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Area forecast discussion 
National Weather Service Lincoln Illinois
904 PM CST Thu Dec 18 2014

issued at 900 PM CST Thursday Dec 18 2014

The surface ridge axis will remain nearly stationary from northwest to southeast
across Illinois for the next 24 hours or so. That will result in
weak northerly surface winds, and a subsidence inversion to keep
clouds lingering across a majority of the area. A few pockets of
clearing have developed directly under the ridge axis roughly along
the I-74 corridor. Additional low cloud cover borders the clearing
area, and light northwest flow in the cloud-bearing layer appears to be
guiding more clouds into the clearing. Forecast soundings do show
a trend toward a thinning of the low level moisture during the day
tomorrow, so the clearing may become more prominent with daytime
mixing and a relaxing of the low level inversion. Will still keep
clouds prevailing in the sky grids overnight, but thin them out
across the north where low clouds have opened up. Fog potential
should be higher in SW Illinois where the very low clouds have
persisted. Forecast soundings do not support much in the way of
fog in our counties, with the better chances mainly toward
Jacksonville and Taylorville. Low temps may remain slightly warmer
than guidance SW of Lincoln where clouds will be persistent.
Otherwise, low temps looked on track. Made minor updates to the
sky and temps for tonight.


Short term...(tonight)
issued at 251 PM CST Thursday Dec 18 2014

Short-wave trough responsible for the light snow earlier today has
sheared out and pushed into Indiana this afternoon, leaving behind
cloudy skies and a few scattered flurries. With another vigorous
wave noted upstream on water vapor imagery over the Desert
Southwest, short-wave ridging will begin to develop across the
Midwest later tonight into Friday. This will ensure dry conditions
and perhaps some thinning of the overcast. 20z/2pm visible
satellite imagery already shows some breaks in the cloud cover
developing from the Bloomington area northwestward into Iowa. As
synoptic subsidence increases in the wake of the departing Indiana
wave, additional breaks will develop further south and east as the
night progresses. Will maintain mostly cloudy conditions overall,
but think some temporary clearing may occur from time to time. Hrrr
hints at this trend as well. Meanwhile further west, a low overcast
will prevail across southern Iowa into Missouri. Will have to keep
an eye on the low clouds and fog to the west, as several high-res
models try to develop fog eastward into the western kilx County Warning Area after
midnight. Will leave out of the forecast for now, as 18z NAM
forecast soundings show little or no restriction to visibility. Low
temperatures will be similar to last night in the teens and lower


Long term...(friday through thursday)
issued at 251 PM CST Thursday Dec 18 2014

Morning vort Max over northern MO that triggered morning south over
southern 1/2 of County Warning Area has weakened and sheared to the east in
confluent upper flow through the day. Result is the end of light
snow for region in afternoon. Moisture channel data depicts next
wave that was over western Arizona at 12z in upper air data, now moved
into eastern Arizona. This wave to continue to track east into the
plains, but has weakened and models keep it south of area.

The main system of interest is the digging of the upper trough into
the Midwest by late in the weekend. Both GFS and eur similar in
closing off low into WI by Mon night. Pcpn still develops and
spreads into area Monday, with a brief early chance of a mix of pcpn
type at the onset. Warm advection ahead of the developing surface
low will then keep pcpn rain through Mon and then a light mix in the
northwest sections possible Mon night. Front to move through by
early tues with cold air following. Therefore a mix of rn/sn in
grids for tues, changing over to sn on tues night. Chance of sn in
the region Wed, but with the upper low in the Great Lakes, pcpn
amounts will be light and only wrap-around light pcpn. Sn amounts
will be light, ending Wed night.


Aviation...(for the 00z tafs through 00z Friday evening)
issued at 534 PM CST Thursday Dec 18 2014

Snow showers and flurries have departed to the east, and VFR cloud
cover has developed across the northern terminals of pia/bmi under
the surface ridge axis. Hrrr and NAM/RUC forecast soundings all
point toward a continuation of at least some cloud cover over the
next 24 hours for all taf sites. VFR cloud heights are projected
to expand from NE toward the SW tonight, with cmi/Dec ceilings
lifting to VFR. Spi will has the best chance of remaining MVFR
based on their proximity to the IFR/LIFR cloud mass. Confidence
is low on how the cloud cover will play out overnight, but will
default to broken-overcast conditions throughout the taf period, and
monitor any holes in the clouds that may develop along under the
ridge axis along the I-74 corridor.

Winds will remain light and variable for the next 24 hours, under
the weak pressure gradient/sfc ridge axis.


Ilx watches/warnings/advisories...


short term...Barnes
long term...goetsch

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