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000
FXUS63 KDMX 212324
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
624 PM CDT Sun Apr 21 2024

 ...Updated for the 00z Aviation Discussion...

.KEY MESSAGES...

- Patchy to areas of frost possible issue tonight east half and
  northeast; mainly in low lying valley areas.
- Weak system arrives Monday; mainly frontal passage with light
  rain chances 40 to 50 percent after midnight.
- Larger wave with severe risk ramping late week/early next week
  with strong warm air advection/frontal lift. Several rounds of
  rain are expected with both systems.

&&

.DISCUSSION...
Issued at 246 PM CDT Sun Apr 21 2024

.Short Term /Tonight through Monday Night/...

Confidence Short Term:  High

Pleasant day today with high pressure over central Nebraska
providing the region with mainly sunny skies and lighter northwest
winds today. Tonight the ridge will continue to slide east southeast
with increasing warm air advection into Iowa tonight. Tonight will
feature only a small risk of patchy frost in the east prior to much
warmer temperatures on the way for Monday. Water vapor imagery this
morning shows the next system crashing through the Canadian Rockies
with a lower level trough now evident on a subjective H850 analysis.
Though the wave is rather impressive and will result in some rapid
development of a lee side trough late tonight, the sprawling high
over our area today and this evening will be limiting much moisture
return in the next 24 hours. The high will continue to move into the
Southern Plains and the Gulf Coast through early Monday. Mins
tonight will still be rather cool; especially east. Some frost may
develop in low lying river valley areas that are more sheltered, but
with southwest winds beginning to increase between 10 and 13z
tomorrow, the coverage will likely be somewhat limited. Meanwhile,
as the trough enters the eastern Dakotas at 12z, southwest winds
will increase through the remainder of the day and push H850
temperatures back to a range of 7 to 10C by 00z. Models continue to
show some shower development in a pocket of strong thetae advection
from central to northeast Iowa by mid to late afternoon, then
farther south into the evening hours. Instability remains rather
limited, but a few rumbles of thunder may occur with the stronger
thetae push over the northeast/southeast during the late afternoon
and early evening.  With the warm air advection partially offset by
clouds/potential showers, highs will be much warmer but not as high
as they could be under complete sun. Highs tomorrow will range from
the mid to upper 60s northeast/southeast to the lower 70s in
central/western sections. With limited moisture return, rainfall
amounts will likely range from a trace to no more than a tenth of an
inch. Rainfall amount probs show chances for greater than a tenth of
an inch only at 15% over most of the region.  They also show some
support for a quarter inch in the far south, but present model
biases of the NAM and HRRR are contributing to a more northerly
displacement and faster return of moisture into Iowa. The
lesser rainfall solutions will be leaned on, which are
reasonable with the Gulf moisture field not supportive of
moisture return here, though some moisture finally arrives along
the trailing cold front as it enters the Ohio River Valley or
at fastest over northern Missouri east to the Ohio River Valley.


.Long Term /Tuesday through Sunday/...

Confidence: Medium

In the medium range, we return to northwest flow for several days as
Canadian High Pressure takes a vacation in lower Michigan until
Thursday morning. For Tuesday, we will have to deal with another
upper level trough at H700 passing southeast across the region;
especially affecting the northeast. The influx of colder air aloft
will likely bring partly sunny skies to a portion of the northeast/
east during the day. Along with that, H850 temperatures fall back to
1 to 3C over the northeast while the central to southwest areas
remain about 4 to 6C. Northwest wind and mixing will bring highs
back to the upper 50s northeast to the mid 60s in the south.
Depending on the timing of the approaching second trough Tuesday
morning, highs northeast might top out to the lower 60s but some
uncertainty remains. Though some instability and light
showers/isolated storms are expected with the trough, most of the
steeper lapse rates continue into Wisconsin at this time. As we move
into the midweek period, the northwest flow will quickly give way to
zonal flow across the Central Plains. Weak high pressure will move
over the area Wednesday with a transition back to lowering pressure
into the Rockies as a stronger H500 wave approaches the west coast.
It appears that some stronger warm air advection late Wednesday
night may spark a few showers over either eastern Nebraska or
far southwest Iowa. For now, our airmass will generally still be
too dry to see any precipitation across our forecast area. We
are likely too generous with PoP and any precipitation into
Thursday morning. Highs Wednesday in the transition period will
remain a bit cooler as the high recedes and southeast winds
return. Upper 50s to lower 60s should rule the day. Thursday
will see the return of moisture; mainly later in the evening and
overnight into Friday. Confidence on overall evolution of the
late week into Saturday system has diminished, even though there
is decent consistency on it mainly being a Friday event and
generally exiting the region by early Saturday morning. The
overnight convection Thursday into Friday and early on Friday is
more likely to be elevated with a second round developing
southwest across Missouri/Kansas then moving northeast Friday
afternoon and evening into southern/eastern Iowa. This would
have a better potential to be surface based. Stronger jet
support is also expected later in the day Friday/Friday evening.
Though much depends again on whether the system begins
occluding or continues to strengthen as it nears Iowa, this will
have at least a similar potential to last Tuesdays event. With
the Gulf now available, some decent rainfall rates/amounts are
again expected. Temperatures will warm from Thursday into the
weekend with a potential for 70s to lower 80s by either Saturday
or Sunday. There remains a signal in both the EC/GFS for a
second system nearly on the heels of the Friday storm; arriving
Saturday night/Sunday with more strong storm potential. Rather
impressive wind fields are forecast with the second system as
well in both model solutions. Lots of time to gear up for both
systems, but will likely concentrate on the Friday to Saturday
system this week.

&&

.AVIATION /00Z TAFS THROUGH 00Z TUESDAY/...
Issued at 622 PM CDT Sun Apr 21 2024

VFR conditions expected to prevail across all sites through the
TAF period. While there are some chances for light
precipitation, chances have trended downward and scattered
nature do not warrant mentions at this point in time. Mentions
may be needed post 00z/just outside of this TAF period.
Otherwise, winds will shift southwesterly and become windy by
mid- late morning with sustained winds around 20kts and gusts to
around 30kts across the sites.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...REV
AVIATION...Curtis