Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 210840

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
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 this afternoon through Saturday morning
     for WIZ056-057-062>064-067>072.



Today/Tonight and Aviation/Marine...MBK
Saturday through Thursday...Cronce is the U.S. government's official web portal to all federal, state and local government web resources and services.