Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 211720 AAA

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
1220 PM CDT Fri Jul 21 2017

Synoptic setup remains favorable for thunderstorms and heavy rain.
Rain and thunderstorms associated with a 925-850mb front, warm
air advection, and increasing moisture transport continue to move
into southwest WI. Severe potential has decreased somewhat by the
convection lingering upstream. Cirrus shield is keeping
temperatures down and limiting instability. The Cape gradient is
closer to the surface front across central Iowa. We could see most
of the severe storms firing just north of this boundary as the
low level jet and moisture transport increases this evening. They
may brush southern WI if this is the case. The latest HRRR
suggests this and the new SPC outlook has shifted the Enhanced
further south, just clipping our southwest. Cape and shear
increase as the day goes on, including 20 knots of 0-1 shear, so
not out of the woods with severe storms.

Heavy rain is still a concern. Warm air advection precip looks
like it will continue across the west as the low level jet and
moisture transport increase and pivot into the region. The 12Z DVN
sounding shows PWs of 1.9 with a freezing level of 14kft. GFS and
NAM show PW up to 2.5 across southern WI today. Even if the
thunderstorm activity shifts south, the axis of heavy rain is
still over southwest WI and much of southern WI will some heavy
rain at times.


Showers will gradually move across south-central WI this
afternoon. Thunderstorms are expected later this afternoon and
evening with the greatest threat for damaging winds and heavy rain
now mainly west and south of Madison. A period of wind shear is
likely for southwest and south-central WI late this afternoon and
evening. It will be a little weaker across southeast WI but still
a concern. Mainly VFR conditions are expected through tonight,
except for with thunderstorms. Fog is likely again tonight with
weak winds and plenty of moisture in place. MVFR or lower ceilings
will develop over central WI Saturday morning and sag toward
southern WI.


.PREV DISCUSSION... (Issued 942 AM CDT Fri Jul 21 2017)

Showers and thunderstorms pushing into the southwest part of the
state are associated with a strong 925-850mb baroclinic zone. This
boundary is expected to move northward bringing showers and
thunderstorms further into the region. The nose of the low level
jet points into northern IA and southern MN where the core of
this activity is.

Any remaining fog should continue to thin this morning, but it
may remain hazy with a humid air mass in place. Light southeast
winds are expected today. Thunderstorms with strong winds and
heavy rain will move in this evening.

PREV DISCUSSION... (Issued 631 AM CDT Fri Jul 21 2017)

UPDATE...Thunderstorms developing in eastern IA may eventually
reach southwest Wisconsin later this morning as short wave ridging
weakens and low level warm air and moisture advection begins to
push northeast.


Patchy fog should quickly thin through mid-morning. Thunderstorms
over eastern IA wl likely move northeast into parts of southwest
WI later this morning.

PREV DISCUSSION... (Issued 340 AM CDT Fri Jul 21 2017)


TODAY AND TONIGHT - Confidence...Medium to High.

Cirrus shield from IA/IL convection over southern WI preventing fog
from becoming more widespread and dense.  GOES-16 night time
microphysics enhancement showing slight warming and thinning of the
cirrus shield in northenr IA, which makes sense as convection wanes
as Mesoscale convective vortex in vicinity of KDVN continues moving
southeast. Removed pops from early this morning in the southwest.
Due to cirrus shield thinning, will continue to mention patchy
fog due to light winds and high low level humidity.

Quasi-station boundary draped across southern IA and central IL will
begin moving northward today in response to upstream short wave trof
over eastern MT moving eastward through the northern Plains. Deep
moisture with precipitable water values approaching 2 inches is tied
to the warm front.  Increasing warm air and moisture advection into
southwest and central WI wl likely trigger scattered thunderstorms
this afternoon.  Influx of low 70 dewpoints will aid in MUCape
values increasing to 2-4000 j while bulk shear increases to 20-
40kts. Thinking a few severe storms may affect northwest CWA this
afternoon.  Can not rule out an isolated tornado with approaching
boundary and low level shear increasing to 20 kts.  SPC Storm scale
Ensemble of Opportunity shows higher updraft helicity and speeds
remaining just west of the area.

