Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 141755 AAB
AFDMKX

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
1255 PM CDT Sat Oct 14 2017

.UPDATE...
Large swath of rain has engulfed much of southern WI as of midday.
Getting a few sporadic lightning strikes, particularly closer to
the WI/IL state line. There should be enough MUCAPE to keep the
status quo through at least the mid afternoon hours.

Will continue to tweak timing of PoPs, with a few more rounds of
rainfall expected. Otherwise, focus is squarely on the surface
warm front still draped across northern Illinois. Recent
convective allowing models question just how far north this
boundary will make it. The most recent SPC Day 1 outlook has
shunted the severe risk further to the south/southeast.
Unfortunately, this will be a mesoscale forecast challenge up to
the point of the surface low passage later this evening. Any areas
that are oriented to the southeast of the warm front will have a
greater risk of severe (damaging wind and a few tornadoes). At
this time, based on the most recent CAMs, an updated area of
greatest concern is areas southeast of a line from Milwaukee to
Janesville. Monitoring the warm front will be a key to the severe
risk for this evening.

&&

.AVIATION(18Z TAFS)...
Rather messy aviation situation across the region. Stratus deck
has remain IFR and will continue to do so well into tonight.
Visibility will be a bit more dependent on rainfall, wind
direction and eventually where a surface warm front ends up moving
to. The forecast remains pessimistic with a few rounds of
rainfall, mainly IFR conditions (primarily ceilings) and mainly
MVFR visibility (some bouts of IFR with drizzle or heavier
downpours of rain). The storm system responsible for these
conditions will accelerate to the northeast this evening. This
will result in gradually improving ceiling and visibility
overnight from west to east. However, there will be southwesterly
low level wind shear to contend with. Heading into Sunday, MVFR
ceilings will persist through the morning and winds will shift to
the northwest and become gusty.

&&

.PREV DISCUSSION... (Issued 1043 AM CDT Sat Oct 14 2017)

UPDATE...

Active weather pattern continues to unfold across the region today
through tonight. A dynamic storm system is gearing up with
shortwave energy diving southeast out of the prairie provinces of
Canada. At the surface, a warm front is draped across northern
Illinois. For the rest of this morning and this afternoon, this
boundary will remain the focus of shower/thunderstorm activity.
This will result in on again/off again bouts of rainfall. The risk
for thunder will increase from south to north from mid afternoon
onward as developing low pressure in Iowa gives the warm front a
nudge to the north.

Will need to keep a very close eye on the evolution of the
mesoscale pattern across the region this evening. As has been well
advertised, precipitable water values look to be around 1.75" or
so (close to 300% of normal for mid October) and the ongoing flash
flood watch is well placed. Of increasing concern is much of the
area ending up in the warm sector for a period of 3-6 hours this
evening. This will open the door to severe storms.

While CAPE values are modest (mixed layer values in the 500-800
J/kg range), shear is rather strong. Model consensus increases 0-3km
shear vectors to 40-50kt from the west-southwest, 0-1km bulk shear
increases to ~45kt out of the southwest and 0-1km helicity values
increase to 400 m2/s2. With low LCL heights expected, there is
definite a concern for wind and tornadoes with any bowing line
segment or supercellular structure.

The window of opportunity for severe is a bit small, but
parameters do come together favorably for the evening hours with
the better risk for areas (roughly) along and southeast of a line
from Port Washington to Johnson Creek to Monroe. Will continue to
hone in on preferred storm type, warm front progression and low
pressure location with the incoming data.

MARINE...

East to southeast winds today will gust to around 20 knots, with
waves building to 2 to 4 feet by late afternoon. Most areas will
stay below Small Craft Advisory conditions through the daytime
hours, though still think a few gusts to 25 knots and higher
waves are possible north of Port Washington through this
afternoon. Thus left the advisory going in the northern zone.

Southerly gusts and waves will likely be a little higher this
evening into tonight, so the advisory remains in effect for the
other zones beginning this evening.

Winds will veer west to northwest later tonight into Sunday and
increase behind departing low pressure. Gust to 35 knots are
likely, so opted to upgrade the Gale Watch to a warning.

PREV DISCUSSION... (Issued 634 AM CDT Sat Oct 14 2017)

UPDATE...Pivoting low level jet to the east helping to focus
showers and thunderstorms to the east and south. Several hours of
dry conditions can be expected before the reinvigorating low level
jet takes aim at southern WI later this morning and afternoon. No
change to the location of the Flash Flood Watch.

AVIATION(12Z TAFS)...Moist easterly flow north of warm front
across northern IL should result in varying ceilings from IFR to
MVFR for most of today. Passing showers and storms may temporarily
raise ceilings for a time.

PREV DISCUSSION... (Issued 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 East to southeast winds today will gust to
around 20 knots, with waves building to 2 to 4 feet by late
afternoon. Most areas will stay below Small Craft Advisory
conditions through the daytime hours, though still think a few
gusts to 25 knots and higher waves are possible north of Port
Washington through this afternoon. Thus left the advisory going in
the northern zone.

Southerly gusts and waves will likely be a little higher this
evening into tonight, so the advisory remains in effect for the
other zones beginning this evening.

Winds will veer west to northwest later tonight into Sunday and
increase behind departing low pressure. Gust to 35 knots are
likely, so opted to upgrade the Gale Watch to a warning.
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 through late tonight for WIZ064>072.

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

     Gale Warning from 4 AM to 4 PM CDT Sunday for LMZ643>646.

     Small Craft Advisory until 4 AM CDT Sunday for LMZ643.

&&

$$

Update...Gagan
Today/Tonight and Aviation/Marine...Gagan/DDV
Sunday THROUGH Friday...Wood



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