Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 250734
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
234 AM CDT Tue Jul 25 2017

.SHORT TERM...

Today and Tonight...Forecast Confidence is Medium...

The weak northwest upper flow increases and becomes more zonal
tonight. Upper level divergence and 700 mb upward motion
increases, especially late tonight. The upstream MCS will likely
weaken and dissipate for the most part before reaching southern
Wisconsin. However some light showers/sprinkles may make it down
through the low level dry air late morning/early afternoon, mainly
northwest areas. Expect some mid/high clouds from this.

A shortwave will move from north of the Dakotas to just north of
lake Superior. An associated 125 knot jet max will move across the
U.S. canadian border, reaching lake Superior late tonight.

The 850 mb winds increase late tonight, but become more west.
There will be a slight chance of showers or a thunderstorm in the
increasing 850 mb moisture convergence.

The receding surface high will be centered over the Great Lakes
today, with a developing south flow. However winds will be
southeast off Lake Michigan with a lake breeze.

Expect Wisconsin river valley fog early this morning, along with
more patchy fog in the other river valleys.

.LONG TERM...

Wednesday and Wednesday Night...Forecast confidence is medium.

A closed upper cyclone will progress across northern Ontario, as a
shortwave trough rides the mean flow east-southeast from central
Minnesota into northern and central Wisconsin. A cold front will
slide southeast across Wisconsin during the period, with showers and
thunderstorms most likely from Wednesday afternoon and Wednesday
night. The primary forecast concerns for this period are the risk
for both severe thunderstorms and heavy rainfall.

As mentioned in previous forecast discussions, several parameters
favor high precipitation efficiency Wednesday evening, including
forward propagating corfidi vectors around 15 knots, deep warm cloud
depths around 4.5 km, and precipitable water (PW) values around
1.75 to 2 inches. From a climatology perspective, these PW values
are certainly high (90th to 95th percentile per NAEFS), but the
return period for such values is not extreme (around 2 years for
1.75 inches). Regardless, there will be plenty of deep moisture
around Wednesday evening.

So let`s discuss the heavy rain threat first, taking a look at
what we know, and what we don`t yet know. We know that showers
and thunderstorms will occur ahead of the cold front from roughly
late Wednesday afternoon into Wednesday night. The previously
referenced moisture parameters suggest that these storms are
capable of producing heavy rainfall. We also know that the
steering flow remains progressive, which minimizes the potential
for training cells (storms repeatedly affecting the same areas),
and suggests that heavy rainfall may remain more localized and
confined to the heavier storms. It appears that rain amounts
averaging 1 to 3 inches are possible where these heavier storms
track, with lesser amounts elsewhere.

Now we arrive at the part of the forecast which lacks clarity: where
these localized areas of heavy rainfall will set up. A perusal
through the model data tonight reveals that the 25.00z
deterministic GFS/NAM guidance have trended back towards a
southerly solution after a waffle north seen on some of the
previous model runs yesterday. This would dive a convective
complex southeast from central Iowa into northern Illinois,
keeping much of the heavy rain threat south of the state line. The
24.12z ECMWF/GGEM solutions remain farther to the north, bringing
the heavier rainfall producing storms over southern Wisconsin.
There appears to be a southward shift indicated by the arriving
25.00z ECMWF data, perhaps suggesting a consensus more towards
the southern (less rainfall for our area) solution. Time will
tell. The bottom line is that heavy rain remains possible in spots
Wednesday night, but we`re not yet able to accurately pinpoint
where those spots will be just yet. Keep up with the forecast.

Severe thunderstorms also remain possible, with the most probable
time from late afternoon into evening. The shear still appears
sufficiently supportive with around 30 to 35 knots in the 0 to 6
km layer, and a veering wind profile. Instability appears modest
with around 1000-2000 J/kg of mixed layer CAPE building by
late afternoon. Shear in the lowest kilometer still appears weak,
so damaging wind and large hail are the primary risks. There
remains some question regarding how widespread the severe risk
will be.

There are now indications we could see an additional shower or
storm Thursday afternoon as the shortwave trough/vort max drops
through. Didn`t include any PoPs during this time just yet, but
this may be necessary in the future if there is a stronger signal
among the model guidance.

Friday through Tuesday...Forecast confidence is medium.

Surface high pressure will migrate from the Upper Midwest into the
lower Great Lakes. This should bring quiet weather, with low
probabilities for any showers or storms. Temperatures will be
seasonal with lower humidity.

&&

.AVIATION(06Z TAFS)...

VFR conditions are expected overnight into Tuesday night across
the area. High pressure will slowly move to the east of the area
during this time.

Light and variable winds are expected into early Tuesday morning.
Light south winds are then expected Tuesday, becoming southeast
near the lake in the late morning and afternoon. They will become
south Tuesday night.

Some high clouds may move through the area overnight into
Tuesday. Scattered diurnal cumulus clouds should develop by later
Tuesday morning and afternoon. Middle to high clouds should
become more extensive Tuesday night.

&&

.MARINE...

The receding surface high will be centered over the Great Lakes
today with a light southeast flow off Lake Michigan with a lake
breeze.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...None.
&&

$$

Today/Tonight and Aviation/Marine...Hentz
Wednesday through Monday...SPM


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