NWS Area Forecast Discussion
FORMAT: Decoded Original

Area Forecast Discussion
National Weather Service Chicago/Romeoville Illinois
320 PM CST Monday january 28 2013

.Discussion,
257 PM CST

Very mild and moist conditions will continue to advect north across
northern Illinois/northwest in this evening, as the atmospheric river
progged to stretch from the western gulf of Mexico north to the Ohio
valley/Great Lakes regions establishes itself.  Temperatures this afternoon have
remained in the mid/upr 40s across the cwfa with a few areas south
of I-80 warming into the low 50s.  Dew points have steadily risen
throughout the afternoon into the low/mid 40s.  Visible imagery shows
considerable cloud cover across the region, however a small break
in the clouds early this afternoon did occur across northern Illinois
stretching north into southeast Wisconsin and central Lake Michigan.  

The warm frontal boundary has already pushed north to central Iowa
stretching east through central Illinois/Indiana.  South of this line across
central Missouri temperatures have warmed well into the 70s with dew points in
the upper 50s, which is exceptionally rare for late jan.  This record
warmth will approach the cwfa overnight, and provide the expected
record highs for Tuesday across the region.  

Overnight the focus remains on the increasing low level moisture and
potent jet.  850mb winds will steadily increase from the
south/southwest to around 30-40kts, then Tuesday will continue to
increase.  Directional shear will be steadily increasing throughout
the night as elevated instability begins to indicate CAPE values
around 200-400j/kg by 09z tue.  Have maintained mention of convection
overnight.  The other challenge remains for this evening through the
overnight, residing with potential for dense fog.  

A few obs this afternoon continue to indicate reduced vsbys around
1sm, with pnt hovering around 1/4sm.  Given the addtl moisture lifting
north advecting across slowly thawing and moist surface conds, it
appears areas of dense fog will redevelop later this eve.  The
challenge is will be fall to dense criteria and for how long with
the expected precip.  So have opted to hold off on issuing a dense
fog at this time, given the lower confidence on placement/timing.  

Confidence, medium.  

Tuesday,
Tuesday appears to be the day that will produce record temperatures with
guidance pushing highs into the 60s, and if skies were to thin
slightly Tuesday afternoon a few areas along and south of a pontiac to
Kankakee line could warm to psbly the mid/upr 60s.  

Temperatures will only be one parameter to be concerned with tue, as
abundant moisture and impressive low level speed/directional shear are
poised to push overhead.  At this time spc continues to have the
south/southeast portions of the cwfa progged for a slight risk of
severe weather tue.  Given such a dyNAMic and contrasting airmass
between the warm/moist and cold, this appears to be such an
anomalous setup that it wouldn/t be too much of a surprise for
severe weather to occur.  The largest concern is for damaging
winds, although with the strong directional low level shear and a moist
boundary in the vicinity of the cwforecast area, a few thunderstorm cores
could see rotating updrafts.  Which opens the door for an isolated
tornado Tuesday afternoon.  While the first half of Tuesday looks to be
quiet, things will rapidly change for the afternoon/eve hours as the
shortwave approaches from the west.  


The other challenge for tue/tue night will be on the potential
for heavy rainfall, which could lead to flooding.  Guidance
continues to show some corrections with respect to the axis of
highest precipitation totals, however it appears to be zeroing in on a
central Illinois/Indiana stretching north through northeast Illinois/northern in.  
The combination of the atmospheric river, pwat values progged to
approach 1.5" which ensembles suggest is 5 to 6 sigma above
climo, have opted to proceed with a Flood Watch.  Further
complicating the flooding potential will be the still moist top few
inches of soil.  While it appears the top few inches of soil will
thaw, it doesn/t appear the entire column will thaw.  Which will
further exacerbate the runoff or ponding concerns.  At this time
guidance points towards quantitative precipitation forecast totals of around 1.00" along and northwest
of a lasalle to lake county Illinois line.  East of this line forecast
precipitation totals ramp up towards 2 inches.  At this time the highest
rainfall totals appear to be along and east of a Ford county Illinois to
lake county in.  

Frontal passage has slowed slightly with the latest
guidance, arriving across the northwest cwfa around 00z
wed, approaching the central cwfa by 6-9z, and should eventually
push through the far southeast cwfa by 12z Wednesday.  

Confidence, medium/high.  

Wednesday through Sunday,
a return to winter is poised to slide back across the region Wednesday and
continue through at least Friday, with a gradual moderation sat/sun.  

High amplitude anomalous 500mb ridge will steadily pivot east, as a
deep trough slowly slides east across the Great Lakes Wednesday and
lingers through the first half of the weekend.  Further west a pac-nw
ridge continues to be advertised to weaken throughout the next few
days, which may aid in further de-amplification of the Great Lakes
trough by sat/sun.  

Much colder air will continue to advect across the Great Lakes
region, as a potent thermal trough of -18 to -20 degree c continues to
push overhead thur.  Guidance has indicated some weakening in the
potency of the cold air.  Much colder air will be displaced to the
north across the upper Midwest/northern Great Lakes, where -24 to
-26 degree c air at 850mb is progged.  This is rather impressive given
how anomalously warm Tuesday will be to transition in a 48hr period from
warm/moist to arctic air thur.  In the later periods confidence
remains high that temperatures will slowly moderate back to
seasonal, possibly leaning above seasonal conds by sun.  

Extended precipitation forecast outlook, after the potent shortwave
progged to traverse the region cyclonic flow appears to linger
through Thursday night.  Then for fri/sat anti-cyclonic flow will begin to
push the prolonged wrap-around moisture to the northeast of the cwfa.  There
appears to be a weak vorticity maximum that may rotate around the 500mb
vort, and slide across the upper Midwest towards the cwfa wed/thur.  
Guidance suggests that enough lift could be around in this timeframe
to push parcels into the dgz, warranting some light accums Wednesday to
early thur.  Then dry air pushes in for the weekend with the
aforementioned temperatures returning to seasonal conds.  

Confidence, medium/high.  

Beachler

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