Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 201703
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1203 PM CDT WED JUL 20 2016

Updated aviation portion for 18Z TAF issuance

.SYNOPSIS...
Issued at 413 AM CDT Wed Jul 20 2016

Becoming very humid and rather hot with thunderstorms at times
through the weekend...then a little cooler and drier next week.

The subtropical ridge anchored by a closed upper high centered
over the Southern Plains dominates much of the CONUS. Within the
westerlies flowing along the northern periphery of the ridge,
troughs are located off the West Coast and over eastern NOAM.
During the forecast period, the westerlies to gradually sag
southward and deamplify as the the upper high center within the
subtropical ridge shifts back toward the Desert Southwest.

Very warm and humid air from the Plains will spread northeast
across the forecast area during the next 24 hours. Once in place,
the warm and humid conditions will linger through the weekend--
though day to day conditions will be moderated by convection and
the movements of frontal boundary expected to linger near the
area. Somewhat cooler and drier air will push in early next week
following the passage of a substantial shortwave embedded within
the low amplitude westerlies. Given the available moisture and
numerous opportunities for convection, precipitation amounts for
the next week are likely to be above normal.
&&

.SHORT TERM...Today...Tonight...and Thursday
Issued at 413 AM CDT Wed Jul 20 2016

The forecast this morning (and likely for the next several days)
is very complex as much of it hinges on the development, location,
and coverage of convection. With weak large scale forcing and
smaller scale features dominating, models are struggling greatly.
The complex of storms expected to develop to the west has yet to
materialize, though some ACCAS and weak returns were finally
showing up on the KMPX radar. It`s hard to see anything very
organized developing out of that, and even if it does,
substantial convective inhibition exists over the forecast area so
it will weaken upon arrival. So backed PoPs for this morning down
to a slight chance across the western part of the forecast area,
and removed PoPs elsewhere. It`s still possible convection may
develop across the area this afternoon or evening once the
atmosphere destabilizes and inhibition is removed. But at this
point do not see anything to focus the storms, so kept PoPs rather
low as coverage would be limited. A severe storm is still
possible, but with coverage expected to stay limited, the Marginal
Risk (5%)on the SPC day 1 Convective Outlook is reasonable.

The very warm and humid air should be in place by tonight, so
raised overnight lows. The best chance of storms tonight will be
across the north, which is at risk of having storms that fire to
the north and west move in overnight. Carried the highest PoPs
there, but still nothing above chance category.

Barring unanticipated widespread convection overnight, Thursday
is likely to be hot and very humid, with heat indices exceeding
100 over the southern 1/2 to 2/3 of the forecast area. Heat
Advisory criteria is daytime apparent temperatures at or above
100. Excessive Heat Warning criteria is at least a 48 hour period
with daytime heat indices AOA 105 and nighttime heat indices
AOA 75. We are unlikely to meet warning criteria as Friday is not
expected to be as hot after a weak cool front passes through the
area Thursday night. So it looks like our heat-related headline is
going to be a Heat Advisory for at least the southern half of the
area for Thursday. But will hold off issuing that for now as
offices to the south and west with Excessive Heat Watches have
decided to maintain their watches due to uncertainty in conditions
in their areas later in the week which will determine whether they
end up needing advisories or warnings.

In addition to the heat, severe storms are possible Thursday,
especially over the Northeast and East, as the tail of a
shortwave passing across Ontario interacts with a weak frontal
boundary sagging into the area. The atmosphere is expected to be
extremely unstable by the afternoon, with CAPES over 4000 j/kg
likely. Weak height falls with the shortwave will tighten the
gradient aloft, likely resulting 35-45 kts of 0-6km shear over far
northeast Wisconsin. The main question is whether or not warm
temps aloft will maintain a cap. Kept PoPs in the chance category
for now since forcing is subtle and might not be able to break the
cap. But if storms develop, the likelihood of severe storms will
be considerable given the combination of instability and shear.
One possible scenario is that storms fire over far northern
Wisconsin and Upper Michigan, then organize and build south into
the reservoir of instablity, with convergence from the outflow
providing the lift to break the cap. Will highlight the potential
in the HWO.

.LONG TERM...Thursday Night Through Tuesday
Issued at 413 AM CDT Wed Jul 20 2016

Models continued to forecast a large upper level ridge over the
Plains through the end of the week. This will lead to warm/hot and
humid conditions across the forecast area through at least Saturday.
Upper flow across the region is expected to be zonal, causing model
timing differences with short waves moving east. The timing
differences led to plenty of slight chances for showers and storms
during the long term forecast.

There is the potential for strong or severe storms on Thursday
night due to the intersection of an outflow boundary, from
convection earlier in the day, and a cold front, combined with
steep mid level lapse rates and veering low to mid level winds.

The period Saturday through Sunday looks to be the best chance
for showers and storms during the rest of the long term forecast.
This is due to upper level short wave energy and a surface low and
associated frontal boundaries making their way across the area.
Storms are likely across the area on Saturday Night.

The cold front will bring cooler, but near normal, temperatures to
the area for early next week.
&&

.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...
NONE.
&&

$$

SYNOPSIS.......Skowronski
SHORT TERM.....Skowronski
LONG TERM......MG
AVIATION.......MPC


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