Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI
FXUS63 KMKX 210145
Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
845 PM CDT TUE SEP 20 2016
.UPDATE...A few storms firing to the north along tightening
baroclinic zone and increasing low level frontogenesis. A few of
these may brush far north later this evening. Still expect main
show to occur from 07z-12z tonight as increasing low level jet
pivots eastward into the tri-state area and weakens slightly.
Expect convection to begin firing in the next 1-3 hours upstream
over southern MN where low level moisture convergence increasing.
Pivoting low level jet and strong surge of low level theta-e
advection and moisture should carry the convection into central
and srn wi later tonight. Will continue high chance to categorical
pops across CWA from east to west. Expect initial surge of late
night convection to progress southeast but redevelopment farther
west and slowing of Corfidi Vectors may result in some heavy
rainfall in the west late. Wl need to see what happens with the
first round of convection before considering posting Flash Flood
.AVIATION/06Z TAFS/...Still expect scattered to numerous
thunderstorms to affect southern WI and taf sites later tonight
into Wednesday morning. Cigs expected to briefly drop to around 1k
feet in some spots later tonight.
.MARINE...Mariners will need to keep an eye on scattered
thunderstorms expected to affect the near shore waters later
tonight into Wednesday morning. Gusty winds and small hail may
accompany a stronger storm. Otherwise, mostly southeast winds are
expected the next several days, with variable winds around
.PREV DISCUSSION... /Issued 332 PM CDT TUE SEP 20 2016/
TONIGHT - Confidence...Medium
Influence of surface high to persist into the evening hours. Then
attention will focus to MCS potential later in the night, especially
after 06z. The meso scale models are coming into a little better
agreement on MCS reaching our northwest CWA 06-09z and then into the
southeast CWA roughly 09-12z. The LLJ will be leaning into the area
and there is an appreciable amount of moisture advection progged.
So the idea that this MCS may sustain itself throughout southern
WI, has plausibility. The greatest severe potential appears to be
in our northwest late where HRRR has proggd elevated CAPE values
over 2k j/kg. So the eastward nudging of the Slight Risk outline
into our northwest CWA makes sense.
WEDNESDAY - Confidence...Medium
Initial MCS or remnants of expected to exit the southeast corner of
the CWA early in the morning. Meso progs already lighting up the
area back to the west vcnty Miss Rvr Vly associated with sustained
LLJ impinging on that area. 850 baroclinic ribbon progged to be
situated close enough to CWA to keep some chances going all day.
Seeing indications of upper divergence aiding the vertical motion
field. Primary west/east jet stream is pretty far north though so
not a classic signature. With 925 temps into the lower 20s celsius
any breaks will allow temps to reach the 80s, but cloud cover/precip
trends will certainly have a say on temp trends.
Wednesday night through Friday...Forecast confidence medium.
Scattered tstorms will likely be ongoing over central WI Wed
afternoon and evening due to persistent thetae advection. MLCAPEs
may build to 2000 J/KG by late Wed afternoon in the warm sector
over srn WI. A vorticity maximum will be moving across central WI
at the time with a more organized shortwave trough approaching
from the west for late night. Overall expect convection to grow
upscale to a MCS...possibly severe...and shift sewd into the nrn
CWA. Very high PWs of 1.5-1.75 inches and a relatively slow
moving MCS, will make heavy rainfall the biggest concern. Issued
a Hydrologic Outlook to address this mainly north of Madison and
For Thu and Thu night, the llj will maintain persistent moisture
transport into central WI while a couple vorticity maximums move
through the area. Thus showers and tstorms will remain likely
across the nrn CWA with lesser chances toward the IL border. By
Fri the precipitation coverage will be shrinking as the llj
weakens and the front moves into nrn IL as a cold front.
Relatively warm and humid conditions will continue with some
cooling on Fri or Fri night.
LONG TERM...Saturday through Tuesday...Forecast confidence
The sharp upper trough over the wrn USA will lift newd over the
weekend and will eventually lift newd into Manitoba and Ontario
Canada along with its surface low. Thus high amplitude ridging
will move over the wrn Great Lakes for Sat-Sun with a weak cold
frontal passage forecast anytime from Sun night into Mon night.
Only small chances of showers and tstorms are forecast for the
weekend with better chances with the fropa.
AVIATION/00Z TAFS/...Expecting VFR conditions well
into this evening as lingering surface ridging dominates. Concern
later tonight into Wednesday morning will be MCS potential riding in
here from MN/IA. LLJ leans in here later with a decent slug of 850
moist advection proggd. At the moment the 20.12 HRRR is the most
aggressive solution spreading remnants of MCS all the way through
CWA by 12z Wednesday. Other models have trended towards the HRRR
idea. All in all will trend conditions to be VFR for most of the
period however an MVFR/borderline IFR cig scenario may unfold in the
wake of any departed MCS. Will keep fog at bay with increasing cloud
cover and higher wind speeds evolving off the deck.
MARINE...Winds and waves will remain below small craft advisory
levels through the week.
TONIGHT/Wednesday AND AVIATION/MARINE...Collar
Wednesday NIGHT THROUGH Tuesday...Gehring