429
FXUS63 KGRB 290325
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1025 PM CDT Fri Jun 28 2024

Updated aviation portion for 06Z TAF issuance

.KEY MESSAGES...

- Thunderstorms are possible late this afternoon through late
   tonight, but thunderstorms are most likely this evening. There
   is a 30-50 percent chance that localized areas south of highway
   10 will see over an inch of rain. Gusty winds and hail are also
   possible all areas, though the overall severe potential is low.

 - Conditions hazardous to beach-goers and mariners will continue
   through late tonight.

 - Some rivers and streams will remain above bankfull or near
   minor flood stage into the middle of next week.

 - The next period to watch for heavy rain or stronger storms will
   be late Monday through Tuesday.

&&

.DISCUSSION...
Issued at 219 PM CDT Fri Jun 28 2024

Short Term...Tonight and Saturday

Extent of showers tonight along with severe potential is the
main focus.

Sharp mid-level shortwave trough is over MT while a secondary H7
wave is over eastern NE. Sfc low ahead of the mid-level trough is
over northern MN with a cold front extending southwest to another
wave of low pressure over eastern NE. Warm front is not well-
defined, but based on sfc obs is somewhere from central MN to
southwest WI. Ahead of these fronts, we have seen intervals of
light showers since mid morning supported by H7 moisture advection
just ahead of a 40-50kt H85 jet. No thunder thus far with NIL
instability. Light showers will continue rest of the afternoon as
additional moisture advection works into the area ahead of the
warm front.

Late this afternoon, MUCAPE increasing over 500J/kg begins to
move in from the southwest which will start to support at least a
chance of thunder. Greater chances for thunderstorms will occur
this evening into the early overnight as mid-level trough moving
to the Dakotas pushes mid-level jet into the upper mississippi
valley which will act upon MUCAPES by this time of 500-1000J/kg.
Yet, the strongest low-level jet will be steadily shifting east of
the area before the greater instability arrives so that continues
to result in only a low chance of seeing severe storms in the form
of large hail and gusty winds. Cannot rule out an isolated strong
storm though given the effective shear over 40 kts. Heavy rain
possible as well as PWATs ahead of the approaching cold front
surge to 1.8 to 2.0 which is near 200 percent of normal and well
over 90 percent of climo. HREF (short range forecast ensembles)
probs point to 30-50 percent chances of local amounts over an
inch this evening with heavier storms especially south of highway
10. FWIW, NBM probs are not near as high, less than 10 percent.
Additional showers and some storms could continue well after
midnight northeast/east-central as the axis of the main mid-level
trough arrives. Lows tonight not too far off from readings late
this afternoon, around 60 north and upper 60s from the Fox Valley
to the lakeshore.

Although the initial cold front will be through the area late
tonight into early Saturday morning, a secondary cold front is
still expected to track through on Saturday. Low-end pops (20-30
percent) cover this scenario, with best chances northern to
northeast WI. MLCAPEs of 300-600 J/kg will result in some thunder
but storms should remain below severe levels. Highs on Saturday
will range from around 70 across far north-central Wisconsin
given the earlier fropa from the second cold front, with highs
reaching the lower 80s east-central WI with the later arrival of
the front.

Long Term...Saturday Night Through Friday

Focus of this forecast revolves around the thunderstorm and heavy
rainfall potential from Monday night through Tuesday night.

Saturday night...A potent shortwave trough will be passing across
northeast Wisconsin in the evening. With such strong forcing,
could see scattered showers lingering along the Lake Michigan
shoreline early in the evening. Instability is negligible by this
point, so the risk of strong thunderstorms is low. Then clearing
will occur overnight.

Monday night through Tuesday night...On the backside of a large
high pressure system, strong warm advection will occur on Monday
night thanks to a 30-40 kt low level jet, pushing precipitable
water values (pwats) over 1.50 inches. Models generate widespread
precipitation in the elevated moisture gradient/convergent zone
from late evening into the overnight hours. Despite the strong
ascent, instability is very weak at 100-200 j/kg, so severe
weather chances look low. But some showers could generate a
moderate rainfall with probability of 1/2 inch of rain from 25 to
50% for the Monday evening through Tuesday morning timeframe.
Then, a second more focused round becomes possible late in the
day Tuesday as another shortwave rounds the base of the trough,
enhancing the overall heavy rainfall potential in the region.
A bit more thunder could accompany this second round as overall
instability and forcing will be better, but the exact details will
depend on how much rain sticks around from Monday morning.

Rest of the forecast...General flow then flattens out across the
region. While this will mean mostly dry conditions for the middle
of next week, a few scattered showers could still cross the region
but will lack the moisture to create heavier precipitation in the
area.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1019 PM CDT Fri Jun 28 2024

The steadier showers should be well east of the region by TAF
issuance, but scattered showers and isolated thunderstorms will
continue to develop ahead of a cold front through the overnight
period. Expect flight conditions to be in the IFR/LIFR categories
until the cold front moves through early Saturday morning.
Partial clearing and rising ceilings should lead to VFR
conditions regionwide by mid to late morning.

A short-wave trough and secondary cold front will bring a chance
of showers Saturday afternoon. There could a few thunderstorms as
well, but confidence in the occurrence or location of storms is
not high enough to include a mention in the TAFs at this time.
Low ceilings will return to north central and far northeast WI
during the late afternoon and evening hours.

LLWS will be ending in far eastern WI early in the TAF period.
Gusty west surface winds will develop in the wake of the cold
front passage, especially Saturday afternoon and evening.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement until 7 AM CDT Saturday for WIZ022-040-
050.
&&

$$

DISCUSSION.....JLA/Uhlmann
AVIATION.......Kieckbusch