Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
1225 PM CDT Friday Aug 26 2016
Short term...(today through saturday)
issued at 245 am CDT Fri Aug 26 2016
A ripple in the upper level flow slated to spin out of the Southern
Plains today, moving across the local area overnight/Sat morning.
West-east running frontogenetic boundary slated to set up around the
Iowa/Minnesota border between 06-12z, lifting northeast by 18z Sat. 25kt low
level jet/850 mb moisture transport nose into northeast Iowa
overnight, working into and across the boundary. Add in some help
from the 300 mb jet's right entrance region and there should be
ample, although not overly impressive, lifting mechanisms for
showers/storms. Little if any instability indicated...mostly
shunted south...so any storm threat may be more isolated to
scattered in nature.
Looking at heavy rain parameters...warm cloud depths via the GFS/NAM
roughly 3500-3800m south of I-90, while pws range from 1 1/2 to 1
3/4. Variables not as high as they were for other heavy rain events
of this Summer, but still high enough where heavy rain could/will be
a concern...again. Quantitative precipitation forecast relatively low in the models...generally
suggesting 1/2 to locally 1 inch as the rain bands move northeast
across the area between 06-18z Sat. That said...northeast Iowa and
southwest WI look the most likely to be under the gun for the
rainfall. While not a lot by most standards, with significant
flooding ongoing for parts of these areas, any additional rainfall
is the last thing they need. Coverage/amounts may not warrant a
widespread Flash Flood Watch, but if signals continue to point to
the hard hit areas, and the heavy rain threat would increase, a
watch could/would be needed.
Long term...(sunday through thursday)
issued at 245 am CDT Fri Aug 26 2016
Fairly messy weather pattern for next week as the upper level flow
transitions from zonal to ridge building over the plains/upper
Mississippi River valley Thu/Fri. The models produce a smattering of
rain chances early in the week as bits of upper level energy transit
through the flow. The ridge building would then favor northern
Minnesota/northern WI for the storm track for the later half of the week.
Ec leans toward a drier solution for the local area compared to the
GFS, although still paints periodic quantitative precipitation forecast through the upcoming week.
That said, certainly some hope for more dry rather than wet
conditions - desperately needed for many across the region. For now,
will stick with consensus solution for rain chances.
Rising heights and warming low level temps will return a taste of
Summer for the end of August. 850 mb temps warm from around 12 c to
18 c for much of next week. Not a huge jump, but should result in
low to mid 80s for highs - a few degrees above normal.
Aviation...(for the 12z tafs through 18z Saturday afternoon)
issued at 1225 PM CDT Friday Aug 26 2016
Good VFR looking to continue thru this afternoon and into this
evening as drier high pressure holds over the taf sites. As the high
departs and deeper south flow spreads over the area, deeper moisture
quickly returns this evening, along with lower clouds, rain showers and sct
thunderstorms and rain. Cigs/vsbys looking to drop into MVFR/LCL IFR range for the
overnight thru early Sat morning hours as an area of rain showers/embedded
thunderstorms and rain and the deeper of the moisture spreads northeast across the
area. By mid to late morning Sat, the deeper moisture and forcing
for rain showers/thunderstorms and rain is expected to be lifting northeast of the area, with
vsbys improving to VFR and cigs rising into the 2500 ft range.
Issued at 245 am CDT Fri Aug 26 2016
Flooding continues on portions of the Turkey and upper Iowa rivers
and their tributaries in northeast Iowa. Warnings continue for much
of this area. Some high water also in Kickapoo river basin in
southwest Wisconsin but that is improving.
Per the "short term" portion of the discussion, heavy rain
parameters not as high as some previous heavy rain events this
Summer, but still high enough that heavy rain will be a threat with
any storms late tonight/Saturday. Modeled quantitative precipitation forecast relatively low
though...generally 1/2 inch with locally 1 inch. In addition...the
forcing mechanisms look to be transitory, moving northeast across
the area. Still, the places hard hit from a few days ago look to get
more rain tonight/Sat. Any additional rainfall could impact clean up
work and slow falls on some river systems. If heavier and/or more
qpf then currently forecast, the threat for flash flooding returns.
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