Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
344 PM CDT Thursday Jul 24 2014
Tonight and Friday...forecast confidence low.
Plan on clear skies and light winds across southern WI this
evening. Lows will be in the middle 50s to around 60.
Showers and thunderstorms should develop over south central
Minnesota around midnight and slide southeast along the 850mb temperature gradient
and with the corfidi vectors. These may clip southwest Wisconsin
late tonight into Friday morning. The low levels will remain dry
through the night...so it is a question of how strong the elevated
thunderstorms approaching the area end up being to determine if they
can saturate from the top down and precipitate. Overall forcing
appears quite weak over southern WI in the absence of a strong
shortwave or favorable position of the upper jet. I
continued the likely probability of precipitation for south central WI Friday morning due to
the 12z synoptic scale models. However...I do not have very much
confidence in this given the WRF-arw and WRF-nmm models and even
early signs in the hrrr.
The nose of the low level jet /llj/ will point into south central Minnesota
around midnight tonight and then veer southeast into south central
WI midday Friday. The low level jet will be the main driver for any
convection and it all looks elevated. Models are all over the place
for precipitation placement Friday afternoon. Persistent 850mb warm air
advection...the nose of the low level jet and weak shortwave activity could be
enough to develop some scattered showers and thunderstorms during
the afternoon. Elevated cape could be up to 1000 j/kg with decent
shear. If storms develop...cannot rule out some half inch to one
Friday night and Saturday...forecast confidence...high
The upper flow becomes zonal with a west/northwest flow as a 100
knot jet maximum over the northern plains pushes east southeast
toward the Wisconsin/Illinois border. Weak upper divergence
exists across southern Wisconsin.
The 700 mb thermal ridge extends into southern Wisconsin Friday
night...then sagging south across northern Illinois Saturday.
Weak 700 mb upward motion moves across southern Wisconsin Friday
night along with a subtle middle level shortwave. 700 mb relative humidity dries
Friday night and is very low Saturday. 850 mb relative humidity is saturated
Friday night with dew points of 16 to 18 celsius and dries from
the west Saturday. Southeast Wisconsin is at the nose of the
40 knot 850 mb low level jet Friday evening before it sags south.
The NAM brings some precipitation to the southeast overnight. The
NAM has almost 3000 joules/kg of elevated cape lifting from around
870 mb. Nsharp hail parameters indicate a severe potential Friday
night. The hail parameters increase Saturday...although a strong
cap at 825 mb would likely prevent any deep moist convection.
Saturday night and Sunday...forecast confidence...high
The zonal flow begins to break down as a middle/upper level
shortwave begins to drop southeast into northern Minnesota
Saturday night....and into the western Great Lakes Sunday. The
upper divergence and 700 mb convergence increases again Sunday.
Nsharp hail parameters still indicate a severe potential Sunday
morning as the cap weakens. NAM increases precipitation during the
Monday through Wednesday...forecast confidence...high
The shortwave over the Great Lakes continues to dig across the
eastern Great Lakes Monday...and south across the eastern U.S.
Tuesday and Tuesday night and Wednesday.
High pressure becomes centered over the eastern plains and upper
Mississippi Valley although the cool air aloft could bring
diurnal showers...especially Tuesday and Wednesday.
A second shortwave drops into the upper Mississippi Valley
Wednesday night into Thursday...retrograting the trough back
across the Great Lakes.
This brings more diurnal precipitation chances and continues the
rather cool temperatures.
VFR conditions through tonight. Warm advection ahead of approaching
shortwave trough brings increasing precipitation chances to mainly the far
western forecast area late tonight into Friday morning with a
diminishing thunderstorm complex. Any precipitation should remain to the
southwest of Madison.
There is a general chance for showers and thunderstorms Friday
afternoon for the whole area. Overall weak forcing...but moderate
instability could lead to small hail if thunderstorms develop.
Tonight/Friday and aviation/marine...mrc
Friday night through Thursday...hentz