Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 171112
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
612 AM CDT Tue Apr 17 2018

...12z AVIATION UPDATE...

.SYNOPSIS...
Issued at 330 AM CDT Tue Apr 17 2018

Latest SFC analysis was indicating elongated ridge sliding east acrs
the MO RVR Valley and into the heart of IA/MO ATTM. The Wednesday
weather-making wave was noted on the current water vapor loop
starting to eject out of western CONUS TROF and acrs eastern NV.
this system to bring about quite a mix of weather to the region from
late tonight through Wed evening. Then dry sensible, but still
somewhat cooler then normal weather on track to end the week out.
Still a low chance for Cedar Rapids to get near a record low this
morning and will keep climate section for reference.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 330 AM CDT Tue Apr 17 2018

Today...Stratocu decks rotating down southward out of western WI not
being handled by the models well, and will have to account for for
at least through mid morning acrs the northeast half of the fcst
area. Then mostly sunny for much of the rest of the day, although
cirrus will continue to stream in from the west and thicken ahead of
the approaching mid week storm system. High temps today in the low
to mid 40s in the far north, to the low to mid 50s in the far south.

Tonight...Strong/impressive wave will look to eject out acrs the
east central plains/MO RVR valley by morning in negatively tilted
fashion. Several 00z run models deepen the sfc low under 1000 MB
acrs north central MO by Wed morning as well. Several indications of
a nice elevated plume of THTA-E/MUCAPES of 200-600+ J/KG swirling up
acrs the CWA from the southeast after 08z on nose of convergent
southwesterly 35+ KT LLJ. Lapse rates in this elevated layer a bit
more marginal, but still plenty there to support sctrd thunderstorms
embedded in a warm air advection wing of elevated showers moving
acrs the southwest and western CWA(or the southwest half of the
CWA)late tonight and toward dawn Wed morning. Some of these storms
to produce small hail, and possibly quite a bit of it, but the
storms also to be progressive and quick moving aloft skirting over
cool llvl laminar layer.

Brisk east to northeast sfc flow north of developing warm front acrs
MO, to continue to cool and dry advect, especially north of I80.
With sfc temps cooling into the lower 30s or even upper 20s in these
areas, some concern for freezing rain and ice glaze especially on
elevated objects/surfaces. Still uncertain how cool the pavement
will get and how much icing could occur on the roads, after Tue
daytime temps help warm the surface. The icing could be the
lead/initial occurrence, before it changes to snow(more on that in
discussion below) to support winter headlines along and north of the
Hwy 30 corridor for Wednesday. For now will advertise a light glaze up
to 0.05 of an inch up to 12z Wed acrs the northwest third of the DVN
CWA, but that could be spread further east and south. Sfc temps
should stay above freezing south of I80 for mainly showers and
embedded thunder late tonight.     ..12..

.LONG TERM...(Wednesday through next Monday)
ISSUED AT 330 AM CDT Tue Apr 17 2018

Main focus is with dynamic and fast moving early Spring storm
Wednesday into Wednesday evening.

In general seeing a bit more southerly and slower trend in the
models with the system, which is fairly typical for these
storms ejecting from the SW. This is resulting in more of our
northeast counties colder and more in the heavier precipitation with
potentially greater impacts.

As for precipitation, anticipate a wintry mix of sleet, freezing
rain and rain lifting up across the cwa Wednesday morning within WAA
wing. Expect there will also be some thunder embedded given such
steep mid level lapse rates, and can`t rule out some hail with MUCAPE
anywhere from 300-800 J/kg on average with sfc-6km 50+ kts and
strong moisture convergence north of lifting warm front. Surface
wet bulb temps are shown to be around or even just below freezing
north of Hwy 30 through the Hwy 20 corridors and therefore would
expect some icing and travel issues for the morning commute Wednesday.
For the afternoon into evening then the deformation zone of the
fast moving potent system will swing through the cwa. Strong dynamical
cooling should erode the warm layer aloft leading to change over to
snow as the dominant precipitation type. Strong omega within the
dendritic growth zone coupled with presence of Conditional Symmetric
Instability (CSI) assessed via Equivalent Potential Vorticity (EPV)
(values less than 0.25 PVU) supportive of potentially heavy snowfall
rates at near 1 inch/hr at times within this deformation zone,
especially over portions of northeast IA within left exit region of
jet. QPF remains a challenge due to strength of system with lots of
moisture to the southwest. Overall trending wetter, but still could
be too low in the northern cwa with around 0.5 inch of liquid over
northeast IA tapering off to less than 0.1 inch south of Hwy 34.
Based on this snowfall amounts of 4-6 inches would appear possible in
far northeast IA with amounts tapering off to less than an inch I-80
and little to no south.

Bottom line, with southerly shift it appears to be a more impactful
storm in terms of wintry side for northern portions of the cwa
with icing followed by period of moderate/locally heavy bursts of snow
with poor visibility and rates near 1 inch/hr for several hours leading
to several inches of accum before winding down by evening. With surface
wet bulb temps still around freezing expect greatest impacts into
northeast IA with snow covered and hazardous travel conditions thereby
leading to decision to hoist a winter storm watch for areas of northeast
IA along the Hwy 20 corridor. To the south through Hwy 30 have opted for
a winter weather advisory for lighter amounts of ice and snow.

Expect the precipitation to wind down quickly late afternoon into early
Wednesday evening from west to east.

Rest of the extended has generally dry conditions with cool conditions
into the weekend before a warming trend. Still appears the weekend
storm will take a far southerly route and not have any real impacts on
our area.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday Morning)
ISSUED AT 605 AM CDT Tue Apr 17 2018

Mainly low VFR BKN to OVC cloud deck will continue to stream down
along and east of the DBQ and MLI TAF sites through mid morning.
Otherwise a VFR Day through this evening with northwest winds
becoming light north to northeast as sfc ridge slides acrs the
area. Tonight, high clouds to thicken from the west ahead of an
approaching storm system, with increasing northeast to east sfc
winds to 8-12 KTs after midnight. A low chance for a wintry
mixture of freezing rain and sleet making it into the CID TAF site
before 12z Wed morning. Other sites such as BRL and possibly MLI
may have to watch out for a band of higher based showers with
embedded thunder streaming in from the southwest late tonight.
  ..12..

&&

.CLIMATE...
ISSUED AT 330 AM CDT Tue Apr 17 2018

Record Lows for Today April 17...

Moline.........18 in 1875
Cedar Rapids...22 in 1997
Dubuque........16 in 1875
Burlington.....23 in 1907

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...Winter Weather Advisory from 7 AM to 7 PM CDT Wednesday for
     Benton-Jackson-Jones-Linn.

     Winter Storm Watch from Wednesday morning through Wednesday
     evening for Buchanan-Delaware-Dubuque.

IL...Winter Weather Advisory from 7 AM to 7 PM CDT Wednesday for
     Carroll-Jo Daviess-Stephenson.

MO...NONE.
&&

$$

SYNOPSIS...12
SHORT TERM...12
LONG TERM...05
AVIATION...12
CLIMATE...12



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