this
boundary. Will continue small pcpn chances for this period -
although a bit broader then would like based on uncertainty of
placement of front later this afternoon.

Both the GFS and the NAM continue to sink the front south tonight as
its parent shortwave slides east across southeast Canada. Another
piece of upper level energy is slated to lift northeast out of the
southern plains, working along and north of the boundary tonight
into Sunday. The 850 mb jet and moisture transport are focused well
south (northern MO), as is the instability. Still, soundings and x-
sections point to enough saturation for the forcing to work on, and
areas of showers and a few showers should result. How far north has
been the question - and continues to be. A lot of dry air in the
north. 900-800 mb dew point depressions north of I-90 for Sun
upwards of 30 C in both the GFS and NAM - significant barrier to
overcome. Will continue shower/isold storm threat across the south
for tonight/Sun.

.LONG TERM...(Wednesday through Friday)
Issued at 130 AM CDT Sat May 19 2018

Nebulous, very weak flow a loft continues into next weekend.
Generally a broad trough over the west coast could/will/maybe spit a
few shortwaves northeast - but where these go, how fast they move is
difficult to pin down. Confidence low in any pcpn chances through
much of the work week.

Models a bit more in step for next weekend with a shortwave trough
in the northern branch of the 500 mb low taking aim on the Upper
Mississippi River Valley. This would provide a good shot for
showers/storms in the Fri/Sat period. No shear to speak of, and no
tap to gulf moisture. However, GFS suggests SBCAPES in excess of
1000 J/kg. Ample to support storm development. Any severe risk still
looks low at this time given the lack of any shear.

Overall, will continue to let the consensus drive the pcpn chances.

For temps, the trends for above normal looks to continue for Tue
through the weekend.


.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 620 AM CDT Sat May 19 2018

Confidence remains somewhat low on details through this forecast
period, especially regarding ceilings and precip chances. Area of
MVFR stratus impacting KLSE early this morning is expected to push
off to the east while another area of IFR/MVFR ceilings near and
behind a cold front across MN approaches from the west. Kept a
similar trend to the previous forecast with MVFR ceilings
developing at KRST (IFR possible) with VFR at KLSE. Model guidance
varies on the eastward extent of the lower ceilings, limiting
confidence. Winds will shift northwest to north behind the front
today and northeasterly later tonight, with some potential for 20
kt gusts. A few showers/storms may develop this afternoon, and
increasing chances tonight. With the highest chances currently
expected south of KRST/KLSE, have not included any precip mention,
but trends will need to be monitored for inclusion in future




LONG TERM....Rieck
AVIATION...JM is the U.S. government's official web portal to all federal, state and local government web resources and services.