Special Weather Statement, NWS Milwaukee, WI

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
343 AM CDT Sat Oct 14 2017

.SHORT TERM...

Today And Tonight...Forecast Confidence Is Medium To High.

Weak pulse of warm air advection and low level jet causing
increasing swath of showers and embedded t-storms across northern
IL, which will be moving into southern WI shortly. Expect this
next batch of showers to weaken and move off to the east by 12-14Z
as low level jet pivots eastward.

Low level baroclinic zone continues to strengthen across northern
IL and southern WI, culminating this afternoon and evening as
vigorous upper level short wave over ID/CO area moves rapidly
eastward across the high plains into the central and northern
plains today, and into the western Great Lakes tonight. Ahead of
this vigorous wave, a rapidly intensifying low level jet will
spread across IA into the southern WI vicinity later this morning
and early afternoon. The jet will further strengthen and remain
anchored across eastern IA pointed into southern WI into the
evening.

During this time, column precipitable water values will increase
to 1.6 to 1.9 inches, which is about one half inch higher than the
SPC climatological record for DVN/GRB sounding sites for today.
The North American Ensemble Forecast System is showing PWAT
standardized anomalies of 3-3.5 units with this evening, with a
return interval also outside the 30 year climate record across
northern IL.

Although Corfidi Vectors and MBE velocity remains progressive,
feel there is a risk of repetitive showers and t-storms depositing
heavy rainfall in a short period of time, affecting the same area
of far southern WI this afternoon and evening. Recent rainfall of
one half inch to one inch has wet the soil, and 3 hourly flash
flood guidance remains in the 2 to 3 inch range in non-urban
areas.

However, this will be a very dynamic system with persistent
strong upper level divergence over the border area during the 12
hour period, very close to the low level baroclinic zone and
strong frontogenetical forcing. Precipitation efficiency will be
high for any showers and storms, easily converting any water vapor
condensates into rainfall. Per collaboration with Chicago and
Quad Cities offices, will pull trigger on Flash Flood Watch for
far southern CWA, including most of the Milwaukee Metro area from
later this morning through the evening.

SPC has bumped risk to enhanced in the far southeast, with slight
across rest of the south and eastern CWA. Rapidly deepening low
pressure over the area will create period of strong low to mid
level shear later this afternoon and evening. Elevated cape values
approach 1000 Joules/kg. Can not rule out wind and/or tornado
threat with this system for a few hours due to dynamics.

Sunday Through Monday Night...Forecast Confidence Is Medium.

Models are in good agreement with the strong 500 mb shortwave
trough sliding to the east Sunday morning. Area forecast soundings
are hanging on to some low level saturation during this time, so
kept the chance PoPs for showers going.

Other issue Sunday is the strong pressure gradient over the area
producing gusty northwest winds during the morning hours. Area
forecast soundings suggest that 30 to 35 knot winds are possible
to mix down during the morning hours, with the strongest winds in
the northeastern parts of the area.

Brought the wind gusts upward to reflect this trend Sunday
morning. They should gradually lower during the afternoon from
west to east across the area, as the pressure gradient weakens.
Cool highs in the lower to middle 50s are expected.

Next issue will be frost potential Sunday night into early Monday
morning. Strong cold air advection occurs with the gusty northwest
winds on Sunday, though it wanes Sunday night. Still, winds will
weaken considerably with clearing skies Sunday night. Passing 500
mb shortwave trough may bring some clouds with it, so there is a
bit of uncertainty with how much clouds there will be. Kept skies
more on the clear side for now.

Expected 925 mb temperatures should bring lows into the middle to
upper 30s away from the lake. The coldest readings should be in
the northwestern portions of the area.

Kept patchy to areas of frost in the forecast for later Sunday
night into early Monday morning. A Frost Advisory may be needed
eventually for this period away from the lake.

Warm air advection will develop again Monday into Monday night
across the area, allowing for milder and more seasonable
temperatures to move in. Dry forecast looks reasonable, as air
column is quite dry.

.LONG TERM...

Tuesday Through Friday...Forecast Confidence Is High.

Models have a general zonal flow at 500 mb set up for Tuesday into
Thursday, before more southwesterly flow develops Thursday night
into Friday. At the surface, south to southwest flow in the low
levels should bring above normal temperatures back into the area
by Tuesday, lingering through the week. Temperatures may rise a
little each day during this period, with highs reaching above 70
by the end of the week. Air column is fairly dry during this
period, so dry forecast looks reasonable.

&&

.AVIATION(09Z TAFS)...

Moist easterly flow and occasional showers and a few storms
should result in widespread low ceilings. Ceilings should start to
improve this aftn and evening, as warm front shifts northward.
Low level wind shear may occur for several hours tonight, as low
pressure rapidly deepens across the area.

&&

.MARINE...

East northeast wind gusts across northern zone expected to gust
to as high as 25 knots today, due to tightening pressure gradient
well north of stationary front across northern IL. Front will
begin shifting northward today as a warm front, setting off
occasional showers and a few thunderstorms into tonight. Few
storms this evening could have damaging winds.

Rapidly deepening low pressure will move across southern WI and
Lake Michigan tonight, with increasing winds and gusts ahead of
and especially in its wake. Will post Small Craft Advisory from
Port Washington to Sheboygan beginning this morning, continuing
until late tonight, and start remainder of near shore waters at
00Z tonight, also running until late tonight.

Gale Watch will remain in effect beginning at 09z Sunday, as
tight pressure gradient behind departing low pressure will likely
cause winds to approach or exceed 34 knots for a time. Not enough
confidence at this time to issue Gale Warning, as some short term
guidance showing wind gust potential remaining just below
criteria.

&&

.HYDROLOGY...

See today and tonight period above, within short term section,
for hydrology impacts for today into this evening.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...Flash Flood Watch from 10 AM CDT this morning through late
     tonight for WIZ064>072.

LM...Gale Watch from late tonight through Sunday afternoon for
     LMZ643>646.

     Small Craft Advisory from 7 PM this evening to 4 AM CDT Sunday
     for LMZ644>646.

     Small Craft Advisory from 10 AM this morning to 4 AM CDT Sunday
     for LMZ643.

&&

Today/Tonight and Aviation/Marine...MBK
Sunday through Friday...Wood

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