Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 181146
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
646 AM CDT THU AUG 18 2016

.SHORT TERM...(Today and tonight)
Issued at 334 AM CDT Thu Aug 18 2016

Latest water vapor satellite imagery shows upper level trough
over the southern Canadian Rockies and ridging over the Northern
Plains and lee-side of the Rocky Mountains. Weak surface ridge
over southern Minnesota...Iowa and western Wisconsin is producing
mostly clear skies across the forecast area. This has formed fog
in river valleys and the favored areas of central Wisconsin.

Main forecast concern are temperatures today. Shortwave ridge builds
into the central United States and southwesterly flow aloft advects
a warmer airmass into the forecast area. The 18.00z GFS/NAM/ECMWF
indicate 925mb temperatures climbing to around plus 23-25 degrees
across the forecast area by 21z today. High temperatures will rise
into the middle to upper 80s and possibly 90 degrees at a few
locations.

Tonight...Shortwave ridge builds into the Great Lakes region by 06z
Friday and southwesterly flow aloft develops over the Northern
Plains states. First impulse embedded in the southwest flow aloft
moves into northern Wisconsin after 06z Friday. Lift/forcing and
associated moisture convergence with impulse looks to be focused
over the northern parts of the forecast area...where the higher
chances of showers/thunderstorms will occur. Next concern is
severe potential with the complex of convection. The 18.00z
GFS/NAM continue to show rapidly decreasing instability after 03z
across the forecast area. The 18.00z deterministic models
indicate 0-3km shear of 20-30 knots by 06z Friday. With the
diminishing instability and convection tracking into the northern
parts of the forecast area this evening...some of the thunderstorms
could be strong/severe with damaging winds...locally heavy rain and
hail.

.LONG TERM...(Friday through Wednesday)
Issued at 334 AM CDT Thu Aug 18 2016

First impulse and associated vertical motion/moisture convergence
moves east of the forecast area Friday morning...subsidence behind
the impulse will allow for a break in convection or a decrease in
coverage of convection Friday morning and possibly into the
afternoon hours.

Late Friday afternoon into Saturday...The 18.00z GFS/NAM/ECMWF
continue to have significant differences with overall evolution of
track/placement/strength of surface features. The last several
runs from the GFS/ECMWF per dprog/dt suggest surface front/low to
track across eastern Iowa into southern Wisconsin. This would
result in a frontogenetical band of precipitation developing over
much of the forecast area Friday night into Saturday. Continued
trend of increasing precipitation chances Friday night into
Saturday. As far as severe potential...confidence remains
low...based on model differences plus on timing/placement of
convection. Trends indicate the possibility of severe will occur
over the southern parts of the forecast area Friday evening into
Friday night...with damaging winds...locally heavy rain and hail
the main threats.

Main forecast concerns Sunday into Wednesday are temperatures Sunday
into Monday. Upper level trough deepens over the Great Lakes region
Sunday into Monday. With the forecast area remaining under cyclonic
flow aloft...this allows colder air to filter into the region.
Unseasonably cooler temperatures are expected Sunday into Monday.
The 18.00z GFS/ECMWF are in agreement amplifying upper level ridge
over the Northern Plains and central United States Tuesday into
Wednesday. However...the models differ on strength of upper level
ridge and this will have impacts on timing of piece of energy
ejecting out of the upper level trough over the western United
States into the Upper Midwest...and precipitation chances.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 646 AM CDT Thu Aug 18 2016

IFR/LIFR visibility/ceilings in fog will dissipate by 18.14Z with
VFR conditions thereafter. Main challenge for later in the period
is the potential for showers and thunderstorms. Best chance for
storms is north of I-90, but with differing forecast guidance at
TAF airfields highlighting timing/coverage uncertainty, will keep
18.12Z TAFs dry for now. Later addition of thunderstorms with
lower visibility/ceilings may be necessary by late afternoon/
evening depending on convective evolution of upstream storms.
Winds will be light from the south-southeast through the period.

&&

.HYDROLOGY...
Issued at 334 AM CDT Thu Aug 18 2016

The latest 18.00z GFS/NAM suggest precipitable water values around
1.5 inches today through Saturday. However...with model differences
on placement and track of convection complex/systems through
period. This will have impacts on exact placement of rainfall
amounts across the forecast area. At this time...the forecast area
could see three quarters to 2 inches of rainfall through the
period...with locally 2 to 4 inches. If storms entrain over any
given area either tonight or Friday evening/night...flooding will
be possible. However...confidence remains low for any flooding
potential.

&&

.ARX Watches/Warnings/Advisories...
WI...None.
MN...None.
IA...None.
&&

$$

SHORT TERM...DTJ
LONG TERM...DTJ
AVIATION...ROGERS
HYDROLOGY...DTJ



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