Weather

996 
fxus63 kilx 060540 
afdilx 


Area forecast discussion 
National Weather Service Lincoln Illinois 
1140 PM CST Monday Jan 5 2009 


Discussion... 
issued 853 PM CST Monday Jan 5 2009 


Latest surface analysis had high pressure centered near the Ohio/PA 
border...with a ridge axis extending west into Illinois. 
Meanwhile...a complex low pressure system was getting organized 
over the plains states. Aloft...a positively tilted trough extended 
from the upper Midwest to Baja California California. Several pieces of 
energy were rippling northeast along the east side of the trough 
toward Illinois. 


A low level dry airmass is in place across the forecast area per 
00z kilx and forecast soundings...with the easterly low level flow 
around the departing area of high pressure helping to reinforce 
the dry low level air. However...waa/isentropic lift is occurring 
ahead of an approaching upper impulse from the plains. The best 
lift is occurring along the Ohio River north to the I-70 corridor. 
This lift and that provided by the impulse itself will cause 
precipitation to break out to our southwest overnight. Expect this 
precipitation to spread into southeast Illinois overnight...but 
the dry air further north should result in a sharp northern edge 
to the precipitation. New 00z NAM soundings suggest that deeper 
moisture/lift will work into locations along/south of I-70 than 
expected earlier...as well as a warmer nose of air aloft. 
So...expect precipitation in southeast Illinois to begin as 
snow...and then rapidly change over to sleet or freezing rain. The 
area threatened is currently under a Winter Weather Advisory and 
this still looks good...but the icing threat appears more 
significant than before. Would not be surprised to see at least a 
tenth of an inch of ice in southeast Illinois by late Tuesday 
morning before surface temperatures warm above freezing. 


Going forecast looks pretty good overall. Main change will be to 
add freezing rain and sleet mention to southeast Illinois. 
Otherwise...plan on tweaking remainder of forecast per the latest 
and expected trends. 


Bak 
&& 


Aviation... 
issued 1140 PM CST Monday Jan 5 2009 


High pressure will continue to slowly lose its grip on the regions 
weather overnight as a storm system lifts toward the region from 
the southwest. The main storm track and heaviest precipitation is 
expected to stay well south of the terminals Tuesday and Tuesday 
night...so did not carry a mention of precipitation in the tafs at 
this time. However...as the low levels of the atmosphere moisten 
Tuesday freezing drizzle or very light snow may develop. The best 
chance for precipitation will be across kdec/kcmi which will be 
closer to the storm track. Forecast soundings suggest the moisture 
depth will be quite shallow Tuesday and may preclude precipitation 
forming at all. The precipitation potential will need to be 
reassessed over the next several hours as the potential becomes 
more clear. 


Bak 
&& 


Previous discussion... 
issued 145 PM CST Monday Jan 5 2009 


High pressure dominates the weather across the Midwest early this 
afternoon with the center of the cold air mass across central Illinois. Early 
afternoon temperatures ranged from the upper 20s to lower 30s with light winds. 
A frontal boundary located across the southern US coupled with shortwave 
energy coming out of New Mexico will help to spread a wintry mix of 
precipitation back north to the southern Midwest starting late tonight with 
precipitation chances highest across the southeast...remaining in the forecast 
into Tuesday night. 


Short term...tonight through Thursday 


Forecast concern this period will be late tonight through about noon Tuesday 
along the I-70 corridor and whether any headlines will be needed 
with the threat for freezing precipitation developing before the morning 
rush hour. 


Models very similar in the short term in developing precipitation from 
south to north aftr midnight as low level warm advection and isent 
ascent increases. BUFKIT soundings off the NAM-WRF suggest a brief 
period of light snow across the far southeast between 06z-09z with 
temperature profiles warming just above zero between 900-850 mb 
which would support a mix or even change to freezing drizzle. 
Soundings suggesting moisture depth not significant enough to 
support much more than drizzle or freezing drizzle along the I-70 
corridor during the morning...with more drizzle occurring during the 
afternoon as surface temperatures warm above freezing by then. Highest probability of precipitation 
will be mainly along and east of I-57 with progressively lower "chc" 
probability of precipitation to the west and north. Based off of the sounding profiles 
further north...may actually see a mix of drizzle and light snow 
from time to time. Soundings indicating even across the north...moisture 
depth not significant enough to support much more than super cooled 
water droplets through the column which would lean more towards the 
drizzle or very light snow. May actually have a little freezing 
drizzle mixed in as well west of I-57 into the early afternoon if surface 
temperatures are slower to warm than what is currently indicated. For now 
will keep the forecast as is further west and north...with chances 
of light snow. Will cut back on quantitative precipitation forecast/snow amts where we do get the 
snow as sounding profiles do not support much if any snow accumulate. 
In areas that do see any snow...looks to be less than an inch. 


Based on the above reasoning and the fact the wintry precipitation will be 
developing late tonight...just before the morning rush tomorrow...have 
decided to go with a winter weather advsy for light icing from 
Cumberland and Clark counties south starting just aftr midnight 
and running it through most of Tuesday morning. 


Models indicate a secondary shortwave dropping into the upper 
level trough late Tuesday night into Wednesday which will continue to support 
or low chance probability of precipitation. The surface low will start to deepen across the 
eastern Great Lakes as this feature helps to close off a 500 mb 
low over southern Wisconsin by noon on Wednesday. This should draw down 
some colder air again into the forecast area with a decreasing 
trend in probability of precipitation Wednesday night into Thursday as the upper trough axis gradually 
shifts east of our area. 


Long term...Friday through Monday 


Leaning more on the European model (ecmwf) this period with temperatures and timing 
of the next precipitation maker into the area later Friday into Friday 
night the main forecast concern. Rather active flow pattern across 
the lower 48 this period as the next significant shortwave races 
southeast into the plains Friday and to our east by Saturday morning. 
Moisture return ahead of this feature will be limited but still 
Worth the low chance probability of precipitation we currently have going. Strong warm 
advection pattern ahead of the shortwave on Friday may bring in 
a brief period of rain or snow followed by some snow showers or 
flurries late Friday night into Saturday morning as much colder 
air tracks southeast into central Illinois. Warmest temperatures across the 
southwest Friday afternoon may not be warm enough as models suggest 
850 temperatures of +5 to +8 degrees c pushing into our west and south 
by afternoon. Will up temperatures...especially across the west and south for 
Friday afternoon in advance of this system. The medium range models 
already showing the next fast moving wave apprchg the area on 
Monday which may bring some precipitation into the area later in the day 
or night. Will let a few more model runs pin down the timing a little 
better with this feature before making any changes to day 7. 


Smith 




&& 


Ilx watches/warnings/advisories... 
Winter Weather Advisory until 11 am CST Tuesday for ilz062-063- 
066>068-071>073. 


&& 


$$ 

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