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Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
643 PM CDT Sat Apr 18 2015

issued at 320 PM CDT Sat Apr 18 2015

A broken line of showers and a few thunderstorms has popped up over
the past couple hours...from about Princeton Illinois to Clinton Iowa...
where convective temperatures were being reached along a backdoor
cold front that had moved in from the Great Lakes. This boundary was
merging with the main synoptic boundary that extended east-NE from a
developing low pressure over western Kansas. Dew points were in the 50s
south of this boundary...while 30s and 40s were found to the north
with actual temperatures in the 70s over the entire forecast area.
12z analysis indicated an upper level low over Colorado with a ridge axis
reaching from the eastern Gulf through the middle MS valley to the
northern plains. Widespread showers and thunderstorms have
continued through the day in the axis of Gulf moisture that was
being pulled north through OK...eastern Kansas into western Iowa/western
Nebraska. The upper low is prognosticated to fill and spread northeast as a
shortwave...merging with a northern a stream shortwave currently over
Saskatchewan...eventually shifting the axis of rain and
thunderstorms eastward through the forecast area through Sunday night.


Short term...(this evening through Sunday night)
issued at 320 PM CDT Sat Apr 18 2015

Main challenge is the timing and coverage of rain and thunderstorms
as the system out west spreads eastward. Models continue to depict a
multi-low complex developing over the central U.S. Tonight into
Sunday...eventually organizing into one primary low over Illinois or
Indiana late Sunday that lifts north into the Great Lakes as the
upper level systems become phased into a longwave trough.

In the near term...the diurnally driven convection along the
boundary through the east-central part of the forecast area will likely
dissipate around sunset...with the primary axis of precipitation
remaining focused on the upper level moisture axis to the west and
incoming upper level difluent zone. Have probability of precipitation trended accordingly
this evening...with areas east of the MS river likely dry from
midnight through Sunday morning. Eastern Iowa and NE MO were kept
with scattered to low end likely coverage for overnight with only
isolated thunderstorms due to limited instability.

Sunday...slightly slower timing and the system fighting dry air at
the low levels will hold off precipitation east of the MS Sunday
morning. Widespread rain and isolated thunderstorm coverage is then
expected for Sunday afternoon through Sunday night. Clouds and
precipitation should limit highs to the 60s...then drier air and
cold air advection on northwest winds toward sunrise Monday will lead to
lows in the 40s. This setup is not favorable for severe storms and
trends still support quantitative precipitation forecast totals in a quarter to half inch range
with possible three quarters to 1 inch west of the Highway 218

Long term...(monday through saturday)
issued at 320 PM CDT Sat Apr 18 2015

..much cooler and mainly dry this week...

The European model (ecmwf)/GFS/NAM all forecast the middle-Mississippi Valley shortwave that
brought rain to the area to phase with a southern Canadian shortwave
and form a cutoff upper level low over the Great Lakes early next
week. With a high amplitude upper level ridge building along The
Spine of the American and Canadian rockies...the upper low will
deepen and remain nearly stationary over the Great Lakes all week. A
northwest flow aloft with near meridional flow by midweek...will
bring much cooler temperatures to the Upper Middle west. As 850 mb
temperatures drop near or slightly below zero...daytime highs will
be in the 50s north to around 60 south with overnight lows in the
30s and 40s with surface high pressure building all the way south to
the Gulf Coast. Monday will have breezy northwest winds with a few
light showers possible north of Interstate 80. A few light showers
will also be possible on Tuesday along and north of the Highway 20
corridor as occasional lobes of streamwise vorticity rotate around
the backside of the upper low. Overall most of the area should be
dry. Patchy frost may be possible from Monday through Wednesday if
skies clear and winds go light. The consensus models continue to
maintain slight chance probability of precipitation for late in the week but model solutions
continue to be inconsistent.


Aviation...(for the 00z tafs through 00z Sunday evening)
issued at 638 PM CDT Sat Apr 18 2015

A weak trough ahead of the main system in the plains is generating
isolated showers and thunderstorms with brief IFR visibilities in eastern
Iowa. Odds favor these storms missing taf sites except perhaps Cid
at least for the next few hours. As the main system moves into the
area tomorrow...rain and low clouds will become more widespread
impacting aviation operations. Isolated to scattered thunder is
possible...though have not included it in tafs at this point
because of the low probability.


Dvn watches/warnings/advisories...


short term...sheets
long term...Kuhl

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