Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 301735
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1235 PM CDT SAT JUL 30 2016

.UPDATE...
Issued at 1235 PM CDT Sat Jul 30 2016

Issued an SPS earlier for the potential of funnel clouds out
toward the I-35 corridor. Current NST parameter continues to
expand across that area within the vicinity of a weakish surface
trough axis, and should increase the next few hours as 0-2km lapse
rates max out. Have already seen a few reports out of northern
Iowa per the Des Moines NWS office and can`t at all rule out
similar reports through the afternoon with any convective cells.

.UPDATE...
Issued at 1054 AM CDT Sat Jul 30 2016

Overall flavor of the going forecast looks pretty good heading
through the evening hours, with the main issue being just where
and how widespread any diurnally-helped shower/storm development
will end up. Current pattern remains a rather nebulous one
featuring broad and weak upper troughing across the Upper Miss
Valley and a weak low level circulation slowly meandering across
the lower Great Lakes, all while broad low level ridging extends
from eastern Ontario back into northern WI/MN with a very weak
wind flow regime intact across the whole region. Satellite trends
the past hour or two tell the story, with a zone of better
boundary layer moisture very slowly lifting back north along and
either side of the I-35 corridor on the backside of the low level
ridge axis and helping pop some thicker cumulus development. Also
starting to see a couple blips on the radar, and with convective
temps right around the 73-75F mark and coolish air in the lower
levels (around 3C at 700mb), it won`t take much to sneak parcels
by weak capping and into an environment characterized by roughly
500 J/kg of skinny CAPE.

As such, have reworked precip chances farther west per trends but
also can`t rule out something similar happening closer to the
river where boundary layer moisture remains still on the higher
side. Farther to the east, it appears low level ridging and slow
drying through the day should mitigate the precip threat,
especially north of I-94, where skies may end up more sunny into
the afternoon. Don`t envision any severe threat with storms today
given pretty much nil shear, but will have to watch the potential
for localized heavy rains given storm motions less than 10 knots
and PWAT values still around 1.25-1.5 inches.

&&

.SHORT TERM...(Today through Monday)
Issued at 237 AM CDT Sat Jul 30 2016

Water vapor satellite early this morning indicates a weak short
wave trough was over North Dakota. The 30.00Z models all bring
this wave southeast and across the region today. As this wave
moves in, it should only be able to produce some very weak pv
advection in the 500-300 mb layer this afternoon. The surface
trough that was over the area Friday is still over northeast Iowa
and is not expected to move much, if at all, today. The 30.00Z
GFS suggests there could be some very weak frontogenesis in the
1000-850 mb layer near the trough and this combined with the weak
forcing from the short wave trough could be enough to generate a
few afternoon showers. These showers should quickly die off in the
evening as the low level surface ridge builds in from the
northeast for tonight. This ridge looks to remain over the area
Sunday for a dry day.

The models are all in good agreement that this low level ridge
will remain over the area Sunday night. Upper level heights are
expected to rise with short wave ridging ahead of a short wave
trough coming across the Dakotas. The higher heights and slower
timing of the short wave trough should keep any rain west of the
area so have removed the rain chances that were over the southwest
part of the forecast area. This short wave trough then looks to
move into the region Monday with the GFS showing some weak to at
times moderate pv advection in the 500-300 mb layer. The low level
ridge still looks to be rather slow to retreat which will keep
some drier air over the eastern sections of the forecast area.
There does not look to be a surface boundary coming in with the
short wave trough to help focus any convection development. There
does not really look to be a concentrated region of low level
moisture convergence. The GFS shows the best axis of moisture
transport over the Missouri River Valley into the western Ontario
but does bring the convergent eastern edge of this into the
western sections of the area Monday afternoon. This, combined with
the forcing from the short wave trough, may be enough to generate
some scattered showers and storms from about the Mississippi River
back to the west.

.LONG TERM...(Monday night through Friday)
Issued at 237 AM CDT Sat Jul 30 2016

The short wave trough will continue to advance east across the
area Monday night with the GFS showing the moisture transport
becoming focused on the area at times. A warm front looks to take
shape across Iowa with some frontogenesis developing in the
1000-700 mb layer. The warm air advection ahead of the short wave
trough kicks in as well to help produce 2 to 4 ubar/s of up glide
on the 305K isentropic surface. This looks to be the best chance
for rain and will have up to a 50 percent chance across a good
share of the area. This system will move past the area Tuesday
with the upper level ridge expected to build back in. However, the
ridge is not all that amplified so that short wave troughs coming
out of the western long wave trough that ride up and over the
ridge will be close enough to possibly bring some more showers and
storms from Tuesday night through Thursday. The upper level ridge
then looks to move east of the area allowing a stronger short wave
trough to come in for Thursday night.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1235 PM CDT Sat Jul 30 2016

VFR conditions continue through the afternoon with some scattered
to broken cumulus, and likely a few showers and maybe a rumble of
thunder around RST. Any showers that develop will be slow moving,
so it is possible that period of lower ceilings and/or visibility
will occur at RST if any showers develop over the site. Otherwise,
high pressure looks to build into the area overnight, with skies
expected to clear off. With light winds in place, suspect there
will be some fog development, particularly across the river
valleys. Not yet ready to commit LSE to LIFR conditions in fog
with low level drying taking place through the night, but will
include a vicinity mention as fog will probably be nearby.
Tomorrow should feature dry weather with just some diurnal cumulus
development, though less than seen today.

&&

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

$$

UPDATE...Lawrence
SHORT TERM...04
LONG TERM...04
AVIATION...Lawrence


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