Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI
FXUS63 KMKX 211451
Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
951 AM CDT WED SEP 21 2016
.UPDATE...No change to Flash Flood Watch. Strongest convection
shifting south into IL feeding off of better surface based
instability. Widespread rain/storms persists with plenty of MU
CAPE noted. Potential for regenerating storms persists today right
into Thursday. Keying on shortwave energy coming out of the
Rockies into the Plains. LLJ will reload and continue to focus
storms along 850 baroclinic ribbon. This already ongoing on
western flank of current MCS. Seeing MLCAPE values increase into
IA and srn MN. As LLJ, shortwave and upper jet divergence help to
retain vertical motion fields, expect the idea of meso progs
lighting up more convection across IA, MN and wrn WI to pan out.
In fact the HRRR and NAMNest show a surge of storms late this
afternoon and early this evening across the northern CWA. Still
some variations in details but best confidence in where focus is
more likely overnight will be outside of our CWA focusing more to
our west and northwest as the night wears on. Seeing some evidence
of storms congealing into another bow echo/line later in the night
into Thursday morning and sweeping across the area in a similar
fashion as to what transpired this morning. But will monitor
afternoon trends as well especially given upstream development in
more unstable air across nrn IA and srn MN.
.AVIATION/18Z TAFS/...Widespread rain/embedded storms persist
across southern WI into at least the early afternoon hours with
slowly exiting MCS. Patchy MVFR cigs noted but by and large
expecting VFR conditions to prevail. Expecting an active period to
persist into Thursday with several bouts of convection. Many of
the models focus the better action north of the TAF sites but some
models showing potential for another MCS to arrive in closer
proximity to TAF sites very late tonight or into Thursday morning.
At this time expecting only the usual potential for MVFR
cigs/vsbys associated with hvy rain/convection and nothing too
.MARINE...Periods of thunderstorms are expected. Any Linear system
could produce gusty winds in the storms and possible wake low winds
behind the line. Northeast winds thursday night and friday could
approach small craft advisory criteria.
.PREV DISCUSSION... /Issued 319 AM CDT WED SEP 21 2016/
TODAY AND TONIGHT...Forecast confidence is medium.
Convection this morning is driven by strong h8-h7 warm and moist air
advection into the Upper Mississippi River Valley coupled with a
weak mid level short wave pushing through the zonal flow at that
level. The ongoing convection and its behavior this morning will
have a big impact on where redevelopment occurs later today and
tonight. This convection will lay out a decent cold pool,
essentially defining the location of the meandering frontal boundary
that will be the focus for repeated thunderstorm development over
the next few days. The mid level flow down to 850mb is southwest
into that cold pool, so the setup for repeated thunderstorm
development is there. The sfc front will likely get shoved south
into Illinois and northeast Iowa this morning and then lift a bit
north once the convection dissipates later this morning. More
activity should then develop along and north of it this afternoon
and evening. Therefore, look for the best chances of showers and
storms over the western and northern portions of the forecast area.
The timing and location of these episodic storms along the edge a
cold pool with persistent southwest flow impinging on it is
difficult to resolve perfectly. But, it`s a scenario that will
result in some heavy rain for some locations. Due to the training
echo potential, have decided to hoist a flash flood watch the
northern and western portions of the forecast area for today through
THURSDAY THROUGH SATURDAY...Forecast Confidence...Medium
Deep moisture will remain overhead through the end of the week.
The highest precipitable water and CAPE values are progged to be
in the north on Thursday as a wave moves through. Highest pops
thus remain in the north.
Precip chances will keep going Friday as southern Wisconsin
remains between upper troughing to the northeast ridging to the
south. It won`t take much forcing to kick off more
The ridge should push in far enough by Saturday to result in a
return of dry weather.
Temps will generally remain above normal Thursday through
Saturday, though close to normal in the northeast half of the
forecast area Friday and Saturday.
SUNDAY THROUGH TUESDAY...Forecast Confidence...Low
Models are in decent agreement in developing a closed upper low
early next week. The problem is the placement and movement of the
low. The GFS brings it overhead on Tuesday, while the ECMWF
lingers it much farther southwest. As a result, the GFS brings the
warm sector and precip through much quicker, while the ECMWF holds
onto the milder air and precip into mid-week. Pretty big
differences in model pops and temps. Generally tried to split the
difference between the solutions for now.
This will be an active period for showers and thunderstorms,
especially around KMSN. A round of convection should be moving
through around 12z this morning, then possibly a lull in the
activity until more redevelopment occurs this afternoon and
continues through the night. Timing of trends in a persistent
pattern that supports repeated thunderstorm development is very
difficult. In general, look for VFR conditions, but this
thunderstorm activity will bring periods of MVFR to IFR conditions
due to restricted VSBYS and briefly lower CIGS. Winds will remain
light, less than 10kts, outside of the convection.
MARINE...Winds and waves will remain below small craft advisory
levels through the week.
WI...FLASH FLOOD WATCH through Thursday evening for WIZ046-047-051-
TODAY/TONIGHT AND AVIATION/MARINE...Davis
Thursday THROUGH Tuesday...DDV