Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Milwaukee/Sullivan WI
917 PM CDT Sat Aug 30 2014

Update...upper level short wave over central WI aided by weak
convergence in vicinity of surface cold front resulting in persistent isolated
-shra near kpdc/kovs. These -shra moving to the east and will affect
portions of Iowa/Sauk counties towards 03z. Infrared imagery showing
cooling cloud tops and weak elevated instability so possible these
-shra will hang on and affect kmsn area around 05z or so.

Otherwise...expect areas of fog to develop toward and after
midnight but may be tempered by possible stratus development as
weak frontal boundary slows down over southern WI later tonight. In
addition...surface observation show quite a bit of stratus upstream over
central WI behind approaching cold front which will likely affect northern County Warning Area
for a time. Hence not enough confidence to expect more widespread
dense fog late tonight but most favored area would be in the south. If
stratus were to develop late tonight...will likely linger into Sun
morning over eastern areas around and north of weak front.


Aviation/06z tafs/...low level convergence along approaching
frontal boundary may trigger areas of IFR stratus later tonight.
Otherwise...higher stratus/middle clouds in the wake of cold front
associated with weak cold air advection may affect taf sites
overnight as well. Before stratus develops...will likely be areas of
fog which could become dense for a time...especially at kenw.


Previous discussion... /issued 304 PM CDT Sat Aug 30 2014/

Short term...

Rest of this afternoon and tonight - confidence...medium
surface trough extends into southern WI from departing low pressure to
our northeast. Low level convergence has been maximized vicinity this
boundary with numerous showers. One shortwave/speed maximum lifting
northeast with another vorticity wrapping east from eastern Minnesota/western WI. Will
linger these rain showers into the early evening before influence of second
vorticity and loss of heating put an end to the precipitation. Then focus shifts
to clearing trend and fog/stratus development later on. Dew points
remain up and low level moisture has increased due to the numerous
rain showers. So with a weak ridge of high pressure sliding across expecting
some fog and stratus to take shape. This played out on low level relative humidity
forecasts and MOS guidance. Some good consensus with the not
much change to going weather grids in this regard.

Sunday - confidence...medium
weak surface ridge shifts east. Southerly flow starts to develop.
Any influence of approaching cold front from the plains will remain
to our west. By days end 925 temperatures modify to 21-24c. So warmer temperatures
with some 80s returning...especially in the western and central County Warning Area.

Sunday night and Monday...forecast confidence is high

A rather strong shortwave exits the plains and moves into the
upper Mississippi Valley. The lead shortwave moves across
Wisconsin Monday with the best dynamics just to the north as a
110 knot jet maximum moves to the north of Lake Superior...with a
second speed maximum pushing across Wisconsin. Strong upper
divergence mainly near and just to the west of the Mississippi
River late Sunday night weakens somewhat and lifts northeast
across Wisconsin Monday. The strongest upward motion pushes into
south central Wisconsin by sunrise and across the southeast
Monday morning. The 850/700 mb thermal ridge moves across southern
Wisconsin late Sunday night and Monday morning. A 50 knot 850/700
mb speed maximum moves across southern Wisconsin...but the best
convergence is to the north.

The surface low moves from just to the west and north with the
cold front exiting the far southeast around noon.

The NAM keeps the heavy rain just west Sunday night and just north
Monday morning. The GFS/European model (ecmwf) brings the band of moderate
precipitation amounts more into into south central and southeast
Wisconsin. The nsharp forecast soundings indicate mainly a
marginal severe potential.

Monday night and Tuesday...forecast confidence is high

The main middle/upper trough axis enters Wisconsin Monday night and
reaches Lake Huron late Tuesday afternoon. There is still some
upper divergence and 700 mb upward motion southeast Monday
evening. The NAM and GFS still have some precipitation...with the
NAM bringing moderate amounts with zero to 1 km mixed layer convective available potential energy
around 900 joules/kg. Middle and low levels then dry. There is still
some fairly steep zero to 3 km lapse rates of 8.4 celsius/kg on
Tuesday...but low level relative humidity is only around 64 percent with middle levels
at 44 percent. Models are dry on Tuesday.

Long term...

Wednesday and Thursday...forecast confidence is high

The GFS and European model (ecmwf) have a zonal flow across the U.S./Canadian
border area as another trough digs a little across the northwest
U.S. The upper flow becoming a little more west/southwest later
Wednesday and Thursday.

Weak high pressure moves across the Great Lakes early Wednesday
with a warm front and precipitation approaching southern
Wisconsin Wednesday night and Thursday on the GFS with moderate
amounts of precipitation.

The 00z European model (ecmwf) is slower and drier...focusing the precipitation
over central and northern Wisconsin Thursday.

Friday and Saturday...forecast confidence is medium

The GFS and European model (ecmwf) have a west/southwest flow across Wisconsin
Friday with a large ridge across the southern U.S.

The 12z GFS and 00z European model (ecmwf) are not as consistent with each other
by Saturday with the shortwaves across Canada.

By Friday night the European model (ecmwf) pushes a cold front into southern
Wisconsin...which is slower on the GFS. Later Friday and Saturday
both models push the front to the south...with the GFS mainly over
central Illinois and the upper Ohio Valley...with the European model (ecmwf)
farther south having the front reaching Kentucky. As a result both
the GFS and European model (ecmwf) have precipitation...with moderate to heavy
amounts Friday...with the European model (ecmwf) drying things on Saturday.

Aviation/00z tafs/...

Low pressure will continue to move northeast away from WI
tonight. A large cumulus field...mostly VFR...persists due to cyclonic
flow aloft. Expect this to gradually dissipate as the evening
wears along. A narrow ridge of high pressure builds in bringing a
lighter wind regime. Fog/stratus development still on track with
the low level moisture...clearing skies and the light wind regime.
With the NAM/GFS MOS in decent agreement brought visibilities down into
the IFR category...with a few LIFR conditions not out of the
question. Influence of approaching cold front on Sunday holds off
to our west during the day so after any morning fog/stratus Burns
off...expect a quiet VFR day before thunderstorms and rain returns Sunday night into
Labor Day.

Marine...hoisted a short term Small Craft Advisory from Wind Point
south to Winthrop Harbor due to southwest winds gusting to 22-25 knots
at times. This in lingering tighter pressure gradient ahead of surface
trough axis. Expect the gradient to become more baggy into this evening
as high pressure draws closer.


Mkx watches/warnings/advisories...



National Weather Service Glossary of Abbreviations