Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
849 PM CDT Thursday Jul 24 2014
issued at 849 PM CDT Thursday Jul 24 2014
Have dried out the forecast for the rest of the evening with
subsidence in the wake of the afternoon/early evening complex in
control. However...something to monitor is a shower developing
west of Alexandria Minnesota. Attention otherwise focuses to a cluster
of storms over northwest South Dakota. 24.23z hrrr and 25.00z rap
take this cluster across northern South Dakota...basically
crossing the upper ridge axis there. As the complex approaches
northeast South Dakota...a low level jet increase is expected
along with veering to point towards the forecast area. In
fact...this increase and turning is already taking place at fsd
where south-southwest 45 knots 850mb winds exist on their radar. A
band of showers and storms is likely to initiate on the nose of
the low level jet...eventually progressing into western portions
of the forecast area between 09-10z and perhaps to the MS river by
Have updated precipitation chances and quantitative precipitation forecast to reflect the
aforementioned scenario. Some hints in the hrrr that the storms
within the showers may fall apart around 12z. If this trend
continues...may have to pull or at least reduce thunder chances.
Update issued at 550 PM CDT Thursday Jul 24 2014
Having some serious concerns with the precipitation chances this
evening...and possibly overnight too.
Currently...water vapor loop and radar imagery showed a complex
of showers and storms dropping south...now into northwest and
north central Iowa. The eastward progression of the precipitation
towards the forecast area has come to a halt due to the complex
motion...along with plentiful dry air over the forecast area.
2030z tamdar sounding from msp showed 10c or higher dewpoint
depressions from 770-600mb and a deep inverted-v at 800mb.
Subsidence in the wake of this complex showing up as a significant
area of clearing over west-central Minnesota...drying in the same
location on water vapor...and surface pressure falls up to 2
Its tough finding a model with a great handle on this current
complex. The 24.21z rap...24.19z hrrr and 24.18z GFS seem to be
the best and all three suggest that the evening may now end up
dry across the forecast area. This seems reasonable as the
subsidence comes into the forecast area in the wake of the
complex. In fact...given the clearing upstream...it\s possible
that clearing comes into the area. Next concern then turns to the
overnight convection possibilities. With subsidence and cold pool
in place over Minnesota and north central Iowa...when the low level jet
increases tonight...the convection may end firing more along the
western Minnesota/Iowa border. The aforementioned models along with the
24.12z European model (ecmwf) and even to some degree the 24.18z NAM all show that
idea. If the convection develops an mesoscale convective vortex...then the convection will
drop southeast and stay mostly west of the forecast area. This is
what the 24.18z NAM and 24.12z hires-arw suggested. However...if
no mesoscale convective vortex forms...a frontogenetical band of showers and embedded
storms would move west to east into the forecast area after
07z...perhaps reaching the MS river by 12z.
For now...have trended precipitation chances down some for the
evening and further decreases are possible. Have not adjusted the
overnight yet...but could see a need to decrease precipitation
chances along/east of the MS river. Needless to say the
precipitation chances for tonight is a fluid situation.
Short term...(this evening through friday)
issued at 320 PM CDT Thursday Jul 24 2014
Main forecast problem was the storms/weather moving in overnight
into Friday morning.
A tale of two air masses across the region this morning in the
observational data. 24.12z radiosonde observations showing steep 700-500mb lapse rates
over the western Dakotas /9c per km/ and middle-level saturation
occurring on the eastern edge of the deeper north-S moist plume in
place. Precipitable water /pwat/ values at kbis/kabr both are both
over 125 percent this morning. Kbis and kabr soundings both show
saturation around 700mb with 500-1000j/kg of elevated cape above
700mb. East of the moist plume...stability reigns with 5-6c/km
500-700mb lapse rates and below normal precipitable water values /65 percent/ at
kmpx. This gradient and moisture transport convergence over the
Dakotas/Minnesota border at 16z matched well with the morning storms and
weather in the Dakotas. This all enhanced by a very visible
shortwave trough sewrd of kbis and another north of the Montana/ND/srn
As the large scale shifts over North America and the long wave
trough over the east weakens...flat ridging builds across the
northern states /through Saturday/. This will promote warming and
Richer moisture to move into the region for a period...before
reamplification of the trough over the eastern Continental U.S. Occurs /Sunday/.
