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Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
537 am CST Wednesday Nov 25 2015

issued at 329 am CST Wednesday Nov 25 2015

Satellite fog channel imagery early this morning showed a large
tongue of stratus associated with moisture advection located
from the western Gulf northward through OK...Kansas and MO into
Iowa...reaching into the western half of the forecast area. This
was in response to the increasing southerly flow between high
pressure over the middle Atlantic region and developing low pressure
Lee of The Rockies. South to southeast winds holding around 10 to
15 miles per hour across the locate area at the surface were providing
enough low level mixing to negate any threat of fog this
morning...and also holding temperatures in a relatively mild range
from the middle 30s to middle 40s.


Short term...(today and tonight)
issued at 329 am CST Wednesday Nov 25 2015

Increasing moisture and eventual middle level lift as the developing
plains frontal system approaches will result in increasing
clouds...and eventually light rain or drizzle over much of the area
by tonight. The best potential for fog now looks to be later tonight
and tonight...mainly over the deepest snowfields and where low level
moisture begins to converge ahead of the approaching front.

Today...based on the rate of moisture return...with surface
dewpoints in the 40s and 50s still well to the south...the delay in
the onset of rain chances to mainly afternoon into evening shown by
the 06z NAM looks reasonable. Thus have slight chance probability of precipitation for light
rain developing in the far northwest this morning...then increasing probability of precipitation
for light rain or drizzle from northwest to southeast during the afternoon as
deeper saturation occurs along with increasing middle level lift ahead
of a weak shortwave in the SW flow aloft. The potential for fog
during the day looks limited to the far north and northwest over the deeper
remaining snow and where any weak low level moisture convergence
is maximized. Quantitative precipitation forecast amounts will be less than a tenth of an inch.
Temperatures will continue to gradually rise...despite most sites
becoming overcast by middle morning. Highs by late afternoon were
kept in a range from the lower 40s north to the middle 50s in the far

Tonight...model consensus has the developing plains cold front
approaching from the a SW to NE orientation...approaching
Independence after 12z. An increasing moisture feed ahead of this
boundary...low level convergence...and lift from middle level waves in
the SW flow will lead to widespread rain over the area. Any fog will
again be most likely held to the north and northwest the forecast
area...but this is not likely to be dense during this period. Quantitative precipitation forecast
amounts will still be rather light...with up to a quarter inch in
the far the strongest forcing and a few
hundredths of an inch in the east and southeast. The moist airmass and
brisk southerly low level flow will keep temperatures nearly
steady through the night in the 40s and 50s.

Long term...(thursday through tuesday)
issued at 329 am CST Wednesday Nov 25 2015

Main forecast concern in the long term are numerous chances for the frozen and liquid form...during this busy
Holiday weekend. At this time it appears that during Thanksgiving
and into Friday all precipitation will be liquid...after this...for the
weekend it becomes more tricky. Frozen precipitation could be possible at
night with liquid during the day as models again are struggling with
the closed low/blocking pattern over The Rockies.

Thanksgiving through Friday afternoon...

Rain should be ongoing at the beginning of the period. Low level
Omega and isentropic ascent suggests that most of the heaviest
precipitation will be west of the river at 12z. Light rain could be ongoing
across the area...however the strongest rain will be located with
the rising motion associated with the front as large scale vorticity
advection is weak. The 00z NAM 4km shows a line of moderate
rain showers... virtually collocated with the front. This model
depicts the band of rain entering our County Warning Area around 18z and marching
through the area and exiting by 09z Friday. Rainfall totals of 1 to
2 inches are possible with the highest amounts located along a line
from dbq to Cid to Marengo. It is safe to say that Thanksgiving will
be wet across our area. As the front moves through the
area....lingering precipitation will encounter cold air at the low levels.
With large scale flow still out of the SW...a 1044 hpa high tries to
build into the area. It will be interesting to see how deep the
cold dome is as models suggest a modest warm nose aloft as the cold
air moves into the low levels. In this situation...precipitation on the
back end...especially north of a line from Marengo to Dubuque
could transition over to freezing rain and then to sleet and even
possibly snow. My confidence is low in this such
added schc freezing rain and sleet across this area in the
northwest. At this not expect any ice or snow
accumulations Friday am. Anything that did fall would melt as highs
are forecast to be above freezing Friday afternoon.

Friday evening through Monday morning...

A high over low block across the western United States provides
models with a tricky forecast for the weekend. As this is a busy
travel weekend...the model discrepancies have my confidence in the
forecast very low. The GFS has a wave that pushes the low to east
faster than the European model (ecmwf). However...the European model (ecmwf) slowly lumbers the low
towards US at the same time. Model consensus forecast moves the
1040+ high through the area quickly this weekend as SW low level
flow advects moisture back into the area. This means that the
chance for precipitation returns to the area Saturday through next week.
This weekend it looks as though some wintry mix will fall at night
with rain during the day. This will lead to difficult forecast
along with possible travel issues come this weekend across our area.
Those traveling back or through the area this weekend should pay
attention to future forecasts.

Monday on...

Major differences in the models make it difficult to have any
confidence in the forecast. What adds to this difficulty is the
proximity of the freezing line....through our County Warning Area at night and just
north during the day. Thermal profiles make it difficult to discern
what type of frozen precipitation is expected. If the low starts to move
from the blocking pattern then inevitably this forecast will become
slightly easier...until then or the models agree on a solution no
changes will be made to the current blended forecast. Expect
specifics to change for the systems next week.


Aviation...(for the 12z tafs through 12z Thursday morning)
issued at 537 am CST Wednesday Nov 25 2015

Low level wind shear conditions will dissipate early this morning. A developing
storm system in the plains will keep south winds from 10 to 20 kts
through the day. An area of stratus advancing northward from the
Central Plains this morning will result in ceilings lowering into
MVFR range by middle afternoon...followed by light rain and drizzle
gradually overspreading the region. IFR conditions with fog and
light precipitation is likely at dbq and Cid...but not as likely
to reach into the mli and brl terminals. For now...will keep mli
and brl MVFR through the night...but fully expect widespread IFR
and possibly LIFR conditions on Thursday at all sites.


issued at 329 am CST Wednesday Nov 25 2015

Long duration rainfall event through Thanksgiving and into Friday
morning could bring rainfall amount from 1 to 2 inches across the
area. With snowpack still on the ground...a large percentage of
this steady rain could go straight to runoff. Ensemble river
forecasts suggest river flooding when the highest quantitative precipitation forecast is used. When
using quantitative precipitation forecast close to what is forecast a few rivers get to action
stage. Current forecasts for Kalona and Marengo take these sites to
flood. At this confidence in this occurring and the
forecast quantitative precipitation forecast occurring is low. We could get robbed of moisture from
the convection to our south. With such low confidence in the quantitative precipitation forecast
forecast decided to hold off on a Flood Watch and issuing flood
warnings for Marengo and Kalona.


Dvn watches/warnings/advisories...


short term...sheets
long term...Gibbs

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