Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 161803
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
103 PM CDT Wed Aug 16 2017

.UPDATE...

The latest guidance from the convection allowing mesoscale models
still supports showers and thunderstorms arriving in our southwest
late this afternoon into early evening. The forecast thinking is
unchanged from the earlier update, which is included below.

&&

.AVIATION(18Z TAFS)...

Prevailing VFR into late afternoon. The first round of showers and
thunderstorms will push from southwest to northeast this evening
into early tonight. Briefly lower CIGs and VSBYs are possible in
the heavier storms. Additional chances for showers and storms are
expected around or a little after midnight. Ceilings will drop
later tonight, likely lowering to IFR levels across northern and
western portions of the area. Visibilities may lower a bit as
well, but there is uncertainty. A low-level jet at 2 kft should
increase to around 25 knots tonight, bringing us close to low-
level wind shear criteria. Southwest to west winds will increase
late tomorrow into tomorrow night with surface gusts to around 25
knots.

&&

.PREV DISCUSSION... (Issued 1121 AM CDT Wed Aug 16 2017)

UPDATE...

Upper troughing was located across the northern and central Great
Plains this morning, with a 60 knot upper speed max rounding the
trough base. Surface low pressure was located in a favorable
region for development along the Nebraska/Iowa border, with
showers and thunderstorms extending east and southeast of the low
center from central/western Iowa into northern Missouri.

The low pressure system will occlude this evening while tracking
from southern Minnesota towards Lake superior on Thursday. An
associated push of low-level warmth/moisture advection will occur
late this afternoon into this evening, as the TROWAL (trough of
warm air aloft) associated with the occluding system passes
through. This will bring strong theta-E advection with a
frontogenesis response to the mid-levels, with additional mid to
upper support provided via jet-level divergence and increasing DCVA.
The meso models are consistent in bringing a broken line of showers
and thunderstorms northeast through the area with this, but differ a
bit on the timing. Thunderstorm chances will increase from southwest
to northeast from late this afternoon into this evening. Deep layer
moisture will quickly increase as well, with the arrival of strong
moisture transport from a 35 knot low-level jet. We should see
efficient rainfall production given favorable moisture parameters,
with any heavier cells capable of dropping a quick 0.5 to 1.0 inches
in a short period. The mean flow should be strong enough to keep the
cells moving along however, so any water issues would be fairly
localized.

The SPC maintains a marginal risk for severe thunderstorms across
our western areas for this afternoon and evening, and this seems
reasonable. CAPE will be rather tall and skinny given the very
moist environment, but low-level shear will increase as the low
deepens to our north. At this time, it appears that any severe
risk would be isolated.

MARINE...

Small craft conditions are anticipated north of Port Washington
later tonight into Thursday afternoon. Small craft conditions are
likely once again for Thursday night into Friday, when gusts of 25
to 30 knots are possible.

BEACHES...

Highs waves are expected north of Port Washington on Thursday so a
Beach Hazards Statement may be needed.

PREV DISCUSSION... (Issued 613 AM CDT Wed Aug 16 2017)

UPDATE...

AVIATION(12Z TAFS)...

There may be light fog in low lying areas at times into the middle
morning hours. Any ceilings should remain VFR category. An area
of showers or a thunderstorm should affect southwestern portions
of the area into the morning. Not sure how far northeast it will
reach, so left mention out of Madison TAF for now.

Light winds will become east to southeast today. Winds will
gradually veer to the south tonight. Low level jet stream at 2000
feet should increase to around 25 knots overnight, but will any
mention of low level wind shear out of TAFs for now.

First round of showers and storms will move northeast into the
area later this afternoon into this evening, affecting mainly the
southwest half of the area. Second round will then move northeast
through the area later this evening and overnight. Visibilities
down to near alternate minimums and MVFR category ceilings are
possible in any storms, along with some heavy rainfall.

Wood

PREV DISCUSSION... (Issued 336 AM CDT Wed Aug 16 2017)

SHORT TERM...

TODAY AND TONIGHT...Forecast Confidence Is Medium.

A few showers may clip northwestern portions of the area into this
morning, with some focused 700 mb warm air advection. In addition,
there are some showers forming to the southwest, in an area with
some 850 mb moisture pooling and convergence. This activity may
push northeast into the southwest counties over the next several
hours. Could get a rumble of thunder as well. May need to adjust
PoPs early this morning to account for this activity.

