Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 191137

Area Forecast Discussion
National Weather Service Des Moines IA
637 AM CDT Thu Jul 19 2018

...Updated Aviation Discussion...

.SHORT TERM.../Today through Tonight/
Issued at 359 AM CDT Thu Jul 19 2018

Convection continues to increase and spread over eastern to
northeast portions of the forecast area early this morning. This
is along and ahead of a stationary boundary with moderate to
strong 850mb and 925mb moisture transport into this location.
Precipitable water values over 2 inches in the east-central
sections of the CWA with warm layer cloud depths approaching 4000
meters. The NAMNest, ARW, and NMM are depicting the current radar
trends the best and thinking have the best handle through at least
12-15z this morning. Even the 19.00z NAM12 is suggesting very
heavy rain in the aforementioned location. Thus, leaned toward a
blend of the NAM12 and the other models above for pop/qpf trends
this morning.

Rainfall rates are fairly stout with dual-pol depicting 2 inch/hour
rates with some brief 3-4 inch/hour rates over portions of Poweshiek,
Mahaska, and Tama Counties. There are already a couple locations
near What Cheer over 2 inches as of 0830z this morning and thus
confident to increase QPF significantly in the eastern part of the
CWA. Appears the shortwave producing the severe convection in
southeast Nebraska will help nudge this convection in the
eastern CWA slightly north-northeast by around 12-14z this
morning, or sooner. Still, the flash flood threat remains possible
until then, especially across the aforementioned counties.

For the rest of the day into tonight, expecting the front to surge
north-northeast through the day and become more oriented northwest
to southeast from say around Pocahontas to Tama Counties.  The main
synoptic surface low looks to track across southern
Minnesota/northern Iowa and becomes nearly vertically stacked by
around 00z Friday. The best forcing for ascent is along and north
of the front from around Highway 20 to the MN border this
afternoon into tonight. The hires models provide a short break and
redevelop convection by as early as 21z this afternoon. Depending
on how quickly this ongoing convection this morning moves east-
northeast will determine how the atmosphere recovers later this
afternoon. Shear becomes more unidirectional late this afternoon,
but b/t 18- 21z, there is a brief for potentially rotating
updrafts with convective initiation. Storms do look to evolve more
into a MCS feature by 00z Friday at the latest, but certainly
some hail possible with the onset and then leaning more toward the
damaging wind threat later in the period this evening.

.LONG TERM.../Friday through Wednesday/
Issued at 359 AM CDT Thu Jul 19 2018

.../Friday through Sunday Night/...

Confidence: Medium to High

Main concerns well advertised the past few days. Strengthening upper
level low will be over eastern Iowa/SW Wisconsin by 12z Friday.
Models differ slightly in the handling of the track but overall have
similar impacts with lingering showers/iso thunder over the east and
northeast along with cloud cover most prevalent over the northeast
two thirds of the forecast area. This will cap temperatures over
that portion of the area in the mid to upper 70s while the central
and south continues in the lower to mid 80s for the afternoon hours
with more but filtered sunshine likely in those areas. By Friday
night the system will depart with a stretch of dry weather expected
Saturday into Sunday night. Behind the system winds will pick up
with northwest winds of 15 to near 20 mph at times and gusts
reaching 25 mph through the afternoon hours. High pressure and
slightly stronger northerly flow will bring pleasant conditions to
the region for most of the area by Saturday into Sunday. There
remains some minor differences between the GFS and Euro. A weak
shortwave rotating on the back side of the closed upper level low
may bring a few clouds and isolated showers to the eastern portion
of the forecast on Saturday, mainly east of Highway 63. Currently
confidence is too low to include any mention. Later in the day on
Sunday easterly winds aloft will return to the western areas along
with slightly cooler temperatures at H850 and building heights at
H500. This will allow for very pleasant conditions over Central

.../Monday through Wednesday/...

Confidence: Medium

Both the GFS and Euro begin the period with the introduction of
another wave over the Northern Plains. As the system nears Iowa...
both models weaken the forcing with the ridge over the region. This
should keep precipitation chances to a minimum across the area and
there still remains some doubt that any showers/iso thunder will
reach even western/northern Iowa by later Monday into Monday
evening. None the less some minor recovery in temperatures aloft
should allow temperatures to recover a few degrees into the
afternoon hours. High pressure will filter through the region
Tuesday with a midweek cool front pushing south from Minnesota along
with a weak wave approaching from the west. This will increase
chances for showers/scattered thunderstorms Wednesday afternoon and
evening. Temperatures during the period will remain near to slightly
below normal with highs in the lower to mid 80s and lows in the
upper 50s to lower 60s.


.AVIATION.../For the 12Z TAFS through 12Z Friday morning/
Issued at 633 AM CDT Thu Jul 19 2018

Ongoing IFR/MVFR ceilings across central Iowa other than OTM.
Expecting the IFR ceilings at ALO/MCW to improve later this
morning to MVFR but there`s a high confidence these two sites will
remain at least MVFR for much of the forecast with the low
tracking over northern Iowa tonight. Lower confidence with an
extended MVFR period for DSM and OTM as the hires models keep the
lower ceilings further north. Plus, after the WAA wing this
morning moves northeast, ceilings should improve to VFR past
14-15z today at DSM.





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