Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
822 PM CST Friday Dec 19 2014

Update...
issued at 821 PM CST Friday Dec 19 2014

Have updated the forecast to downplay the fog and freezing drizzle
coverage for overnight. IFR conditions and fog potential appear
greatest mainly west of the Highway 218 corridor through the rest of
the night. There...there will be less influence from the drier
air on light southeast winds out of the high pressure center moving ever
so slowly eastward across Central Lake Michigan. The axis of dewpoints
in the lower 30s from Arkansas/OK north across MO into S central Iowa will
slowly spread north and then east over the next 24 hours. This
could still result in patchy light drizzle or freezing drizzle in
the far S and SW late tonight as a weak middle level trough provides
weak lift in the saturated layer below 800 mb on forecast
soundings.

&&

Synopsis...
issued at 312 PM CST Friday Dec 19 2014

Upper-level low will slowly move from southern Manitoba
this afternoon to western Ontario through Saturday. The trough axis
attendant to this feature extends into northern Nebraska and will
swing through Iowa Saturday morning. There is a lot of low-level
moisture located in the southwesterly flow preceding the trough
where widespread stratus is found across the eastern plains and
Midwest.

&&

Short term...(this evening through saturday)
issued at 312 PM CST Friday Dec 19 2014

Tonight...trend after midnight will be for increasing clouds
with ceilings lowering to near or below 1000 feet above ground level as low-level
trough approaches and increases relative humidity in the near surface-850 mb layer. NAM
and sref produce a few hundredths of precipitation through the
overnight period but feel this is overdone. If light precipitation were to
occur...it would most likely be light patchy freezing drizzle since
saturated cloud layer is at -8/-9 c or warmer...although a few
snowflakes would also be possible. At this point...not expecting
widespread -fzdz. However with temperatures in the middle 20s to near 30 f
definitely a situation to monitor. Have slight chance for -fzdz
after 3 am across the area with the slight chance continuing
through early Saturday morning.

Saturday...defined vorticity maximum will move east of the area by
midday which will significantly dry out mid-levels. Large-scale
subsidence will develop an inversion between 900-850 mb...trapping
lower level moisture beneath it. Thus it will be difficult to get
much clearing through the afternoon and into the nighttime. 925-850 mb
warm air advection will provide weak lift within a saturated environment...
therefore patchy drizzle will be possible during the middle/late afternoon.
Highs will be in the middle 30s on average.

Long term...(saturday night through friday)
issued at 312 PM CST Friday Dec 19 2014

Forecast problem of the day remains the storm system approaching
the area early this week and what kind of sensible weather we will
experience. Like yesterday there is good agreement between the
models that the Midwest and Great Lakes will see a storm system
Monday th ru Christmas evening...but specifics are dynamic and thus
hard to nail down.

The same as yesterday...a series of shortwave troughs and vorticity maxes
will move through the flow this weekend with a trough approaching
the area at the beginning of the extended period. Guidance is in
good agreement with the possibility of measurable quantitative precipitation forecast with the
system on Saturday night/Sunday am. At this time model soundings
suggest a mist event with freezing mist across the northern and
western zones. Temperatures warm to above freezing on Sunday
afternoon and turns to all rain. Impacts from this system could
linger through early afternoon on Sunday.

On Monday the better vorticity maximum digs a trough into the central United
States leading to an active week in terms of weather. Models
agree on one thing and that is there is a chance for precipitation Monday
through Christmas evening. The way we get there is different with each
model. Current consensus guidance suggests mist transitioning into
rain through Tuesday 6z..then a chance of snow mixes in with the
rain in our western zones. As the system occludes...we see snow on
the back end. Winds look to be gusty...especially on Tuesday PM
into Wednesday. Looking at specific models....the Gem has the
initial low in north WI and then it occludes and develops a low across
Arkansas and Tennessee. This solution would suggest that more snow across our
eastern zones would be possible. The GFS has an elongated low
over Iowa and Minnesota...and then develops another low over Chicago...in
this case the whole County Warning Area would see snow. The European model (ecmwf) is even more
different where it GOES negatively tilted on Wednesday and drops most of
the snow to our east. What im trying to say is that we are no
closer to a model consensus than we were yesterday. The sensible
weather is still up in the air with the models changing. I still
have low confidence in the sensible weather for the system as any
change in storm track...development et cetera..could lead to a change in
the affects. Again...there will be a system affecting the area
early this week and those with travel plans should continue to pay
attention to future forecasts.

Looking to Christmas day...the day looks dry. Another storm system
approaches the area on Friday. This system could bring US a chance
of snow for the latter part of the week.

&&

Aviation...(for the 00z tafs through 00z Saturday evening)
issued at 543 PM CST Friday Dec 19 2014

A very slow moving ridge of high pressure over WI and Illinois will
slowly move east tonight...allowing low clouds and MVFR
conditions to its west to gradually build eastward tonight and
Saturday. Confidence in the timing of this transition remains
low...but this appears most likely to occur between 03z and 06z
this evening. The flight restrictions will more likely due to
lower ceilings from 1500 to 2500 feet above ground level...while there is a lower
potential for light fog to develop and lower visibilities into a
4sm to 6sm range as well. There remains a low possibility for
light drizzle or freezing drizzle...but confidence of any significant
precipitation is too low to mention in the forecast. The low
clouds and MVFR conditions will linger through Saturday while
light surface winds turn more consistently from the south.



&&

Dvn watches/warnings/advisories...
Iowa...none.
Illinois...none.
MO...none.
&&

$$

Update...sheets
synopsis...uttech
short term...uttech
long term...Gibbs
aviation...sheets

National Weather Service Glossary of Abbreviations