Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
702 am CDT Wednesday Aug 27 2014
Short term...(today and tonight)
issued at 255 am CDT Wednesday Aug 26 2014
Shortwave trough sparking areas of showers and a few thunderstorms
across northern Minnesota/WI early this morning. This feature will slip
quickly east...already moving over northeast WI by 15z. Locations
mostly north of I-94 will see a few showers through daybreak.
With the exit of the shortwave the forecast area will be under the
influence of weak high pressure. The much talked about lingering surface
front sits well south...across northern MO...and won't be a factor
for the local area until tomorrow. Thus...expecting a dry day for
most /Post the shortwave/.
Mesoscale convective system finally firing up over the plains is expected to hold together
through the day as the low level jet/warm air advection stick with
it. Should be approaching the forecast area overnight...with rain
likely west of the mississipppi river toward 12z Thursday.
Long term...(thursday through tuesday)
issued at 255 am CDT Wednesday Aug 27 2014
NAM/GFS/ECMWF continue to point to a soggy end to the work week...a
consistent signal although some details have varied here and there.
One change over the past few days is that the upper level shortwave
may take a bit more time making it to the region...which would
result in another round of heavy rain on Friday.
Main points to the forecast remain...
1) surface front. This remains a key player in where the
precipitation chances will lie. Currently it sits across northern MO
into central ill. Its prognosticated to lift northward on Thursday as an upper
level shortwave trough moves into the Central Plains. Originally the
thought was that this boundary would clear east/southeast on
Friday...getting a shove from the shortwave. However...recent model
trends bring the trough in slower...and would linger the front into
Friday...probably into Sat.
2) shortwave trough. Models have been very consistent with tracking
a 500 mb shortwave trough from the southern rockies into the
central/Southern Plains at middle week. As mentioned...trends have been
slowing down the speed on this feature...holding off on bringing it
into the region until Saturday afternoon...then shifting it
east/northeast by Sun morning.
3) low level jet/moisture transport. The nose of the 850 mb
jet/moisture transport has been the main kicker for nocturnal
convection. It should serve to fire off an mesoscale convective system in the Nebraska/western Iowa
region late tonight...and then instead of waning during the
day...turn its attention on the local forecast area...with a strong
surge of lower level moisture on a 40 knots jet nosing in by 18z Thursday.
This continues to shift north and east on Friday.
4) warm air advection. Strong push of 925-850 mb warm air advection
in the 18z Thursday - 06z Friday time frame...accompanying the low level
jet/moisture push. Will be a big player in the likelihood of rain
during this time.
5) instability/wind shear. Neither look to have significant
contributions to storm threats into the weekend. BUFKIT soundings
point to relatively meager...elevated...skinny cape...not supportive
of strong updrafts. The deeper wind shear tends to be displaced
north of the better instability...in the colder air. There is some
overlap on Thursday...but this doesn/T look like it will lead to a
hail/wind threat. Overall...outside of heavy rain...the severe risk
looks minimal at best.
So...tossing all these factors into a hat we end up pulling out
periods of showers and thunderstorm for the region Thu-Fri...which
could linger into Sat.
Low level jet/warm air advection/moisture should persist the
plains/S mesoscale convective system from this morning into Thursday...helping it track into the
forecast area between 12-18z. Bits of upper level energy tossed out
of the main trough will aid convection...as will the surface front.
Could see more development south of the mesoscale convective system - across eastern Iowa -Thu
The upper level shortwave starts to move in Friday...but likely
doesn/T spin across the local area until early Sat. It will help
kick off more showers and storms...inconcert with the still
lingering surface front. Moisture transport and low level warming will
have shifted east by this time. Overnight Friday-Saturday morning
shaping up to be the period with the highest precipitation chances associated
with the trough...although still some question on shortwave speed.
Heavy rain is likely through the period...which could bring some
Hydro related issues. More on this treat in the Hydro section below.
Aviation...(for the 12z tafs through 12z Thursday morning)
issued at 658 am CDT Wednesday Aug 27 2014
A 6-8k deck of clouds associated with a short wave trough moving
through northern Wisconsin will move out of the taf sites by
27.16z. Skies then will be VFR through 28... then lower
into 4-8k range. Showers and storms may produce IFR/MVFR ceilings
and visibilities after 28.10z.
issued at 255 am CDT Wednesday Aug 27 2014
Very favorable setup for efficient...heavy rain producers over the
next few days. Warm cloud depths continue to hover near 4000 M /if
not higher/ while precipitable Waters Peak around 2 inches Thursday-Thursday night. What
instability theres is will likely go into rain production rather
than hail potential. Shower/storm tracks look more west-east Thursday-Thursday
night...leading to training storms. Orientation might be more
southwest-northeast later Friday into Sat. Run total quantitative precipitation forecast from the
ec/GFS/Gem through Sat range from 3 to 4 inches. Locally higher
amounts would seem likely. That said...the anticipated heavy rain
region for Thursday looks to be north of the one Fri-Sat...which helps
decrease the flash flooding/river rise potential. 3 hour ffg is
generally 2.5-3 inches. At this moment...think there could be quick
rises on some rivers...but should stay within bank. As along as
storms move...and areas Don/T get repeated hits...the flash flood
threat is reduced. All said...with the high likelihood of heavy
rain...and the potential for significant amounts...will issue a
hydrologic outlook to highlight the threats. Might need a Flash
Flood Watch if the anticipated heavy rain regions Thursday-Sat look like
they will overlap.