Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI
FXUS63 KMKX 231542 AAA
Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
1042 AM CDT SAT JUL 23 2016
An area of storms continues to approach from the west, driven by
some warm advection, arrival of deeper moisture, and a weak
shortwave. Given these ingredients will continue to push eastward
into the afternoon, expecting these storms will make their way
through much of the forecast area. The latest HRRR agrees with
this idea, though it does show a weakening trend as the storms
head east. Another round of storms is possible this evening into
tonight ahead of the main shortwave.
Kept the Heat Advisory as is for now, with the east having the
best potential to hit the higher heat index values due to more
sunshine this morning. Some locations are already in the 95 to 100
range. Less confident in advisory levels in the west due to the
extensive cloud cover and approaching storms. Still, if the sun
pops out for even a short while later on, it will not take long
for heat indices to shoot up given such high dewpoints. Overall
though, will probably need to bump temps and heat indices down
some more in the west for this afternoon.
Seeing some MVFR ceilings trying to develop as daytime heating has
resulted in some cumulus. Should see mainly VFR for the afternoon,
though lower ceilings and visibilities possible in thunderstorms.
This first round of storms will likely move through late this
morning into the afternoon given latest radar and mesoscale model
trends. The storms may weaken some as they move eastward. Another
round of storms is then possible this evening into tonight ahead
of the main shortwave.
It looks mainly dry then Sunday as the cold front moves through
and high pressure begins to build in behind the front.
Dense fog has persisted in the northern zones. Given these trends,
dewpoints in the upper 70s over the cool lake waters, and weak
onshore winds, have extended the marine dense fog advisory to 4 pm.
Despite some areas of dense fog in the southern zones, chose to
leave them out given the more patchy nature to the fog in those
areas. Expect some showers or storms to reach the lake by
mid-afternoon, which should help scour out the fog.
.PREV DISCUSSION... /Issued 330 AM CDT SAT JUL 23 2016/
TODAY AND TONIGHT...Forecast Confidence Is Medium.
As an upper jet max moves across the upper Great lakes, a weak
upper ridge moves across Wisconsin today. Then a jet max moves
toward the Upper Mississippi valley late tonight.
GFS had the strongest upper divergence and 700 mb upward motion
later tonight, but the model likely has convective feedback
problems. 700/850 mb dew points rise today across south central
Wisconsin, and across all of southern Wisconsin tonight.
A southwest to west 700 mb speed max pushes into southern
Wisconsin late tonight, increasing the low level moisture
convergence. GFS precipitable water increases to over 2.25
inches, and with surface dew points in the mid 70s, any MCS will
likely produce heavy rainfall. The MCS should be fairly
progressive, so will not go with a flash flood watch ATTM.
Zero to 1 km CAPEs rise to around 3000 joules/kg on the forecast
soundings this afternoon, with 2000 joules/kg tonight. SPC also
has a Slight Risk for south central Wisconsin, and a Marginal Risk
for the southeast. Most likely timing for this evening and
Highs today will be up near 90 most places. The exceptions to
this will likely be in the east, due to onshore winds, especially
north of Milwaukee. This may be true as well in the southwest,
due to higher possible cloud cover is any showers/thunderstorms
move in this afternoon. Will be another hot and humid one, with
heat indices in the 95 to 99 degree range most places, except for
near Lake Michigan. Few areas may reach a heat index of 100
As far as headlines, it looks like Sunday will have Heat index
values of 95 to 99 again. That makes 4 consecutive days with 95 to
99 or above, and results in a Heat Advisory being issued for today
SUNDAY THROUGH MONDAY NIGHT...Forecast Confidence Is Medium.
GFS/ECMWF/Canadian models are quicker with the cold frontal
passage across the area than the NAM for Sunday. Prefer the
quicker frontal passage of the GFS/ECMWF/Canadian models.
Mesoscale models are showing convection from overnight tonight
exiting the eastern counties early Sunday morning. More
convection develops with the cold front in the far east or to the
southeast of the forecast area by 00Z Monday.
Some smaller 500 mb vorticity maxima shift through the area later
Sunday afternoon into the evening. Area is under the right
entrance region of the 250 mb jet streak, providing added upward
vertical motion. However, not seeing much else to provide more
upward motion. Some uncertainty here with if and how widespread
any convection may be.
Thus, will continue with chance POPs in the morning, lowering to
slight chance in the afternoon. Storm Prediction Center has most
of the area in the Marginal Risk for severe storms, meaning an
isolated severe storm is possible.
Mean layer CAPEs may get to 2000 J/kg or perhaps higher, with
deep layer bulk shear of 25 to 30 knots, so Marginal Risk is
warranted. Precipitable water vales are high as well, and
relatively slower storm motions suggest heavy rainfall is
Any convection that develops during the day Sunday may affect
forecast heat index values and subsequent Heat Advisory, but
given the lower end POPs, feel more confident with the heat than
thunderstorms for Sunday at this point.
Models then bring high pressure into the region for later Sunday
night into Monday night. This should bring dry conditions with
somewhat lower temperatures and dew points.
TUESDAY THROUGH FRIDAY...Forecast Confidence Is Medium.
ECMWF/GFS generally show zonal to west northwest flow at 500 mb
during this period. They bring several weak vorticity maxima
through the area, with a more significant one for Thursday night
or later Friday. Differences with timing exist in this regard.
The models suggest the best chances for showers and storms looks
to be Wednesday into Thursday, as more moist air moves back into
the region. Still a lot of uncertainty this far out in the
forecast, so will keep the chance POPs going for Wednesday into
Thursday. It looks to be a warm and humid period, with onshore
winds at times near Lake Michigan.
Some IFR/LIFR fog/stratus through just after sunrise. Then brief
MVFR cigs. VFR conditions are then expected later this morning
into the evening.
May see a few storms at times in the west or southwest forecast
area this afternoon. Better chance for storms across southern
Wisconsin looks to be this evening and overnight. IFR conditions
with the storms.
Could see some fog at times through Saturday, due to higher
moisture lingering over cooler Lake Michigan waters.
Will likely see storms return by Saturday evening or night, with
a chance lingering into at least Sunday morning.
WI...HEAT ADVISORY until 7 PM CDT Sunday for WIZ046-047-051-056>059-
LM...DENSE FOG ADVISORY until 4 PM CDT this afternoon for LMZ643-644.
TODAY/TONIGHT AND AVIATION/MARINE...Hentz
Sunday THROUGH Friday...Wood