Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 211933

Area Forecast Discussion
National Weather Service Green Bay WI
233 PM CDT Fri Jul 21 2017

Forecast discussion for routine afternoon forecast issuance

.SHORT TERM...Tonight and Saturday
Issued at 228 PM CDT Fri Jul 21 2017

Forecast issues center on precipitation trends for the rest of
the afternoon into tonight.

Area radars this afternoon show convection mainly with the 850 mb
warm front over southern Minnesota into northeast Iowa and
southwest Wisconsin. Initially this convection was running into
an upper ridge drifting east of the area this afternoon and
evening. Surface warm front with very unstable surface based capes
from central Iowa into northern Illinois. Convection passing north
of this warm front was also producing an outflow boundary over
southwest Wisconsin and may hold up the warm front. So convection
will likely continue to bubble up in this area this afternoon and
evening for a focus of heavy rain.

Short range model trends are trending with heavy rain staying
more south and southwest of the area for the rest of this
afternoon and early evening into the southwest half of the state
in a region which has received several rounds of heavy rain this
week. Pwats of around 2.00 inches are progged to work into parts
of central and east central wisconsin this evening as an 850 mb
short wave trough tracks across the state. GFS and NAM attempt to
develop convection around MSP early this afternoon and track it
into the state this evening producing 1 to 2 inches south of a
line from AUW to GRB. This has not verified so far, but needs to
be watched as mid level lapse rates were on the increase over the
north half of the state and deeper layer shear was just to the
northwest, where convection was becoming more active over northern

Anticipate some break in the convection Saturday morning as the
overnight system departs to the east leaving plenty of stratiform
clouds across the area. Will then increase pops in the afternoon
from the north as the mid level trough drops into the northern
Great Lakes region. Potential of severe weather remains as
instability will be available in the warm humid air mass while
deep layer shear is on the increase from the north.

.LONG TERM...Saturday Night Through Friday
Issued at 228 PM CDT Fri Jul 21 2017

Forecast focus begins with shower and thunderstorm potential
Saturday evening through Sunday evening, followed by timing of
precipitation during the midweek as a surface front moves through.

Shower/thunderstorm chances persist Saturday evening with the area
still under the influence of a surface low, approaching mid-level
shortwave, and right rear quadrant of the upper-level jet. There
is potential for some stronger/severe storms in the evening with
MUCAPE anywhere from 1000-2000 J/kg and surface to 6km shear
around 40 kts. With wet- bulb zero heights still somewhat high
(around 11000 to 12000 ft), would expect main concerns to be
damaging wind and brief heavy rain (PW is around 1.00 to 1.50
inches). The greatest threat is across the north, closer to the
large scale forcing, during this time.

Instability decreases overnight, then redevelops Sunday during
the late morning and afternoon. This combined with a 500mb
shortwave will once again bring the chance for showers and
storms, mainly during the afternoon. PW is closer to 1.00 inches
and shear is weaker, but WBZ heights are lower (around 9000 ft).
Some large hail and gusty winds are possible before the forcing
and instability move east of the area.

Surface high pressure and mid/upper level ridging takes over for
the start of the work week, which will also bring humidity to more
comfortable levels for a couple days. Moisture returns Tuesday
afternoon with model uncertainty when it comes to the timing of a
surface cold front and associated precipitation. GFS now the
quickest bringing pcpn back to the area Tuesday afternoon, while
the EC and Canadian hold off until Tuesday evening. Due to these
difference stuck with a blended solution which features chance
thunderstorms from Tuesday afternoon through Wednesday evening. It
remains too early to determine any severe potential, which will
likely be influenced by the timing of the front.

Long term models agree in weak surface high pressure building in
on Thursday, then diverge on the influence of a surface warm
front south of the area on Friday. Included southern/western areas
in a slight chance pcpn on Friday for now.

Temperatures will be below normal Sunday and Monday, especially
across northern Wisconsin, and then return to near normal for the
rest of the work week.

.AVIATION...for 18Z TAF Issuance
Issued at 1138 AM CDT Fri Jul 21 2017

For the afternoon hours, Thickening high level clouds
will lower to mid level during the afternoon as convection over
southeast Minnesota, Iowa and southwest Wisconsin late morning,
gradually shift eastward. Convection has been weakening while
approaching Wood county late this morning, but still anticipate
some debris type precipitation to drift across parts of central
and east central wisconsin along with patchy mvfr cigs. Trend of
most model runs this evening into Saturday morning is to shift
the heavy rain potential southward. Still anticipate cigs lowering
to mvfr and possibly ifr for a period later tonight as the
convection passes through. These conditions may linger Saturday
morning before improving late Saturday morning into the afternoon.

Issued at 228 PM CDT Fri Jul 21 2017

Short range progs have trended with the heavier rain to slide over
southwest then southern Wisconsin through tonight due to the
surface warm front appears stalled. Still a potential of heavy
rainfall of an inch or two with any storms which develop tonight
across central and east central Wisconsin. Much of the area has
missed the heavier rainfall, so an inch or two will not impact
rivers much. An inch or two falling quickly over urban area may cause
some minor flooding.



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