Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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650
FXUS63 KDVN 192130
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
330 PM CST Fri Jan 19 2018

.SYNOPSIS...
Issued at 325 PM CST Fri Jan 19 2018

Warming flattened/zonal flow pattern continues acrs the upper half
of the CONUS, while late weekend/early next week storm system looms
just off the CA coast. regional snow cover taking a beating despite
the waves of cirrus. Moistening of the boundary layer to hasten Sat
night into Sunday ahead of the large closed low rolling into the
southwestern plains, eventually fueling drizzle and fog the second
half of the weekend before more organized rains arrive by Sunday
night. After whatever storm system can affect the region through
early week, behind it not much of a cool down for mid week/close to
normal, before temp moderation begins again late week ahead of
another inland striving long wave.

&&

.SHORT TERM...(This Evening through Saturday Night)
ISSUED AT 325 PM CST Fri Jan 19 2018

Tonight...Current water vapor loop showing long fetch mid/upper
moisture feed from well southwest of CA, streaming northeastward
acrs the upper Midwest. Interesting interaction noted as this feed
crosses the east central Rockies, with the mountains making an
enhanced plume streaming east acrs the central plains and MO RVR
Valley...even making it locally acrs the mid to upper MS RVR Valley.
Ongoing southwest llvl feed and this cloud cover to make for a
seasonably mild night. Much of the area may stay in the 30s, and
will go with upper 20s north of I80 and lower 30s south of I80. But
guidance low temps have been too cold the past few nights, so
suspect that these fcst values and even adjusting up them up a bit
may still not be mild enough unless the cirrus thins more than
expected later tonight.

Saturday...The NAM is holding on to snow cover too long, and too
quick to saturate the sfc layer and thus will ignore it`s early
onset of low stratus and fog generation late tonight and Sat
morning. But increasing south to southwest warm and moist conveyor
should start to have it`s effects as the day progresses. Mid and
high clouds to thicken during the morning and midday, then the
stratus watch will be on streaming up from the south during the
afternoon and into Sat evening. This cloud field already organizing
acrs eastern TX and lower MS RVR Valley. Despite the increasing
cloud cover and sfc DPTs, will lean toward the milder of guidance
highs, with areas along and south of I80 making it into the mid 40s
at least, with some upper 40s possible in the south if the stratus
doesn`t arrive too quickly...otherwise the mid CWA/I80 corridor may
be the warmest. Will keep it dry through 00z Sunday.

Saturday Night...LLVL Stratus and fog to thicken under ongoing warm
wedge aloft/stout inversion as the evening progresses, and prefer
the 12z GFS milder sfc/BL into the overnight as well as further
north boundary placement. This will be important, as increasing lift
will start to wring out drizzle out of the low stratus, and the
question will be if any of it can freeze/sfc temps get cold enough.
If the GFS has it`s way and that`s the more favored solution for now,
sfc temps will be too mild/above freezing even in the far north/
northwest when the drizzle breaks out and limit the light icing
potential. The NAM again seems too cold with the sfc layer and
breaks out freezing drizzle with some ice accums along and north of
the Hwy 30 corridor after midnight, if not even further to the
south. With model blend with the sfc temps, for now will keep some
freezing drizzle mention for late Sat night going in these areas for
one more fcst package, with the threat fine tuned or removed by
following fcst shifts. The fog could be dense and more of a
widespread impact into early Sunday morning, unless the LLVL
moisture depth increases enough for more of a light rain breaking
out(as opposed to drizzle) which could knock the fog density down
some. Will advertise a seasonably mild sfc temp regime Sat night
along and south of the favored boundary placement...some of these
areas may not dip out of the upper 30s.    ..12..

.LONG TERM...(Sunday through next Friday)
ISSUED AT 325 PM CST Fri Jan 19 2018

Sunday and Sunday Night

Strong, closed off upper low will be crossing the Four Corners
region early Sunday before reaching central Kansas by Sunday night.
With no arctic air in place downstream of the low, mild air will
entrain into the east side of the system leading to rain as the
dominant precip type for the DVN forecast area. However, there are a
few caveats:

1) There is a chance for freezing drizzle or light freezing rain
approximately along and north of highway 30 continuing out of late
Saturday night through the day on Sunday due to ENE sfc winds
potentially holding temps near freezing. Confidence on this
occurring remains relatively low since the NAM 12km and 3km are
the only models cold enough for a period of ice. But it`s at least
worth a mention.

2)Chance for mixed precip or a changeover to snow around midday
Monday across the northwest. The ECMWF has been trending south with
the track of the upper-level low (the center of which is forecast to
pass slightly south of the Quad Cities), the sfc low, and the 32 F
sfc wetbulb contour. Our current forecast has not trended toward the
ECMWF yet because the GFS/CMC are quite a bit warmer.

Precipitation:

Models have now increased QPF across E Iowa/NW Illinois, which will
be falling on frozen ground. The heaviest amounts of 0.50 to 1.00
inches are favored over the east and north. Not too concerned about
ice jams or river flooding. Stream flows are below average for this
time of year, and temps are only expected to briefly reach into the
40s before falling back below freezing. Main concern is for ponding
and standing water in low-lying areas.

Late Sunday night into Monday morning, MUCAPE values are forecast
over 100 J/kg, so wouldn`t be surprised if we have isolated elevated
thunderstorms during the period of strongest mid-level isentropic
and frontogenetic forcing. Lastly, areas of fog are likely, and
could become dense at times.

Monday

The sfc low begins to occlude as it moves from NW Missouri through
eastern Iowa. After a dry slot and lull in the precipitation, a
changeover to snow is likely into Monday afternoon or evening,
continuing into the night. Light accumulations are possible at this
time, but given the strength of the upper-level low and presence of
a TROWAL feature at 850mb there could be heavier snow showers
embedded within the light snow.

The 12Z ECMWF has been consistently further south with the low
track. If it verified, a period of a mix or quicker changeover to
snow would be possible across the N/NW forecast area. Plenty of time
for other models to trend toward the ECMWF/NAM or vice versa.
Therefore, changes to the forecast for Monday can be expected as we
get closer to this event.

Tuesday On

Dry conditions for the middle of next week with warming temps into
the 30s and 40s by Thursday. Uttech

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday Afternoon)
ISSUED AT 1142 AM CST Fri Jan 19 2018

Don`t buy the quick onset of lower stratus and fog that the NAM is
developing late tonight into Sat morning, and thus will advertise
another VFR TAF cycle with persistent southwesterly sfc winds and
passing high clouds. More of note may be a marginal low level wind
shear scenario setting up as early as this evening, if sfc winds
under sharp inversion can drop below 8 KTs. FCST soundings
advertise 40+ KTs at 1800 to around 2000 FT AGL from 240 to 250
degrees this evening and into the overnight. This flow aloft is
expected to decrease by mid Sat morning, if not earlier.   ..12..

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...NONE.
IL...NONE.
MO...NONE.
&&

$$

SYNOPSIS...12
SHORT TERM...12
LONG TERM...Uttech
AVIATION...12



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