Area Forecast Discussion
Issued by NWS Green Bay, WI

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49

847
FXUS63 KGRB 201949
AFDGRB

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

Forecast discussion for routine afternoon forecast 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 18Z TAF Issuance
Issued at 1202 PM CDT Wed Jul 20 2016

Another low confidence forecast.  As a tropical airmass moves into
the state, isolated storms will be possible through the taf period.
Awfully tough to estimate timing and coverage of any storms, but
appears that higher coverage of storms will be possible over far
northern WI on Thursday morning.  Probability of thunderstorms is
too low to include in the tafs.

The other issue is the potential for low clouds and fog late
tonight.  The highest threat will be over the northwoods.  Winds
should remain between 5 and 10 kts tonight, which should limit fog
potential, and increase low stratus potential.  Widespread low
stratus exists over northern Minnesota, so included a tempo for RHI
for late tonight.  If stratus does form, indications from upstream
observations suggest cigs will be slow to improve on Thursday
morning.
&&

.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.......MPC



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.