Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI
FXUS63 KMKX 191529 AAA
Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
1029 AM CDT MON SEP 19 2016
Not much has changed with the expected storms later today.
Moisture/instability will continue to increase into the afternoon
as a shortwave approaches. Plenty of sunshine so far this morning
to heat things up. Latest mesoscale models have storms firing in
the northwest forecast area by around 3 pm, reaching the southeast
5-6 pm. May see trailing convection lingering into the evening
hours, not exiting the southeast until 10 or 11. Given the timing
of the best instability, seems the highest severe threat will be
from the time the storms fire this afternoon through late
afternoon or early evening.
Thunderstorms are expected to develop this afternoon as a
shortwave moves through the forecast area. Some of the storms
could be strong to severe.
Could see some lower visibilities with the storms this afternoon
and evening. Otherwise, VFR conditions are expected through
Tuesday as high pressure quickly builds in behind the trough.
A Small Craft Advisory is in effect until 6 pm for gusty
swly winds ahead of an approaching cold front. Light winds are
expected during and after the frontal passage this evening and into
.PREV DISCUSSION... /Issued 347 AM CDT MON SEP 19 2016/
TODAY AND TONIGHT...Forecast Confidence Is Medium.
Showers and thunderstorms across central Lake Superior and U.P.
of MI. This is on the northern end of narrow instability channel
ahead of cold front, extending from the Arrowhead region of MN
southwest to near FSD. This is in a region of focused 700-850 mb
warm advection and differential CVA, with a strong vort max
rotating through the base of a negatively-tilted short wave
Instability axis shifts southeast into forecast area this
afternoon, with 850 mb thermal ridge nosing into srn WI on
relatively weak 20 to 25 knot southwesterly flow. Shear increases
with veering and strengthening winds above. Above normal high
temperatures in the upper 70s to lower 80s help to mix out the cap
around 850 mb.
Forecast 0-6 km bulk shear values climb to 35-40 knots, as the
cap breaks by mid-late afternoon, with steep mid-level lapse
rates producing 1500-2000 Joules/kg of CAPE. All are supportive of
organized, severe convection.
SPC has included most of the southern half of the CWA in a Slight
Risk...with the rest in a Marginal Risk for severe thunderstorms.
Hail and damaging winds will be the main threat, but 0-1 km shear
of 25 knots brings potential for a tornado, with SPC putting a 5
percent probability in the Slight Risk area.
Showers and thunderstorms will settle southeast with the cold
front late this evening and overnight, with cold air advection on
northwest winds dropping lows into the 50s.
TUESDAY THROUGH WEDNESDAY NIGHT...Forecast Confidence Is Medium.
Models are in decent agreement with high pressure sliding slowly
east of the area Tuesday into Tuesday evening. The influence of
the high should keep quiet weather across the area. Warm
temperatures are expected to continue, with highs in the upper 70s
to around 80 inland. Onshore winds should keep highs near Lake
Michigan cooler Tuesday.
A more active period of weather is then depicted by the models for
later Tuesday night into the end of the week. Lee cyclogenesis
over the central high Plains develops Tuesday night into
Wednesday night, with a quasi-stationary front extending northeast
from the low into far southern Minnesota and central Wisconsin.
The front sags slowly southward into or just south of the area,
depending on the model solution. The upward vertical motion with
the frontogenesis with the front is modest at best.
There is some focused warm air advection later Tuesday night into
Wednesday across the area. The main low level jet nose remains
west of the area later Tuesday night into Wednesday morning, with
the nose trying to lean over into central Wisconsin Wednesday
afternoon. It remains focused to the north and east of there
Wednesday night. In addition, there is some consensus of a 500 mb
vorticity maximum crossing the area Wednesday into Wednesday
All in all, look for on and off chances for showers and
thunderstorms during the late Tuesday night into Wednesday night
period. The best chances seem to be Wednesday into Wednesday night
in northern portions of the area. There is a good amount of mean
layer CAPE during this time, with 0 to 6 km bulk shear values in
the 20 to 30 knot range. SPC marginal severe risk seems reasonable
for Wednesday into Wednesday night.
Rising precipitable water values, and parallel orientation of
surface front to mid level flow, suggests some heavy rainfall
would be possible as well. Warm and humid conditions are expected
Tuesday night into Wednesday night.
THURSDAY THROUGH SUNDAY...Forecast Confidence Is Medium.
ECMWF and GFS are showing the surface front sagging slowly south
of the area Thursday into Friday, with easterly winds developing
over the area and lingering into the upcoming weekend. Given the
close proximity of the surface front, with passing 500 mb
vorticity maxima Thursday, continued PoPs for Thursday into
Mainly tall skinny mean layer CAPE with weak deep layer bulk
shear is depicted by the GFS during this time. This would limit
any severe risk. High precipitable water and other favorable index
values, with the surface boundary parallel to the mid level flow,
suggests heavy rainfall may continue to be an issue into Friday.
Too early to tell at this point about the heavy rainfall
potential this far out. Still something to watch for. Warm and
humid conditions should remain Thursday into Friday.
Continued PoPs into the weekend, though models differ quite a bit
with timing of cold front into the region Sunday and Monday.
Will continue to carry low-level wind shear at KMSN until daytime
warming mixes out low-level inversion around 14Z.
VFR conditions with gusty southwest winds up to 20 knots expected
from mid-morning to mid-afternoon. Showers and thunderstorms
develop later this afternoon and evening, along an approaching
There is an increasing potential for severe thunderstorms, with
hail and damaging wind gusts from mid-late afternoon through the
mid-evening hours. Expecting MVFR cigs and vsbys with the storms,
which will exit the far southeast with the front between 09Z and
Still looking for gusty southwest winds ahead of the cold front,
though possibly not as robust as earlier runs, with model
soundings showing mixing just getting into 23-25 knot winds within
2k ft of the surface. Steep low-level lapse rates will allow
enough downward momentum transfer for gusts of 22 to 25 knots from
mid-morning through mid-afternoon to keep Small Craft Advisory
Winds turn northwest behind the front tonight, and north on
Tuesday, but speeds low enough to keep both winds and waves below
Small Craft Advisory criteria.
LM...SMALL CRAFT ADVISORY until 6 PM CDT this evening for LMZ643>646.
TODAY/TONIGHT AND AVIATION/MARINE...REM
Tuesday THROUGH Sunday...Wood