Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 211804
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
104 PM CDT Thu Mar 21 2024

...18z Aviation Update...

.KEY MESSAGES...

- Scattered flurries early this morning, with no impacts.

- Mixed precipitation and accumulating snow likely early Friday
  across northeast Iowa and northern IL. Snowfall rates between
  0.5" and 2"/hr expected after 6am, creating slick spots on
  roads and sidewalks during the AM commute. A winter weather
  advisory has been issued for the Highway 20 corridor.

- Rain will be the predominant precipitation type along and
  south of Highway 30 tonight-Friday AM, with surface and
  pavement temperatures remaining above freezing.

- Active weather early next week, with a strong mid-latitude
  system bringing much needed precipitation the area, as well
  as strong winds.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 311 AM CDT Thu Mar 21 2024

Morning radar mosaics show light returns across northern and
east central IA. This is occurring with a weak shortwave moving
in the northwest flow aloft across the Dakotas. Surface
observations show these returns are falling out of cloud decks
greater than 5kft and with our 00z DVN sounding showing a very
dry sub cloud layer, none of it is reaching the ground. DOT
webcams also confirm this. In any case, have kept scattered
flurries in the forecast through 13z. Expect temperatures to
reach the mid to upper 40s today which is a a few degrees warmer
than yesterday, with less wind. Attention then turns to our
next weather maker for tonight currently onshore across the
Pacific Northwest per latest water vapor imagery.

Tonight...strong clipper on track to move across the Pacific
northwest and northern Rockies today and into the Dakotas/northwest
IA tonight. The accompanying surface low will move from northwest NE
to northwest MO by 12z Friday. Strong forcing shown on RAP13 925-
850mb fgen progs under strong upper level divergence from increasing
speeds aloft (50kts at 500mb 10z) will support precipitation quickly
developing after 06z tonight initially over northern IA and
towards the Highway 20 corridor in eastern IA. What has changed
from the previous forecast is that the bulk of the models has
shifted further south with the snow ptype and increased snowfall
amounts particularly on the HRRR and the NBM models, with the
only models keeping the bulk of the snow north of the CWA is the
00z NAM 3km and the 00z ARW. Thermal profiles suggest
precipitation will fall initially as a rain/sleet/snow mix,
before quickly changing over to snow primarily along and north
of Hwy 20 after 09z. While there will be marginal
surface/pavement temps to support much accumulation initially,
strong lift/omegas in the DGZ and high RH in this layer suggests
that snowfall rates of 1-2"/hr will be possible overwhelming
the pavement for accumulation to occur. The 75th percentile 1-hr
snowfall rates off the 00z HREF support this notion and show a
2-3 hr period of 1+ inch snowfall rates at any given location.
If that wasn`t enough, steep lapse rates show that thunder is
not out of the question in this environment.

With all this in mind, combined with time of day and that it has
been a while for the area seeing any snow, have issued a winter
weather advisory for the Friday AM commute through 1pm. Total
snowfall accumulations of 2-5 inches is possible in the advisory
areas with the higher amounts expected in the northern portions of
the counties.

Friday...snowfall and mixed precipitation will be mainly occurring
north of a line from Vinton, IA to Savanna, IL line through 8am,
with rain the dominant precipitation type south. Expect snowfall
rates to decrease quickly after 15z and progress east into northeast
IL. With surface temps increasing slightly above freezing, some
melting is expected on roads through the day. Total QPF amounts for
this event have also increased some from the past 12hrs and now most
areas should see 0.2" liquid from this system. All precipitation
should end from west to west Friday afternoon. Highs will be a
challenge and may be overdone in the north where snow falls. In any
case, forecast highs will range from the upper 30s in northeast IA,
to the lower 50s in northeast MO, southeast IA, and west central
IL.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Issued at 311 AM CDT Thu Mar 21 2024

Friday night-Sunday...High pressure acrs the upper MS RVR Valley
Friday night through Sat with east-northeast winds to bring a mainly
dry but cool below normal temp day. Elevated return flow/WAA aloft
acrs the plains to east of large organizing upper trof acrs the
Intermountain West, may try to spill some spotty elevated bands of
light snow or rain-snow mix Sat night. Digging upper trof base into
northern Mexico by the latest ensembles shows the scope of the size
of this trof complex, with lee-side southwesterlies eventually
sprawling out acrs the upper Midwest by Sunday night.  Sunday looks
breezy with increasing east to southeast pressure gradient/winds
while sfc cyclogenesis really blossoms acrs the southwest plains.
Some warming component as compared to Saturday, but not a huge jump.
More of an organized elevated warm air advection(WAA) surge should
drive some precip acrs western and northern portions of the DVN CWA
by Sunday afternoon, with it possibly being wet snow or rain-snow
mix with minor snow acumms possible on grassy-elevated sfc`s.
Impressive warm moist conveyor signal still there as the upper trof
shears acrs the northern plain and MO RVR Valley through late
Monday. Continued slowed and further west upper trof axis trend in
the ensembles suggest that while Sunday night will be windy and not
as cool, the main south-to-north precip stream may still be off to
the west with the local area just getting in on some spotty quickly
moving light showers or even an isolated thundershower.

Monday through next Wednesday...Monday, the pre-trof conveyor of
showers with even a little embedded thunder should be realized acrs
the area by morning or even being held off until the afternoon.
Strong south to southeasterly LLVL wind fields and shear, but hope
for saturation to limit severe potential but it will have to be kept
in mind especially on the nose of any dry slot trying to wrap into
the back of the system Monday afternoon or night. Now with ensemble
signs of a near 990 MB sfc low rolling acrs MN by Tue, dry in-
wrapping and widening dry cooling slot/cool conveyor may engulf much
of the area by later Tue for a drying trend that day. Current
ensemble precip handling has the comma wrap around rain/snow now
more off to the north acrs MN/WI Tue night. Still appears 1-2+
inches of rain may fall acrs much of the area by the time the system
occludes and exits, with localized higher amounts possible acrs the
drought-ridden areas of IA. Then wave embedded westerlies look to
maintain an active weather period for much of next week, with a type
of system possible every other day.  ..12..

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1255 PM CDT Thu Mar 21 2024

Mid deck with some embedded non-consequential flurries this
afternoon with east winds of 5-10 KTs. Tonight, still a
challenging fcst walking in a rather vigorous clipper system
with an associated precip shield. The precip may not make it to
MLI and BRL until mid Friday morning...mainly rain and MVFR
conditions at BRL and possible rain-snow-sleet mix at MLI after
10 AM. The DBQ site will look to be the most impacted with a
wintry mix changing over to a heavy wet snow by 12-13z or so.
The most intense snowfall rates at over an inch and hour and
doing accumulation/low VSBY restrictions will look to occur
from around 8 AM to 1100 AM or so Friday morning. Sharp snowfall
gradient to the south and CID may only experience light snow or
rain-snow mix getting more widespread from 11-12z Fri and on
into the morning. Another factor to complicate things, is enough
elevated instability to produce some isolated lightning
flashes/thunder acrs much of the area late tonight and
especially on Friday.   ..12..


&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...Winter Weather Advisory from 1 AM to 1 PM CDT Friday for
     IAZ040>042.
IL...Winter Weather Advisory from 1 AM to 1 PM CDT Friday for
     ILZ001-002.
MO...None.

&&

$$

SHORT TERM...Gross
LONG TERM...12
AVIATION...12


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