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Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
1118 PM CST Thursday Dec 18 2014

issued at 311 PM CST Thursday Dec 18 2014

At 245 PM CDT...a ridge of surface high pressure remained in place
across the region. A few light flurries lingered in parts of eastern
Iowa and western Illinois...but were decreasing in coverage as middle
level low pressure pushed east. Middle afternoon temperatures were in
the 20s.


Short term...(this evening through friday)
issued at 311 PM CST Thursday Dec 18 2014

The main forecast issue in the short term period is cloud trends
and impact on temperatures overnight into Friday.

In the near term...isolated flurries will continue to diminish in
coverage as forcing from a middle-level short wave pushes east. Model
layer relative humidity forecasts point to potential for some mostly clear to party
cloudy periods across northern and eastern portions of the
forecast area. These areas have only risen into the 23-38 degree
range this afternoon. With some partial clearing expected...believe
undercutting guidance north and east is the way to go. Low level
moisture will be greatest overnight in the extreme south...and
have added patchy fog from 09z through 14z Friday.

On Friday...surface ridging and stagnant low level winds expected
to continue with persistent cloudy to mostly cloudy skies. 850mb
temperatures are forecast to be near to slightly warmer than Thursday.
Believe afternoon highs will only be modestly warmer than the upper 20s to low 30s...cooler than the consensus
blend guidance.

Long term...(friday night through thursday)
issued at 311 PM CST Thursday Dec 18 2014

Main concern in the extended period is the storm system that will
affect the Midwest the end of this weekend through midweek next
week. Computer model solutions regarding this system are different
and as such this makes for a difficult forecast. At this time the
system looks to bring some rain to the area...changing over to snow
at the end of the forecast. This forecast is subject to change as
guidance begins to agree on a solution.

A series of shortwave disturbances will transverse through the flow
this weekend. On Saturday night into wave will move
through the area...bringing with it the chance of freezing drizzle
across northern portions of the County Warning Area. Temperatures look to rebound
above freezing on Sunday afternoon. This means that any impacts
from the freezing drizzle should be short lived. In the wake of this
system southwest flow will help to warm temperatures until another
wave moves through on Sunday night.

On Monday...a much more robust system with better vorticity
advection and magnitude will move into the central Continental U.S.. models are
in good agreement with the broad details of the forecast.
However...the surface features and subsequent sensible weather are
hard to nail down due to the model inconsistencies. Current blended
guidance suggests that this system will be more of a rain and then
rain/snow mix system...with snow on the back end of it. Looking at
individual models and the surface field evolution leads one to
realize there are a lot more things that could happen. For
one...some guidance develops a low to our south over the southern
Mississippi River valley and then advects it to the NE...merging
with the low over north WI. This low could act to rob US of deep
moisture and lead to an outcome with lower chances of precipitation and
overall less quantitative precipitation forecast. The Gem model has the initial surface low over
southern Canada and develops a low to our east. The GFS has the
initial low to our west...moves it over La Crosse and into north
WI...then develops a surface low to our east. The European model (ecmwf) is similar
to the GFS and then develops a secondary low even further to the
east across the Atlantic states. What this means is that the
specifics of this forecast will change and there is low confidence
in the current forecast for sensible weather. There is high
confidence that there will be a sensible weather producing system
across the Midwest early next week. As far as the event being all
rain...rasn...or snow...this is hard to nail down and subject to
change. Those planning travel for next week should pay attention to
later forecasts.


Aviation...(for the 06z tafs through 06z Friday night)
issued at 1118 PM CST Thursday Dec 18 2014

Nearly calm winds and variable cloud cover will remain in place
through the rest of the night as a ridge of high pressure lingers
overhead. A deck of lower clouds with MVFR ceilings will affect
primarily the dbq site tonight...before more widespread lower
clouds looming just west of the ridge axis spread into eastern Iowa
and northwest Illinois during the day Friday. Confidence in the exact timing of
this transition to MVFR conditions remains low...but there is a
good likelihood that all sites have ceilings at or below 2500 feet by 06z
Friday night.


Dvn watches/warnings/advisories...


Synopsis...rp Kinney
short term...rp Kinney
long term...Gibbs

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