Scientific Forecaster Discussion
fxus63 kmkx 210517
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
1117 PM CST Wed Nov 20 2019
As the showers and embedded thunderstorms continued to move east/NE
into southern WI they moved into a more stable environment. Will
leave the slight chance thunder until just after midnight then
drop all mentions of thunder as lapse rates won't support storms.
The more widespread rain is quickly approaching the area from the
SW and the current forecast handles these trends well.
VFR cigs will give way to MVFR cigs as showers spread in from west
to east tonight. An embedded thunderstorm could impact the msn
terminal through 09z but chances were too low to include in the
tafs at this time. As a strong low pressure near the area winds
just off the surface will begin to increase and wind shear is
possible. Brief drops to IFR cigs/vis are possible under heavier
Winds during Thursday afternoon switch to the west/northwest but due to an
inversion we expect the MVFR cigs to hold through much of the day
and into Thursday evening.
Previous discussion... (issued 933 PM CST Wed Nov 20 2019)
Radar this evening showed a narrow band of showers with embedded
thunderstorms. These appear to be forming along the leading edge
of the low level jet and also an axis of warm air advection aloft. While this warmer air
axis will continue to shift over southern WI, the lapse rates
aloft are not as steep and therefore the available instability
will be lower. So, really only expecting a rumble or two as this
initial band of showers moves through. The rest of the forecast is
The low pressure system continues to approach the area from the SW
and winds have been slowly increasing through the night. The
current forecast handles this well and no changes are needed at
Previous discussion... (issued 523 PM CST Wed Nov 20 2019)
The low clouds have been slow to break up but have finally started
to Peel north this evening. With lighter winds and high level
clouds, temps could fall down into the upper 30s for a brief
period this evening. Those will likely end up being the lows for
tonight. As the strong system nears the area late tonight, winds
and warm air advection ramp up and temps could warm through sunrise. No major
changes were made at this time.
MVFR cigs linger at a few locations across central WI but the bulk
of these clouds have pushed north. Brief period of VFR cigs are
expected before rain moves in later this evening. A system is
forecast from central Iowa into southern WI late tonight into
Thursday. Winds shift to the south as the system approaches and
increase considerably as a 45-55 knot jet at 4,000ft moves across
the area. Cigs/vis drop as rain moves in with some drops to IFR
possible at times. Even though rain ends from west to east
Thursday afternoon, expecting the MVFR cigs to hold through the
end of the taf period.
The strong jet will result in wind shear at all terminals tonight
as winds go from 20-25 knots at the surface to upwards of 45 knots
Previous discussion... (issued 348 PM CST Wed Nov 20 2019)
Tonight and tomorrow...forecast confidence is medium/high...
Quiet conditions are expected through the early evening however,
winds will steadily increase from the south as a system approaches
from the west. This system will be forced primarily by an upper
level low but will also be helped by strong warm air advection in the low and
mid levels. The strong warm air advection will be due to a strong southwest low level jet
from 50 to 70 kts. We will also have a very moist column during
this period with precipitable water around an inch, which is
approximately 200% higher than normal for this time of year. Thus
the period from 6z to 18z will be especially wet.
With temperatures well above freezing for this period we will see
all rain with amounts around 0.7 to 0.9 inches expected, though
slightly lower amounts are expected in the far southeast. This
system will be a quick hitter with periods of moderate rainfall at
times but primarily overnight with the system mostly out of the
region by the mid to late morning. Otherwise south winds will
remain breezy through the period. Some light/patchy fog will also
be possible overnight through much of the day tomorrow.
Thursday night through Friday... forecast confidence is high.
Low pressure will depart the Great Lakes Thursday night. Gusty
northwest winds will gradually diminish through the night as
colder air spreads into the region. While dry air will spread into
area, moisture may linger in the low levels, so low clouds may be
slow to clear. This will keep temperatures from tumbling too low.
Lows in the mid 20s are expected.
Stratocumulus clouds will quickly fill in Friday morning if there
is any clearing overnight due to continued cold air advection and
steep low level lapse rates through the morning hours. The
pressure gradient and advection rate will relax for the afternoon
as high pressure noses into the Midwest and southern Wisconsin.
High temperatures on Friday will be in the upper 30s.
Saturday through Sunday... forecast confidence is medium.
The next system of interest stems from a closed upper low over the
southwest U.S. That weakens as it slides into the middle of the
country for the weekend. Precip (rain) associated with this low
pressure system is expected to remain just to the southeast of
Wisconsin, over northeast Illinois and far Southern Lake Michigan.
Our area will see some of the clouds from this system. There is a
better chance for sunshine on Sunday. Saturday and Sunday highs
will be around 40 for southern WI with lighter westerly winds.
Monday through Thursday... forecast confidence is medium.
There is a clipper system that should miss southern WI Monday,
although the 12z ecwmf deterministic model suggests a small chance
for some sprinkles or flurries Monday afternoon and night. The
next low pressure system of interest will track through the center
of the country on Tuesday. Warm frontal precip is expected
somewhere between southern WI and central Illinois. There are
significant differences between the GFS and ecwmf on the strength
of the low, with the GFS showing a much stronger low with a more
northerly solution that would impact Minnesota and WI Tuesday through
Given that the GFS is stronger with the mid week system, it takes
longer for the next system to arrive, thus that has southern WI in
quiet weather on Thanksgiving. On the contrary, the ecwmf is
quicker to bring in the next low pressure system that is laden
with a lot of moisture. This would suggest at least a wintry mix
for southern Wisconsin Thursday afternoon and night. We will
continue to watch this system as it is still a long way out and
there will be a lot of changes to this forecast.
Some MVFR ceilings continue to linger in the region this afternoon
but will slowly push northeast out of the region with VFR
conditions behind. Mid to upper level VFR ceilings will then continue
through the early evening.
By the mid to late evening will see an increase in southerly
winds as well as some low level wind shear around 2 kft from the
southwest at 40-50 kts through much of the taf period. Overnight
we will also see rain push in that will include some MVFR visibilities
with falling ceilings. IFR ceilings are expected for at least a period
overnight into Thursday morning throughout southern Wisconsin.
Winds will slowly increase from the south into this evening as a
system approaches from the west. Increasing winds from the
southeast will increase waves as well especially along the
shoreline thus a Small Craft Advisory is in effect beginning at 3z
this evening. Winds are then likely to approach gales by 10z and
thus a Gale Warning is in effect through 9z Friday. During this
period the low will pass over the region that will switch winds
from the south to the northwest. Winds will weaken as the system
pushes out Friday.
WI...Lakeshore flood advisory from 3 am to 3 PM CST Thursday for
Lm...Gale Warning from 4 am Thursday to 3 am CST Friday for lmz080-
Small Craft Advisory until 6 am CST Friday for lmz643>646.