National Weather Service Raw Text Product

645 
FXUS63 KDMX 191135
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
635 AM CDT Sun Sep 19 2021

...Updated 12z Aviation...

.DISCUSSION.../Today through Saturday/
Issued at 345 AM CDT Sun Sep 19 2021

Sunday:

Trough making landfall along the Pacific Northwest clearly present 
on GOES-16 IR imagery early this morning with a few perturbations 
already ejecting ahead of the main PV anomaly across the Rockies. 
This has pushed the amplified ridge axis across the central Plains 
over the past 12 hours, and passed through Iowa overnight. A deep 
surface low has developed over southern Canada associated with the 
main trough. With the perturbations ejecting eastward from the main 
system, two more areas of Lee Cyclogenesis are starting early this 
morning. The first is near the CO-KS-NE region of the High Plains, 
with the second area just west of Black Hills extending into the 
northern High Plains. Surface analysis depicts a warm front that 
extends from central North Dakota southeastward into the mid 
Mississippi River Valley. With the development of the surface 
cyclones over the central and northern High Plains, southwesterly 
low-level flow across Iowa will strengthen throughout the afternoon 
and continue to push the warm front toward the Great Lakes Region, 
with plenty of warm air working across Iowa from the Central Plains. 
This alone will help to increase temperatures this afternoon across 
the forecast area at least into the upper 80s. GFS and RAP soundings 
are favoring deep layer mixing this afternoon, extending to around 
800mb by the 18-21z timeframe this afternoon. Winds at the top of 
this will in the ballpark of 20-25 kts. In addition to the WAA with 
the strong southerly flow, dry soil conditions and deep mixing will 
likely push some areas above 90 degrees this afternoon. Temperatures 
have been increased to around the 90th percentile of NBM guidance. 
Most guidance is depicting dewpoints to be in the mid 60s this 
afternoon, but this may be a tad overdone with the potential for 
robust mixing. Thus, not expecting heat index values to greatly 
exceed the air temperatures this afternoon. In addition to the heat, 
winds will be quite breezy, with the deep mixed layer also aiding in 
increasing wind gusts. Winds across northwest portions of the 
forecast area may be able to hit 30 MPH a few times, with the rest 
of the forecast area seeing gusts closer to 25 MPH. Even with these 
dry and warm conditions, RH values should remain above 30 percent 
for most of the afternoon, thus fire weather concerns will be 
minimal for today. Precipitation does not enter the picture for the 
forecast area until very late this evening, where far northwestern 
areas may see some shower activity. However, the main show with rain 
showers and thunderstorms is shaping up to take place on Monday 
morning, and will continue to spread across the rest of the forecast 
area Monday afternoon through late evening. More details in the next 
section.  

Monday:

