274
FXUS63 KGRB 211808
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
108 PM CDT Tue May 21 2024

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- Impactful strong to severe storms are anticipated across
  portions of the area today into this evening. The areas most
  likely to see severe weather will be central to east-central
  Wisconsin. Damaging winds, large hail, and a few tornadoes will
  be possible with the stronger storms.

- Several rounds of heavy rainfall today and this evening may
  also pose a flooding concern.

- Chance of showers and thunderstorms across northern Wisconsin
  Thursday afternoon into Thursday evening.

- Better chance for showers and thunderstorms over all of
  northeast Wisconsin Friday afternoon into Saturday morning.

&&

.DISCUSSION...
Issued at 232 AM CDT Tue May 21 2024

Short Term...Today...Tonight...and Wednesday

Despite cloud cover, areas of dense fog have formed across much of
the area early this morning with abundant low level moisture and
light winds across the region. Although some areas may not receive
dense fog this morning, overall the dense fog should be ubiquitous
enough to continue the Dense Fog Advisory through this morning
across the entire region. The dense fog should lift this morning
with daytime heating as the boundary layer winds mix to the
surface.

Attention then turns to the convection expected today and this
evening. Hi-res models indicate there will be two rounds of
convection; one moving from west to east from late morning through
early this afternoon and another moving through this evening.

The first round of showers and thunderstorms will track through
later this morning into the early afternoon north of a warm front
lifting north across the western Great Lakes region. These initial
storms are not expected to be severe as they will mainly be
elevated with limited instability.

The bigger concern are the storms that are anticipated this
evening. This activity will be driven by a pronounced negatively
tilted mid level trough and attendant shortwave along with a
surface cold front. Although some supercells are possible ahead of
the main activity, the convection is expected to quickly develop
into a QLCS that will track through the area from west to east
this evening. Although instability isn`t overly impressive with
MUCAPEs around 1000 J/kg, bulk shear values are off the charts at
40 to 60 knots. Hodographs indicate quite a bit of low level
shear and helicity, especially across central Wisconsin, which
will bring increased probabilities for tornadoes (10-14%) across
this area. North and east of this area for enhanced tornado risk
the probabilities are lower but are certainly not zero. The most
prevalent severe weather type is expected to be straight line
winds given the linear nature of these storms and very strong
winds, 60-70 knots, just above the surface. The probabilities for
wind damage are rather high given these factors with 30-44% across
central and east-central and 15-29% across north-central and
northeast Wisconsin. Large hail will also be a threat as there is
some cooling and enhanced CAPE in the hail growth zone from these
deep updrafts.

The main area of concern will be central Wisconsin for all threats
as supercells will have a better chance of developing or tracking
through this region before conglomerating into a QLCS. Central and
east-central will be the primary areas of concern for the wind
threat and possible QLCS tornadoes where instability and shear
will be maximized with the best chance to be in the warm sector
for surface based storms. Further north, although the severe
weather threat is lower, this area could also see strong to severe
storms.

In addition to the severe potential, flooding could be an issue
from these storms with two rounds of heavy rainfall expected today
and this evening. Despite the fast moving nature of the storms,
PWATs of 1.4 to 1.6 inches are expected, representing the
climatological maximum for this time of year, along with long
skinny CAPE profiles.

Behind the cold front, cooler temperatures and lower dew points
are expected on Wednesday. Some showers could linger, especially
across north-central Wisconsin, under cyclonic flow as a shortwave
tracks through the western Great Lakes region.

Long Term...Wednesday Night Through Monday

Models indicate a progressive pattern to exist through early next
week as a series of Pacific upper lows/shortwave troughs move
across the northern tier of states. This would bring periodic
precipitation chances to northeast WI. Timing of these systems
will be the main forecast challenge. Temperatures for the most
part to average near normal.

Wednesday night and Thursday...
Dry conditions are expected Wednesday night as the large system
responsible for the stormy weather continues to lift north over
western Ontario and weaken. A prevailing southwest wind at 5 to
10 mph should keep fog at bay through the night despite mostly
clear skies. Min temperatures to range from the middle 40s north,
upper 40s to lower 50s south. There is a concern that a weak
frontal boundary will drop south and potentially stall over
northern sections of the Great Lakes on Thursday. There may be
enough convergence along this boundary to generate a chance of
showers and a few thunderstorms across northern WI by the
afternoon. Have added a pop to the forecast along with more cloud
cover. Otherwise, partly cloudy skies can be expected over the
rest of the forecast area with max temperatures in the upper 60s
to around 70 degrees north, lower 70s near Lake MI and middle to
upper 70s south.

Thursday night and Friday...
This boundary is parallel to the mean flow aloft which means it is
not going anywhere fast. May need to carry a small pop over the
north Thursday night, but any precipitation would be spotty and
light. The rest of the area should remain partly cloudy with min
temperatures near 40 degrees far north, upper 40s to around 50
degrees south. A better chance for showers will arrive on Friday
as a mid-level shortwave trough moves into the Upper MS Valley,
preceded by a cold front that is forecast to reach western WI late
in the day. Marginal instability may allow for some thunderstorms
to develop, but too early yet to discern severe storms potential.
Max temperatures on Friday to be in the middle 60s near Lake MI,
mainly lower to middle 70s inland.

Friday night and Saturday...
Showers/storms will continue through at least Friday evening as
the shortwave trough moves northeast toward Lake Superior and the
cold front sweeps east across the rest of WI. The far north and
far eastern part of the forecast area may see this precipitation
linger past midnight before ending. Early indications show that
most of Saturday appears to be dry with weak high pressure in the
vicinity. Models are waffling a bit with this high pressure, thus
confidence for a completely dry Saturday still has some doubt.
Little change in air mass will bring max temperatures on Saturday
into the middle to upper 60s near Lake MI, around 70 degrees north
and lower to middle 70s south.

Saturday night and Sunday...
Model differences continue into Saturday night with the possible
existence of a fast-moving, weak shortwave trough. If this trough
comes to fruition, it would be a chance of showers to eastern WI
so have kept a small pop in the forecast for now. This weak system
would exit the region by Sunday morning and with little synoptic
forcing evident, it appears that most of Sunday would be dry. Max
temperatures on Sunday appear to mirror Saturday which would be
close to normal.

Sunday night and Memorial Day...
The next prominent upper low/shortwave trough is progged to move
from the northern Plains into the Upper Midwest on Memorial Day.
Latest timing among the models would keep any morning festivities
precipitation-free, but expect precipitation chances to increase
over central WI in the afternoon. There is potential for this
system to slide south enough to keep any showers away, so stay
tuned. Max temperatures to remain near normal with upper 60s
lakeside, around 70 degrees north and lower to middle 70s south.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 108 PM CDT Tue May 21 2024

VFR to MVFR conditions will be in place for the afternoon and
evenin, but IFR/LIFR conditions are possible at times as showers
and thunderstorms cross the area. Initial convection early this
afternoon is not likely to be severe, but will have thunder and
the potential for small hail or a brief wind gust.

As we get into the late afternoon and evening hours, storm
strength and potential increases, with a strong round of storms
expected to cross Wisconsin in the 23Z-05Z timeframe. Timed this
thunderstorm round with a set of TEMPO groups for the area TAF
sites for now. Damaging winds, large hail, and a few tornadoes are
possible with these storms. After the storms pass, active weather
should return for the late overnight but winds will likely remain
gusty. The passing system will also bring a period of LLWS that
will likely last through the overnight hours.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION.....Kurimski/Kallas
AVIATION.......Uhlmann