FXUS63 KDMX 261144

Area Forecast Discussion
National Weather Service Des Moines IA
644 AM CDT Fri Apr 26 2019

...Updated Aviation Discussion...

.SHORT TERM.../Today through Saturday/
Issued at 422 AM CDT Fri Apr 26 2019

The main forecast concern is obviously focused on a potent
clipper system ejecting out of the Northern Rockies and bringing
the potential for impactful winter weather to portions of Iowa
Saturday. Models remain in disagreement with the surface low
track, the ECWMF is tracking the low across southern Iowa
(basically along Highway 34) while the GFS is the furthest north
tracking roughly b/t the Highway 20/Highway 30 corridor. The
26.00z NAM is more along the Interstate 80. The 26.00z HREF has
trended closer the NAM/ECMWF with the southern track. Needless to
say that the southern tracks would favor more snow in the northern
sections of forecast area and utilized a blend of the
HREF/GFS/NAM. The timing of this system looks to be from 09z
Saturday to late Saturday afternoon (~20-23z), so fairly quick
moving and dynamic system.

High confidence in the very strong winds with this system and the
potential for a small heavy band of snow to set up over northern
Iowa. Just a shift in 50-100 miles of the low track will
significantly alter the snowfall forecast and the potential for a
"bust" is medium to high...at least wrt to the higher snowfall
amounts. Yes, the ground has been warm but the snowfall rates should
overcome this and the fact that very strong winds coupled with the
falling snow should lead to potential for significant travels issues.
Confident enough to hoist a Winter Storm Watch across the north for
4+ inches and the strong winds. Potential for an advisory further
south as the snow/rain gradient will be cutting a few counties in
half where northern portions may see snow accumulations and the
southern section all rain.

Sharp gradient of temps from north to south tomorrow and trended
cooler with temperatures throughout the day. A midnight high is
likely or at least prior to 12z Saturday with temperatures slowly
dropping throughout the day across northern Iowa. The NAEFS mean
850mb winds shows a 50-55 knot jet punching into southern Iowa by
12z Saturday and the strong WAA is concerning as it may help nudge
the low to track slightly further north closer to the GFS solution.
No doubt a considerable concern for the low to track a bit further
south and change the heavy snow band location. However, only talking
about a county or two difference where the heavier snow sets up, not
a major shift expected attm.

There is an immense amount of frontogenetical forcing, especially
between 850-700mb setting up over northern Iowa from around 12z
Saturday and continues to through 21z. Plus, very strong forcing
within the dendritic layer during this time and even with the
aforementioned WAA, there doesn`t appear to be a warm layer aloft to
deal with and thus the precipitation type is either rain or snow.
Confidence is increasing that the snowfall rates look to overcome
the warm ground temperatures and the surface temperatures for
accumulations to begin from 12-15z Saturday across the far north.
There is even some negative EPV suggested via the NAM/GFS and could
release CSI resulting in upright convection. Thus, cannot rule out
the potential for thundersnow and this scenario would only add to
the snowfall rates Saturday morning.

The potent PV anomaly punches into the state b/t 15-18z Saturday
providing a significant trop fold and intense winds late Saturday
morning into the afternoon.  Increased winds and gusts Saturday and
with the falling snow coinciding with the snowfall across the north,
certainly some nasty travel impacts from the visibility restrictions
and quickly accumulating snow.

.LONG TERM.../Saturday through Thursday/
Issued at 422 AM CDT Fri Apr 26 2019

/Saturday Night through Monday Night/

Confidence:  Medium to High

Quick and intense storm that hits the region tomorrow is expected
to be just east of the area by 00z Sunday with winds relaxing
quickly behind the storm as it races east. High pressure bubble
will settle into the area but with little separation between the
exiting system and another fast moving northern stream storm
entering the Northern Rockies...it`s likely that skies will remain
at least partly cloudy for the night. Lows Saturday night will be
quite cold over the new snow cover in the far north. In fact much
of the area will likely experience freezing temperatures as far
south as southern Iowa. It`s possible that freezing conditions/
frost may develop if winds decouple enough between 09-12z.
Temperatures should be in the mid 20s north to the lower 30s in
the south. On Sunday, the next system will quickly bring stronger
thetae advection back into Iowa with decent isentropic lift
overcoming the dry air...resulting in showers and possible
isolated thunder in the west during the afternoon hours and into
the overnight. As the system then pulls east...a break in the
rainfall will develop between Monday morning and Tuesday morning.
With the stronger push of warmer air Monday, highs will push back
into the 50s north and in the lower 60s in the south. This should
help melt a the remainder of the snow over northern Iowa.

/Tuesday through Thursday/...
Confidence: Medium

There is better agreement/consensus today with the med range models
from Tuesday into Thursday of next week.  Both the GFS/Euro are now
holding the more significant longwave trough back in the southwest
until Tuesday night...then beginning to kick it east/northeast as a
stronger northern stream H500 wave drops south into the Northern
Rockies. The pattern remains complex with several pieces of energy
ejecting from the system and tracking toward the Central US. With
the Gulf available for increasing moisture input, showers and
thunderstorms are likely to quickly redevelop and lift north into
southern Iowa by Tuesday afternoon into Tuesday night. The first
leading shortwave will move from the southern Plains into northeast
Illinois by 00z Thursday. This will bring a swath of rainfall over
Iowa during the day Wednesday eventually tapering off by afternoon.
A trailing system will take a more southerly track and just clip
southern Iowa with some additional rainfall Thursday afternoon. With
clouds and showers being the predominate mode of weather for midweek
into Thursday, temperatures may need to be lowered on Wednesday. For
now and some uncertainty will use a blended approach.


.AVIATION.../For the 12Z TAFS through 12Z Saturday morning/
Issued at 643 AM CDT Fri Apr 26 2019

VFR conditions are expected today into tonight across all TAF
sites. Introduced MVFR visibility and ceilings at FOD and MCW
toward the end of the TAF period with the incoming storm. The
other TAF sites won`t see the impact of the storm until past 12z


Winter Storm Watch from Saturday morning through Saturday
evening for IAZ004>007-016-017-026>028.



SHORT TERM...Podrazik

NWS DMX Office Area Forecast Discussion