Area Forecast Discussion
Issued by NWS Central Illinois
FXUS63 KILX 011746
Area Forecast Discussion
National Weather Service Lincoln IL
1146 AM CST Sat Mar 1 2014
ISSUED 1011 AM CST Sat Mar 1 2014
Cold front has reached the I-70 corridor as of 10 am, and will
continue to move through the southeast CWA into early afternoon.
High temperatures further northwest have already occurred, and
falling temperatures will be the trend through the afternoon as
Arctic air upstream pushes southeastward. Large area of light
snow has set up over the north half of Iowa and will be tracking
eastward much of the day, before taking a southeast nudge later
this afternoon. Have adjusted PoP`s for the afternoon to focus
them on the 3-6 pm time frame as a result. Other adjustments to
hourly temperature trends have been sent.
Will hold onto the Winter Storm Watch until all of the 12Z model
guidance has arrived, before making the decision on how to convert
ISSUED 1146 AM CST Sat Mar 1 2014
Complicated scenario for this TAF set. MVFR conditions this
afternoon have briefly popped up to ceilings over 3000 feet in
some places and into IFR range in others. Also have had some
recent reports of some freezing fog just northwest of KSPI. Main
onset of widespread IFR conditions will be this evening, as snow
rapidly spreads across the TAF sites in the 01-05Z time frame.
Latest high-res models diminish the heavier snow for a time around
KPIA/KBMI late tonight before the next surge northward during the
daylight on Sunday. Have added blowing snow mention after 12Z as
winds ramp up into the 15-20 knot range.
ISSUED 328 AM CST Sat Mar 1 2014
SHORT TERM...Today through Sunday
At 08Z/2AM Arctic cold front was just entering the NW CWA
accompanied by a narrow band of light snow, which has briefly
dropped vsbys to 3-5 miles. Latest trends and short range models
indicate what snow there is will dissipate as the front continues
to shift southeast this morning. Did include low chance pops this
afternoon across the northern third where models show weak omega
behind front off weak dpva.
As the Arctic front shifts south towards the Ohio Valley this
evening, a strong 140+ kt 300mb jet sets up over the Great Lakes.
This in conjunction with a tightening mid-level baroclinic zone to
our south sets up good moisture transport into a region of strong
frontogenesis over Iowa and northern IL. As this zone gradually
shifts south this evening, expect snow to increase in coverage and
intensity. Given the strength of projected frontogenesis and a
hint of negative EPV above this zone would expect some banding and
enhance snowfall rates overnight. 00Z guidance has trended more
progressive the the band of stronger forcing, shifting this south
of I-72 Sunday morning. This seems reasonable given the fact this
is an open wave at 850/700 mb. Snow will likely continue over the
southern half Sunday afternoon, and into Sunday night far SE, as
additional shortwave energy ejects from southern plains trough.
Thermal profiles off consistent GFS/ECMWF show snow for much of
the CWA, except south of I-70 where a sleet/freezing rain mix is
possible for several hours after onset. Have discounted aggressive
warmer NAM thermal structure (which would bring mixed precip north
of I-72) due to open wave feature. This does lead to lower overall
confidence in p-type than would be desired at 24 hrs. Given
precipitable water values 0.6-0.9 and projected forcing, model qpf
raning from 0.3 NW to 0.75 SE looks reasonable. Used 12:1
snow:liquid ratio, except 10:1 south of I-70 due to thermal
profiles. For the northwest half of the CWA projected totals of 4-6"
in 18+ hours fall short of warning criteria (6"+ in 12 hours or
8"+ in 24 hours) and have issued winter wx advisory for 6 PM
tonight through 6 PM Sunday. For areas southeast of a Shebyville
to Monticello to Rantoul line, 5-8" forecast over a 24+ hour period
also falls short of warning criteria. The potential for mixed
precip does complicate things, as warning sleet/ice accums would
be possible under NAM solution. With uncertainty remaining in
this area will keep winter storm watch going (midnight tonight
through 6 AM Monday morning), and let day shift make the call
after looking over 12Z data.
LONG TERM...Monday through Friday
Bitterly cold air moves into the region behind the departing
storm. Late season Arctic high pressure will produce temps 20-25
degrees below normal, with lows likely dropping below zero across
the north Monday and Tuesday mornings. A weak shortwave dropping
through in northwest flow behind the high, will bring a chance of
light snow to the northern half of the CWA Tuesday. With low
moisture availability have kept pops to slight chance. While temps
will remain below normal through the week, readings will gradually
moderate as mid level flow flattens. Dry weather is expected as
split flow keeps major systems well to our north and south.
WINTER STORM WATCH from late tonight through late Sunday night
WINTER WEATHER ADVISORY from 6 PM this evening to 6 PM CST
Sunday FOR ILZ027>031-036>038-040>043-047>053.