Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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000
FXUS63 KDVN 241745
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1145 AM CST Sat Feb 24 2018

.UPDATE...
Issued at 1144 AM CST Sat Feb 24 2018

Late morning update to expire winter weather advisory. With road
surface temps warming, the threat for any additional significant
weather has ended. A light mix will linger along the highway 20
corridor early this afternoon, with patchy light rain/drizzle to
the south, before the next round arrives from the west late this
afternoon.

UPDATE Issued at 559 AM CST Sat Feb 24 2018

Radars show a solid axis of what is likely freezing rain with
some sleet continuing to spread north across southeast Iowa and
west central IL. While it is encountering drier and eroding on
the northern edge, it will likely continue to advance and produce
at least a glazing of ice up into the I-80 corridor before
temperatures rise this morning.

Have thus extended the advisory another row of counties northward
with an expiration time of 15z, when temperatures should exceed
32 and the precipitation is expected to weaken.

&&

.SYNOPSIS...
Issued at 351 AM CST Sat Feb 24 2018

Elevated warm advection ahead of a system lifting out of the
southern plains was spreading freezing rain, and possibly some sleet
and snow into northeast Missouri, southeast Iowa and west central
Illinois early this morning. At the surface, a north to
northeasterly flow out of high pressure over the MN and WI was
feeding a relatively dry airmass over the forecast area with
temperatures ranging from the mid 20s north to lower 30s south. An
upper level trough over the Rockies will lift northeast through the
plains into the Great Lakes today tonight, lifting a surface low out
of the Rockies through the forecast area, spreading precipitation
through the region.

&&

.SHORT TERM...(Today and Tonight)
ISSUED AT 351 AM CST Sat Feb 24 2018

Initial challenge is the wintry mix across the south early this
morning, then attention turns to showers and thunderstorms with the
fast moving system later today through this evening.

Headlines: A winter weather advisory remains in effect for roughly
areas south of I-80 from the Quad Cities westward through late
morning. The current area of light wintry precipitation will
continue to spread north-northeast, but dry air at the low levels
will delay the onset until after sunrise along the I-80 corridor. It
still appears that temperatures should climb sufficiently to
transition the mostly light mixed precipitation to all rain showers
by late morning and no changes to the advisory coverage have been
made.

This afternoon will have brisk southeast winds as the pressure
gradient tightens ahead of the approaching surface low. Scattered
showers will become more numerous by late afternoon with
only elevated instability available for isolated thunderstorms. The
main surface low is generally depicted passing just southeast of the
forecast area, possibly touching the west central IL counties as it
passes early in the evening, on its way to central WI  by midnight.
This would keep marginal SBCAPES just southeast of the area,
limiting any strong storm potential. The intercept of higher
moisture along axis of heavy rain continuing well to the south,
along with fast movement of the system, should keep QPF well below a
quarter inch in most areas.

Temperatures will climb through the day, with highs around 40 north
to mid 40s south occurring early in the evening as the low passes
through. Strong forcing with the upper level trough will likely
drive an organized area of showers and thunderstorms through the
region early in the evening. Current model timing of this
strengthening shortwave suggests faster timing and more abrupt
ending of the precipitation tonight from previous forecasts. This
will keep the potential for any mixed or wintry precipitation on the
back edge very low as strong subsidence moves in after midnight. The
surface low will be deepening as it exits into the upper Great
Lakes. The resulting strong surface pressure rises and tight
pressure gradient will produce strong west winds of 20 to 30 mph
with possible gusts over 35 mph overnight as temperatures fall back
into the 20s.

.LONG TERM...(Sunday through Friday)
ISSUED AT 351 AM CST Sat Feb 24 2018

Sunday

Breezy with WSW gusts of 20-30 mph as a strong 985 mb low crosses
Lake Superior and heads into central Ontario. Temperatures will
remain relatively mild in the 40s, except for mid to upper 30s far
north.

Monday and Tuesday

High pressure in control. Lack of snow cover and late February sun
angle will allow daytime highs to reach the 40s and 50s Monday, then
50s and 60s on Tuesday.

Wednesday through Thursday

A period of widespread precipitation is likely, especially from
Wednesday evening or night through Thursday. The GFS, GFS ensemble
mean, and CMC are similar in the track and evolution of a closed
500mb low, taking it on a path from Arizona to the southern Great
Lakes. If this scenario played out, Gulf moisture would entrain into
the system and significant precipitation would occur across the
forecast area (GFS PWATs are over 0.75"). The latest ECMWF is a big
outlier and doesn`t phase the Arizona mid-level low with a northern
shortwave until they reach the East Coast. Overall, confidence is
lowest on precipitation amounts due to the much weaker ECMWF.

Thermal profiles are mild ahead of this system which supports rain
as the dominant precipitation type initially. Depending on the
amount of cold air the low can wrap in, and on the degree of dynamic
or evaporative cooling, a period of snow is possible on the backside
of the low, mainly across the north.

Friday On

Expect seasonable temperatures and dry conditions to end the week
with a 500mb ridge over the Central U.S. Uttech

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday Afternoon)
ISSUED AT 1134 AM CST Sat Feb 24 2018

Expect mainly drizzle, low stratus, and MVFR fog this afternoon.
Another round of precipitation will arrive this evening, with a
4-5 hour window from west to east. P-type at TAF sites is expected
to be predominately rain, although cannot rule out a brief mix at
KDBQ this evening. Surface winds will become strong and gusty from
the west overnight, in the wake of the exiting surface low, as
ceilings and visibilities improve through daybreak.

&&

.HYDROLOGY...
Issued at 1144 AM CST Sat Feb 24 2018

Rock River

Major flooding continues to affect Joslin and Moline along the Rock
River. Joslin has reached crest and has begun slowly falling over
the past few hours, with the current forecast taking it below major
flood stage by late Sunday afternoon. However, changes in river
levels are possible until ice completely breaks up.

Pecatonica River

A crest into major flooding is expected at Freeport early morning
Monday. Also, renewed rises on Yellow Creek are possible, so people
affected by the creek should remain alert.

&&

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

$$

UPDATE...RP Kinney
SYNOPSIS...Sheets
SHORT TERM...Sheets
LONG TERM...Uttech
AVIATION...RP Kinney
HYDROLOGY...Speck


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