Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 181527 AAA
AFDMKX

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
1027 AM CDT SUN SEP 18 2016

.UPDATE...

Mostly sunny skies and highs a few degrees above normal are
expected today. No significant updates to the forecast are
anticipated through the afternoon.

&&

.AVIATION/18Z TAFS/...

VFR conditions will continue through at least the evening. The NAM
is hinting at some lower clouds in the west later tonight into
Monday morning as a weak trough approaches, but leaning toward
keeping the forecast VFR through Monday.

There will be a chance for showers/storms late tonight and Monday
as the wave moves through.

A period of low level wind shear tonight still looks possible per
the 12Z NAM.

&&

.MARINE...

Light wly winds this morning will give way to a lake breeze and
ssely winds by late morning or early afternoon. Swly winds will then
prevail tonight and Monday. Winds gust will approach small craft
advisory conditions from late Monday morning through the afternoon.
A Small Craft Advisory may be needed. A cold front will pass Mon
night with light winds on Tue as high pressure moves across the
region.

&&

.PREV DISCUSSION... /Issued 340 AM CDT SUN SEP 18 2016/

TODAY...Forecast Confidence Is High.

Quiet weather today into this evening with short wave ridging
behind a weak short wave trough exiting the forecast area this
morning. Northern flank of surface high pressure slides across the
region this morning, shifting east through the day. As the high
shifts east, winds back to the southwest and then south ahead of
approaching northern stream wave and surface low crossing southern
Canada. While not quite as gusty as Saturday, could see some 20
MPH gusts over portions of South Central WI this afternoon.

925 mb warm advection jumps temperatures at that level almost 5C
between 12Z and 00Z Monday. This should allow some 80 degree highs
over the western CWA, with upper 70s elsewhere. Weaker pressure
gradient over the east will allow a lake breeze to develop, and
push inland a few miles, holding highs lakeside in the mid 70s.

TONIGHT...Forecast Confidence Is Medium.

Models overall agree in keeping stream of short wave energy well
north of the forecast area, close to the upper jet pushing into
far northern WI in zonal flow overnight. 00Z NAM is most vigorous
in generating stronger temperature and moisture advection ahead of
sharper 925 mb and 850 mb troughs that dig farther south into the
state. This leads to higher QPF amounts ahead of surface trough
trailing from the southern Canadian low.

GFS not as robust with the forcing or moisture return through 12Z,
with only light QPF holding off to the the west of the CWA, and
ECMWF keeps QPF north of the forecast area closer to the better
dynamics. Will go with lower consensus PoPs that limit slight
chance/low chance PoPs to the far northwestern CWA. Warm advection
and increasing clouds will hold lows in the upper 50s to lower
60s.

MONDAY THROUGH TUESDAY NIGHT...Forecast Confidence Is Medium.

Models are indicating a warm and humid airmass will be in place
across the area on Monday into Monday evening, as southwest winds
strengthen. Highs should reach into the lower to mid 80s across
most of the area. A weak cold front is forecast to slide east and
then southeast across the area Monday into Monday evening. The low
level frontogenesis response with the front is rather weak. There
is some focused warm air advection on Monday as well, though the
passing 500 mb vorticity maximum remains well north of the area.
Think any convection will be scattered in areal coverage at best.

Forecast soundings and ECMWF mean layer CAPE values get into the
1000 to 1600 J/kg range in general, with 0 to 6 km bulk shear
values in the 40 to 50 knot range. This would indicate the
potential for strong to severe storms Monday into Monday evening.
There is somewhat of a cap on the forecast soundings in some
areas.

SPC has about the southeast half of the area in a marginal severe
risk for Monday into Monday evening, meaning an isolated severe
storm is possible. Given the expected mean layer CAPE and shear
values, will have to watch later model runs for a potentially
greater severe risk than SPC currently has for our area.

High pressure then shifts east into the area later Monday night
into Tuesday, before shifting east Tuesday night. The high should
bring quiet weather for most of this period, with continued warm
temperatures.

Models are showing focused warm air advection and low level
frontogenesis helping develop an area of showers and thunderstorms
over the Minnesota/Iowa border area later Tuesday night. Models
have some of the QPF with this sneaking into the western counties
by 12Z Wednesday. Kept low end POPs in that area for later Tuesday
night for now.

WEDNESDAY THROUGH SATURDAY...Forecast Confidence Is Medium.

ECMWF and GFS generally show low pressure developing in the
central high Plains Wednesday into Thursday night, before shifting
northward into the western Dakotas Friday and Saturday. This is in
response to a deep 500 mb trough shifting northeast across the
central Rockies.

The warm front that extends from the low in the central high
Plains northeast into north central Wisconsin Wednesday becomes
stationary and then shifts southeast as a cold front Thursday into
Friday. There is some timing and placement differences with the
front during this period.

Thus, warm sector airmass Wednesday into Thursday would be
replaced with east to northeast winds and cooler temperatures for
Friday into Saturday. Chances for showers and thunderstorms would
persist each day as well, with the front nearby and some mean
layer CAPE lingering.

Kept PoPs for showers and thunderstorms going for most of the
Wednesday through Saturday period. Temperatures should remain warm
at least into Thursday. Onshore winds Friday into Saturday may
bring some cooler temperatures, though did not lower them just
yet in forecast, until better model agreement can be reached.

AVIATION/12Z TAFS/...

Any patchy IFR/MVFR fog will dissipate by mid-morning, with VFR
conditions through the remainder of the forecast. Could see some
gusty southwest winds in the western CWA this afternoon, with a
tightening pressure gradient ahead of an approaching trough of low
pressure.

Clouds will be on the increase tonight, with a chance for showers
and a few thunderstorms along and ahead of the trough, reaching
the far western forecast area west of Madison by 12Z Monday. Later
shifts may have to add low-level wind shear into the KMSN TAF, as
southwest winds rise to around 38 knots at/near 1500 feet.
However, clouds and mixing should keep surface winds up enough to
keep the speed differential with height below criteria.

MARINE...

Expect west winds to back southwest late in the morning, then
south and increasing to 10 to 15 knots this afternoon, as high
pressure slides to the east. Southwest winds could approach Small
Craft Advisory levels on Monday, as the pressure gradient tightens
ahead of approaching low pressure, especially in the northern
marine zones. Offshore fetch will keep wave heights below
criteria, with the higher waves toward open water.

&&

.MKX Watches/Warnings/Advisories...
WI...None.
LM...None.
&&

$$

UPDATE...DDV/Gehring
TODAY/TONIGHT AND AVIATION/MARINE...REM
Monday THROUGH Saturday...Wood



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