Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
323 PM CDT Sat Aug 1 2015
Tonight - confidence...low
main focus will be pinning down convective trends as the night wears
along. 850 low level jet becomes more favorable aiming into County Warning Area with increasing
warm air advection and moisture advection. Models are showing varying solutions for
mesoscale convective system track. 1000-500 millibar thickness lines/mesoscale convective system maintenance
composite index from Storm Prediction Center mesoscale Page favor more of a 00z European model (ecmwf) solution.
The 12z NAM is the furthest north with precipitation into central/southern WI. The 12z
European model (ecmwf) has come more in line with other models showing convection
arriving after 6z across the County Warning Area. Given the proggd elevated
cape/consensus of low level jet forcing and middle level shortwave will still
carry high end chance probability of precipitation for whole County Warning Area but watching for a
potential southeast curve to any mesoscale convective system that might take shape that could leave
parts of southern WI more on the northern fringe of a southeastward diving mesoscale convective system track
favoring NE Iowa into northern Illinois. Watching afternoon vsbl animation thinking
more likely scenario is we will be seeing our activity originate from
western Minnesota into eastern ND which would favor southern WI a bit more. Overall model
solutions including latest hrrr favor some convection a bit later
in the night across southern WI however there is some loss to the
organization and coverage in the latest hrrr run. And this reflected in
the latest swody1 with the removal of the slight risk from southern WI.
Sunday - confidence...medium
could still be dealing with lingering morning convection
associated with the low level jet/500 millibar shortwave. Then will have to
see how the mesoscale details play out. Even the GFS sounding is
generated convective available potential energy over 2k Sunday afternoon. 850 jet is proggd to
focus more to our east and southeast as the day wears on. However
surface/850 front will still be hanging back to the north and a
west-southwest low level flow will reinforce moisture instability as the day
wears on. Cwasp showing solid middle 70s plus on all forecasts with the
GFS showing 80 plus by 00z. So given pronounced low level forcing
approaching with surface/850 front swody2 slight is warranted with
the greatest threat later in the afternoon.
Sunday night... forecast confidence medium.
Synoptic and mesoscale models are in fair agreement with
thunderstorms developing along the cold front as it tracks from
central WI down through southeast WI and northern Illinois late Sunday
afternoon through Sunday evening. I went with likely probability of precipitation in
southeast WI Sunday evening... but it may just be a broken line.
Showers and storms should wind down quickly after midnight as dry
air settles in behind the front.
Expecting high cape and shear just ahead of the front... enough to
produce large hail and damaging winds with any strong thunderstorms.
A tornado is a remote possibility given the limited low level shear.
Monday through Wednesday... forecast confidence high.
Northwest flow will dominate the Great Lakes region for the first
part of the work week as an upper low remains stationed over Quebec.
High pressure at the surface will allow for fairly light winds.
Temperatures will be around seasonal normal Monday and Tuesday... then
slightly cooler in the middle 70s Wednesday due to northeast breeze off
Thursday through Sunday... forecast confidence medium.
Models try to generate an mesoscale convective system over the northern plains that tracks
southeast through the Midwest Thursday and possibly another one into
Friday. The GFS is maintaining precipitation chances in southern WI with
this system and appears to have some convective feedback issues...
while the 12z European model (ecmwf) shifted the main chances farther south.
Expect dry high pressure to settle back into the Great Lakes region
by Friday night and keep precipitation chances minimal through the weekend.
Temperatures should be in the 70s.
Aviation/00z tafs/...VFR conditions at least into this evening.
Main focus for convective development arrives later tonight into
Sunday morning as more pronounced low level jet aims into southern WI. Still some
track uncertainty on mesoscale convective system but overall moist advection with 850 low level jet
and shortwave activity warrants the chance of storms later tonight
into Sunday morning. Pockets of MVFR ceilings/visibilities with any stronger
storms. Mesoscale details after overnight/early morning storms will
dictate storm evolution/severity on Sunday afternoon/evening as
surface low and cold front interact with potentially very unstable
airmass. In addition with low level jet strengthening overnight some
low level wind shear is possible.
Marine...Small Craft Advisory hoisted for Sunday as pressure
gradient tightens up ahead of approaching surface low pressure
system. SW winds should ramp up pretty quick in the morning and get
rather gusty. Cold front may spawn some strong thunderstorms late
Lm...Small Craft Advisory from 9 am to 6 PM CDT Sunday for lmz643>646.
Tonight/Sunday and aviation/marine...collar
Sunday night through Saturday...mrc