Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Milwaukee/Sullivan WI
1256 PM CDT sun Jul 13 2014
A cold front is dropping through southern WI this afternoon. The
leading edge of the frontal boundary is led by weak 500mb
vorticity advection and the 850mb thermal boundary. It is evident
on visible satellite by a line of broken clouds. Winds are gustier
out of the west behind this first boundary...along with scattered
to broken cumulus clouds. Then a secondary boundary will drop
southeast through southern WI later this afternoon...also apparent
on satellite by a more distinguished line of clouds. Behind the
back edge of this frontal boundary...there are much lower
dewpoints...fewer clouds and a wind shift to the northwest.
I am going to leave the isolated shower mention in the forecast for
early afternoon until that second boundary and drier air gets in
here. The dewpoints are still in the 60s with decent convergence.
Shallow cloud depth is the inhibiting factor for getting many
showers...but there have been a few higher returns on the radar in
the Sheboygan and Manitowoc areas already today.
Temperatures are on track to reach around 80 today before the wind shift
and cooler/drier air come in.
A brief -shra is possible along a cold front moving southeast
through southern WI this afternoon. Scattered to broken VFR ceilings
through late afternoon...becoming clear this evening.
Previous discussion... /issued 325 am CDT sun Jul 13 2014/
Today and tonight...forecast confidence - medium.
Lingering low level moisture and increasing boundary layer winds
causing scattered-broken stratus formation over most of WI. Stratus expected
to continue into the early morning before drier air aloft causes
stratus to lift and thin. Stronger surface cold front over northwest WI will
sweep southeast and move across southern WI during the afternoon. Enough low
level moisture and frontogenetical forcing along this front to set
off at least isolated -shra in the vicinity of the front. Before front
passes through...stratus should give way to partly to mostly sunny
skies which should help temperatures climb back into the upper 70s
to lower 80s.
A quiet night lies ahead as the first wave of cooler air settles
into southern WI. Second surge of cool air and deeper moisture will
accompany approaching upper low on Monday.
Monday - confidence...medium
more pronounced intrusion of vorticity as an unseasonably strong
upper trough of low pressure digs southward into the Great Lakes.
Differential cyclonic vorticity advection combined with decent
lapse rates and some cape should result in potential for rain showers. 850
temperatures tumble to the 5-7c range by 00z. The European model (ecmwf) has caught up to
the quicker GFS and NAM with respect to the pace of the cold
Tuesday - confidence...medium
500 millibar closed low will be well to the east and northeast of
WI. However lingering cyclonic flow suggests a few ripples to
contend with. All models show light quantitative precipitation forecast in this regime especially after
18z. Will have some slight probability of precipitation in the west and chcy in the east.
Low level thermal trough modifies a bit. 925 temperatures in the teens
celsius so will go with middle/upper 60s.
Wednesday through Saturday - confidence...medium
upper flow still showing a cyclonic curvature Wednesday though
surface high draws closer and airmass dries out. May need a small
chance or sprinkles especially in eastern and NE County Warning Area as we get closer but for
now will go with the dry collab with surrounding offices. Then
fairly benign northwest flow transitions to a weak zonal regime for end
of the week. Not ready to buy into the light...spotty and random
nature of quantitative precipitation forecast being thrown around by the GFS. Prefer the European model (ecmwf)
with overall low level ridging dominating into Saturday with the
dry look. Any renewed warm air advection/low level jet induced precipitation looks to hold off
until beyond this forecast period.
Aviation/12z tafs/...sct-bkn IFR stratus should thin and dissipate
early this morning as boundary layer winds continue to increase and
mix drier air to the surface. A brief -shra may affect taf sites later
this morning and afternoon as stronger cold front sags through southern WI.
Marine...webcams not showing any significant fog on the Lakeshore
areas since winds have picked up from the northwest. Will cancel marine
dense fog advisory with new near shore issuance. However patchy fog could
continue at least through this morning over the near shore waters
as dewpoints will linger in the low to middle 60s...which is likely 5
to 10 degrees warmer than water. Stronger cold front will pass across near
shore this afternoon with developing pressure gradient behind front
resulting in northwest winds approaching Small Craft Advisory levels
Monday into Tuesday.
today/tonight and aviation/marine...mbk
Monday through Saturday...collar