Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 141153
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
653 AM CDT Fri Jul 14 2017

.SHORT TERM.../Today through Tonight/
Issued at 320 AM CDT Fri Jul 14 2017

Pleasant summer weather is anticipated through the period with
seasonal temperatures and highs just a tad below Thursday`s. The
only minor concerns would be early morning fog today and then small
chances for very weak convection early this morning and again toward
daybreak Saturday. The HRW NMM, ESRL HRRR and a few NCAR ensemble
members actually depict isolated weak echoes with the spotty 4-6kft
ceilings developing from Orange City to Clarion in weak warm
advection between the Nebraska H85/H7 ridge and the thermal trough
at the same level dipping into the I35 corridor. No mention yet
however as development chances are low and anything that occurred
would be quite isolated. Patchy fog has also developed over
southwest and south central portions of IA and will include a
targeted mention as needed there driven by latest surface obs and
GOES 16 RGB nighttime microphysics imagery.

The current Great Lakes upper low will then drift away today with MN
stratus associated with the weak low level cyclonic flow likely only
brushing northeast sections before becoming cellular into peak
heating. The trailing surface high should reach IA around 18z and
only slowly reach the MS River by the end of the period. Weak return
flow to the west will then very slowly increase with warm/theta-e
advection spreading into the northern Plains ahead of the current
compact Saskatchewan short wave as it reaches Ontario by daybreak.
There will be a sharp thermodynamic forcing couplet with this
feature along the MN/Canada border overnight with a much weaker lobe
dropping southward into IA. Most of the parameterized models are dry
with insufficient moisture or lift for precip, however it is
interesting to note that the HRW NMM and NSSL WRF both suggest weak
elevated convection early Saturday morning west of I-35. Much like
this morning, confidence of occurrence and the isolated nature of
anything that would develop has precluded anything in the forecast,
but something to watch into tomorrow.

.LONG TERM.../Saturday through Thursday/
Issued at 320 AM CDT Fri Jul 14 2017

After relatively cooler and less humid air on Friday, a return to
summery warmth and humidity is on tap for the weekend into next
week. Upper level ridge axis will be centered near 110 degrees West
on Saturday with a shortwave trough rounding the ridge and coming
down from Manitoba into Ontario and the Great Lakes by Saturday
evening on the nose of a 70 knot upper level jet. At the surface, a
cold front will approach the area late in the day and interact with
an environment characterized by MLCAPEs between 2000 and 3000 J/kg
and 30 to 35 knots of deep layer shear sufficient for storm
organization. A limiting factor to more widespread storm formation
will be a warm layer around 800mb as depicted in the NAM and GFS
forecast soundings at KMCW and KALO. Storms that are able to form
may become severe. Large hail would be possible with lapse rates
of 7 to 7.5C/km along with plenty of CAPE in the hail growth
region. Dry mid-levels will lead to large DCAPE values supportive
of strong wind gusts. Overall, the best forcing and upper level
support will be northeast of central Iowa so thunderstorms at this
point look limited in coverage over the central part of the state.

The front will hang up over Iowa on Sunday and this may keep
isolated showers or storms chances around, though it appears the
most likely area will be over eastern Iowa. Both the GFS and ECMWF
have the bulk of their precipitation over this area. In addition,
a longwave trough coming ashore the Pacific Northwest/British
Columbia will begin to flatten the western US ridge. This will
result in the ridge starting to expand eastward toward the
Midwest. With this beginning to impinge on central Iowa, have
opted for a dry forecast. Temperatures will be slightly cooler
north of the front, but humid conditions will envelop the state.

500mb heights in rather good agreement on Monday with Iowa being on
the eastern side of the upper level high. 850mb temperatures nothing
terribly impressive between 20 and 21C compared to other warm spells
we`ve seen this summer, but still temperatures will be in the upper
80s to near 90 degrees over many parts of central Iowa.

Beyond Monday and especially beyond Tuesday, confidence begins to
decrease as models diverge due to the evolution of the upper level
high and its placement. The 00z global model suite shows
precipitation into Iowa on Tuesday as a cold front approaches the
state. The ECMWF focuses the precipitation over northern Iowa while
the GFS and CMC are farther south. The ECMWF is centered more
longitudinally on Iowa and a bit farther north, which helps to keep
the rainfall over the northern part of the state, while the GFS and
CMC are a bit farther west and south with the placement of the
high. Into Wednesday and Thursday, rain and storm chances persist
over the state with the GFS and CMC the main players to this
solution. This would be the not as hot weather scenario as well.
Meanwhile, the ECMWF also has rain and storms, but moreso over the
northern half of Iowa or north of Iowa with a more possible hot
weather pattern. Still, have temperatures into the 90s over at
least the southern part of the state close to or slightly above
guidance, but could be higher depending on details already
discussed. With dewpoints well into the 60s to around 70 degrees,
heat indices well into the 90s to a few degrees above 100 will be
possible over this part of the state on Tuesday and Wednesday.
Ultimately, confidence in temperatures ranges from low over
northern Iowa to higher over southern Iowa. The opposite is true
for rain and storm chances with higher confidence that northern
Iowa will have better rain and storms chances and lower confidence
over southern Iowa.

&&

.AVIATION.../For the 12Z TAFS through 12Z Saturday morning/
Issued at 652 AM CDT Fri Jul 14 2017

There is fairly high confidence in VFR conditions into the
evening. Some ceilings may develop north, but would be above 3K
ft. Confidence decreases slightly by early Saturday morning
however. Some models suggest fog developing but that was the case
this morning as well and nothing has occurred. Thus will leave VFR
for the entire period for now until confidence improves or trends
suggest otherwise.

&&

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

&&

$$

SHORT TERM...Small
LONG TERM...Ansorge
AVIATION...Small



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