Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
341 am CDT sun Aug 30 2015
Short term...(today and tonight)
issued at 341 am CDT sun Aug 30 2015
Most immediate concern is patchy dense fog early this morning.
Because not all low cloud from yesterday dissipated and the fact
that 0.5 km southwest winds are increasing from now until
sunrise...widespread dense fog is in question. There have been
some sites across the region that have reported one-half mile or
less in fog...but not consistently as visibility GOES up and down
over relatively short periods. For now...will cover fog threat
with a Special Weather Statement...but monitor carefully for more widespread persistent
low visibility that could necessitate an advisory.
Whether stratus of fog...most areas should wake up to gray/foggy
skies. However...low-level moisture should begin to mix out after
sunrise as boundary layer deepens to between 1500 and 2000 feet by
midday. Should finally begin to see a fair amount of sun by early
to middle afternoon. With a slow start to warming for the day...
dropped temperatures a few degrees with highs now expected in the
middle to upper 70s. For areas that see a bit more sun...highs may
warm into the lower 80s.
Expect mostly clear skies tonight with light southerly winds. Will
keep patchy valley/bog fog mention...but its occurrence is a bit
less certain. Model soundings showing near surface saturation...
but also 15 to 20 kts immediately above the nocturnal inversion.
This set-up may be more favorable for low stratus than fog...but
something to watch. Lows tonight will drop into the 60s.
Long term...(monday through saturday)
issued at 341 am CDT sun Aug 30 2015
Monday through Wednesday...500 hpa ridge moves into the eastern
Continental U.S....although perhaps a bit flatter than previously forecast
given weak short-waves moving through southwest flow aloft. Main
impact will be warming temperatures with daily highs in the 80s...
although some valley locations could approach 90 degrees given
favorable south-southwest surface Wind. Ridge axis retrogrades
back across the Southern Plains for late in the week...resulting
in Thursday/Fri/Sat possibly being the warmest days of the week with
many locations into the upper 80s...perhaps 90 degrees.
Will keep periodic 20-30 probability of precipitation through the week...although think
dry conditions will be the rule since axis of maximum middle-level
moisture transport remains mainly west of the forecast area and
deep layer shear is weak. Best chance for more widespread showers
and thunderstorms holds off until late Saturday into Sunday as
eastern Pacific trough/cold front finally push far enough east to
impact the upper MS River Valley. Will introduce high end chance
probability of precipitation for this period. With clouds/possible rain and approaching
cold front...could see temperatures begin to retreat as early as
Aviation...(for the 06z tafs through 06z Sunday night)
issued at 1130 PM CDT Sat Aug 29 2015
Very challenging forecast.
Fog...stratus...or both remains the question and latest models runs
and trends aren/T giving much confidence one way or the other. That
said - the impacts on aviation could be the same as flight
categories from fog or low stratus could be the same.
Low stratus has been reluctant to exit the area at late
evening...slowing down/stopping. There are even some hints of
northward development over northern Iowa. Rap model suggest this will
occur in earnest...with low ceilings spreading over southern Minnesota
overnight. Klse would hold steady or drop a bit in heights...but
likely stay IFR.
If the stratus does not advance into krst...fog is the likely
outcome...and sub 1sm is a possibility. Some locations across
southern Minnesota/northern Iowa already have visibility restrictions. If the
stratus holds at klse...the impact on visibilities will not be as extreme as
Confidence is high in ceilings and/or fog impacts at krst/klse overnight
through most of Sun morning. Which one is more dominant is where the
confidence is shaky. Going to stay the course as a result...holding
with the previous forecast for the most part. Expect updates
overnight as confidence increases in the more likely outcome.