Beginning overnight...the Dakotas air mass will advect eastward
and into the area. This airmass is characterized by higher
moisture but also very warm temperatures aloft as an elevated
mixed layer moves in. While this has steep middle-level lapse
rates...the 700mb temperatures warm from 8c to 12-14c
Friday...providing a Stout capping inversion to surface based
convection by afternoon. This translates to a 600 j/kg sbcin. This
cap building has very good consensus amount the deterministic
models...and have lowered rain chances Friday afternoon
Prior to the elevated mixed layer building in Friday...low-level
warm advection and moisture transport increases with height falls
and increased southerly flow overnight. This evening there is some
signal for middle-level moisture transport convergence /700mb/ to
move in from the west during the evening and lapse rates would say
some middle-level towering cumulus could develop some showers
/100-300j/kg MUCAPE/. Have been monitoring the convection entering
western Minnesota and while MUCAPE values have increased a few hundred
from 15-18z...much of Minnesota is still quite stable. However...the
northern side seems to be tied to the 700 mb transport convergence
and shortwave trough approaching Fargo. Would expect echo to
weaken as it moves toward I-35...although there is weak
forcing...the air mass is very dry. Will have to monitor...but
timing looks to fit the evening increase of precipitation chances.
By midnight...as the ridge builds east...moisture transport and a 40-
50kt low-level jet at 850 mb feed the eastern edge of the moisture
plume up the . By 09z...a mesoscale convective system is expected
to enter the western forecast area on very strong moisture
transport convergence at 850 mb /50kts decreasing to 15 kts...kdsm
to klse/. This signal is very consistent among the 24.12z models
and latest rap guidance. Corfidi vectors suggest southeast
movement to the stronger convective elements...and this could pose
some problems with the complex missing to the southwest. With the
moisture transport convergence into The Heart of the forecast
area...would think a miss is probably unlikely. This would favor
the areas southwest of I-94...especially into northeast Iowa and
little change to the rain chances were made.
Precipitable water values increase rapidly overnight with over
1.75 inches in the area by morning and warm cloud depths of 4 km.
Thus heavy rainfall rates will occur in the system...but many
areas can use the rains. Some brief city flooding is possible.
Isolated hail and gusty winds have been added to areas west of the
miss river in the hazard weather outlook due to the magnitude of
convergence/forcing and about 1500 j/kg of MUCAPE. Storm Prediction Center has also
shifted their day 1 convective outlook to near our western
forecast area edge...which is appropriate. Capping should then
build in and the system moves east/southeast in the afternoon.
Long term...(friday night through thursday)
issued at 320 PM CDT Thursday Jul 24 2014
Friday night poses an interesting problem in southern Wisconsin as the
cap is still fully in place with no surface based convection
allowed per all guidance /600 sbcin and 700mb temperatures near 14c/.
Yet a handful of deterministic models continue a signal of quantitative precipitation forecast
across southern Wisconsin in the evening. Have kept a small rain chance
there as it would be some middle-level instability that would have to
be tapped north of the surface low shifting through eastern Iowa. The
24.12z NAM does have some 900-800mb fronotgenesis working within
the capped area...and possibly some isentropic lift generates
stable showers. Prefer the 24.12z NAM with rainfall south of the
forecast area...but have kept a slight chance of rain to cover
this situation Friday evening.
A drying trend then in northwest flow after midnight...through
Saturday provides a very nice dry day. The high precipitable water
airmass should push south to near I-80 corridor.
Strong qg convergence forcing moves in Sunday with the strong
trough moving through Minnesota/WI as the long wave amplifies again over
the eastern Continental U.S.. forcing comes in later Saturday night and is
strongest Sunday with some instability. Have increased rain
chances for Sunday. Rain chances may need to be pulled further
southwest as well depending on the wave track. A cloudy breezy
showery day for most areas it seems.
Monday through Thursday have below normal temperatures and mostly
dry weather under the influence of the longwave trough.
Aviation...(for the 00z tafs through 00z Friday evening)
issued at 631 PM CDT Thursday Jul 24 2014
The aviation forecast has become more uncertain...primarily with
shower and thunderstorm chances. Much of this uncertainty stems
from a complex of showers and embedded thunderstorms that recently
dropped south through western Minnesota and Iowa.
Both taf sites currently sit in a plentiful dry airmass which
should keep conditions VFR through the evening. There could be
some brief gusts at rst of 20-25 knots prior to 01z as the decaying
portion of the complex moves through. During the overnight...
showers and thunderstorms look likely to form again in Minnesota and Iowa
and eventually progress east-southeastward through Friday morning.
However...where they form and if they can develop into a complex
greatly affects the tafs. The location may be farther west thanks
to the current storm complex...which would lean to a better chance
the taf sites stay drier. Therefore...have changed the -tsra to
thunderstorms in the vicinity and removed any lower ceiling potential at lse. Still
maintained the MVFR visibility at rst given the higher chance for
precipitation to occur there vs lse. Any precipitation should move
away from the taf sites by 17z with VFR conditions thereafter.
Winds could get gusty at rst again briefly in the late morning and
afternoon in the wake of the morning storms.