Scientific Forecaster Discussion
fxus63 kmkx 191943
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
243 PM CDT Thu Sep 19 2019
Tonight and Friday...forecast confidence is moderate.
The challenge in this short range period is figuring out whether
we'll see more convection and what the trigger will be. The
atmosphere will remain juiced with precipitable water values
hovering around 1.5-1.7in. Quite high for mid to late September.
Ridging dominating in the mid levels and at the surface tonight
into Friday. But, the pattern is weak and any lingering low level
boundaries could help to fire off isolated activity, so will hang
onto small pops. But, just not seeing anything organized.
The concern would be if something does pop, the weak flow means
slow moving activity. The areas in western Sauk and especially
Lafayette County got pounded last night with heavy rain. They
can't take much more. If heavy rain does develop, it could lead to
some flooding. Given the serious lack of confidence on anything
developing, and its location, no flood headlines will be issued at
Friday night through Sunday night...forecast confidence is
Models are continuing to show warm and humid conditions lingering
across the area Friday night until the cold frontal passage
Sunday. Breezy south to southwest winds are expected with a
tightening pressure gradient Saturday into Sunday, ahead of the
front. The southwesterly low level jet will flow into the area
during this period, continuing to bring plenty of deep moisture.
The main 500 mb trough also pushes east through the region Sunday
into Sunday night.
A round of showers and thunderstorms should move through the
region ahead of the front later Saturday into Saturday night, with
more showers and storms with the front itself for later Saturday
night into Sunday. At this time, Saturday night into Sunday
morning appears to be the most likely period for showers and
Heavy rainfall and some flooding is possible Saturday into Sunday,
with forecast soundings showing tall skinny mean layer cape with
moist adiabatic profiles. Our far western and southern counties
have been hit the hardest with heavy rainfall over the past week,
so more heavy rain would cause flooding issues.
Warm cloud depth, precipitable water and other heavy rain
parameters are favorable as well for heavy rainfall. There may be
a few rounds of showers and storms, which would bring the
possibility of repeated rounds of heavy rainfall. Gefs plumes are
showing a trend of precipitation amounts of one inch or greater by
Sunday evening, with some variance. Will continue to monitor the
heavy rain and flooding potential as this weekend draws closer.
Storm Prediction Center has a marginal severe risk for Saturday/Saturday night in the
western parts of the area. Given the look of the moist adiabatic
profiles and modest deep layer shear in the forecast soundings,
most storms should remain below severe levels.
The cold front should exit the area by Sunday night, with somewhat
cooler and drier air working in.
Monday through Thursday...forecast confidence is moderate.
High pressure passing by to the south of the region Monday, and to
the southeast on Tuesday, should bring a period of quiet weather
to the area. Temperatures should be near seasonal normals.
Models then differ more with trends for Tuesday night into
Thursday. Warm air advection returning to the region Tuesday night
into Wednesday may bring chances for showers and storms. Trends
after that vary widely. They range from a dry period with high
pressure passing to the south of the region per GFS/Canadian, to
a warm front developing across the region per the European model (ecmwf). The gefs
plumes show a decent amount of variance in precipitation amounts
this far out, which is not surprising. Kept pops and temperatures
from blended models going for now.
Aviation(21z taf updates)...
Looks like VFR conditions through the taf period. We have a small
chance of showers or storms through the taf period. Nothing
obvious to latch onto for timing or organization, etc. We're just
under a very moist airmass and any subtle boundary could trigger a
shower or thunderstorm. This would mainly be west of
kmke/kenw/kues. Winds will remain light southerly through the taf
Light winds will continue across Lake Michigan through Friday
night, as high pressure lingers east of the Great Lakes with low
pressure out west. The low will finally push toward the region by
Saturday. This will bring increasing southerly winds by Saturday
afternoon, persisting into Sunday, but veering to the southwest
Sunday morning. A Small Craft Advisory will likely be needed by
mid day on Saturday, continuing through Saturday night. An
associated cold front finally sweeps through the area by Sunday
evening, bringing westerly winds in its wake. High pressure
sliding overhead will bring light winds again later Monday