Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 171525

Area Forecast Discussion
National Weather Service Des Moines IA
1025 AM CDT Sat Jun 17 2017

Issued at 1025 AM CDT Sat Jun 17 2017

Have sent out an update for the showers moving across the area
pushing slightly further south than previously anticipated.
Additionally this should keep temps likely a few degrees cooler
this afternoon across the central/north so have dropped highs a
bit. Also latest severe weather outlook pushes the chance for
strong to severe storms to the far southeast, so have made some
adjustments on timing for that activity which could be more toward
the late afternoon/early evening timeframe across the
south/southeast. Also went with showery wording this morning with
the lack of thunder, and isolated thunder across the north through
the afternoon/evening.


.SHORT TERM.../Today through Tonight/
Issued at 333 AM CDT Sat Jun 17 2017

Low confidence forecast today.  We continue with a severe threat
today...but many questions remain given the amplitude of last nights
convective complex plowing through central Missouri at 0745z.
Forecast upper level dynamics have not changed much and surface
analysis at 06z shows weak low near Ottumwa with associated wind
shift/boundary sitting over southern Iowa southwest into northern
Kansas and northeast toward Chicago.  Airmass south of the boundary
less worked over...but dewpoints still holding in the mid to upper
60s in central Iowa now. Most of the synoptic scale models take the
boundary southeast through the morning hours...generally near the
border or just south of Iowa by 00z. This "corridor" surrounding
the boundary will be the favored area of surface moisture
convergence today (higher dewpoints). Along with that the H850
thermal trough has also dipped south a bit by 18z. There really is
little change on the timing of the rather strong H500 wave tracking
east southeast but the better height falls and speed shear arrives
from near 00z to 06z and catches up the the H850 moisture plume as
it continues south of the forecast area. Another more northern
stream upper level trough swings into Minnesota by 00z tracking
further east through 06z. both the GFS/NAM show a plume of 3000 to
5000 j/kg MUCAPE over the southeast quarter by 18z but both models
also suggest an H700 cap through a portion of the afternoon. The
Euro continues to keep the boundary south of Iowa and the better
MUCAPE south of Iowa today as well with little if any convection
developing in our area over the far south through evening. By 19-21z
the GFS/NAM show 0-6km Bulk shear rapidly increases to 50-60kts
across the southeast. Both models also show some minor strengthening
of the weak sfc feature as the upper level height falls catch up to
the the surface trough.  The main threats once again would be rather
large hail and possible intense wind gusts with a small threat for
an isolated tornado.

Today will likely be uneventful for most of the morning with rapid
changes taking place in the south by mid to late afternoon/early
evening. There is currently a small area of showers over South
Dakota/north central Nebraska that will likely remain light/weaken
as it crosses into Iowa this morning.  Timing for the onset of any
thunder today will be somewhat have kept a small
chance of some convection north/south through about 2 pm...then
increase chances quickly from 20 through 00z over the far southeast.
We continue outlooked for an Enhanced Risk of Severe in the
southeast today and this evening with a slight surrounding this
area. Much depends on if the weak surface boundary over southern
Iowa now tracks south of Iowa by afternoon. Conditions remain
favorable for higher end severe weather...but it may end up forming
slightly south of Iowa later this afternoon nearer the boundary and
weak surface low. None the less...once it begins to develop it
should quickly go severe whether in southeast Iowa of just
south/east of the area.  Because of this...we may see some
adjustments to the convective outlook later this morning...and this
is the reasoning for the low to medium confidence rating on todays
forecast.  Highs will reach the mid to upper 80s south with the
upper 70s to lower 80s north. Tomorrow the relief arrives over the
entire forecast area.

.LONG TERM.../Sunday through Friday/
Issued at 333 AM CDT Sat Jun 17 2017

Summary...Cool down to start the period with highs falling into the
70s for Sunday then slowly warming much of the week back into the
80s/near 90 at times. Weather will be generally quiet early in
the week before multiple opportunities for storms return mid-week
and to round out the week.

Sunday through Wednesday AM...This period will be highlighted
primarily by cooler and generally quiet conditions as a trough
pivots through the state Sunday and through the Great Lakes to start
the week. The majority of energy associated with the upper wave will
miss the CWA to the northeast, though could see a few scattered weak
showers/isolated thunder Sunday as models depict weak instability
remaining. Then again Monday and Tuesday in the far northeast as
vort maxes work their way through. By Wednesday as the trough pushes
further east, 500mb patter will begin to flatten, returning
southerly winds and a level of moisture return.

Wednesday PM through Friday...Models continue to be in fair
agreement on the large scale pattern, but lose each other in the
details and surface reflections. Regardless, periods of storms will
return to the region as multiple waves of WAA and frontal passages
move through while the 500mb pattern remains generally zonal.  While
hard to pinpoint the best opportunities due to model differences in
details, Euro/GFS show decent agreement in development of a weak
trough developing north of the area and dragging a cold front
through during the Thur-Fri time frame. Should it remain consistent
in subsequent model runs, it may also become the next fair
opportunity for severe weather.


.AVIATION.../For the 12Z TAFS through 12Z Sunday morning/
Issued at 646 AM CDT Sat Jun 17 2017

Main early concerns remain patchy fog east of I35 through 14z and
also some MVFR/IFR stratus near KALO and KOTM through 15z. Models
continue to advertise one more push of convection aft 18z with
potential for some higher end storms again between 20-04z. Have
continued VCTS to categorical at KOTM where chances for severe
convection highest. Hires models and Convective Allowing Models
are beginning to focus the development of the strongest storms and
more widespread coverage south in Missouri around 00z. Trends
suggest less impacts here...but will have day crew monitor for
changes. For now will continue mention but still have lowered
confidence on higher end event here in central Iowa at this time.





LONG TERM...Curtis
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