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Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
639 am CDT sun Apr 26 2015

Synopsis...
issued at 308 am CDT sun Apr 26 2015

The latest surface analysis was indicating a broad ridge occupying
much of the upper Midwest...holding frontal systems at Bay across
the northwestern High Plains and down across the Southern Plains to
Tennessee Valley regions. This system also squeezing northeasterly
drying cooling flow across the local area...with lower stratocu
continuing to get shunted to the south as the early morning
progresses. Aloft...mid/upper level moisture flow patterns on the
latest water vapor loop indicating a deep trough across the southern
great bsn...with Omega blocking upper ridge axis holding firm across
the plains.

&&

Short term...(today and tonight)
issued at 308 am CDT sun Apr 26 2015

Today...there may be some patchy frost in low lying areas of the
northeastern third of the County Warning Area early this morning...but enough
northeast surface winds to keep it at a minimum despite low/dry dewpoints
advecting into those areas out of WI. After the clouds get eroded
out of the southern County Warning Area this morning...expect a mainly sunny day
today. But with deepening northeast fetch off the Great Lakes/Lake
Michigan...some moisture draw may help fuel at least sctrd cumulus
development across much of the northeastern half of Illinois and possibly
westward to the MS river. Cumulus rule parameters also picking up on this
scenario. Otherwise...forecast soundings showing deep mixing in an
unseasonably cool airmass...possibly up over 850 mb mb which would
support widespread highs in the upper 50s to around 60 especially
taking into account late April insolation.

Tonight...surface ridge maintains across the upper Midwest into the
western Great Lakes making for mainly clear skies as long as there
are not any lake fetch stratocu getting trapped nocturnally under an
inversion. Bulk of short range models keep some type of wind
component/northwest to northeast/ going of at least 3 to 6 miles per hour
into the early Monday morning hours and keep guidance low temperatures
mainly in the middle 30s tonight. But dry down-mixed surface dewpoints
possibly lower than what the models suggest will happen...as well
as the possibly of surface winds decoupling under an inversion
warrant a lean toward the coolest of guidance lows or even
undercutting those values a few degrees. For now will go with low
to middle 30s north of i80 with patchy frost possible...and middle 30s
to around 37 degrees to the south.

Long term...(monday through saturday)
issued at 308 am CDT sun Apr 26 2015

Main challenge is temperatures as a surface ridge axis keeps a
seasonably dry airmass in place under a split flow upper level
pattern for much of this week. A warming trend is still on track for
late week into the weekend...when returning Gulf moisture will bring
the next chances for rain.

Monday and Tuesday...light northeast winds out of a ridge over
central Canada continue to channel a somewhat cool and dry Great
Lakes airmass into the region...while shortwave ridging aloft keeps
skies mostly clear. The result will be an extended period of dry
weather with large diurnal temperature swings. Verification of
current dewpoints and forecast trends suggest models may overall be
a bit too moist at the low levels. Have thus undercut lowest model
dewpoints and trended toward the cold side for overnight lows and
warmer guidance for daytime highs. Highs should reach the lower to
middle 60s Monday...then middle to upper 60s Tuesday. Monday night into
early Tuesday morning...temperatures will drop into the 30s...with
far northwest Illinois possibly near or just below freezing as center of the
passing ridge axis suggests light and variable winds. Later
forecasts will need to monitor this area for a potential late season
frost.

Wednesday through Saturday...an eastward migrating western U.S.
Upper level ridge will result in a gradual warming trend with
temperatures rising from the lower 70s middle week to at least the middle
and possibly upper 70s by Saturday with lows in the 40s...reaching
the 50s by the weekend. Slight to low end chance probability of precipitation for showers
and thunderstorms were brought in for Friday night into Saturday.
European model (ecmwf) and GFS agree with a developing low level southerly flow and
moisture return in this timeframe...but there remain significant
differences with the timing and placement of weak surface fronts and
upper level shortwaves...keeping forecast confidence low at this
time.

&&

Aviation...(for the 12z tafs through 12z Monday morning)
issued at 633 am CDT sun Apr 26 2015

A large region of high pressure across the upper Midwest will
produce VFR conditions at all taf sites through Monday morning.
There is a chance for 3-5k feet above ground level cumulus of sctrd coverage across
northern Illinois and may get in the vicinity of mli by this afternoon.
Otherwise mainly clear skies with north to northeast surface winds of
5-15 kts through the taf cycle.

&&

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

$$

Synopsis...12
short term...12
long term...sheets
aviation...12

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