Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 242338

Area Forecast Discussion
National Weather Service Des Moines IA
638 PM CDT Mon Jul 24 2017

.SHORT TERM.../Tonight through Tuesday/
Issued at 302 PM CDT Mon Jul 24 2017

Confidence: High

Few concerns through Tuesday. Warm air advection begins to increase
across the region later this evening with some mid level clouds
expected mainly over the northwest/north. Best thetae advection
should remain west/northwest of our area through Tuesday as an
analyzed 12z H850 trough lifts northeast into western Minnesota by
Tuesday morning. This will keep any thunderstorms in Minnesota
through 00z Wednesday.  Lows tonight will be on the rebound in the
south and west as winds swing back over to a more southerly
direction. The receding high will still influence northeastern and
eastern areas with light winds and cooler overnight lows. Again some
hint that light fog may develop in the valley areas of the Cedar
River and perhaps the Des Moines River over eastern sections of our
forecast area though confidence remains low at this time as to a.
areal coverage and duration.   Tuesday the warmer air returns with
afternoon highs reaching the lower 90s in the south/west with slower
recovery northeast where both cooler overnight lows and residual
morning cloud cover will slow afternoon heating for a bit longer.
Highs Tuesday will reach the lower 80s east to the upper 80s in the
west/south.  Heat indices may reach the mid 90s south and west
Tuesday with mid to upper 80s in the east. Lows Tuesday night will
be warmer and more humid with 70 to 75 common across the region.

.LONG TERM.../Tuesday night through Monday/
Issued at 302 PM CDT Mon Jul 24 2017

Tuesday Night through Thursday:

Confidence: Medium

Though dewpoints have dropped and upper level moisture fields have
been displaced to the south/southwest...the deep moisture is not all
that far away from Iowa this morning. H850 subjective analysis shows
large area of +15C H850 dewpoints from eastern Kansas southeast to
the southeast US along the boundary that moved through our area
Friday evening. The southwest US monsoonal moisture plume is also
evident...though more or less confined to that area with little if
any teleconnection here at this time. As the northern stream trough
lifts northeast...the deeper moisture will be pulled back into the
Central Plains/Upper Great Lakes setting the stage for an active
period from late Tuesday night into Thursday.  The models are coming
into better agreement regarding the timing/evolution of the boundary
as it crosses Iowa Wednesday.  Both the GFS/Euro now suggest some
convection entering NW Iowa by 12z...spreading slowly east southeast
during the morning...covering the northwest two-thirds by 00z
Thursday.  At this point the area is outlooked for a slight risk of
severe weather. Judging by the parameter space...we will probably be
on the lower end of a slight risk unless something changes
dramatically in the next 24 hours. Bulk shear forecasts 25 to 35kts
in general through the event with the higher MUCAPE of 3000 to 3500
j/kg out ahead of the main shortwave at H500/h700 which never really
amplifies much as it moves east.  Most of the Bufr soundings today
show tall/skinny cape which suggests slower development throughout
the event. With all of the moisture return occurring in the next 24
to 36 hours aloft with rises in surface dewpoints...GFS forecast
PWATs increase to 2 to 2.25 inches along the boundary. The models
continue to favor higher rainfall amounts from eastern Iowa
southwest into Missouri...which makes sense given the better
instability expected between 21-03z in those areas.  At this
point...despite the generous available moisture through the
column...rainfall totals may end up higher across the northeast/east
and then south of the area as the late evening low level jet
increases into Missouri and instability is also maximized east/south
of our area. None the less...up to 1/2 to 1.5 inches of rainfall is
possible...possibly locally higher in some areas. Highs Wednesday
will reach the 80s to lower 90s south with afternoon heat indices
south into the mid to upper 90s.  No headlines for heat are expected
due to both the shorter duration and lesser impactful heat indices
through the period.

Thursday Night through Monday:

Confidence Medium to High

Overall....confidence is on the high side except for Friday.
Once the front washes south/southeast of Iowa...another
refreshing area of high pressure will build across the Northern
Plains and upper Great Lakes. Cooler and drier northeast flow will
set up driving H50 temperatures into the mid teens Thursday and
lower to mid teens Friday. Both the Euro and GFS are indicating a
secondary weak upper level system passing over the area Friday.
This may create just enough instability through the column to
promote isolated showers or thunderstorms on Friday prior to
another high settling into the area for the weekend. For now
confidence remains limited regarding the coverage/potential for will leave the Friday forecast dry at the moment.
Over the course of the 4 day period...highs will remain comfortable
in the 70s to lower 80s with only a warming trend indicated by
Monday. Little if any additional precipitation is forecast through
the period.


.AVIATION.../For the 00Z TAFS through 00Z Tuesday evening/
Issued at 637 PM CDT Mon Jul 24 2017

VFR conditions through this evening. Potential for MVFR cigs or
fog to develop late tonight into Tuesday morning over mainly
northeast Iowa including the ALO/MCW areas. Widespread VFR and
breezy southeast winds are expected by mid morning and continue
the remainder of the day.




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