Area Forecast Discussion


391 
FXUS63 KGRB 220039
AFDGRB
Area Forecast Discussion
National Weather Service Green Bay WI
739 PM CDT Tue May 21 2019
Updated aviation portion for 00Z TAF issuance
and new information added to update section
.UPDATE...
Issued at 715 PM CDT Tue May 21 2019
The primary push of the rain band around the mature cyclone over
the Plains is to the north. That combined with a feed of dry air
from the east at low-levels should limit far eastern WI to just 
some sprinkles until at least midnight. Despite the delay, the 
band will eventually overspread the entire area overnight, then 
exit off to the northeast tomorrow morning.
There are some indications that strong and possibly even severe
convection could develop tomorrow afternoon in the wake of the
main band. By early to mid-afternoon, a strong mid-upper level
speed max will be pushing into the area from the southwest. The
LFQ of the speed max is likely to cut across central Wisconsin
during the late afternoon, providing organized large-scale lift. 
The models indicate steep mid-level lapse rates (7-8K/km) across 
the area, and deep layer shear around 50 knots. The main 
uncertainty will be the amount of destabilization that occurs. 
There would likely need to be a period of at least partial
sunshine in the wake of the primary rain band for a severe threat
to materialize. Computed MUCAPES off the models (for central 
Wisconsin) were generally from 800-2000 J/kg, though the higher
end values are likely overdone and a result of an overestimate of
surface dew points. In any case, the situation tomorrow afternoon 
will need to be monitored closely. The position of central 
Wisconsin relative to the upper low center tomorrow afternoon will
be similar to where northeast Kansas was located this afternoon 
when a band of low-topped supercells developed. 
Will tweak the HWO for tomorrow to introduce the possibility of
strong storms. The forecast itself already contains a chance of
thunderstorms, and PoPs are probably okay since coverage of the
storms would likely be scattered.
&&
.SHORT TERM...Tonight and Wednesday
Issued at 236 PM CDT Tue May 21 2019
The main forecast concerns to be timing of rain showers to
overspread the area tonight and extent of thunderstorms Wednesday
afternoon.
The 19Z MSAS surface analysis showed the center of high pressure
situated over northeast sections of Lake Superior with low
pressure located over southwest KS. Gusty east winds between these
two weather systems with some gusts greater than 30 mph over parts
of central WI. A warm front also extended eastward from the low
pressure into the Ohio Valley.
The nearly vertically-stacked, strong system is forecast to lift
north-northeast across the central Plains tonight as the warm
front moves north toward southern sections of the Great Lakes. An
initial band of WAA-induced light rain showers this evening to be
replaced by lift from the approaching front with a more widespread
light to moderate rain showers after midnight. Instability looks
too weak to carry any thunder. However, heavier rainfall could
transport stronger winds aloft to the surface, thus a few gusts
later tonight could reach the 30-40 mph range. Min temperatures to
be in the lower to middle 40s north, middle to upper 40s south.
The weakening system is forecast to move northeast into the Upper
MS Valley, while the warm front shifts into the Great Lakes on
Wednesday. Shower activity will come to an end across central/
east-central WI during the morning with a continued chance of rain
showers across the north. A concern for Wednesday afternoon is the
chance for additional showers and some thunderstorms to redevelop
over the area as a 140 knot upper jet to be overhead and mid-level
lapse rates steepen. Do not see any of these storms turning strong
or severe, but small hail and gusty winds will be possible. High
temperatures to range from the upper 50s to lower 60s near Lake
MI, around 60 degrees north and lower to middle 70s far south.
.LONG TERM...Wednesday Night Through Tuesday
Issued at 236 PM CDT Tue May 21 2019
Main concern in the extended forecast will be the potential for 
additional rainfall and continued elevated flows on area rivers. In 
addition, temperatures are expected to warm to above average levels 
for the last half of the work week with increasing chances of strong 
to conditionally severe thunderstorms.
Wednesday night into Thursday: A surface low and vertically stacked 
upper-level low center will shift eastward, crossing northwest WI 
and much of Upper MI. The better/deeper moisture and better forcing 
will lift north of much of the CWA, keeping most of the showers and 
thunderstorms across northern MN/Upper MI/Lake Superior for this 
time period. The best chance of seeing continued showers and a few 
thunderstorms would be across mainly the north half of the area, and 
especially close to the WI/UP border during the evening hours. High 
pressure will then quickly build across the area late Thursday into 
Thursday evening as upper ridging builds into the area. This will 
allow for a break in the precipitation overnight through the day 
Thursday. Overnight lows will be in the upper 40s to low 50s with 
high temperatures warming into 60s north-central and the 70s 
elsewhere on Thursday.
Thursday night into Friday: The next low pressure system is expected 
to lift northward through the Plains once again, lifting a warm 
front toward southern WI late Thursday night through the day 
Friday. This will lead to another round of showers and 
Thunderstorms from Friday morning through late Friday afternoon. 
If the clouds clear from early day showers and thunderstorms, then
stronger afternoon thunderstorms will become increasingly likely 
as instability increases and shear values increase. This will be 
largely dependent upon how far north the warm front lifts. In 
addition, there will also be some upper-level jet support during 
the late afternoon hours. Models have trended a bit slower and a 
bit farther to the south with the warm front positioning. This 
would place the better chance of seeing some stronger 
thunderstorms/possibly severe over portions of central and east- 
central WI, with the best chances over southern WI. This would be
near the warm front and ahead of the cold front as it passes 
through during the afternoon/early evening hours. Overall details 
will need to be fine- tuned as we get closer to the end of the 
week, but it will be worth keeping an eye on over the next few 
days. These details will make all the difference in the severe 
potential. Temps will continue to be above normal with afternoon 
highs running into the 70s (warmest central/east-central) with 
overnight lows only dropping into the upper 40s and low 50s.
Friday night through Saturday night: High pressure is expected to 
pass across the area through this time period, allowing for another 
break in the precipitation. Highs will be in the 70s with overnight 
lows in the 40s and 50s.
The rest of the extended: A weak system/cold front is expected to 
pass through the area Sunday night into Monday morning. This may 
give a small chance of showers and possibly a thunderstorm during 
this time period, but not expecting a widespread wash out. Perhaps a 
more widespread precip event may occur for Tuesday into the middle 
of the week. Generally went with a consensus of the models during 
this time period, giving highs in the 60s and overnight lows in the 
40s.
&&
.AVIATION...for 00Z TAF Issuance
Issued at 715 PM CDT Tue May 21 2019
A large rain band wrapping around a mature cyclone lifting NNE
from the central Plains will overspread the area tonight. A feed
of dry air from the east at low levels will allow ceilings to 
hold up initially, but expect they will eventually trend down to 
MVFR and IFR (central WI) by daybreak. The primary rain band will 
shift off to the northeast tomorrow, but scattered showers and 
thunderstorms will probably develop during the afternoon. LLWS 
will develop rapidly this evening.
&&
.HYDROLOGY...
Issued at 236 PM CDT Tue May 21 2019
Even through most rivers and streams have crested and started to
slowly fall, additional rainfall tonight and again later this 
week will keep the rivers elevated. In addition, the release of 
water from dams along the Wisconsin and Menominee rivers will 
generate fast flows downstream. Anyone living near rivers and
streams should keep aware of the latest weather information and 
be prepared to seek higher ground should any flooding occur.
&&
.GRB WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
UPDATE.........Skowronski
SHORT TERM.....Kallas
LONG TERM......Cooley
AVIATION.......Skowronski
HYDROLOGY......Kallas