-- Discussion containing changed information from previous version are highlighted. --
183
FXUS63 KDMX 181128
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
628 AM CDT Tue Jun 18 2024

 ...Updated for the 12z Aviation Discussion...

.KEY MESSAGES...

- A few severe storms possible late this afternoon and early
  evening, mainly west of Interstate 35.

- Heavy rain possible, especially into central Iowa tonight and
  Wednesday. This could lead to flooding or flash flooding in
  some areas and headlines may still be needed.

- Unsettled weather continues into the weekend.

&&

.DISCUSSION...
Issued at 422 AM CDT Tue Jun 18 2024

Surface low pressure is over northwest Nebraska early this morning
with a trough extending north/northeast into southeast North Dakota
and far northwest Minnesota. Much of the thunderstorm activity is in
that region currently and in advance of the approaching short wave
trough moving through Montana. This leaves Iowa firmly in the warm
sector where 3 am temperatures were well into the 70s. Strong
southwest flow aloft is over Iowa this morning as well due to that
short wave trough moving into the Northern Plains and the large
upper high over the eastern CONUS. That upper level flow will remain
southwesterly into Wednesday but it will relax a bit as the eastern
upper high tilts and ridges into the Southern CONUS. This all will
help allow the cold front to move into northwest Iowa later today
then will settle slowly south into the day Wednesday.

Strong moisture advection is ongoing this morning from Texas
northward into Kansas and northward into the western Iowa and
eastern South Dakota. Areas of stratus are already beginning to
develop from eastern Kansas and southward within this band of the
moisture/theta-e advection and a few storms are beginning to bubble
further south into Oklahoma and Texas. This is all of note as it is
a possibility into Iowa yet this morning. Model proximity soundings
show a warm layer aloft near 3.5-4 kft and with the influx of
moisture arriving below this layer, stratus may develop into Iowa.
In addition, some elevated weak convection may develop above this
layer with enough saturation and forcing near 700 mb for parcels to
begin to lift from that level. Do not have much for precipitation in
the forecast for this morning but trends suggest that PoPs may need
to be increased for the period soon. Finally, mixed layer winds
below the warm layer are 35 kts or higher and this will likely
result in windy conditions at times today with gusts of 30-40 mph
possible. High temperatures today will continue to follow the trend
of not reaching NBM forecast values as the bias correction continues
to push temperatures too high. In addition, limited mixing depth,
potential increasing clouds and cooler 925 mb temperatures to begin
the day all suggest cooler than Monday temperatures for most areas.

The main forecast challenge will be thunderstorms that will have an
early attendant severe weather threat with damaging winds large hail
and eventually transitioning to a heavy rain threat as profiles
saturate. The cold front should reach the far northwest forecast
area by mid to late afternoon and then will move slowly southeast
overnight. By mid evening, the threat with any storms should have
shifted to heavy rain potential. PWAT values along the instability
axis will be in the 2 inch range and these values and the
instability axis do extend north/cool side of the surface boundary
so post boundary convection is likely and expected. The mean storm
motions should be near parallel to the boundary therefore training
of storms is likely as well. Warm cloud depths of 13 kft or higher
will promote warm rain processes leading to efficient rain producing
storms. There are a couple things to monitor for the heavy rain
potenitial. One, the surface low will lift rapidly northeast and will
be over Ontario tonight. The cold front is linked to another area of
low pressure currently over southeast Colorado which will settle
south into New Mexico as high pressure moves into from the west.
This will stretch the boundary and forcing thin and the main low
level jet will become more focused into Kansas later tonight.
Second, the Gulf moisture flow will not have a direct route to Iowa
as surface high pressure ridged into central Texas and a tropical
system in the western Gulf, focus much of the moisture
advection into southeast Texas with any remaining moisture
taking the long detour route into Kansas. Those two areas
certainly have very heavy rain potential so how much of a rich
moisture source will remain available later tonight into early
Wednesday. Still expect a fairly widespread 1-3 inches of rain
with potential for higher amounts in some areas. Several
guidance sources are targeting heavy rain into the Des Moines
Metro area. No hydro headlines for this period as some
uncertainty remains but this still may turn into flooding/flash
flooding into some areas especially parts of central Iowa.

The boundary will being to lift back northward on Thursday which
will bring addition storm chances to the region. Friday should be
mostly dry and warmer again. Another system moves through later
Saturday and Saturday night which will bring renewed storm chances.
The high temperature forecast Friday through early next week is
currently dominated by the NBM, which is in the 75-90th percentile
range and is likely running to warm in most cases through this
period. With the attention on the near term, no changes were made to
these temperatures at this time.

&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
-- Changed Discussion --
Issued at 626 AM CDT Tue Jun 18 2024

MVFR stratus continues to develop and expand over the area this
morning. Some uncertainty remains to the extent and how long it
will linger today. Some areas, it may remain much of the day but
could rise with diurnal mixing. A cold front will arrive
northwest Iowa late this afternoon and move slowly southeast
overnight. Widespread storms are expected in vicinity of the
boundary and may impact sites for a prolonged period.
-- End Changed Discussion --

&&

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

&&

$$

DISCUSSION...Donavon
AVIATION...Donavon