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000
FXUS63 KILX 080545
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1145 PM CST Fri Mar 7 2014

.DISCUSSION...
ISSUED 855 PM CST Fri Mar 7 2014

A cold front continues to slowly sink across central Illinois this
evening. The front is expected to clear our counties in southeast
Illinois by early Saturday morning. The latest surface analysis
has the front lying roughly along the Illinois River Valley.

A very dry low/mid level airmass, per 00Z ILX and area forecast
soundings, is having the front come through dry. However, deeper
moisture trails the front by a few hours, as noted by the lower
cloud ceilings upstream from the front. Additional post frontal
short wave energy should be able to squeeze some light
precipitation out of this deeper moisture late tonight into
Saturday as it spreads across the area. While the precipitation
should be very light, the type is a little less certain. When the
precipitation threat first arrives, forecast soundings suggest
enough of an elevated warm layer to support mostly rain at the
surface, with surface temperatures above freezing as well. By late
tonight/early Saturday, the elevated warm layer will be largely
eroded in most areas through evaporational cooling, trending the
precipitation toward snow (although some freezing rain/sleet can`t
be ruled out).

Going forecast is in good shape overall. Updated forecast
primarily to slow the temperature drop tonight, which also delayed
the risk of freezing rain in the north until well after midnight.
Other tweaks were mostly minor and revolved around the expected
hourly trends through the night.

Bak
&&

.AVIATION...
ISSUED 1145 PM CST Fri Mar 7 2014

A cold front continues to cross central Illinois late this
evening, having already cleared the KPIA and KBMI terminals. It
will pass through the remaining local TAF sites within the next
few hours. The front will initially come through dry, with the main
change a wind shift from southwest ahead of the front to northerly
in its wake. VFR conditions should prevail until FROPA, but MVFR
conditions are expected to develop a few hours after FROPA and
linger through much of the remaining 00Z TAF valid time. It is
still unclear how much precipitation will accompany the lower
cigs, but overall it is not expected to be much. Any precipitation
that does fall could be in the form of snow, freezing rain, or
rain. However, since the expected coverage and amounts of
precipitation are likely to be so low, have not gone above a VCSH
in the TAFS.

Bak
&&

.PREV DISCUSSION...
ISSUED 200 PM CST Fri Mar 7 2014

SHORT TERM...Tonight through Monday

Main forecast concerns this period include precip timing and type
late tonight thru Saturday morning, and then how warm to go with
temperatures as we head into early next week.

Water vapor loop early this afternoon showing our next upper trof
getting itself organized over parts of western Colorado. Models are
in good agreement with this trof and attendant cold front which will
swing across our area late tonight and Saturday. Forecast soundings
across the west and north where precip will arrive first after
midnight tonight starts our rather dry but as the night wears on,
forecast soundings suggest the 850-600 mb layer should moisten
sufficiently enough for some very light snow or freezing rain to
occur. Forecast soundings west of the Illinois River support more
very light snow, with areas east of the river seeing a higher
potential for a brief period of very light freezing rain by dawn
before soundings support more of a mix of very light snow and rain
thru the noon hour. Further south, more of a rain and snow sounding
early Saturday morning before transitioning over to a chance of light
rain for the afternoon hours. Best 700-500 mb layer QG forcing and
250mb jet support suggest the time frame for this light precip would
be from 10z-16z, and then after 16z across the remainder of the
forecast area when soundings indicate mostly light rain. Total QPF
thru this event looks to be less than a tenth of an inch with any
lingering threat for precip in our southeast counties exiting by
late Saturday evening as the better upper forcing shifts away from
our area.

Pacific high pressure settles in for the remainder of the weekend
bringing quiet weather along with near normal temperatures on
Sunday afternoon. After a rather chilly start to the morning on
Sunday, we should see afternoon temperatures climb into the mid
40s north to the lower 50s south and west. The fair weather system
will shift to our south Sunday afternoon allowing a return flow to
set up by late afternoon setting the stage for a very mild Monday.
A northern stream wave will streak across the northern Great Lakes
on Monday with a southwest wind flow ahead of the system in our
area helping to push afternoon temperatures into the mid 50s north
to the mid 60s south and west! As the upper wave shifts off to our
northeast late Monday, a cold front will shift southeast into the
lower Great Lakes Monday night and through our area by Tuesday
morning.

LONG TERM...Tuesday through Friday

Focus this period will be on the system forecast to push across
the middle of the country late Tuesday into Wednesday. Models still
have difficulties with respect to amplitude of the southern stream
energy and any phasing that may occur with the more dominant northern
stream trof. The last few runs of the ECMWF have consistently shown
a more phased look to the trof, while the GFS, UK and GEM models
were showing a more progressive look to the northern stream wave
with very little if any phasing occurring. More often than not, the
ECMWF seems to perform better with these split flow patterns and
phasing issues compared to the other models. However, the system is
still almost 5 days out so am not ready to jump on just any one
solution at this point.

With a lean more towards the ECMWF solution, will go with the slower
onset of precip across our area with the main threat for rain starting
Tuesday night with any transition to snow or a mix of rain and snow
holding off until very late Tuesday night far northwest and over the
central sections of the forecast area Tuesday. Temperatures by midweek
will cool down significantly but it appears this cool down will not
be around for very long as the pattern deamplifies for a time by the
end of the week thanks to a powerhouse jet, with models depicting speeds
of 180-200 kts across the central Pacific by the end of the week. With
a more zonal flow pattern expected late in the period, our air masses
will be more of Pacific origin rather than Polar or Arctic. So temps
by Friday should be back to slightly above normal. After our precip
threat on Wednesday ends, we will keep the extended dry through Friday.

Smith


&&

.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$