Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 182137
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
337 PM CST Sun Feb 18 2018


.SYNOPSIS...
Issued at 336 PM CST Sun Feb 18 2018

Yesterday`s snow cover, evident on satellite imagery over the
northern half of the forecast area, has been slow to melt so far
today. Along and north of the I-80 corridor, the snow has held
temperatures in the 30s to lower 40s, despite south to southeast
winds from 20 to 30 mph. Further south, temperatures over
southeast Iowa, northeast Missouri and west central Illinois have
reached the upper 40s to around 50. The strong winds were
occurring in a tight surface pressure gradient ahead of a
strengthening frontal system reaching from central MN into the CO
front range. This system will affect the region over the next
several days, providing several rounds of rain, thunderstorms and
possible ice accumulations as it very slowly moves through the
region.

&&

.SHORT TERM...(This Evening through Monday)
ISSUED AT 336 PM CST Sun Feb 18 2018

Challenges tonight through Monday include the potential for more
dense fog and the onset of drizzle and/or showers with at least
isolated thunderstorms thrown in as well.

South to southeast surface winds will diminish to roughly 5 to 15
mph by midnight. This continued warm advection flow will likely
lead to more fog, especially over the lingering snow cover and
colder surface conditions along the I-80 corridor counties
northward late tonight. Stronger elevated warm advection on a 40
to 50 kt southwest flow at 850 mb, combined with approaching weak
shortwaves in the upper level flow, will lead to at least
scattered showers by late this evening, continuing overnight. Have
gone with a slower onset based on short term high res models
supported by what looks to be initial dry conditions immediately
upstream evident by the minimal cloud cover over the Central
Plains. There may be enough low level jet convergence and elevated
instability to produce a few thunderstorms, which will be mainly
limited to the south and southeast toward morning. Temperatures
will likely remain steady or fall only a few degrees from current
readings overnight.

Monday, fog may linger or even become more widespread as wind
fields weaken and become more convergent ahead of the approaching
front. Increasing low level moisture from light precipitation will
also reinforce the fog, which for now have kept going in the
forecast over mainly the north until at least noon. Otherwise,
continued mid level forcing acting along the baroclinic axis, or
frontal zone, setting up over the area will keep showers and
possibly isolated thunderstorms through the day. All models have a
surface low riding northeast along the front oriented over or just
west of the forecast area, to act as a focus for showers and
thunderstorms. This is shown pulling springlike warmth into far
southeast IA, northeast MO and west central IL, where highs may
reach the lower 60s. On the flip side, the highway 20 corridor is
forecast to hold in the upper 30s to lower 40s. Deep saturation
on forecast soundings, a lack of sufficient low level
instability, weak mid level forcing, and mostly unidirectional low
level shear looks to preclude any severe thunderstorm threat.


.LONG TERM...(Monday Night through Sunday)
ISSUED AT 336 PM CST Sun Feb 18 2018

The main forecast concern are precipitation type and
surface temperatures Tuesday into Wednesday.  The models are in good
overall agreement through the period but the CMC Northern Hemisphere
appears to be too cold with this storm system.

The long term period is characterized by general 500 hpa troughing
across the western U.S. and ridging across the eastern U.S. This
will keep eastern Iowa, northwest Illinois and far northeast
Missouri in the zone of active weather in the general
southwesterly flow aloft through the long term period.
Temperatures will remain near to slightly above normal through the
period.

The biggest period of concern is from 00z Tuesday into 12z
Wednesday but specifically 00 to 12z Tuesday..  Multiple pieces of
energy will eject out of a deep western U.S. trough toward the Upper
Midwest. This will bring precipitation to the area Tuesday into
Tuesday night. The best lift and heaviest precipitation during
this time period will be southeast of a line from Cedar Rapids to
Dubuque. A deep warm layer aloft and surface temperatures well above
freezing will bring rain with embedded thunderstorms.
However, northwest of this line is another
story. A pronounced warm layer aloft characterized by wet bulb
temperatures of +5 to +10C over a surface layer in the 30 to 34
degree range. The big question is will temperatures be at or above
freezing during the 00z to 12z Tuesday time frame. The Canadian
Northern Hemisphere is being disregarded as too cold during this
period despite having similar overall surface fields to the other
models, just colder surface temperatures. The GFS is colder than the
ECMWF and NAM which keep that area at or above freezing during the
whole time. Have leaned the forecast toward a GFS/ECMWF, and NAM
blend for this reason. Farther to the northwest will see the entire
event with surface temperatures below freezing and a prolonged
freezing rain event. Right now there is almost a tenth to two tenths
of an inch of freezing rain in the grids for that time period. A
slight shift in the track of this system, especially with regard to
surface temperatures, could have significant impacts on where
and how much ice accumulates. Please stay tuned for further updates.
Freezing rain will transition to all rain as
temperatures warm on Tuesday morning.

A cold front will move across the area during the late morning and
afternoon on Tuesday. The strongest cold advection lags the front
with precipitation type transitioning from rain to freezing rain or
sleet. Drier air moving into the region will result in precipitation
quickly coming to an end after 00z.

High pressure will build into the region for Wednesday with
slightly below normal temperatures and breezy northwest winds.

After Wednesday, the active weather continues with multiple
shortwave troughs ejecting out of the western
U.S. trough towards the Upper Midwest Thursday through Sunday.
Timing of each individual wave is difficult at this time but
there will certainly be dry periods to the forecast under
shortwave ridging. Each wave will bring the
chance for precipitation either snow or a rain and snow mix at this
time.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday Afternoon)
ISSUED AT 336 AM CST Sun Feb 18 2018

...18Z Discussion Updated for Wind Shear...

South to southwest winds from 45 to 50 kts beginning roughly in
the 1200 to 1600 ft agl layer will result in low level wind shear
conditions this evening. This will begin roughly 23z to 01z this
evening as surface winds from the south decrease to less than 15
kts. All forecasts have been updated to include LLWS, which
is expected to become less of an issue after midnight when these
winds aloft decrease and become more aligned with surface winds.

Strong south winds this afternoon will gust as high as 30 to 35
kts. Otherwise, conditions will remain VFR. Tonight, fog will
become an issue as winds diminish ahead of an approaching frontal
system. This front will also bring drizzle, then showers and
possible thunderstorms going into Monday morning. All sites have
been trended with IFR and then LIFR conditions due to lowering
ceilings and visibilities. The probability of thunderstorms
affecting any given site was too low to include in the current
forecasts. Confidence is high that conditions will remain IFR
through late Monday morning, but confidence is low on trends with
possible dense fog and associated LIFR or even VLIFR, which could
linger through 18z at many sites.

&&

.HYDROLOGY...
Issued at 1142 AM CST Sun Feb 18 2018

River flood watches have been issued for the Rock River at Joslin
and Moline due to forecasts of moderate flooding at both
locations. These forecasts are based on the 48 hour rainfall
potential of 1 to 1.50 inches. These forecasts and products will
change based on both forecast and observed rainfall. Please see the
latest river forecasts and watches for the most up to date
information.

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...Winter Storm Watch from Monday evening through Tuesday morning
     for Benton-Buchanan-Delaware.

IL...NONE.
MO...NONE.
&&

$$

SYNOPSIS...Sheets
SHORT TERM...Sheets
LONG TERM...Cousins
AVIATION...Sheets
HYDROLOGY...Cousins



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