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Area forecast discussion
National Weather Service Lincoln Illinois
1131 PM CST Mon Jan 22 2018

Update...
issued at 857 PM CST Mon Jan 22 2018

The surface low is projected to track from near the Quad Cities at
9 PM to central Michigan by 6 am/12z. Our area will remain in the
warm sector for quite a bit of the rest of the night, which will
lead to mainly rain showers the rest of the evening, and several
hours after midnight. Spotty snow showers are projected to start
mixing in after midnight. The coldest air looks poised to wrap
into central Illinois starting in our southwest counties and
advancing to the northeast, not the typical northwest to southeast. A complete
change over to snow may occur at a few locations late tonight and
Tuesday morning as the deformation band precip reaches Illinois,
but snow accumulations would be minimal due to limited precip
amounts after change-over. We could see a couple tenths of an inch
of snow north of I-72, but confidence is low that any snow will
stick other than on grassy surfaces. However, slippery conditions
could still develop in any areas that dip below freezing for a
period of time late tonight into Tuesday morning. That should be
mainly west of I-55.

Winds will be gusty in our southeast counties for at least a few more
hours, due to the pressure gradient remaining tighter farther
from the center of the surface low. Winds will generally remain
at or below 10kt north of Peoria as the surface low passes from
Davenport to Chicago. West to northwest winds will increase
tomorrow morning, starting in western Illinois and progressing
eastward.

Updates this evening were mainly to pops and weather, with a
slight drop in lows for our western counties. A slight increase to
winds tonight was done across our southern counties. Updated
forecast info is already available.

&&

Short term...(this evening through tuesday)
issued at 225 PM CST Mon Jan 22 2018
deep 996mb low over south central Iowa this afternoon responsible
for the ongoing weather across the plains and the Midwest. This
mornings showers moved off to the east, leaving significant
cloud cover over the eastern half of the state, tempering any
early warm up. However, a wide expanse of a clear slot opened up
over the western half of the state and the mid Mississippi River
valley. The deepening low resulted in an increasing pressure
gradient. Sustained southerly winds 20-25kts all day, gusting to
over 30kts, ushered plenty of warm air into the region. The warm air advection
countered the cooler air wrapping around the occluded system. The
warm up resulted in temperatures in the mid to upper 50s. Not much
in the way of surface based instability, but the upper low
provided enough vertical motion to start shower/thunderstorm
development. The precipitation is ongoing and likely to scatter
out and lose the thunder after sunset. Scattered showers will
continue through the overnight. As colder air moves in with
proximity to the low center, expect a change over to snow or a
rain/snow mix. Ground temperatures have been relatively warm, and
considering the surface temperatures are lingering just over the
freezing mark, not much in the way of accumulation is expected.
Though some of the more dynamic showers may end up resulting in a
couple tenths. Tomorrow, temperatures much cooler as the winds
come around to more northerly behind the low exiting to the NE.
Temperatures will likely be almost steady in the 30s through
tomorrow. Cyclonic flow aloft will likely result in a cloudier sky
than some of the guidance has optimistically calculated.

Long term...(tuesday night through monday)
issued at 225 PM CST Mon Jan 22 2018

Clearing Tuesday night will result in temperatures dropping into
the 20s, still a couple degrees above seasonal normals. Forecast
settles into a northwesterly flow regime with temperatures closer
to seasonal normals. Models still pulling a wave through the
region on Wed morning, but so far the wave is moisture starved
aloft and sliding over a surface high...producing zero qpf...with
the exception of the 12z European model (ecmwf) that has painted a small amount
across south central Illinois. Not enough to warrant a mention just
yet...but something to watch. Behind that wave, high pressure
ridge slips eastward, starting another warm advection pattern
through the end of the week. Highs are back to the low 50s by
Friday. A developing storm system in the desert SW will push
moisture and precip chances northeast into the Midwest to start
the weekend. Another rainy Saturday shaping up so far...warmer but
rainy.

&&

Aviation...(for the 06z tafs through 06z Tuesday night)
issued at 1131 PM CST Mon Jan 22 2018

The initial surface low in southeast Iowa has begun occluding, with
another surface circulation developing in far NE Illinois. An
expansive area of light rain showers will continue overnight, with
a transition to snow showers over time. The change to snow looks
to develop from SW to NE across the area, as cold air wraps into
Illinois from the W-SW. Any period of light snow later tonight
could produce some slippery travel surfaces, especially where air
temperatures drop to or just below freezing. The coldest areas in
central Illinois should be west of I-55, as cold air flows into
the area. Precipitation should diminish quickly tomorrow morning,
as cold air overspreads Illinois.

Surface winds will start the 06z taf period from the west-SW at
10-15kt. As the elongated surface low departs into Michigan,
surface winds will shift to the west and eventually northwest over the next
12 hours. Winds will eventually become gusty from the northwest to 25kt
tomorrow afternoon, as the low departs to the E-NE.

Cloud heights look to drop to MVFR and possibly IFR later
tonight, as the lower lcl's just south of the surface Low Pass
across the area. Ceilings should improve back to MVFR across the
board during the day tomorrow, with VFR conditions by tomorrow
evening.

&&

Ilx watches/warnings/advisories...
none.
&&

$$

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