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Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
324 PM CDT Wednesday Apr 23 2014

issued at 233 PM CDT Wednesday Apr 23 2014

12z upper air analysis showed upper ridging that ran from the
Southern Plains all the way north into Canada with a large trough
extending from British Columbia into Montana. Several shortwaves
were noted with one just east of The Rockies and another over
Southern California this morning.

18z surface analysis depicted low pressure systems east of The
Rockies with a warm front extending south from a South Dakota low
all the way down into Louisiana. Latest radar imagery showed
elevated showers just entering the western sections of the County Warning Area.
These returns have had a hard time reaching the ground across Iowa
due to the dry low levels in place over the region.


Short term...(this evening through thursday)
issued at 233 PM CDT Wednesday Apr 23 2014

Extensive middle-high level cloud cover has kept temperatures several
degrees lower today than yesterday. Do not expect readings to rise
much more today with highs topping out mainly in the 50s. Strong
warm air advection aloft will help keep elevated showers moving east across the
County Warning Area this afternoon and early evening with light quantitative precipitation forecast amounts under a
tenth of an inch. There will likely be a lull in the precipitation chances
tonight prior to arrival of the main forcing associated with the low
pressure system and cold front nearing the County Warning Area Thursday. Latest
model runs have slowed the progression of the cold front somewhat
and do not expect much in the way of precipitation prior to sunrise
Thursday. Thursday morning lows to range from the low 40s
northeast to low 50s southwest.

Thursday...showers and thunderstorms will be seen throughout much
of the day as a cold front moves across the region. Model soundings
completely saturate by 12z as front begins to push across Iowa.
The best coverage for rain appears to be from the middle morning to the
middle afternoon hours as low level convergence/omegas/moisture
fields are the strongest during this time. These storms are
expected to remain below severe levels. Rain amounts between a
half inch and three quarters of an inch appear likely by Thursday
evening and will be welcomed by many. The latest NAM/Gem/ECMWF
models all show a secondary low forming across the southeast
portion of the County Warning Area Thursday afternoon which would support
additional rain development in the south sections that would
linger into the evening. Highs Thursday will be below normal due
to the expected cloud cover and rain with readings topping out in
the upper 50s to lower 60s.

Long term...(thursday night through wednesday)
issued at 233 PM CDT Wednesday Apr 23 2014

The main feature in the middle and longer range forecast is the blocked
middle level flow pattern that all models develop early next week.
This will likely lead to and extended period of rain for a large
portion of the region along with well below normal temperatures
early next week. While the overall synoptic pattern is shown by the
all of the operational models...the details on the location of the
upper low and area of maximum rainfall differ considerably among the
solutions leading to a fair amount of uncertainty in the finer
points of the forecast.

For late this week the upper trough that/S moving out of the western
U.S. Will be moving through the middle Mississippi Valley Thursday
evening. We should see some lingering light rain mainly along and
east of the river before 06z with the rain moving well east of the
area during the early morning hours. Clearing early Friday morning
is expected in the subsidence behind the trough...allowing temperatures to
cool into the upper 30s to middle 40s. Friday should be a relatively
mild day with dry northwest flow and highs in the upper 60s to middle
70s. Friday night with the upper ridge over the northern
plains...the models have been showing a jet streak dropping
southeast through the region producing some light rain late Friday
night. The 12z runs were keeping this feature further north than the
06z runs introduced some low probability of precipitation for northwest Illinois.

Saturday a deep middle level trough moves through the western U.S.
Pumping up middle level ridging over the central U.S. This should bring
one more mild and dry day to the region. By Saturday night the
upper low is closed off and begins to lift northeast. This will set
up a good feed of Gulf moisture ahead of the upper low...into the
Midwest along with a diffluent upper air pattern. This should place
the region in an area of prolonged period rainfall as the upper and
surface lows tracks slowly east...staying south of the forecast area
through Tuesday. While it/S fairly certain that some locations will
receive substantial rains from this system over a 48 hour or more
period...there/S still uncertainty where the maximum rain area will set
up. This mornings GFS was favoring southern Iowa and northern MO
Sunday through Tuesday while the 12z European model (ecmwf) kept the heaviest rains
over western Iowa into Missouri. The middle Mississippi Valley region
had a relative min in rainfall thanks in part to a strong easterly
flow feeding dry air in from eastern Canada. The threat of severe
weather looks relatively low at this time with the region remaining
well on the cold side of the system. The biggest impact from the
the system may be river flooding especially if the more northern GFS
solution verifies.

The blended guidance kept relatively mild temperatures in place
Sunday into Monday as the system approaches the region then well
below normal for Tuesday in the cold easterly flow and possibly
widespread rains north of the upper/surface lows. The current
forecast has highs in the upper 40s to middle 50s Tuesday. If the
precipitation sets up over the region then some areas could struggle to get
out of the upper 30s.

The GFS is more progressive in moving the stacked system east and
drying out the middle Mississippi Valley by Wednesday. The European model (ecmwf) though
lingers the upper low over the region through the end of the week.
For now have gone with the more progressive and dry GFS.


Aviation...(for the 18z tafs through 18z Thursday afternoon)
issued at 1242 PM CDT Wednesday Apr 23 2014

High based clouds generally above 9kft to persist over the area this
afternoon and into the evening. A few light showers or sprinkles
will be possible in the afternoon with conditions remaining VFR.
Winds to remain out of the east southeast through much of the
overnight under 15 kts with a few gusts over 20 kts at the Cid
terminal in the afternoon. Stronger winds expected Thursday
morning as a low pressure system and associated cold front move east.
Showers and a few isolated thunderstorms will develop ahead of the
front beginning around 10z/24 across eastern Iowa and spreading
east during the day on Thursday. Conditions will likely drop to
MVFR levels in rain.


Dvn watches/warnings/advisories...


short term...gross
long term...dlf