Strengthening low level jet will result in rapid convection
initiation upsteam over southeast MN/western WI during the evening.
This will transition to wind and hail event which will likely track
along the warm front which by that time should be draped across
southern Wisconsin.  Low level jet pivots east and south carrying
strong convection across southern WI overnight.  Axis of heavy
rainfall has shifted slightly northward more into southern WI so wl
expand Flash Flood Watch farther northeast to include Marquette,
Dodge, Waukesha and Milwaukee counties.  Many areas could receive
overnight rainfall exceeding 2 inches. 3 hour flash flood guidance
in this area is less than 2 inches. Also these areas received heavy
rain in the recent past so antecendent conditions remain moist. For
more discussion on tonight`s rainfall, check out below hydrology


Saturday through Sunday... Forecast confidence is Medium.

We will lose our stronger forcing Saturday morning, but the front
may take a few more hours to sag south of the WI/IL border after
the main round of storms exits. It`s a small chance but there
could still be a few showers and weak storms that fire up along
any lingering outflow boundaries or the front through the
afternoon into the early evening. The rest of Saturday night
will be dry as a deeper layer of dry air tracks over the area.

A more amplified and robust upper trough is expected to roll
across Lake Superior on Sunday. The ECMWF is fastest with this
feature. The associated surface low should move across northern
WI that drags a cold front through southern WI midday Sunday.
There could be a few showers and storms associated with this
Sunday afternoon and early evening, but moisture and instability
are still questionable.

Monday through Thursday... Forecast confidence is medium.

High pressure will keep us dry and relatively cool for the first
part of next week. Cyclonic flow may push some low clouds into
southeast WI Monday morning along a secondary cold front. Thus
highs are only expected in the lower 70s.

Another upper low will roll through Ontario Tue night into
Wednesday and extend a surface frontal boundary into central WI.
This stalled boundary will be the focus for our next chance for
storms Wed through Fri morning as it wavers over the area.

AVIATION(06Z TAFS)...Cirrus shield from IA/IL convection over
southern Wisconsin preventing fog from becoming more widespread and
dense.  GOES-16 nighttime microphysics enhancement showing slight
warming and thinning of the cirrus shield in northern IA.  Expect
thinning to continue through the early morning as convection to the
south wanes.  Hence light fog wl continue to affect southern WI
through the early morning with the fog becoming localized dense,
especially in the northern CWA.  Scattered thunderstorms will
develop over south central WI this afternoon and transition to
heavy rain/wind event overnight.


Light winds will allow patchy fog to develop closer to the shore
early this morning.  The fog should be shallower and quickly
dissipate.  Recent MODIS imagery measured Lake Michigan sea surface
temp has warmed into the upper 60s to lower 70s as recent winds have
remained light resulting in little upwelling.  Mariners main concern
will be a period of thunderstorms tonight.  These thunderstorms may
produce damaging downburst winds and large hail. Most favored time
is 03Z-10Z.


Will continue Flash Flood Watch across southern WI and expand into a
few counties farther east including Marquette, Dodge, Waukesha and
Milwaukee.  3 hour Flash Flood Guidance is less than 2 inches in
these areas.  Precipitable water will increase to around 2 inches
this afternoon and tonight, which is well above the normal for mid-
July.  These values are causing the North American Ensemble Forecast
System standardized anomolies to close in on 3 units tonight. Return
Interval for these elevated PW is getting into the one day every 1-2
year. With strength of low level jet and passing right entrance
region of upper level jet, significant flooding event could occur
tonight somewhere in southern Wisconsin.  Fortunately, Corfidi
vectors remain progressive most of the night, but hint at possible
back-building late, most likely after MCS has passed off to the
east.  Biggest hydrology concern is for those areas in northwest CWA
which received heavy rainfall Wednesday night/Thursday morning
including Sauk, Iowa and Dane counties.  Rock Springs on Baraboo
River already forecast to get close to Major flood.  Will be
watching hydrographs closely in next 24 hours.


WI...Flash Flood Watch from 4 PM CDT this afternoon through Saturday
     morning for WIZ046-056>058-062>072.



Today/Tonight and Aviation/Marine...MBK
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