FXUS63 KGRB 201953
Area Forecast Discussion
National Weather Service Green Bay WI
253 PM CDT Wed Sep 20 2017
Forecast discussion for routine afternoon forecast issuance
.SHORT TERM...Tonight and Thursday
Issued at 253 PM CDT Wed Sep 20 2017
The main forecast concerns include convective trends, the
potential for severe thunderstorms during the late afternoon and
evening, and fog potential late tonight into early Thursday.
Scattered thunderstorms were developing from southwest into
central WI, in an area of increasing moisture convergence just
ahead of a cold front, where SBCAPE was 2000-2500 j/kg, and the
cap was eroding. Low clouds have persisted across the northwest
half of the forecast area, and have held temperatures in the
middle 60s to lower 70s, so instability is much weaker there.
Expect the best severe threat to occur south of Highway 29 and
west of the Lake Michigan shoreline, where temperatures have
warmed into the mid 70s to mid 80s, and dew points were in the
upper 60s. Shear was sufficient for severe thunderstorms, and
possibly a few supercells, with 0-6 km shear of 30-45 kts and
0-3 km SRH of 250-350. The NAM12 seems to have a decent handle
on the situation, and focuses the strongest convection across
central and east central WI between 21z/Wed-03z/Thu.
Convection should taper off across our southeast counties late
tonight, as the cold front exits the region. High pressure
will build into our northwest counties, setting the stage for
widespread fog development over north central and far northeast
WI late tonight into early Thursday. Will have a wide range of
temperatures overnight, with lows from the middle to upper 40s
northwest to the lower to middle 60s southeast.
Lingering fog/low stratus should eventually mix out Thursday
morning. The cold front will shift back north as a warm front,
and should approach central WI later in the afternoon. Models
show CAPE climbing to around 1500 j/kg in the vicinity of the
boundary, so have added some small pops for our southwest
counties. High temperatures should range from the middle 70s
to lower 80s.
.LONG TERM...Thursday Night Through Wednesday
Issued at 253 PM CDT Wed Sep 20 2017
The pattern will be relatively stable across the continental U.S.,
and feature a deep trough over the west, and expansive ridging over
the east. The medium range models show that the region will be on
the warm side of this pattern through early next week. Will use a
blend of the ecmwf/gfs.
Thursday night through Saturday...The front will be returning north
as a warm front on Thursday night. Will have weak instability and
steep mid-level lapse rates along the boundary, so think at least a
slight chance of thunderstorms is warranted over northern WI. Mid-
level heights will then build across the region for Friday thru
Saturday as the front moves into Lake Superior. Despite a hot and
humid airmass south of the front, focus for showers/storms will
reside well upstream over the northern Plains. 925mb temps suggest
highs near 90 degrees for both Friday and Saturday. Heat indices
should peak in the low to mid 90s during the afternoon hours.
Rest of the forecast...Well above normal temperatures will continue
for Sunday and Monday, though temps will probably cool somewhat as
upper level ridging shifts to the east coast. Will see increasing
chances of showers and storms from west to east on Monday and
Tuesday as a cold front slowly approaches from the west. More
seasonable temps will return behind the front for next Wednesday.
.AVIATION...for 18Z TAF Issuance
Issued at 1239 PM CDT Wed Sep 20 2017
Plenty of low clouds still in place ahead of the approaching
cold front, with most locations reporting MVFR/IFR ceilings.
The first round of showers and storms dissipated as it moved into
far northeast WI, but additional storms should redevelop along a
cold front as it moves into north central and central WI later
this afternoon. These storms should work their way into eastern WI
by early evening, then weaken before midnight.
Expect areas of fog to develop over most of north central and
far northeast WI overnight as high pressure builds into the
region. Have added a TEMPO group to the RHI TAF for LIFR
conditions in fog late tonight/early Thursday. With the frontal
boundary lingering just to our south and east winds, suspect
that low stratus will remain over parts of the region through
Beach Hazards Statement until 10 PM CDT this evening for WIZ022-
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