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Area forecast discussion 
National Weather Service La Crosse WI
1114 PM CDT sun Apr 20 2014

issued at 644 PM CDT sun Apr 20 2014

Becoming a bit concerned that the current showers and
thunderstorms across northern Iowa into far southwest Wisconsin
may not move much tonight. Many models struggled this afternoon to
have the convection where it was. Most thought it would be closer
to I-90 instead of northern Iowa. Believe the southward
progression was a result of cool air outflow...which can
definitely be seen in temperatures between La Crosse and Dubuque.

Now there is a warm front re-inforced by that cool air outflow
situated along US-20. Models are in agreement that a low level jet
will develop this evening...not strong...but one nonetheless. This
low level jet could help to propel the warm front north.
However...the low level jet will likely help to maintain
convection north of the boundary...with its cool air outflow
holding back the front. Thus the concern that the convection
ongoing over northern Iowa may not move. It should be noted that
the 925-850mb flow is southwest until about 08-09z...then it turns
westerly. So it would appear until 08-09z is the time for the
front to move north.

Precipitable water values in place are on the order of 1-1.1
inches while MUCAPE stands between 500-1000 j/kg being lifted into
the boundary. The MUCAPE should slowly fall through the night...
but plenty enough to keep convection going. The MUCAPE certainly
is not high enough to have a deep updraft...nor is the
precipitable water high enough to get big rains. Still...the
training concern to the precipitation does raise some situational
awareness of hydrology issues. So far rivers have not responded to
recent rain which is good news. We\ll just have to keep a monitor
for any heavier precipitation cores.


Short term...(this evening through Tuesday night)
issued at 121 PM CDT sun Apr 20 2014

The main forecast problem is with the rain chances through

Surface analysis this afternoon indicates the stationary front
extends from western Iowa into southern Wisconsin and is over the
very southern parts of the forecast area. Local and regional
radars shows the most concentrated area of rain north of the front
from northeast Iowa into western Wisconsin with more scattered
activity back into the north central Iowa.

Given the movement of the current activity...expect that most of
this will have moved east of the forecast area by 00z. How much
additional activity will form along the front is rather uncertain.
The forcing does not look to be all that strong with the area
split between the northern stream system moving across southern
Canada and a short wave trough getting ejected out of the system
over the Southern Plains. The indications are that this wave
coming out of the southwest will track across southeast Iowa into
northern Illinois. The best pv advection in the 500-300 mb layer
will be with the Canadian system with some very weak pv advection
across Missouri and Illinois with the southwest wave. Weak to at
times moderate frontogenesis will be in place north of the front
in the 1000-700 mb layer tonight and then shifting southeast
across the area Monday morning as the front moves out with the
short wave trough. Isentropic up Glide of 3 to 5 ubar/S will occur
into and over the front on the 300k surface into Monday morning.
The moisture transport on this level is not very strong and is
parallel to the boundary and not into it. With the weak forcing
along the boundary...confidence on how much additional activity is
going to develop overnight is not very high. Have continued the
trends from the previous forecast of showing the highest chances
along and north of the boundary with a band of 50 to 70 percent
chances. Will also continue to mention some thunder for most of
the area...except northeast of Interstate 94 where it looks like
there will be little to no cape in place. The rain chances will
persist through Monday morning before diminishing through the
afternoon as the front moves away from the region.

High pressure will then build over the area for Monday night and
persist into Tuesday night. Quiet weather with some slightly
cooler air with overnight lows dropping into the 30s with some 20s

Long term...(wednesday through sunday)
issued at 121 PM CDT sun Apr 20 2014

An extended period of wet weather looks to be possible starting
Wednesday and persisting through Friday with additional chances
then for the upcoming weekend. The upper level ridge will get
pushed east of the region as an upper level low comes across the
northern rockies. A lead short wave trough is expected to come out
of the upper level low and ride up the backside of the departing
ridge. This will help generate the first rain chances as low
pressure should develop over the High Plains with strong warm
advection over the region ahead of this surface low and short wave
trough. The next short wave trough rounding the upper level low is
forecast to become very strong and cause a new upper level low to
form over the upper Midwest as the first upper level low weakens
and dissipates. This new upper level low will then move slowly
from the upper Midwest across the Great Lakes keeping the
precipitation chances going. Differences then between the 20.12z
European model (ecmwf) and GFS on how strong the subsequent ridging will be with
the European model (ecmwf) weaker allowing more short wave energy to move over the
region for the precipitation chances next weekend.
Unfortunately...temperatures look to be cool enough Friday and
Saturday night for the possibility of a rain/snow mix or light


Aviation...(for the 06z tafs through 06z Monday night)
issued at 1114 PM CDT sun Apr 20 2014

Showers and embedded thunderstorms across northern Iowa...situated
just north of a warm front along US-20...are inching their way
northward towards the taf sites. It\s possible for these showers
to affect the taf sites from now through 14-16z when a cold front
is slated to move across. Since confidence of these showers
hanging at either taf site for more than 1 or 2 hours is low...
have only went with a vcsh. Also did not include thunder as the
amount of lightning has dropped off quite a bit with loss of
instability. There has been some br around the taf sites...enough
to knock lse down to MVFR. MVFR visibilities should be the general
rule until cold frontal passage. Plan on ceilings to stay VFR.
After the cold front passes...drier air comes in on gusty
northwest winds to ensure VFR conditions. The winds could gust
upwards of 25 knots. Plan on the wind gusts to diminish around 01z
with loss of daytime heating...but a 10-15 knots sustained wind looks
likely to persist associated with cold air flowing in.


issued at 644 PM CDT sun Apr 20 2014

Flooding along the lower reaches of the Mississippi River will
continue through the first part of the week as runoff from
upstream snow melt and recent rain runoff moves downstream.
Rainfall overnight into Monday is not anticipated to be enough to
produce any additional flooding concerns. However...potential
training of precipitation tonight will need to be
monitored...especially with any heavier precipitation cores.


Arx watches/warnings/advisories...


short term...04
long term...04