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Area forecast discussion
National Weather Service La Crosse WI
401 am CDT Thursday Apr 24 2014
Short term...(today and tonight)
issued at 401 am CDT Thursday Apr 24 2014
Main forecast concerns this package...rain chances/amounts today...
cloud trends tonight...temperatures.
06z data analysis had high pressure over the central Great Lakes and
low pressure lifting northeast across Kansas. Stronger pressure falls
ahead of this low and troughing northward into the Dakotas were over
eastern Kansas to Minnesota. Broad scale ascent ahead of this low and the
strong middle level trough moving across the plains was producing
rather widespread -shra/isolated thunderstorms and rain across northern Iowa/southern
Minnesota/WI as Gulf of mex moisture spread northward into the upper
Midwest. Even with all the clouds/developing rain showers temperatures across the
region were on the cool side early this morning.
No problems noted with 24.00z model initializations. A rather tight
consensus among the models for today/tonight...toward the earlier
European model (ecmwf) runs...as the middle level trough approaches/passes. However
trend is toward stronger/more energy in the southern portion of the
trough as is moves through the middle MS valley. This stronger trend is a
bit slower than the earlier runs by 12z Friday. Check of observation vs. Model
data at 06z showed models were good with the Kansas low and troughing
into the Dakotas. Per WV imagery...all appeared reasonable with the
trough/shortwave energy from the Southern Plains to alb/saskat. A
blend of the models looked best with the 00-06z precipitation across the
plains/upper MS valley. With the model/ensemble consensus looking
good at 06z...favored the compromise/blend of the models. Short-term
forecast confidence remains good this cycle.
In the short term...strong surge of Gulf of mex moisture in the 925-
700mb layer will continue into the area this morning. This an area
of MDT/strong lower level Thermo-dynamic forcing/lift and under
broad lift/divergence aloft between jet maxes and ahead of the
approaching middle level trough/shortwave energy. Precipitable water values around 200
percent of normal coupled with the deep layered forcing/lift...the
100 percent or nearly so rain chances this morning are well trended.
Stronger/deeper of the moisture/lift slowly translate east across
the area this afternoon with a downward trend in the rain chances
west of the MS river. Latest model consensus has the surface-700mb
trough axis near the MS river at 00z Friday. The slight slowing trend
among the models now has the middle level trough axis passing this
evening. In response to this...generally raised rain chances this
afternoon into this evening especially over the east half of the
forecast area. Cape still looks to remain very limited today but given
the deep moisture/strong lift will continue the isolated/embedded
thunderstorms and rain mention. Once the surface-500mb trough axis passes...strong signal
for deep subsidence and sharp/rapid drying in the 925-500mb layer
over the southwest 2/3 of the forecast area. Will trend grids for this
but have to delay it a bit due to the slight slowing trend.
All the lower clouds/rain longer into the afternoon today will limit
temperatures and the downward trend of highs seen in the model/ensemble
consensus looks good. With decreasing clouds/clearing expected for
much of the forecast area later tonight...cooler lows per the consensus
look good. If winds end up light/decouple...will have to watch for
fog development with the decreasing clouds/clearing later tonight
and plenty of moisture from the rains today to remain in the
boundary layer. Left late night fog out of the grids for now but
will pass along to day crew to keep an eye on the potential.
Long term...(friday through wednesday)
issued at 401 am CDT Thursday Apr 24 2014
Main forecast concerns for Friday through Saturday night...small
-shra potential Friday night/Sat...returning rain chances Sat night...
Rather tight consensus among the models for Friday with a faster exit
of today/S trough and any secondary shortwave on the back side to
pass well North/East of the forecast area. Signal remains strong for
rising heights/ridging aloft to build into the upper Midwest Friday
night/Sat with the area east of the ridge axis. Improving consensus
for this ridge axis to pass Sat night as strong over The Rockies
ejects into the central/Southern Plains. With the improving between
model and run-to-run consistency out to 12z sun...especially with
the troughing digging through The Rockies...forecast confidence for Friday
through Sat night is trending good this cycle.
Generally dry/quiet weather Friday as a weak back-door cold front
slides into the area. Bulk of the lower level cooling behind this
boundary remains North/East of the forecast area. Model soundings
showing mixing to around 800mb on Friday...with 850mb temperatures in the +2c
to +10c range. With some sunshine for much of the area and MDT west
to northwest winds for mixing...Friday trending to be a warmer day with
highs southwest of I-94 in the 60s. This weak frontal boundary sags
south of the area Friday night and becomes the focus for lift/precipitation
production later in the weekend as a warm front. Can high pressure
noses into the area behind the front for Friday night/Sat. However...
signal among the models for a weak shortwave to sneak through the middle
level ridge and across the area Friday night. Some 850-700mb warm
advection/moisture increase looks to accompany this wave...with
baroclinic zone north of the weak front helping provide some lift.
