Area Forecast Discussion
Issued by NWS Green Bay, WI

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000
FXUS63 KGRB 190951
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
451 AM CDT Sat May 19 2018

Forecast discussion for routine morning forecast issuance

.SYNOPSIS...
Issued at 446 AM CDT Sat May 19 2018

Scattered showers at times through tonight. Turning breezy and
cooler this afternoon and evening as a cold front drops southeast
across the area.

The primary westerlies will remain across Canada and the far
northern U.S., with troughs near the West Coast and over eastern
Canada surrounding a ridge over the intermountain West. A weaker
branch of the flow will persist across the CONUS, with the
forecast area remaining near the confluent zone between the
branches.

The seasonably mild air that is across the area this morning will
be replaced by cooler Canadian air starting this afternoon.
Readings will warm back to above normal by the middle of next
week, then remain there into at least the start of the Memorial
Day weekend. The primary opportunities for precipitation will be
today and tonight, and possibly again toward the end of the
period. But since these events will probably consist of scattered
showers rather than widespread rains, the best guess is that
rainfall totals at most locations will end up AOB normal for the
period.
&&

.SHORT TERM...Today...Tonight...and Sunday
Issued at 446 AM CDT Sat May 19 2018

Lots of forecast details to work out today through Sunday. The
scattered showers across the south and east this morning were
generated by an upper system lifting out of the lower Ohio Valley.
They will persist for a while, but should shift east by mid-late
morning.

The next feature of concern is a rather strong cold front
currently entering northwest Wisconsin. It should accelerate
southeast once the Ohio Valley system begins to shift east. The
front should reach north-central Wisconsin by early afternoon, and
appears strong enough to result in temperatures dropping with
FROPA. The pressure gradient behind the front aided by mixing with
CAA will support gusty surface winds. It`s not entirely clear
whether the boundary will accelerate down the lake and bay
accompanied by strong gusts like the last couple cold FROPAs. On
one hand, it`s a strong front. On the other, it will have more of
a southwest/northeast orientation. The fronts that typically surge
down the lake and bay are usually more east-west oriented. The
southwest-northeast orientation may result in winds immediately
behind the boundary staying more northerly rather than turning
northeast, which is typically what is needed for a strong push
down the bay. Raised winds some near the lake and bay, but further
adjustment may be necessary.

The models were considerably more aggressive in generating
precipitation along and over the boundary than yesterday. That
seemed reasonable given lingering moisture advected into the area
by the Ohio Valley system. There will also be low-level
convergence near the boundary. Raised PoPs from the previous
forecast, bringing mid-upper end chance PoPs (35-50%) southeast
during the afternoon and evening. The LDS indicated that the
initial development along the front in Minnesota yesterday evening
resulted in numerous lightning strikes. Given similar mid-level
lapse rates across our area today, added a chance of thunder in
the noon-midnight time frame.

Precipitation with the front should be exiting the southeast by
about midnight, but models suggest a convective cluster may race
east in the frontal zone late at night and Sunday morning. The
NAM and GFS kept it south of the area, but the ECMWF and NMM Hi-
Res WRF were farther north, clipping the southern two tiers of our
counties. What`s concerning is that all the models showed some QG
forcing across the area late at night and into Sunday morning as
the area will get clipped by the tail of a strong shortwave moving
east across southern Canada. It`s easy to see how the
precipitation could end up being farther north if the front is at
all slow to clear the area. For that reason, lingered low PoPs
across the far south through early Sunday. Sunday is likely to be
another day where clouds and northeast winds keep the southern
part of the forecast area considerably cooler than the north.

.LONG TERM...Sunday Night Through Friday
Issued at 446 AM CDT Sat May 19 2018

Fairly zonal pattern at 500mb expected through the period.
Still some discrepancies with timing and location of the
next weather maker on Monday. The NAM/ECMWF are farther
north and would bring rain to the area, while the GFS was
slower and a little farther south, and the Canadian model
was well south of the area and misses us completely. Took
a compromise of the northern solutions for this time period.

Tuesday should be dry and mild. Afternoon relative humidity
readings across the north should drop below 30 percent,
and this trend should continue again Wednesday and Thursday
afternoons. Low confidence in the small chances of showers on
Wednesday due to model differences in timing and location of
the system that could bring rain to the area. The next chance
of a widespread rain would be on Thursday night and Friday.
Raised high temperatures a few degrees each day from Tuesday
through Thursday. Meanwhile, lowered minimum temperatures a few
degrees each night especially in our typical cold spots across the
north from Monday morning through Thursday morning.
&&

.AVIATION...for 12Z TAF Issuance
Issued at 446 AM CDT Sat May 19 2018

Ceilings and visibilities will probably continue to edge down
through daybreak, then should begin to improve by mid-morning.
Anticipate primarily VFR ceilings and visibilities this afternoon
through tonight, with brief MVFR conditions accompanying the
scattered showers.
&&

.CLIMATE...
Issued at 446 AM CDT Sat May 19 2018

Abnormally dry conditions were introduced across the north with
the latest drought monitor last week. If we do not see much rain
over the next day or two across the north, the next best chance
of rain would not be until Thursday night or Friday. Warm and dry
conditions are expected across the north on most days through
much of next week. With that in mind, the dry conditions are
expected to worsen until the rain arrives late next week. The main
impacts of the dry weather will be lowering water levels on area
rivers and lakes, along with a slower than normal green up due to
the lack of rain and an increase risk of fires.
&&

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

$$

SYNOPSIS.......Skowronski
SHORT TERM.....Skowronski
LONG TERM......Eckberg
AVIATION.......Skowronski
CLIMATE........Eckberg



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