Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 220553

Area Forecast Discussion
National Weather Service Des Moines IA
1153 PM CST Mon Jan 21 2019

.DISCUSSION.../Tonight through Monday/
Issued at 339 PM CST Mon Jan 21 2019

Overview: A wintry mix of precipitation will bring impactful weather
causing hazardous travel to the state starting tonight through
Tuesday night. Then, there will be several chances for snow later
this week into this weekend as quick moving clippers zip through the
region. Frigid air is expected toward the end of the week into early
next weekend as well.

Strong and gusty winds this afternoon have been causing areas of
blowing snow, causing snow to spread/drift across east/west roads.
Expect this to continue into this evening before winds decrease.
Then attention turns to a trough that is centered over the
Intermountain West as seen on GOES-East clean IR imagery this
afternoon that will spread eastward tonight into Tuesday night. Of
importance to tonight`s forecast of precipitation type is the
GOES- East cloud top phase, which is showing the loss of ice
introduction spreading from central into eastern Nebraska this
afternoon. As warm air advection/isentropic lift on the 290K
surface increases tonight over Iowa, soundings show that the ice
loss seen on GOES-East will spread into our forecast area as dry
air aloft enters the ice introduction layer. This will limit snow
during the first part of this storm. Therefore, freezing drizzle
will be the dominant precipitation type tonight across central
Iowa except for any initial snow up north. The freezing drizzle
will develop and spread east/southeastward through the overnight
hours as the low levels saturate, though it may not reach our far
southeast counties until around or shortly after daybreak. A few
hours before sunrise Tuesday, soundings from the RAP, GFS, and FV3
show saturation beginning in the cloud layer near the Minnesota
border with the NAM later towards mid-morning. So expect a
transition from freezing drizzle to snow during the predawn into
the early daylight hours over far northern Iowa roughly along and
north of Highway 3. This transition from freezing drizzle/freezing
rain will slowly shift southward toward the Highway 30 counties
through the remainder of the morning. With freezing drizzle
expected to reach the Des Moines metro towards morning rush hour,
this could have big implications for morning travel.

Big question remains how far north does the warm air get into
southern and central Iowa, which will impact precipitation types
during the day Tuesday. There is a high degree of variability on
temperatures tomorrow. Just using Des Moines` high temperature for
example, the suite of model guidance including bias corrected, raw,
National Blend, and MOS range from 24.5 to 39 with an interquartile
range of 31 to 35. At least for the southern 2 to 3 tiers of
counties, enough warm air will allow for a changeover to all rain
ahead of surface low pressure that will track from south central
into east central Iowa. The transition zone looks to be between
Highway 30 and Interstate 80 counties with periods of wintry mix of
snow, freezing rain, or rain from late morning into the afternoon.
During the afternoon hours, a dry slot will also poke up into the
state. This typically causes a loss of ice in the cloud layer so
would not be surprised if a period of freezing drizzle or drizzle
occurs as that slides across central and southern Iowa from west to
east in the afternoon into the early evening.  As the low moves east
of the forecast area, will see a deformation zone wrap around the
back side with a quick, ending shot of snow.

For snow amounts, enhanced snow rates with dendrites are possible on
Tuesday given cross sections from the NAM and GFS showing good
saturation within the dendritic growth zone and strong lift. Dynamic
cooling to the wet bulb not out of the question, which may cause the
transition zone to slip a bit quicker southward than expected.
Frontogenetical forcing is maximized in the 925-850mb with lesser
forcing in the 850-700mb near and north of the wintry mix/snow
boundary. Narrow banding aside, snow amounts of 4 to 5 inches around
and north of the Highway 20 counties with some spots close to 6
inches toward Mason City. The combination of snow and ice has
resulted in an upgrade to a winter storm warning for much of this
area. Elsewhere, snow amounts will range from an inch or so near I-
80 to around 3 inches near Highway 30. Ice amounts will be maximized
south of Highway 20 and north of Highway 34 with up to around a
tenth of an inch or slightly more possible in this good freezing
drizzle/rain environment. Lesser ice amounts are possible north and
south of this area.

With subsidence spreading across the state on Wednesday, will have
brighter skies than the first part of this week. As northwest flow
aloft develops later Wednesday, this will set the stage for fast
flow clippers to dive across the region. Confidence is low beyond
late Wednesday with timing and spatial differences, though models
seem perhaps in slightly better agreement than yesterday. First
clipper is set to cross the region Wednesday night into Thursday as
an arctic cold front races through the region. The 18z Sunday and
today`s 12z run from the GFS have backed off the aggressive QPF
solution of the 12z/Sunday GFS. Q-vector convergence is the
strongest in the GFS with weak convergence/neutral in the ECMWF. So,
could be a brief period of light snow for parts of central Iowa.
More important will be the gusty winds that develop and could blow
around falling or already fallen snow. Frigid air will spill into
the state on Thursday with temperatures dropping through the day
across much of the area as a 1035mb high slips into the region.
850mb temperatures will drop into the middle teens below zero to low
20s below zero Thursday afternoon into Friday night. This will
result in low temperatures Friday morning below zero across central
Iowa and around -20 over far northern Iowa. While winds will
decrease Thursday night into Friday morning, they will still be
elevated enough that wind chill values would necessitate wind chill
headlines. A possible complication to this very cold night is the
timing and placement of the next clipper that might arrive toward
Friday morning (GFS/CMC) or later in the day (ECMWF) and bring
clouds keeping temperatures up a bit. Temperatures will begin to
slowly rise and moderate into the weekend, but still will be quite
cold Friday night into Saturday morning over the northeastern half
of the state and may once again need a wind chill headline.
Additional, nebulous clippers with snow chances are possible with
uncertainty remaining high into next weekend.


.AVIATION.../For the 06Z TAFS through 06Z Tuesday night/
Issued at 1152 PM CST Mon Jan 21 2019

Conditions haven`t changed much since 00z, and may have even
improved slightly with ceilings nudging up to VFR in spots.
Conditions are expected to deteriorate through the early morning
hours however low level moisture increasing into -FZDZ and/or
-SN developing and MVFR or IFR visibilities or ceilings. These
conditions should persist through the end of the period.


Winter Weather Advisory until midnight CST Tuesday night for

Winter Weather Advisory until midnight CST Tuesday night for

Winter Weather Advisory from 6 AM Tuesday to midnight CST
Tuesday night for IAZ050-061-062-074-075-084>086-095>097.

Winter Storm Warning until midnight CST Tuesday night for

Winter Storm Warning until midnight CST Tuesday night for IAZ027-



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