Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 190331

Area Forecast Discussion
National Weather Service Green Bay WI
1031 PM CDT THU AUG 18 2016

Updated aviation portion for 06Z TAF issuance

Issued at 246 PM CDT Thu Aug 18 2016

Wet the next couple days, then cooler and less humid for the
latter part of the weekend and early next week.

The large scale pattern will undergo amplification the next
several days, as a strengthening trough swings through the
northern Plains and Great Lakes region. Some flattening of the
flow is likely next week, but with the mean trough position
lingering from Hudson Bay to the northern Plains.

Seasonably warm and humid conditions at the start of the period
will give way to cooler weather during the weekend and early next
week as an early autumn-like air mass sweeps into the area. The
cool down will be short-lived with temperatures bouncing back to
a few degrees above normal by the middle of next week. With ample
moisture in place, precipitation as the upper trough and cold
front cross the area the next couple days should result in above
normal amounts for the period. The next opportunity for
substantial precipitation after that will probably arrive during
the middle of next week.

.SHORT TERM...Tonight and Friday
Issued at 246 PM CDT Thu Aug 18 2016

The latest RAP analysis and satellite/radar imagery show low
pressure centered over the central SD/NE border early this
afternoon.  A warm front extends east from the low across southern
Minnesota into northern IL.  North of this warm front, a weakening
MCS is traveling just south of due east over eastern Minnesota.
Several warnings were issued earlier today with this complex, but
only isolated warnings have been issued since then. ML capes of 800-
1200 j/kg extend east across southern Minnesota through southern WI,
so potential for strong storms to develop later this afternoon in
this area. Until this complex becomes closer to the region, more
modest instability may produce isolated shower/thundershower
activity later this afternoon into early this evening. Uncertainty
is rather high with this forecast, and revolves around precip trends
and low severe weather chances.

Tonight...No shortage of complexities in this forecast. Models are
not handling the current convection well, so followed a combo of the
RAP and HRRR, which appear to be handling the situation the best.
The models bring the ongoing MCS into central WI during the evening,
before weakening/breaking it up over east-central WI overnight.  If
there is a severe threat, the best chances would occur during the
evening as the MCS arrives. Bulk shear values are rather marginal,
around 20 to 30 kts depending on the model.  But with mucapes
upwards of 1000 j/kg over central WI this evening, appears that
there will be an isolated threat of damaging winds. Severe
convection lingered into the morning hours today, so not out of the
question that east-central WI could also see an isolated threat, but
it does not look likely at this time.

Convection looks to move out late tonight at the same time as a cold
front moves into north-central WI.  Spotty showers will be possible
along it, but think it will serve to prolong low clouds/patchy fog
through the rest of the night.  Muggy low temps in the mid to upper

Friday...Uncertainty about tonights portion of the forecast creates
fairly low confidence in the significant weather tomorrow.  If the
convection moves across the forecast area as planned, should have a
stable situation during the morning ahead of the front due to cloud
cover and spotty shower activity.  That may change during the
afternoon as the front moves into eastern WI.  If sufficient heating
occurs during the afternoon, then upwards of 2000-2500 mlcape could
generate storms along the front.  0-6km remains marginal, but cannot
rule out an isolated severe storm from developing.  Shower activity
may also become more widespread behind the front over central and
north-central WI as mid-level fgen increases.  Highs ranging from
the mid 70s northwest to the mid 80s southeast.

.LONG TERM...Friday Night Through Thursday
Issued at 246 PM CDT Thu Aug 18 2016

Though the evolution of the large scale pattern is pretty straight
forward, small scale details that will have significant impacts on
the forecast remain elusive. The timing of the southward push of
the frontal boundary across the area remains in doubt. We could
start the long-term period with it already to our south, or it
could slide through during the night. The NAM and Canadian don`t
bring the front through until after the wave lifts through the
region, but those models seem overdeveloped with the cyclone and
were discounted. There may be a lull in the precipitation,
especially early Friday night, but expect at least scattered
showers across the area. The most significant rain is likely to
occur Saturday as the may wave rides northeast up the front. Took
PoPs up to categorical for Saturday.

The system is likely to produce a widespread soaking rainfall.
QPFs have edged down since yesterday, and as long as totals
remain under 2 inches the area rivers should be able to handle the
water. Of course if any locations get heavy rainfall tonight or
Friday, the potential for flooding with the added rain from the
Saturday system will be increased. Will attempt to detail the
situation in the HWO.

Somewhat blustery and cooler weather is expected by Sunday as the
wave departs off to the northeast. There was some indication of
lingering light precipitation, however, so kept low PoPs in
throughout the day.

.AVIATION...for 06Z TAF Issuance
Issued at 1031 PM CDT Thu Aug 18 2016

Convection this evening tracked from southern Minnesota to
northwest Illinois along and near the cap edge and best
instability. Isolated showers were also passing over Upper
Michigan and far northern Wisconsin near a weak mid level short
wave tracking east to northeast. Weak high pressure drifted over
the forecast area this evening to limit convection even though MU
capes were around 1800 j/kg as per spc analysis and GRB evening
raob. Anticipate convection to be on the increase overnight with
the waa pattern increasing, but confidence on the coverage is
low. Convection which passed over southwest Wisconsin likely
intercepted the best instability. Convection was developing again
over southeast Minnesota late this evening but not showing much
push east. Fog may be an issue overnight but mid to high level
clouds were filtering across. Convection timing for Friday
continues to be an issue as well due the location of a cold front
dropping over the state. Latest progs showing some agreement with
lifting a surface wave over the state Friday morning between 12z
and 18z or so. This low is currently initialized over southeast
South dakota near the stronger convection. Will adjust pcpn timing
in the tafs for this feature.



LONG TERM......Skowronski
AVIATION.......TDH is the U.S. government's official web portal to all federal, state and local government web resources and services.