Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 171136

Area Forecast Discussion
National Weather Service Des Moines IA
636 AM CDT Sun Sep 17 2017

Issued at 631 AM CDT Sun Sep 17 2017

Updated aviation discussion for 12z TAFs below.


.SHORT TERM.../Today through Tonight/
Issued at 400 AM CDT Sun Sep 17 2017

Primary forecast challenges in the short term are: 1)lingering
showers and thunderstorms early this morning SE; 2)cooler
temperatures today; 3)quick return to precip again overnight

Today...Forecast Confidence: High

00Z upper air analysis and water vapor imagery indicated a
seasonable strong upper trough pivoting newd through the northern
plains with another trough coming onshore over CA.  At the
sfc...MSAS analysis indicated a sfc low over far nrn MN with a
trailing cold front thru MN/WI/IA.  A cool, Canadian high was
developing across the Dakotas into Neb. Radar mosaic as of 09Z
showed residual post frontal showers and thunderstorms continuing
over eastern and southeastern IA.  No severe wx reported overnight,
although many areas picked up beneficial rainfall ranging from a
quarter locally over one inch.

By early Sunday morning /12Z/ much of the precipitation is expected
to have cleared the forecast area except perhaps in the far south
and east.  In its wake, sfc high pressure will move into IA from the
west resulting in a return to dry conditions with a coolish
north/northwest wind.  850mb temps will drop into the mid-teens
which should result in high temps in the upper 60s north to the mid
70s south. Initial deck of post frontal stratus clouds should clear
by early morning, although soundings and cross sections do show the
potential for occasional mid and high level cloudiness this
afternoon...but still expecting partly cloudy skies.

Tonight...Forecast Confidence:  Medium

The aforementioned sfc high will quickly translate eastward into the
Great Lakes as sfc low pressure reforms over the central Plains in
response to an approaching upper shortwave trough/PV anomaly.  Good
agreement among the 17/00Z operational models showing the re-
emergence of the LLJ after 06z which will allow moisture to surge
back into the region from the southwest.  Soundings and cross
sections showing decent lift/saturation after 06z which should allow
precip to breakout over western Iowa after midnight.  Various CAM
solutions then show at least scattered showers and thunderstorms
moving eastward into central and perhaps eastern Iowa by daybreak
Monday morning. Instability is looking fairly modest at this time
/MUCAPE values at or below 500 J/kg/ so not anticipating any severe
thunderstorms at this time.  Temperatures overnight should fall into
the 50s.

.LONG TERM.../Monday through Saturday/
Issued at 400 AM CDT Sun Sep 17 2017

./Monday through Tuesday/

Confidence: Medium to High

Southwest US shortwave will quickly arrive between 06 and 12z Monday
with southwest flow still in place aloft.  This will result in a
quick return of moisture as the wave again taps into a tropical
connection from stretching from the Pacific across Mexico into the
lower to mid Mississippi River Valley. Forecast PWAT by Monday over
the area indicates a quick rise to +1.5 inches by 18-06z over the
southeast portion of the forecast area. Height falls at H500
combined with increasing isentropic lift through H850-H700 will
result in showers and thunderstorms over the area through the
morning and afternoon hours.  Another round of potential 0.25 to
nearly an inch of precipitation is possible.  Have raised PoP to
likely/categorical in the far south with likely over most of the
north. Once again the wave will be in/out fairly fast by Monday
evening with only weak lift and increasing warm air advection across
the area into Tuesday.  The better lift will move north of the
region by Tuesday morning resulting in little if any chance for
precipitation.  H700 temps also warm to over 10C and will aid in
capping any possible development through the day.  With that said...
Monday will be rather wet/cloudy resulting in cool afternoon highs.
By Tuesday though...H850 temps quickly vault into the lower 20s with
strong H850 winds especially into the mid to late afternoon hours.
This will help push afternoon highs into the 80s once again with
summerlike dewpoints also returning.

./Tuesday Night Through Saturday/

Confidence:  Medium

The warming trend will continue Tuesday night into Wednesday.  There
is continued consensus between the GFS/Euro regarding a strong
northern stream shortwave brushing across Iowa late Tuesday night
into Wednesday...but less consensus as to how far south any
precipitation/thunder develops during the day. Confidence in the
arrival time of the convection is also limited due some model
uncertainty.  With the better forcing over MN/Dakotas...the overall
impact regardless of the exact timing will be less over Iowa than
areas north. Upper level wind fields will support severe storms over
the Dakotas/MN and possibly into northwest Iowa...but overall the
threat for our area remains rather limited.  Though H850
temperatures are not quite as warm Wednesday...afternoon highs the
warm start will likely result in highs again in the 80s over most of
the area.  Confidence lessens moving forward through the period with
similar challenges to yesterdays.  By mid to end of next week a
large H500 trough digs into the Pacific Northwest and western CONUS
by 12z Friday. Both the GFS/Euro have similar solutions with a
rather high amplitude pattern setting up.  A strong blocking high
over the eastern US will help strengthen south southwest flow over
the Plains resulting in unseasonably warm air through the period. increasing fetch of moisture will reach into the northern
states. Jose and Maria are also becoming part of the forecast
equation...not with regard to direct impacts here...but with a large
blocking high at H500 over the Great lakes by Saturday 9/17...Jose
nearly trapped somewhere near the northeast coast and Maria edging
toward the southeast US...there should be little movement east of
the large west CONUS H500 trough.  This results in rather limited
confidence as to when we will first see any precipitation next
weekend. Weak impulses may bring some scattered showers/storms to
the region either Thursday or Friday...but the main activity is more
likely to be delayed until late Saturday or Sunday.  Currently
consensus in our forecast has Saturday...but the latest models...
especially the Euro is now slower.  With this in mind...highs in the
lower to mid 80s with more summerlike humidity may extend all the
way into next weekend and only cool once the active front arrives in
the second half of the weekend.


.AVIATION.../For the 12Z TAFS through 12Z Monday morning/
Issued at 631 AM CDT Sun Sep 17 2017

Showers over southeastern Iowa continue to depart, so all
terminals should be rain free by 12z. Still seeing some residual
IFR/MVFR CIGs in the post frontal airmass over ALO/DSM/OTM. These
should quickly transition to VFR by 15z. Otherwise, a light
north/northwest wind and VFR conditions expected through late
tonight. Next round of precipitation - and corresponding lower
CIGs and VSBYS should move into area after 06z tonight.




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