Scientific Forecaster Discussion
fxus63 kmkx 252328
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
628 PM CDT sun Aug 25 2019
Update...initial weak mid-level wave along with some mid-level
moisture advection producing scattered light showers over south
central WI into northern Illinois. Showers should become more isolated
this evening, but upstream stronger mid-level wave over northern
MO is still expected to take a northeast track overnight into
Monday morning as deeper column moisture moves into the area.
This will likely bring a period of more widespread showers to the
area, especially southeast areas. Precipitable water roughly
doubles overnight into Monday morning to around 1.5". Instability
remains negligible through Monday morning.
Aviation(00z tafs)...isolated to scattered light showers expected
this evening, will become more widespread later tonight into
Monday morning across much of the area. Expect VFR ceilings to
lower to MVFR and possibly IFR late tonight and Monday morning.
Previous discussion... (issued 345 PM CDT sun Aug 25 2019)
Tonight through Monday night...forecast confidence is medium to
The persistent high pressure that was over the area this weekend
has shifted off to the east. To our west a few shortwaves exist
and will play a role in rain chances tonight through Monday night.
Winds at the surface have shifted to the southeast while winds aloft have
backed to the south and southwest. This has allowed moisture
mainly above 700mb to start to stream back into the area. The
first shortwave is lifting into Minnesota and subtle lift associated with
this feature has resulted in some light returns. However, drier
air still lingering near the surface is preventing some of this
rain from reaching the ground. Kept in a slight chance as this
narrow axis of rain shifts east.
As we head into the overnight hours, a system moves out of Kansas/NE
towards central Illinois. Moisture transport with this feature will be
better and rain chances increase through the night as the
shortwave nears the area. Forecast soundings would suggest an
almost completely moist adiabatic profile overnight so have kept
thunder out of the forecast. This is a shift from previous
guidance, that were advertising this system would stay to our
south and east keeping the area mostly dry tonight. But all the
new guidance made a shift westward with the track of the rainfall
and based on the current movement of activity to our SW this shift
seems reasonable. So, have modified pops and quantitative precipitation forecast for tonight to
reflect this change.
Latest trends in model guidance would suggest a less favorable
setup for strong to severe storms on Monday. The shift in the
overnight into Monday morning rainfall would likely mean that we
won't be able to heat up as much as previous guidance was
suggesting. The timing of the cold front has also been delayed
with the latest runs putting it into the area overnight Monday
into Tuesday. As the main upper level system moves into WI it
pivots north/NE towards northern WI Monday night. This would take the
stronger forcing away from the area and we may not have as much of
a focus for showers and storms along the front. There is also a
signal in guidance for some type of splitting of precip over
southern WI. This could be the result of the better forcing going
north, and any storms that do develop along the front are going to
shift east/southeast towards the better instability, which would be away
from the area. Lots of mesoscale details to be worked out and
won't be able to really pinpoint those until the showers tonight
move through the area. So, for now have kept higher chances of
showers with only slight chances for storms for Monday into Monday
Tuesday through Wednesday night...confidence is medium.
Dry but breezy conditions are expected as a low pressure system
exits the area to the northeast. Another upper level disturbance
will swing a trough axis through Tuesday evening into Wednesday
morning. This could spark off an isolated shower, but with a lot
of dry air in place it is not likely. The main story will be
strong westerly winds winds that move in for Wednesday behind the
mid level trough axis. 850 mb winds will be around 30 to 40
knots. At this point believe we will see some 30 knot gusts mix
down to the surface Wednesday afternoon over land and over Lake
Michigan. Winds will then decrease overnight as we head into
Thursday through Sunday...confidence is medium.
Upper levels seem a bit active with multiple shortwave troughs
moving through. The main timeframes look to be Thursday evening
into Friday and again on Sunday. The question will be is there
enough moisture in the atmosphere to develop showers? At this
point it looks like the best moisture remains to our south.
Therefore have a dry forecast in place through the weekend. If
there is a trend for more moisture to be available later on this
week, will probably have to add in rain chances.
For temperatures we will see highs in the 70s and lows in the 50s
Wednesday through Sunday.
Increasing moisture tonight will result in ceilings gradually
lowering towards MVFR. A stray light shower is possible this
afternoon and evening but rain chances start to increase towards
Monday morning. Winds stay out of the southeast through the period and
it's possible that a few gusts near the lake could be upwards 20
to 25 knots Monday afternoon. If showers become more widespread
Monday morning, cigs could end up borderline IFR/MVFR with
visibilities of 2 to 3 miles.
A stronger system approaching the area from the west will result
in increasing winds and waves tonight into Monday. The east/southeast winds
we've had the past few days have kept waves between 2 to 3 feet.
Winds ramp up to 20 to 25 knots by Monday morning and this will
result in waves building towards 3 to 5 feet with some 6 foot
waves possible. A Small Craft Advisory is in effect for Monday.
A cold front moves through the area Monday night shifting winds to
the west, which will allow waves to come down. Another system will
move across the lake on Wednesday bringing winds to near gales.
Gusty southeasterly winds expected tonight through Monday night
will result in building waves through the period. Waves of 2 to 3
feet this evening will build to 3 to 5 feet with occasional waves
as high as 6 feet Monday afternoon. A high swim risk is in effect
for all Lake Michigan beaches tonight through Monday night.
WI...beach hazards statement from Monday morning through late Monday
night for wiz052-060-066.
Beach hazards statement from 1 am CDT Monday through late Monday
night for wiz071-072.
Lm...Small Craft Advisory from 1 am Monday to 1 am CDT Tuesday for
Small Craft Advisory from 7 am Monday to 1 am CDT Tuesday for
Small Craft Advisory from 7 am Monday to 7 am CDT Tuesday for