Area Forecast Discussion

FXUS63 KDMX 222047
Area Forecast Discussion
National Weather Service Des Moines IA
347 PM CDT Thu Mar 22 2018
.SHORT TERM.../Tonight through Friday/
Issued at 347 PM CDT Thu Mar 22 2018
Bottom Line up Front... 
Confidence remains high in winter system to impact Iowa Friday 
through Saturday. Accumulating snow and strong winds likely in 
northern Iowa, and moderate to heavy rainfall in central to southern 
20z GOES-R Water vapor imagery picking up on huge upper low still
spinning off the British Columbia coastline. On the southern side
of this low, an impulse is being ejected onshore in California. 
Models have been in good agreement with propagating this upper low
through the Four Corners tonight, and into the Plains States by 
Friday afternoon. Models also continue strong agreement with 
developing a sfc low in response... placing the low over western 
KS by 00z Sat and northern Missouri by 12z Sat. 
Overall, there have been little changes to the overall mesoscale 
features of this event. There is still a huge slug of low-level 
frontogenetical forcing, especially evident at 900mb that sets up 
over southern to south-central IA. With this system tilting 
northwestward with height, there is good response with 700mb 
frontogenetical forcing further northward along the I-80 corridor. 
Confidence increasing slightly in being able to pin down a slug of 
strong isentropic adiabatic ascent propagating through southern 
Iowa at some point during the 06z Sat to 12z Sat timeframe. Thus, 
may see some higher localized rainfall rates and even some 
Precip type across northern Iowa will be very touchy. Very 
temperature-dependent. Trend is to be slightly cooler with sfc 
temps, which would drop accumulating snow slightly further south,
closer to the Highway 30 corridor. Snow ratios also touchy with 
this event. Obviously, the cooler temps would support higher snow 
ratios... but if the track shifts 40 miles northward, snow ratios 
could drop to 4:1 range... which would hammer snow amounts. For 
this reason, have held current Winter Storm Watch as is, 
anticipating overnight shift making final adjustments. Our far 
northeastern counties may need to be upgraded to a Winter Storm 
Warning, while the rest of the Watch area could be transitioned to
a Winter Weather Advisory. Factoring into impacts will be 
easterly winds in the 15 to 25 mph range. This would reduce 
visibility to one mile or less... possibly even 1/4 mile during 
periods of more heavy snow. 
Going backwards in time, slight concern event start time may need to 
be pushed earlier. The GFS is especially picking up on band of 
isentropic ascent phasing with deep enough low-level moisture for 
cloud thicknesses to support light precip by 15z Fri. Will watch in 
next shift, but cannot rule out earlier start time of any headlines.
.LONG TERM.../Saturday night through Thursday/ 
Issued at 347 PM CDT Thu Mar 22 2018
System will have departed by Saturday evening with weak surface 
ridging sliding into the state.  Model sounding continue to indicate 
plenty of low level moisture lingering, especially the south and 
west, therefore cloudiness is expected to persist into the overnight 
and Sunday for much of the area.  Exception may be northeast areas 
where drying will extend a bit deeper into the boundary layer.  Warm 
advection begins into Sunday as next large trof moves onto the west 
coast and into the intermountain region.  Increasing isentropic lift 
into the afternoon produces some deeper saturation across the far 
west by later in the afternoon with some sprinkles/light rain 
Strong theta-e advection and moisture transport arrive late Sunday 
night into Monday which should spur shower development across 
central Iowa during the early morning hours on Monday.  Elevated 
instability will increase across the area on Monday with the strong 
southerly flow with some isolated thunderstorms expected with the 
shower activity.  By late Monday afternoon, low pressure will be 
approaching southwest Iowa near the Omaha area with surface 
instability edging into the southwest CWA.  Models would indicate 
some potential for stronger deep convection, nearly surface based in 
eastern Nebraska into Kansas by late afternoon.  Some of this may be 
able to reach the far southwest portions of the forecast area by 
Monday evening, although this time of year, weakening usually occurs 
after sunset limiting overall severe threat.  Much of the rest of 
the forecast area will see widespread showers and some 
thunderstorms as the surface front passes through the state Monday
night. Lingering showers will persist into the southeast on 
Tuesday before the system departs and subsidence increases. Some 
quieter weather is expected into the middle of next week with 
seasonable temperatures expected.
.AVIATION.../For the 12Z TAFS through 12Z Friday morning/
Issued at 647 AM CDT Thu Mar 22 2018
Thin band of light rain possibly mixed with a frozen mix moving
across IA through 19z associated with warm air advection. Mid
level clouds will remain through the period though a few breaks in
the cloud cover will be possible. Winds remain generally east
through the period as well and under 12kts. Major changes on the
way for Friday 03/23...especially aft 18z. Those concerns will be
address in next few packages. /rev 
Winter Storm Watch from Friday evening through Saturday 
afternoon for IAZ006-007-016-017-026>028-038-039-050.
SHORT TERM...Kotenberg