Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 211144
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
644 AM CDT Wed Jun 21 2017

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Thursday
Issued at 347 AM CDT Wed Jun 21 2017

Patchy ground fog should mix out quickly shortly
after sunrise. Sunshine will fade behind increasing middle and
high clouds today as warm advection begins ahead of a warm front.
Dry air currently in place should be able to hold off any rain
until early evening. Highs today will be close to normal.

Showers and thunderstorms are likely tonight as a warm front moves
slowly north from southern Wisconsin. The storms will be elevated
above a stable layer near the ground, but a few storms could produce
some strong wind gusts, especially across central Wisconsin. Wet bulb
zero heights are rather high, so do not expect significant hail.
Heavy rains are possible across the southern half of the forecast
area, which could produce rises in the Wolf River, which is
already just above flood stage in some places.

There could be a break in the action for awhile Thursday, as a
wave along the warm front moves east into Michigan. Additional
thunderstorms are likely Thursday afternoon as the warm front
makes some progress north across the region while a cold front
approaches from the west. Instability and wind shear looks to
support some severe storms.

.LONG TERM...Thursday Night Through Tuesday
Issued at 347 AM CDT Wed Jun 21 2017

Models are fairly consistent in showing the mean flow to consist
of an initial upper ridge from the eastern Pacific to the
southwest CONUS, another upper ridge from FL into the western
Atlantic and a broad upper trough that will stretch from the
northern Rockies to the eastern Great Lakes. By early next week,
the Pacific part of the ridge is forecast to be replaced by a
modest upper trough, while the upper ridge re-orientates itself
over the western CONUS. Meanwhile, the upper trough will be
focused over east-central North America. A prevailing west-
northwest flow over WI will continue to send systems through the
region at regular intervals, thus more unsettled/below normal
temperature conditions can be expected.

Showers/thunderstorms appear at least likely across central/east-
central WI during Thursday evening as the cold front slowly pushes
across the region. The front will be slowed by both a surface wave
moving along the boundary and the front becoming increasingly
parallel to the mean flow aloft. Plenty of lift will be available
between the cold front and the right entrance region of the upper
jet. Plenty of moisture will be available with dew points in the
50s north/60s south and PW values pushing 2.0" over central/east-
central WI. Lastly, even though instability is expected to weaken
during the evening, 0-3km bulk shear is progged to range from 35
knots (north-central WI) to 50 knots (east-central WI), thus a few
storms could become strong to severe. Will also have to watch for
flooding potential (both flash and river) as the ground is
saturated from previous convection. All of this activity will
gradually end from northwest to southeast as the front sags
farther south later Thursday night. Min temperatures to range from
the upper 40s to lower 50s north-central, to around 60 degrees
east-central WI.

After a dry start to Friday, models indicate a weak shortwave
trough embedded within the broad upper trough would move into WI
Friday afternoon. This little feature, combined with daytime
heating, could spark a few showers/possible rogue thunderstorm
over northeast WI. Otherwise, look for more clouds than sun on
Friday with max temperatures in the upper 60s to around 70 degrees
north-central, middle to upper 70s eastern WI.

The departure of the shortwave trough and loss of daytime heating
will end any further precipitation threat by early Friday evening.
The rest of Friday night should be quiet under partly cloudy skies
as the region to reside between systems. Look for min temperatures
to be in the upper 40s to around 50 degrees north, lower to middle
50s south. The next shortwave trough is forecast to sweep into the
western Great Lakes region on Saturday and bring yet another
chance for showers/thunderstorms to northeast WI. Much like
previous systems, this event will be hit and miss with not
everyone seeing precipitation. However, those caught under any
thunderstorm could see a brief downpour. Max temperatures on
Saturday to only reach the middle 60s north-central, upper 60s to
lower 70s elsewhere.

The process basically repeats itself for the latter half of the
weekend with any showers or thunderstorms ending Saturday evening,
then redeveloping on Sunday as another weak shortwave trough moves
through the cyclonic flow within the broad upper trough aloft.
Once again, the precipitation will be spotty with dry hours in
between. Temperatures will remain below normal Sunday with middle
60s north-central, upper 60s to around 70 degrees elsewhere.

By the start of the new work week, the mean flow to consist of a
western CONUS upper ridge and an eastern CONUS upper trough. Even
with the cyclonic flow over WI gradually weakening, still cannot
rule out additional showers/stray thunderstorms on Monday. Max
temperatures are expected to edge upward a bit on Monday as the
coolest air shifts east. Look for readings to be around 70 degrees
north/lakeshore, 70-75 degree range elsewhere.

Forecast becomes more uncertain by next Monday night/Tuesday as
the GFS swings a cold front through WI, while the old ECMWF showed
high pressure in control of our weather through Tuesday. Have
followed the model consensus solution which only brings a slight
chance of showers Monday night, but keeps Tuesday essentially dry.
Temperatures should continue to slowly moderate with readings
mainly in the lower to middle 70s on Tuesday.
&&

.AVIATION...for 12Z TAF Issuance
Issued at 644 AM CDT Wed Jun 21 2017

good flying weather through the afternoon or early
evening hours, then showers and thunderstorms will arrive from
southwest to northeast during the evening hours and continue
overnight. There may be a break in the activity for awhile
Thursdaymorning, but more showers and thunderstorms are expected
Thursday afternoon and evening.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM.....RDM
LONG TERM......Kallas
AVIATION.......RDM


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