Previous grids carried a small -shra chance across the southeast end
of the forecast area Friday night and left this for now. NAM/European model (ecmwf) try to
produce some light quantitative precipitation forecast across the northeast part of the forecast area
Sat but forcing for this not well focused and 850mb layer prognosticated on
the dry side. Left Sat dry for now.
By Sat night trend of the tighter model consensus is more progressive
with the middle level troughing moving into the plains and more
northerly with the Central Plains surface low. With the middle level ridge
axis moving east of the area Sat night...deeper south to southwest
flow is allowed to spread more moisture north/northeast into the
upper Midwest...over the now warm frontal boundary near the MO/Iowa
border. Precipitable water values over the area prognosticated to quickly increase to an
inch or more Sat night. With more clouds/low level warm advection
Sat night...trended lows upward. Lows still looks to support a -sn
chance over the northeast end of the forecast area late Sat night...
but 850mb temperatures in the +4 to +8c range for what would be a cold
rain late Sat night/early Sun morning. Removed -sn mention from
these periods. Used the model/ensemble consensus highs/lows for
Friday through Sat night.
For Sunday through Wednesday.../days 4 to 7/...
Main forecast concerns are rain chances and temperatures through the
Model runs of 24.00z trend more progressive/further north with the
large middle level trough/low in the central Continental U.S. Much of the sun-Wednesday
period. Movement of this system promises to be slow with it still
looking to be the western limb of a central/eastern noam Omega
block...with generally cool and moist period of weather for the area.
Forecast confidence in the sun-Wednesday period is average.
With the broad surface-500mb circulation looking to very slowly work its
way across the middle section of the Continental U.S. Sun-Wed...the forecast area
remains on its east/northeast side and under the moist warm conveyor
belt through at least sun/Monday and likely through Tue/Wed. Model soundings
show the column to be too warm for any -sn through Monday and removed this
mention from the late night/early morning grids of sun through Monday.
Increasing rain chances for sun through Monday per the model consensus
quite reasonable. Some 925-850mb cooling to occur for Monday night into
Wednesday as the surface-700mb low tries to slide east through the middle MS valley
and some drier low level air...allowing for evaporative cooling...
wraps southwest around the middle MS valley circulation. Continued the
mention of late night/early morning -ra/-sn across mainly the north
portions of the forecast area Tue/Wed. May be enough lower level dry air
to wrap in Tuesday/Wednesday to limit precipitation...but left the 20-40 percent
consensus precipitation chances as is for now. Model/ensemble consensus
highs/lows generally 10f below normal appear well trended at this
time with plenty of clouds expected and low level flow from an east
to northerly direction.
Aviation...(for the 06z tafs through 06z Thursday night)
issued at 1100 PM CDT Wednesday Apr 23 2014
The first band of precipitation has shifted east of klse late this
evening...while another band is moving northeast across IA/neb. This
next batch will be showery in nature at the onset...becoming more
widespread toward 09z per latest hrrr/ruc13/nam12. Ceilings will fall
into the IFR/MVFR categories overnight...and likely hold there
through Thursday night. The rain won't exit until close to 00z Friday.
Mostly forecasting MVFR visibilities with the rain...but a period of 1-2sm
Tight pressure gradient will keep winds fairly breezy from the
east/southeast through most of Thursday...with a swing to the
west/northwest as the low passes to the east toward 00z Friday. BUFKIT
soundings also suggesting about a 3 hour window where low level wind shear could be
a concern...roughly in the 10-13z time frame. Its borderline right
now...so will leave out of the forecast. Would be ws020/17048kt.
issued at 401 am CDT Thursday Apr 24 2014
Rain amount of 1 to 1.5 inches early this morning and today have the
potential to create runoff and MDT to strong within bank rises on
many rivers and streams across the forecast area. Potential for some
minor flooding along portions of the Kickapoo river if the rain
amounts/runoff end up on the heavier side of what is expected. With
many river still running high from recent rains/snowmelt...will
issue and esf for the possible rises/flooding from the rains today.
Also of concern is the next round of rain expected Sat night through
Monday...with potential for 1 to 2 inches of rain during this
period. These rains if they materialize/fall would create
additional rises and flooding threats well into next week.