Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI
FXUS63 KMKX 201519
Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
1019 AM CDT TUE SEP 20 2016
.AVIATION/18Z TAFS/...Areas of dense fog have burned off that had
affected the KMSN TAF earlier. So 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 are slower and weaker
and further north of TAF sites. All in all will trend conditions
to be VFR into Wednesday with SREF only hinting at MVFR
conditions. Will keep fog at bay with increasing cloud cover and
higher wind speeds evolving off the deck.
.MARINE...Light winds and low wave heights expected.
.PREV DISCUSSION... /Issued 329 AM CDT TUE SEP 20 2016/
TODAY...Forecast Confidence Is High.
High pressure centered over the area at sunrise this morning will
slip off to the east by evening. The light winds, clear skies and
wet ground from yesterday`s rain has lead to dense fog across much
of the area, but this is expected to clear out by mid morning. The
rest of the day looks fantastic with highs in the upper 70s to
around 80, and abundant sunshine. Dew points should also dry out
TONIGHT...Forecast Confidence Is Medium.
As the high slips off to the east, another trough of low pressure
and warm front approach from the west. There is a rapid return of
deep warm air advection, in an increasing south to southwest
flow. The moisture return is also impressive. We should see
thunderstorm development upstream across southern MN and northern
Iowa this evening, with the potential of an MCS rolling east late
tonight into Wisconsin.
Even without an organized MCS, the warm, moist air advection
should lead to the development of showers and storms across the
area, mainly after 3 am. There is a marginal risk for severe
thunderstorms over south central Wisconsin for the potential of an
MCS, but there is lots of uncertainty with that scenario.
WEDNESDAY THROUGH THURSDAY NIGHT...Forecast Confidence Is Medium.
Low pressure will remain over the central high Plains during this
period, with a quasi-stationary surface front extending northeast
across the Iowa/Minnesota border and into central Wisconsin.
Synoptic models vary a bit with the exact position of the front.
The low level jet at 850 mb points northeastward into southeast
Minnesota into western Wisconsin on Wednesday into Wednesday
night. It does weaken a bit Wednesday night. In addition, several
500 mb vorticity maxima push eastward through the region Wednesday
into Thursday, with some timing differences among the models. The
warm air/moisture advection also continues across the area
Wednesday into Thursday.
Adjusted area forecast soundings and ECMWF output show a good
amount of mean layer CAPE for Wednesday into Wednesday evening,
with modest at best deep layer bulk shear. Mean layer CAPE and
deep layer bulk shear decreases later Wednesday night into
Thursday night. Precipitable water values are high during the
Wednesday to Thursday night period, with other parameters for
heavy rainfall favorable at times.
Remnants of possible MCS may affect the area Wednesday morning,
though there is still uncertainty with if and where this may
occur. Given where the low level jet nose and surface boundary
set up, it seems that the best shot for repeated rounds of heavy
rainfall will be across west central to central Wisconsin
Wednesday into Wednesday night. This area remains most favorable
for weaker but additional convection Thursday and Thursday night.
Kept likely PoPs for the most part across northwestern and
northern portions of the forecast area Wednesday into Thursday,
with chance PoPs lingering Thursday night. Kept heavy rainfall
wording for Wednesday into Wednesday night in the northwest and
far north, as 1.00 to 1.50 inches of rainfall are forecast, with
higher amounts in the far northwest counties. Warm and humid
conditions are expected Wednesday into Thursday night as well.
FRIDAY THROUGH MONDAY...Forecast Confidence Is Medium.
ECMWF and GFS are trying to show a diminishing trend with QPF for
Friday into Saturday, taking it slowly northward and out of the
area by later on Saturday. Kept PoPs for this period, though they
may need to be reduced if this trend continues in later model
runs. Warm conditions are expected to continue.
There is still disagreement with timing with the next system for
Sunday into Monday. The GFS is quicker with bringing a cold front
east across the region Sunday night, accompanied by a passing 500
mb shortwave trough. The ECMWF is slower and does not bring the
front and associated QPF through the area until Monday. The 500 mb
shortwave also is more amplified to the west of the region.
Given the model differences, will continue to use consensus blend
of PoPs and temperatures for Sunday into Monday.
IFR cigs/vsbys due to fog will clear up by around mid-morning.
Thereafter, the rest of today will be VFR with scattered clouds,
as surface high pressure slips off to the east during the day.
Warm and moist air advection will return quickly tonight, as low
pressure approaches from the west. We expect showers and storms to
first develop over southern Minnesota and northern Iowa this
evening, then slowly push east into the area late tonight, mainly
after 08z Wed, then continue through Wednesday morning. We could
see brief MVFR conditions within any thunderstorm activity,
otherwise it should remain VFR tonight.
Light and variable wind this morning will shift to the southeast
this afternoon, then south to southwest late tonight into
Wednesday. Winds and waves will remain below Small Craft Advisory
levels through the week.
TODAY/TONIGHT AND AVIATION/MARINE...Davis
Wednesday THROUGH Monday...Wood