Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
253 PM CDT Monday Apr 27 2015
Tonight and Tuesday...forecast confidence is high.
Diurnal cumulus clouds should diminish soon after sunset across
eastern portions of the area. Gusty winds near Lake Michigan should
also subside by sunset...with loss of low level mixing.
Models are in good agreement with high pressure moving slowly
southeast into Wisconsin tonight and Tuesday. The high will continue
to bring quiet weather to the area. Expect clear skies tonight into
Tuesday morning. May see diurnal cumulus development Tuesday
afternoon in western portions of the area...along with some middle
to high clouds late in the day.
Lows tonight should drop into the middle 30s in most areas...though may
get a little colder in low spots with light winds. Slightly milder
temperatures are forecast for Tuesday...with middle to upper 60s well
inland. Onshore winds will continue to keep Lakeshore areas in the
upper 40s to lower 50s.
Tuesday night through Wednesday... forecast confidence is medium
There is good agreement that a deep 500 mb closed low will drop
south through Wisconsin and Michigan on Wednesday afternoon. The
most recent guidance has this low forecasted to get considerably
deeper than what has been shown in recent days. Closer to the
surface...an inverted trough/low will swing south out of Canada
through lower Michigan grazing eastern Wisconsin. There will be
considerable low level moisture associated with the low as it
moves through the Great Lakes...but there is uncertainty regarding
the western extent of the moisture.
The GFS...NAM...and Gem are more aggressive with the strength of the
system both in the upper and lower levels. In these solutions the
low is able to get more moisture into eastern Wisconsin. The GFS
and NAM both bring measurable precipitation to the forecast area on
Wednesday afternoon. However the Gem is dry across the forecast
area and the European model (ecmwf) has a weaker low...with the best precipitation chances
south and east of southeastern Wisconsin. Given the previously dry
forecast and poor run to run continuity for all guidance...will be
keeping rain out of the grids for now. However...if models
continue to keep higher precipitation chances...rain will need to be
included in future forecasts.
Thursday and Friday...forecast confidence is medium
High pressure at the surface with the upper level flow transitioning
to a more zonal pattern should bring pleasant weather to the
forecast area for the end of the week. Portions of the middle-levels
become saturated at times on Friday...but due to the overall lack of
forcing...this moisture will manifest itself as clouds rather than
precipitation. Weak gradient flow at the surface should allow for a
lake breeze to develop on both days.
Friday night through Sunday...forecast confidence is medium
There is generally good agreement among the guidance that the upper
level flow will remain largely zonal through the weekend with a few
modest shortwaves pushing through the region. However...while the
there is general consensus on the upper level pattern in this
period...there is considerable disagreement on what will occur at
Overall it appears that some sort of surface boundary will push
through the region over the weekend...but there is still uncertainty
regarding the the exact timing and intensity of the feature.
Accordingly...have at least slight chance precipitation running through the
weekend to account for all plausible outcomes. As we get closer to
the weekend...guidance should consolidate on a more specific time
for rain chances. Finally...there is modest instability in both the
European model (ecmwf) and GFS on both Saturday and Sunday afternoon...so have
included a slight chance of thunder on both days.
Monday and Monday night...forecast confidence is medium
The European model (ecmwf)...and especially the GFS are still showing a chance of
showers and thunderstorms on Monday. Both models begin to break down
the zonal flow aloft by bringing a high amplitude shortwave
through the region. Additionally...both models bring a surface low
through southern Wisconsin with mu cape in excess of 1000 j/kg in
the afternoon. Therefore have included showers and thunderstorms
High pressure will slowly slide southeast into the region tonight
and Tuesday...continuing the quiet weather. Diurnal cumulus clouds
should linger until around sunset at Waukesha. Gusty east winds at
the eastern sites will diminish by sunset.
Light winds are expected tonight into Tuesday...becoming east at the
eastern sites later Tuesday afternoon. May see diurnal cumulus
development near Madison in the afternoon...mainly scattered in
Northeast winds of 10 to 20 knots are expected until sunset mainly
south of Wind Point. These winds would generate waves of 2 to 4
feet. No Small Craft Advisory is needed. The winds and waves will
weaken early tonight.
Very low relative humidity values of 25 to 30 percent are expected
once again inland Tuesday afternoon...higher near the lake with
onshore winds. Winds will be light across the area. Relative
humidity values may climb slightly Wednesday...but should still be
in the 30 to 35 percent range in the afternoon.
Tonight/Tuesday and aviation/marine...wood
Tuesday night through Monday...bsh