Once this activity moves out or dissipates, a good portion of the
day should be dry. There will be some sunshine in the southeast
counties, given upstream partial cloudiness at best. There may be
more clouds in the south central counties. Broken diurnal cumulus
clouds should develop as well by afternoon. Clouds will gradually
thicken by middle to late afternoon. A warm and humid day is
expected, with highs in the upper 70s to around 80 expected.
Cooler values will be found with onshore winds near the lake.

Mesoscale models show first wave of showers and thunderstorms
moving northeast into the southwest half of the area later this
afternoon into early this evening. This is driven by more focused
warm air advection and 500 mb differential cyclonic vorticity
advection.

Second wave of showers and thunderstorms then moves northeast
through the area in the late evening and overnight, with continued
focused warm air advection and differential cyclonic vorticity
advection.

The low level jet will continue to feed very moist air into the
area, with precipitable water values around 2 inches. Deep warm
cloud depth, with tall skinny mean layer CAPE, and modest deep
layer bulk shear, suggest mainly heavy rainfall being the main
hazard with this convection overnight. Continued high PoPs for
later this evening into the overnight hours.

LONG TERM...

Thursday through Friday... Forecast confidence is medium.

The upper trough will cross WI on Thu with the center of the
surface low tracking through northern WI. Steep low level lapse
rates north of I-94 and closer to the low should have higher
afternoon storm coverage than areas toward the IL border. Not
expecting strong storms. Showers will diminish during the evening
as the low moves off to the east.

The upper low and corresponding surface low are expected to deepen
as they track across the Upper Great Lakes Thu night. This will
lead to a strengthening pressure gradient over southern WI and
thus increasing winds. Persistent southerly winds will lead to
high waves north of Port Washington on Thursday morning so a
Beach Hazards Statement may be needed. Winds will turn to the
southwest Thu afternoon and west Thu evening. Gusty west winds are
expected through Friday afternoon.

Friday night through Sunday... Forecast confidence is medium.

Another shortwave trough will track through southern WI Friday
night into Saturday morning and bring a chance for scattered
showers and storms. Moisture is questionable with this system.

Saturday precip chances are trending lower, so do not expect an
all-day rain. Sunday is looking dry, warm and humid with highs in
the lower 80s.

Monday through Wednesday... Forecast confidence is medium.

A front will slowly sag southeast through WI Monday through
Tuesday. This does not bode well for clear skies for a great view
of the eclipse on Monday. However, the front should be located
toward northern WI so as long as there are not excessive storms
that would have a large cloud extent, there is hope for a decent
view. Stay tuned.

Conditions will remain favorable for scattered storms through
Tuesday evening until that front gets south of the IL border.
There could be some severe storms ahead of this front.

AVIATION(09Z TAFS)...

There may be light fog in low lying areas at times into the early
morning hours. Any ceilings should remain VFR category. A few
showers or a thunderstorm may clip western portions of the area
into early this morning.

Light winds will become east to southeast today. Winds will
gradually veer to the south tonight. Low level jet stream at 2000
feet should increase to around 25 knots overnight, but will any
mention of low level wind shear out of TAFs for now.

First round of showers and storms will move northeast into the
area later this afternoon into this evening, affecting mainly the
southwest half of the area. Second round will then move northeast
through the area later this evening and overnight. Visibilities
down to near alternate minimums and MVFR category ceilings are
possible in any storms, along with some heavy rainfall.

MARINE...

Small Craft Advisory is in effect from 09Z to 21Z Thursday for
areas north of Port Washington. Waves in this area will reach the
3 to 5 foot range, with increasing south winds later tonight into
Thursday. Some gusts to around 22 knots are possible at times
Thursday. The rest of the waters should see 2 to 4 foot waves and
gusts to around 20 knots Thursday.

Winds will veer to the west northwest Thursday night and Friday,
and should be rather gusty at times. Frequent gusts to 25 knots
are possible during this period. Any high waves will remain over
the open waters. A Small Craft Advisory may be needed for Thursday
night into Friday.

BEACHES...Persistent southerly winds will lead to high waves
north of Port Washington on Thursday so a Beach Hazards Statement
may be needed.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...Small Craft Advisory from 4 AM to 4 PM CDT Thursday for LMZ643.

&&

$$

Update...SPM
Today/Tonight and Aviation/Marine...Wood
Thursday THROUGH Tuesday...Cronce



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