Area Forecast Discussion
Issued by NWS Green Bay, WI

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164
FXUS63 KGRB 270825
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
325 AM CDT TUE SEP 27 2016

Forecast discussion for routine morning forecast issuance

.SYNOPSIS...
Issued at 318 AM CDT Tue Sep 27 2016

Cool and showery today and tomorrow, then warmer for the end of
the work week.

A small (longitudinally) area of split flow progressing eastward
across North America has reached 90W. While the northern branch of
the split will continue on to the east, further progression of
the southern branch of the split will end due to increased lower
latitude blocking downstream. That will result in the deep,
vertically stacked cyclone over Ontario shifting south across the
area the next couple days, and possibly returning north again this
weekend. It should finally get kicked east of the area late in the
period as the westerlies increase downstream from a trough
developing over the West.

A cool Canadian air mass sweeping across the area will result in a
couple days of below normal temperatures (especially daytime
highs) before readings rebound back to near or above normal for
the rest of the period. Precipitation chances will be tied to the
cyclone affecting the region. Showers are likely early in the
period as the system digs south across the area, and are possible
again this weekend depending on how far west the remnants of the
cyclone track when returning north. Though almost all areas will
get some rain, amounts will probably end up AOB normal for the
period.
&&

.SHORT TERM...Today...Tonight...and Wednesday
Issued at 318 AM CDT Tue Sep 27 2016

Cool, blustery, autumn like conditions are expected the next 36
hours as the cyclone over Ontario shifts south across the area.
Given precipitation noted on surface obs and radar mosaic, felt
comfortable raising PoPs to categorical for at least a time today.
Showers should increase from north to south as the system begins
to shift south today. In addition, shortwave rotating around the
base of the system will swing across the southern part of the
forecast area this afternoon and early this evening. Models
indicate increasing QG forcing with this feature, so expect the
most widespread shower activity will occur at this time, with the
coverage of the showers decreasing later in the evening and
overnight. Precipitation is likely to become lake-enhanced by
late tonight and on Wednesday morning as the center of the cyclone
settles to our south, and strengthening north-northeasterly flow
develops across the area. Hi-res models all suggest a convergence
band over western Lake Michigan and a sufficient water/air
temperature difference to support lake-effect showers. So, raised
PoPs over the east back to likely for a time during the morning,
before tapering them back in the afternoon. Destabilization may
be sufficient to support some thunder in the east, though held off
adding that to the grids for now to allow later shifts to make the
final decision.

The clouds, precipitation, and cool air wrapping around the
cyclone will limit the diurnal temperature range, with daytime
temperatures staying below normal.

.LONG TERM...Wednesday Night Through Monday
Issued at 318 AM CDT Tue Sep 27 2016

Forecast uncertainties revolve around the motion of the upper low,
currently spinning over the Great Lakes all the way through the
weekend. Most guidance agrees to push the low into the Ohio Valley
during the Wed nite-Thu time period. Then models are trending toward
the ECMWF in returning the upper low towards the central Great Lakes
late Friday night into Saturday.  Will give a slight edge to the
ECMWF.

Wednesday night through Thursday night...As mentioned above, the
upper low will be slowly spinning over the Ohio Valley for much of
this period.  However, will continue to see north to northeast low
level flow, with some moisture lingering, and marginally unstable
conditions off Lake Superior.  Therefore, could see some clouds
lingering Wednesday night, especially over far northern WI, and
possibly eastern WI too.  Eventually though, should see clearing
take place which should lead to partly cloudy to mostly sunny
conditions for Thursday as high pressure builds into the region.
Some clouds may try to wrap around the low and back into eastern
portions of the state on Thursday night, but moisture should not be
deep enough for precip.  Sided with the warmer temps for Thu based
on low level temps and decent mixing.

Rest of the forecast...Although high pressure will be across the
area for Friday, may still see increasing clouds, especially over
eastern parts of the state with a small chance of a shower over east-
central WI. Plenty of uncertainty for this weekend depending on the
track of the upper low. Confidence is rather low by this time, but
at least plan to keep scattered to broken cloud cover over eastern
WI for much of the weekend. Precipitation chances look minor and any
impacts should be low. As the low moves away finally, should see
relatively quiet conditions into early next week.
&&

.AVIATION...for 12Z TAF Issuance
Issued at 318 AM CDT Tue Sep 27 2016

Gradually lowering ceilings are expected today and tonight as
deep vertically stacked cyclone drops south across the region.
&&

.MARINE...
Issued at 318 AM CDT Tue Sep 27 2016

Mixing over the land will increase some today, and when combined
with destabilization and mixing as the cooler air flows over the
warmer waters of the Lake and Bay, anticipate wind gusts remaining
sufficient to support the current Small Craft Advisory. Though
with the gradient gradually weakening with the approach of the
upper low, gusts likely won`t be as strong as yesterday. The 00Z
expiration of the SC.Y may be a bit early, but is close enough to
stick with for now.

Conditions are likely to become favorable for waterspouts over
Lake Michigan Wednesday morning if the convergence band indicated
on most of the high-res guidance sets up over western part of the
lake and shifts toward the shore. Added the chance of waterspouts
to the grids and will mention in the HWO.

Another SC.Y will probably be necessary tomorrow as winds increase
from the northeast when the cyclone shifts south of the area.
&&

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

$$

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



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