Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus63 kdvn 201720 

Area forecast discussion
National Weather Service Quad Cities Iowa Illinois
1120 am CST Wed Nov 20 2019

..aviation update...

issued at 1026 am CST Wed Nov 20 2019

Winds have increased over northwest Illinois and dense fog has now
cleared up as of 1015 am. The dense fog advisory has been

Update issued at 843 am CST Wed Nov 20 2019

Winds in our northeast have not yet increased and switched to the
south/southeast. Thus, the dense fog dissipation is not likely
going to occur by 9 am, and I have extended the advisory through
11 am, which matches the latest hrrr trends on wind/visibility.


Update issued at 538 am CST Wed Nov 20 2019

Conditions have improved considerably over the past couple hours
with the arrival of a band of mid level clouds over eastern Iowa
and west central Illinois. Surface reports and web cams no longer
show any dense fog and so far and fog has not been redeveloping
so far along the back edge of this cloud deck. Have thus cancelled
the dense fog advisory for southeast Iowa, west central Illinois
and the I-380 corridor of east central Iowa.

Earlier, an expansion of the dense fog advisory was made to cover
Bureau and Putnam counties, where dense fog set in after 3 am.
Based on current trends, this fog may also dissipate ahead of
schedule and may be able to cancel these areas early as well.


issued at 318 am CST Wed Nov 20 2019

Dense fog developed overnight over much eastern Iowa into west
central Illinois under the surface ridge axis. This was enhanced by
the lingering NE to SW thermal and moisture gradient between the
persistent stratus field across the northeast and the clearing
that occurred over much of Iowa Tuesday. Early morning
temperatures were in the lower to mid 30s with a few pockets where
temperatures have cooled into the upper 20s. In the large scale,
an upper level ridge axis was over the central U.S. With a
shortwave evident on water vapor imagery over Baja California. Widespread
cloud cover was seen ahead of this feature in the strong warm air
advection and developing moisture return over the Southern Plains.


Short term...(today and tonight)
issued at 318 am CST Wed Nov 20 2019

The initial concern is the dense fog this morning. Focus then turns
to warmer, breezy conditions and the onset of widespread rain
tonight as the advancing shortwave sends a developing low
northeastward, reaching northern Iowa by Thursday morning.

This morning, advancing thick high clouds topping the upper level
ridge, will likely delay the dissipation of the dense fog. These
same clouds, most dense from central Iowa into southeast Iowa, will
likely prevent any further westward development of the current area
of fog and will likely be able to keep southeast Iowa and northeast MO out
of the dense fog advisory. Elsewhere, it will likely take a
tightening pressure gradient behind the departing ridge and
strengthening southeast winds to mix out the fog. This may take as
long as 9 or 10 am, as shown by the hrrr visibility fields and will
thus keep the dense fog advisory going until 15z.

Through the rest of the day, incoming high cloud cover will fill in,
just as the current stratus field over northern Illinois continues to
retreat east of the area. The filtering of weak November
sunshine, moist ground and unfavorable southeasterly winds, will
likely keep temperatures somewhat in check as warm air advection
commences. Have thus stayed closer to cooler guidance with daytime
highs from the mid 40s northeast to the mid and upper 50s central
and southwest. If cloud cover thins out more than expected,
readings could indeed push into the lower 60s over northeast MO
into southeast Iowa.

Tonight, rain overspreads the area in the evening with the onset of
strong upper level lift ahead of the shortwave and strong 850 mb
moisture advection. Models are in decent agreement with roughly
25 to .75 quantitative precipitation forecast overnight, from south to north overnight as the
system works with the feed of anomalously high precipitable
water. Will mention isolated thunderstorms toward morning over the
southwest, where models have marginally steep mid level lapse
rates and MUCAPE values. The tight pressure gradient in the
system's warm sector passing through the area will provide breezy
to windy conditions overnight with steady or even rising overnight
temperatues in the 40s and 50s.

Long term...(thursday through tuesday)
issued at 318 am CST Wed Nov 20 2019

Key messages:

1) rain/isolated thunder Thursday morning

2) generally quiet and dry Friday through Monday

3) potential for active weather mid next week


Rain will be ongoing across much of the area for the beginning of
the long term period with continuing Theta-E and moisture advection.
As mentioned in the previous discussion, widespread precipitation
will likely come to an end from west to east sometime around 9 am as
as isentropic lift begins to wind down. This will be followed by the
cold front, which should be past the Mississippi River by 18z (all
deterministic guidance in good agreement of this). With this front
there will likely be a second round of rain showers, and perhaps an
isolated thunderstorm with the enhanced lift. Latest runs of
mesocale models including the namnest and rap continue to suggest an
influx of MLCAPE (mostly elevated) across the area, giving some
support to the later. Will make mention that if an updraft is
somehow able to fire up in the forcing with the front, very high
values of low-level shear would support rotation. However, this
threat continues to remain fairly negligible at this point.

High temperatures are a challenge and very much depend on the timing
of the front. Latest deterministic guidance and blends have come in
cooler, especially across the south. Where there were once
widespread low 60s have changed to upper 50s. Think this is valid
especially given how much moisture and cloud cover will be in the
low levels. These highs will occur during the morning, with
temperatures falling behind the front Thursday afternoon and
evening. Low temperatures for the day will likely occur just before
midnight Friday.

Storm total quantitative precipitation forecast will range from around half an inch over northeast
Missouri and west central Illinois, to near one inch around the
Highway 20 corridor nearest to where the low tracks. If a updraft is
able to develop, then quantitative precipitation forecast may be enhanced over areas that see a

Friday through Monday...

Other than a slight chance of some snow showers for areas south of
Highway 34 early Saturday morning, the period will be dominantly dry
with high pressure as our influence. Friday will be the coolest day
of the period due to cold air advection behind the front. As we head into the
weekend, temperatures will be rise again, and will be near to
slightly above normal by the end of the weekend.

Tuesday on...

Global guidance remains mixed on potential solutions, but are in
agreement of the weather turning active as a deep trough builds into
the western rockies. The GFS and European model (ecmwf) are in agreement of a system
developing over the eastern Great Plains and Midwest, but differ
greatly with regards to timing and track of this system, which will
be important for determining precipitation types. Being this will be
a busy travel time for the Thanksgiving Holiday, it will be
important to monitor the latest forecasts.


Aviation...(for the 18z tafs through 18z Thursday afternoon)
issued at 1117 am CST Wed Nov 20 2019

Improving conditions will exist the rest of today, as a warm front
has lifted north of the area, with mainly VFR conditions expected
until rain arrives this evening. Rains will saturate from top
down, resulting in light showers from high cloud bases through 04z
tonight, then saturating to a moderate rain with lower clouds
around 1000 to 2000 ft through the overnight hours. Rains should
push east of the area around 13-15z Thursday, with scattered lower
clouds and light showers behind it. Winds will increase tonight,
with low level wind shear at 2000 ft/45kts from the south. By morning as the rain
ends, winds will switch from south to southwest, then towards 18z,
a shift to west / northwest will occur as the low pressure passes
by the region.


Dvn watches/warnings/advisories...



National Weather Service Glossary of Abbreviations