Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 221732
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1232 PM CDT Mon Apr 22 2019

...Updated for 18z Aviation...

.SHORT TERM.../Today through Tonight/
Issued at 308 AM CDT Mon Apr 22 2019

Confidence: Medium to High

Low pressure  and an associated vorticity maximum over eastern
Nebraska at 07z continues to shift east northeast overnight and by
12z the surface low will be a weaker, broader trough extending from
near Mason City back southwest to about Omaha. Showers and scattered
thunderstorms continue along the warm front moving north now and
will be lifting into northeast Iowa by morning. Another cluster of
showers and thunderstorms over Kansas has fired southeast with some
new weaker and more widely scattered development now taking place in
southwest Iowa. As the low rotates northeast today, showers and some
thunderstorms will develop in the warm sector this morning ahead of
the low, but coverage is not expected to be too widespread.  As
daytime heating increases later this morning into the afternoon,
coverage of thunderstorms in the warm sector will increase through
the afternoon hours as the low and cold front shifts east toward
Dubuque this afternoon. A more pronounced shield of deformation
showers will develop over northern Iowa as the surface low again
consolidates into one over southwest Wisconsin. The period of more
widespread precipitation will be from 18-00z today and especially
across the north half of the forecast area north of US30.  Have
raised PoP from chance/likely this morning to categorical over the
north this afternoon. Severe chances today look rather limited. Most
of the area this morning and early afternoon continues to see a
decent cap through about 2 to 3 pm.  From a line running near
Marshalltown north to Waterloo there may be a small window between
about 4 pm and 6 pm of a stronger storm or two. Though the cap has
essentially eroded, cape is only about 600-700j/kg and the cape is
tall/skinny. Possibly a hail threat. The better instability will
have moved off into western Illinois and southern Wisconsin prior to
this small window forming farther west over Iowa.  The HRRR
continues to show a thin line of storms forming between 20-23z over
the east, strengthening by 21z near the previously mentioned
corridor.  Highs today will be chillier in the northwest dropping
into the lower 50s back near Estherville while mid to upper 70s will
again be possible in southeast sections.  As the low pulls out of
the area tonight, northwest winds will increase into the evening
hours with winds of 20 to 25 mph with gusts to 35 mph this evening.
By Tuesday, sunshine will return to the area with comfortable highs
in the 60s along with north winds at 10 to 15.

.LONG TERM.../Tuesday through Sunday/
Issued at 308 AM CDT Mon Apr 22 2019

Tuesday night...Maintaining a persistence forecast for Tuesday
night with the surface low and frontal boundary east of the
forecast area while aloft, a deep shortwave digs into the desert
southwest with a northwest to westerly flow over Iowa. The Euro
develops a fairly substantial shortwave across Iowa Tuesday night
into Wedensday morning but this was not present yesterday and as
such is somewhat of an outlier. The Canadian does develop a weak
wave but it is east of the forecast area and not nearly as strong
as what the Euro is suggesting. With the absence of a boundary and
considerably drier air in place, even if a wave were to
develop/move across the CWA, it`s doubtful we would see much
precip from it. This will be monitored to see if there is more
consistency with time regarding the development of the shortwave
and whether it will be able to produce anything more than clouds.

Wednesday through Wednesday evening...The upper flow moderates some,
becoming more zonal and as such, temps warm some 5 to 10 degrees
from Tuesday.  A shortwave will drop through the Northern Plains
brushing northern Iowa Wednesday night and Thursday.  There may be
some light showers with this feature across the far north but impact
will be inconsequential and most of the forcing and instability with
the wave will be in MN so not really looking at thunder with
this...just some showers.

There is some difference between the models for Thursday night
through Friday with respect to deepening the shortwave that passes
by on Thursday.  The GFS keeps it fairly muted while the Euro
deepens it just as it passed into eastern Iowa and tries to phase it
with a strong wave across the Southern Plains.  The Canadian is
more of a blend of the two models...keeping the deeper southern wave
and a far weaker, but still present, deepening wave over IL/IN so
further east.  For now given model uncertainty and the lack of
moisture per forecast soundings, I kept the period dry.

Friday night, a better shortwave drops out of the Pacific Northwest
region with a surface low developing over the Rockies.  A warm front
develops from the low and extends into the Central Plains.  It is
unclear whether there will be enough moisture, theta-e advection or
forcing to do much but model consistency for this far out is quite
good and they do break out precip sometime late Friday or Friday
night across the far northwest.  By Friday night and Saturday, the
Euro has that shortwave becoming a little stronger as it comes out
of the Rockies but the surface low also deepens and nears northwest
Iowa at this time with a warm front somewhere between northern Iowa
and southern MN,  Better moisture and forcing is in place by this
time so showers and thunderstorms look like a reasonable forecast
though timing and location are still somewhat in question given the
difference in location of the surface low/frontal boundary.

Sunday into Monday model discrepancies deepen even further with the
GFS developing a big upper ridge coming into the Upper Midwest while
the Euro has a more zonal flow across the region with another
shortwave/surface low developing over the Plains and bringing
another shot of showers across Iowa Sunday. Since we had this going
and model confidence is low, I kept with persistence though rain is
looking less likely for Sunday at this point.  Somewhat cooler temps
are expected late next weekend either due to cloud cover/precip or a
more northwesterly flow.

&&

.AVIATION.../For the 18Z TAFS through 18Z Tuesday afternoon/
Issued at 1227 PM CDT Mon Apr 22 2019

Main forecast challenge through the current TAF period is
shra/tsra chances through this evening, along with the development
of widespread mvfr to ifr cigs. Best chance for tsra activity
should be north of KDSM, but latest model guidance has suggested
the potential for isolated tsra near the terminal between 19z-21z.
Have included vcts to highlight the potential, although confidence
is quite low. Higher spatial coverage is more probable further
north, with prevailing tsra warranted at KALO. Low cigs have also
continued to slowly creep south and east today and will eventually
overtake all TAF sites. Cigs into IFR category are likely at
times, especially across northern Iowa. Improving conditions back
to VFR is expected by early to mid Tuesday morning for most of the
area.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...REV
LONG TERM...FAB
AVIATION...Martin


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