The synoptic picture becomes a bit messy heading into Monday, as 
there will be a few short-wave perturbations to watch out for ahead 
of the main trough/PV anomaly. H5 flow will turn southwesterly by 
early Monday morning, with a few shots of CVA traveling across the 
central and northern Plains into the upper Midwest. These will 
largely be in control of the two previously mentioned surface 
cyclones. A stronger cold front will be developing behind the 
surface cyclones, but may stay ahead of the main trough with this 
active pattern. The first surface cyclone currently over the CO-KS-
NE region is expected to dip southeastward toward the middle and 
lower portions of the Mississippi River Valley, following closely to 
southern area of stronger CVA. The northern cyclone will move across 
the Dakotas and eventually into Minnesota by late Monday morning. A 
few isolated showers or thunderstorms may reach the Estherville and 
Storm Lake areas by early Monday morning, but the preceding dry 
conditions may limit the coverage and intensity of these showers. 
The main focus for the forecast are will be convection initiating 
along the strong cold front ahead of the main PV anomaly. This will 
cross into the forecast area around the 14-16z timeframe. Widespread 
rain showers and thunderstorms are expected across most of Iowa 
along the cold front. There is still potential for severe storms 
during the later afternoon to early evening hours after peak 
heating. There is still question as to if the kinematic fields will 
align favorably with the thermodynamic fields. Current model 
forecast soundings are not showing signs of a stronger EML, despite 
the stronger WAA and southwesterly flow ongoing in the 850-700mb 
layer from the previous day. For surface based parcels ahead of the 
cold front, this limits the capping inversion substantially, and 
with the strength of convergence along the front, any CIN that maybe 
around would not be an issue. However, mid-level lapse rates will 
not be overly robust, struggling to exceed to 6.8 C/km across most 
of the forecast area. Lapse rates in the boundary layer may become 
steep with insolation Monday afternoon, but if mixing continues to 
remain robust and moisture transport is not overly strong, this may 
limit the surface-based CAPE. 00z HREF mean surface CAPE is around 
1500 J/kg for most of central and northern Iowa, but many of the 
CAMs that comprise the HREF may be a bit over on their dewpoint 
forecasts. Therefore, robust updraft development will largely depend 
on mechanical lift from the cold front. The next factor is deep 
layer shear. The highest bulk shear values are expected to be post-
frontal, closer to the main trough axis where the H5 jet will be 
around 80 kts. However, depending on how the few short-wave 
perturbations that have ejected eastward from the main system play 
out, it may be able to help achieve 0-6 km bulk shear values around 
35 kts just ahead of the cold front for a few hours. If this is able 
to materialize, convection may be able to organize along the leading 
edge of the cold front, and augment the severe potential. With the 
strong convergence, would expect the mode to be mainly linear, but 
should the deep layer shear vector orient perpendicular along a 
portion of the front, this could foster an embedded supercell 
thunderstorm. If the shear remains as more unidirectional speed 
shear, expect mainly a linear system or cluster with strong wind 
gusts. The occurrence an embedded supercell is highly conditional on 
getting some deep layer in the warm-sector though. If this does not 
occur, the severe threat will be diminished. The surface cyclones 
and cold front are appearing to be rather progressive in nature, and 
this could result in the cold front quickly undercutting a lot of 
the convection by late afternoon. However, if these updrafts are 
advected rearward and are not completely torn apart, there may be 
enough post-frontal shear to support stronger organized elevated 
activity for a few hours. However, potentially over estimated of 
dewpoints in the 00z CAM solutions that have tried to depict this 
may not be completely reliable.

If the parameters come together in time ahead of the front, the main 
hazard will be damaging wind gusts. Updrafts within the warm sector 
will likely have moderately strong theta-e deficits between the 
surface and the top of the CBL, which any strong downdrafts with 
rapid evaporational cooling could pack a stronger punch. There is 
some potential for hail, but with skinny CAPE profiles noted in 
current forecast model soundings, and questionable deep layer shear, 
the hail threat does not seem overly significant. For northern Iowa 
and areas closer to the center of the surface cyclone in Minnesota, 
backed surface winds may create some modest turning in the low-level 
hodographs to support a brief tornado. However, Iowa may be too far 
south for this. If elevated convection develops behind the front and 
realizes the strong cloud-layer shear, damaging winds and hail could 
develop, but again this may be largely limited by meager lapse rates 
and skinny CAPE profiles. If these parameters do not come together, 
than anticipate decent rain showers and thunderstorms along the 
front, and may be able to produce locally heavy rainfall. With this 
system being rather progressive across the upper Midwest, no hydro 
issues are anticipated. 

Extended:

After the cold front passage Monday night, temperatures will be more 
seasonable throughout the rest of the week. A ridge develops across 
the Front Range as this stronger trough finally catches up to the 
short-wave perturbations and becomes long trough feature that extends 
from the Central Great Lakes region into the central Ohio River 
Valley. This will place Iowa in an area of stronger northwesterly 
flow through the much of the week, which will help to keep 
temperatures on the cooler side. With clear nights, strong 
raditional cooling will aid overnight temperatures dipping into the 
lower 40s across northern Iowa. Toward the end of the week, there 
still divergence in long term model guidance solutions with a strong 
closed low system. If this were to dip further south into Iowa, this 
would introduce some rain chances toward the end of the week. 
However, not many ensemble members are doing this, thus will 
maintain a dry forecast for now as it appears the forcing will stay 
further north of the area into the next weekend. 

&&

.AVIATION.../For the 12Z TAFS through 12Z Monday morning/
Issued at 631 AM CDT Sun Sep 19 2021

South to southwesterly wind gusts of 20 to 25 kts are expected
this afternoon across most of central Iowa. Winds will remain
breezy overnight. VFR conditions forecast for the 12z TAF period.
Monday, cold front will bring rain showers and thunderstorms to
most of central Iowa. 

&&

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

$$

DISCUSSION...Krull
AVIATION...Krull