Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Milwaukee/Sullivan WI
1014 PM CDT Monday Mar 10 2014
cold advection to not increase until after sunrise Tuesday when northerly
winds increase in response to cyclogenesis over the central Great
Plains. 18z and 00z NAM continues to track strong shortwave trough
currently over the intermountain west farther northward than other
models. There is strong interaction with the upper trough digging
southward from Canada resulting in a strong middle level confluent zone
and frontogenesis over southeast WI and northern Illinois. Heavy snow is the result.
This however remains an outlier so no changes to light rain
changing to snow with 1-3 inches forecast...higher amounts in far
southeast WI. The midnight shift will assess newer models and make
Aviation/06z tafs/...VFR conditions through Tuesday am...then ceilings of
1.0-3.0 kft developing in the afternoon along with light rain as
upper level front develops over southern WI. Northerly winds will cool temperatures
so rain will change to light snow in the late afternoon or
evening. 1-3 inches of accumulation.
Previous discussion... /issued 322 PM CDT Monday Mar 10 2014/
Tonight and Tuesday...forecast confidence medium
Clouds will increase this evening as surface high pressure departs
the region. Despite the NAM and GFS being too cold today synoptic
models are in better agreement with tonights low temperatures.
Middle level frontogenesis associated with low pressure moving south
of the state Tuesday may bring some precipitation to the northern County Warning Area
Tuesday morning and then more widespread precipitation across southern
Wisconsin starting during the afternoon.
NAM and GFS continue their cold bias so favored guidance with higher
temperatures of upper 30s north to around 40 in the south. Therefore
precipitation should stay all rain until the late afternoon or evening.
Light rain may start as early as 7 am as a west to east band in the
northern County Warning Area as indicated by some of the mesoscale models. This is
consistent with an area middle level frontogenesis. This forcing is
shown to shift south and intensify across southern WI during the
afternoon and evening. Divergence associated with the right entrance
region of an upper level jet also increases during the day. As a
result the greatest chance for precipitation across the area is during the
late afternoon and the evening. Southern Wisconsin may not see
precipitation until later in the evening.
Tuesday night and Wednesday...forecast confidence...medium
A shortwave drops into the upper and middle Mississippi Valley
Tuesday night and reaches the western Great Lakes Wednesday.
Southern Wisconsin is in the right entrance region of the
departing upper level jet Tuesday evening. This results in mainly
weak...upper divergence...but is up to moderate values toward
Kenosha around midnight.
700 mb upward motion is weak Tuesday evening...but is moderate
along the Illinois border region and over the far southeast into
midnight. 700 mb motion become neutral or weak downward late
Tuesday night. Saturated 700 mb relative humidity exists over the far southeast
through midnight. Higher relative humidity exists in the 925/850 mb layer during
The models are depicting more precipitation over the south mainly
before midnight...but extends longer toward Kenosha. As a result
up to an inch of snow is possible...with maybe two inches toward
The main surface low moves across the Ohio River region with weak
high pressure across Wisconsin. With the proximity of the middle
level trough there could still be some very light instability snow
showers on Wednesday. However NAM soundings show a shallow low
level inversion with a fairly strong inversion around 4 thousand
feet...so any precipitation would be isolated and very light. Cold
air advection occurs Wednesday.
Wednesday night and Thursday...forecast confidence...high
The main middle/upper trough moves across the northeast U.S.
Temperatures should drop Wednesday night before warm advection
begins Thursday as low pressure moves across south central Canada.
Most models have a around a hundredth of precipitation Thursday
morning until and a bit longer toward Sheboygan.
Thursday night and Friday...forecast confidence...high
A northern stream shortwave moves into the upper Great Lakes by
Friday morning should start mild before weak cold advection
Friday afternoon as the low moves across the upper Great Lakes
or southeast Canada.
Saturday through Monday...forecast confidence...high
A large upper level trough extends over the Hudson Bay area that
extends farther south into the Great Lakes and upper Ohio Valley.
The GFS moves this trough to the east Monday...but the 00z European model (ecmwf)
keeps the trough axis back across the Great Lakes.
Weak surface high pressure extends into the Great Lakes...with
colder temperatures. The 12z GFS brings precipitation into
southern Wisconsin Sunday...but the 00z European model (ecmwf) and 06z dgex are
both slower or farther south...and are dry. Prefer the drier
VFR conditions will continue through tonight under middle to high level
clouds. Precipitation will slide into the area Tuesday morning...likely
lingering into Tuesday night. Biggest questions are with precipitation type
and amounts...as there are still decent differences among models.
Looks like it will likely start as rain...then transition to snow in
the afternoon or by early evening...with an inch or so of
accumulation by Wednesday morning. Highest accumulations will be in
the southeast toward Illinois.
With some lingering dry air in the low levels...some question as to
when lower ceilings will develop tomorrow. VFR may linger into the
afternoon...but at least MVFR likely by late afternoon or evening as
the low levels saturate and precipitation transitions to snow.
Expect quiet conditions through the day Tuesday...with Small Craft
Advisory conditions possible Tuesday night and Wednesday as north to
northeast winds increase. Latest satellite shows mainly open water
across the nearshore waters...so some waves possible again for now.
Northeast winds tomorrow night may push some of the ice back into
tonight/Tuesday and aviation/marine...ddv
Tuesday night through Monday...hentz