Area Forecast Discussion
Issued by NWS Green Bay, WI

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338
FXUS63 KGRB 202317
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
617 PM CDT WED JUL 20 2016

Updated aviation portion for 00Z TAF issuance

.SHORT TERM...Tonight and Thursday
Issued at 248 PM CDT Wed Jul 20 2016

The latest Rap analysis and satellite/radar imagery show a strong
mid-level sub-tropical ridge building north into the north-central
US early this afternoon.  850-700mb thetae advection in combination
with ill-defined shortwaves are producing isolated or small clusters
of showers and storms over western and central WI.  This threat will
continue through the rest of the afternoon, given the weak forcing
but ample instability in the 1500-3000 j/kg range.  If storms
continue to fire, a storm or two could become severe.  Looking
upstream, no significant shortwaves to note. A cold front is
positioned over the western Dakotas into south-central Canada.
Forecast confidence remains low given the lack of large-scale
features and recent poor performance of the models.  Forecast
concerns revolve around precip trends and high heat indices.

Tonight...Mid-level thetae advection will shift east of the area
during the evening hours, though model guidance continues to show
modest moisture transport persisting for much of the night.  This
mid-level moisture transport appears to be maximized over northern
WI. In addition, models also indicate that a weak impulse will
arrive over western Lake Superior and the western Upper Peninsula
late tonight as well.  With the mid-level ridge flattening somewhat
overnight, any weak shortwave impulse will have potential to
generate an isolated or cluster of storms given the elevated
instability that will be present.  As a result, will show only
slight chances of thunderstorms in the forecast until chances
increase over the north ahead of the weak shortwave that arrives
late.  Potential for severe weather is limited due to weak shear and
forcing.  Heavy rainfall will be the main threat.

Other concerns tonight revolve around low clouds and fog given the
moist airmass in place.  Plenty of low stratus is persisting across
northern Minnesota early this afternoon.  MOS guidance suggests the
possibility of low clouds and fog developing after midnight over
northern WI.  Think stratus is a better bet since will have a 5-10
kt synoptic surface wind.  Have therefore increased sky cover late
tonight over the north.  A muggy night with lows only falling into
the 70s.

Thursday...Potential for convection will carry over across northern
WI during the morning as the weak shortwave moves across the Upper
Peninsula.  If this convection materializes, outflow from the storms
could generate additional storms further south across north-central
and northeast WI during the later morning or afternoon hours.  To
complicate matters further, a more potent cold front will be pushing
into northwest WI very late in the afternoon.  Models are more
enthusiastic about morning convection than afternoon convection for
what its worth, but given the absence of upstream features to latch
onto, think storm chances are higher in the afternoon due to
tremendous instability above 4000 j/kg and 0-6km shear of 30 kts.
These parameters would support severe weather if thunderstorms
develop.  The main threats would be large hail and damaging winds.

The other story will be hot and humid weather, along with high heat
indices.  Afternoon max temps in the mid 90s with dewpoints in the
mid 70s equates to feels like temperatures between 100 and 105 from
Marathon to southern Oconto on south.  Will issue a heat advisory
for these counties.  Some threat of heat indices this warm further
north into Marinette county, but given the possible morning
convection, will let the next shift take a look.

.LONG TERM...Thursday Night Through Wednesday
Issued at 248 PM CDT Wed Jul 20 2016

The upper level high pressure block over the U.S. will flatten
out by mid-weekend. This pattern change will enhance the
frequency of precipitation for northeast Wisconsin through next
week. Conditions will remain hot and humid until the upper level
ridge breaks down and the best chance for severe thunderstorms
for our region will be Thursday night. The next significant
chance for showers and thunderstorms will be on Saturday into
Sunday morning.
&&

.AVIATION...for 00Z TAF Issuance
Issued at 609 PM CDT Wed Jul 20 2016

Scattered showers and thunderstorms early this evening extended
from near Tomahawk to Medford to 20 miles south of Marshfield.
This cluster of storms should slide southeast and exit the area
later this evening. Did add a chance of showers and storms to the
KAUW/KCWA til 02z for this cluster. Otherwise, an isolated storm
is possible over the remainder of the region this evening. Chances
too low to add to the KGRB/KATW/KMTW/KRHI 00z tafs. Attention then
turns to low clouds overnight with mvfr/ifr cigs. Also, latest
HRRR model now suggesting clusters of thunderstorms across North
Dakota early this evening will move into portions of north-central
and central Wisconsin towards daybreak and across northeast
Wisconsin during the morning. Will make last minute coordination
with offices to the west and north. May need to make a mid evening
update if consensus develops if storms will make it into the
region late tonight and Thursday morning. Hot and humid conditions
are expected on Thursday which could lead to another round of
strong or severe storms Thursday afternoon and evening. Stay
tuned!

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Heat Advisory from noon to 8 PM CDT Thursday FOR WIZ020-030-031-
035>039-045-048>050-074.
&&

$$

SHORT TERM.....MPC
LONG TERM......ML
AVIATION.......Eckberg



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