Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 162035
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
335 PM CDT Sun Jul 16 2017

.SHORT TERM.../Tonight through Monday/
Issued at 333 PM CDT Sun Jul 16 2017

Confidence: Medium to High

Main concern will be heat along with potential convection this
evening south along with cloud cover Monday. Subjective surface
analysis at 13z shows weak boundary along the US34 corridor with a
weak low near the Quad Cities. This feature will continue to drift
south today into northern Missouri this evening. The boundary is
more of a wind shift than airmass change...especially in the
boundary zone. Dewpoints really don`t drop off much until you reach
northern Iowa. Meanwhile values from US300 south are still in the
lower to mid 70s with temperatures in the lower 90s. This will keep
heat indices in the upper 90s/lower 100s in south central/west
southwestern areas through about sunset this evening. Isolated
thunderstorms are already breaking out over eastern Iowa. Hires
models still suggest some lingering activity this evening over the
south which should diminish after sunset. Lows tonight will remain
quite warm especially west of I35 and in the lower 70s as the
boundary pivots across the region. As low pressure develops over the
northern Plains...the weak surface boundary will begin to become
more north/south oriented by 12z Monday. Weak surface convergence
overnight into Monday morning along the north/south boundary will
likely help stratus clouds to thicken and become more widespread.
Hires models suggest some convective development overnight...but
H700 temperatures remain at 10-11C which should inhibit convection.
Monday should not be as warm as today. The lingering cloud cover
into the late morning/early afternoon hours should limit
temperatures initially...though if the area can go partly
cloudy/mostly sunny by afternoon we may recover into the lower 90s
once again. The area of most likely recovery will be west of I35.
Though instability increases again Monday afternoon...H700
temperatures near 12C are likely to create enough inhibition that
the day will be storm free.


.LONG TERM.../Monday night through Sunday/
Issued at 333 PM CDT Sun Jul 16 2017

Synopsis...
Extremely Broad area of anticyclonic flow at 500mb centered near the
Four Corners region...with influence reaching from the
Mississippi River through the Baja of California. By Monday, core
of anticyclonic flow moves towards the Ozarks/Oklahoma region...
serving to push the DMX CWA into return flow. Long-range model
guidance trying to suggest core of high pressure to push into the
Deep South by next weekend. As suggested in yesterday afternoon`s
AFD... models have slowed this progression... keeping Friday in
warmer temperatures. Will probably see Saturday`s temperatures
bumped up as this feature`s departure may stall an additional day.


Above Normal Temperatures/Excessive Heat...
Confidence: Very High

With 850mb temperatures around +18C to +22C under scattered clouds,
much of the DMX CWA reached into the low 90s for Saturday afternoon.
850mb temps dropped a few degrees for Sunday, however, cloud cover
was less. The result was fairly persistent temperatures both days.
850mb temps should remain in this range for Tuesday...perhaps a few
degrees warmer initially suggesting comparable to slightly
warmer temps to this past weekend, pending cloud cover and precip.

Wednesday and Thursday... 850mb temps jump in a range of +21C to
+24C across the DMX CWA. Tracking dewpoints, no reason to see much
departure from persistence forecast from this weekend... meaning sfc
dwpts in the low to mid 70s seems plausible. Combining
temperature and dewpoints, apparent temperatures should be well
into the triple digits both Wednesday and Thursday. With minimum
temperatures bottoming out in the low to mid 70s near and south of
Highway 20, Heat headlines will likely be issued during
tomorrow`s update for Wed and Thu. Could possibly add Tuesday, and
may need to drag heat headlines into Fri as well if ridge
departure slows. Given potential for 4 consecutive days of heat
adv conditions, cannot rule out headline being in form of
Excessive Heat Watch/Warning. If anything, max temps for
Wednesday-Friday are conservatively cool and could be bumped up
several degrees in subsequent afternoon forecast package updates.


Strong to Severe Thunderstorm Potential...
Confidence: Medium

Somewhat similar to last week, instability/CAPE off the charts each
afternoon with sfc dwpts into the 70s. For sustained convection to
generate, location of trigger/focusing mechanism key. With steep
low- level lapse rates, large hail will be primary threat.
Damaging winds also a primary threat, as DCAPE reaches 2000 J/kg.
At this time, it appears that each day Tue-Fri, will feature a
weak shortwave racing eastward through and near to the IA/MN
border. Given the relatively small size of these shortwaves,
expect them to be highly variable in terms of timing and location.


With run-to-run consistency starting to present, can highlight
northern Iowa for Tuesday evening into Wednesday morning.
Reinforcing rounds of theta-e advection coincide with a slug of
1000mb-850mb moisture transport. Depending on flavor of model, PWATs
in this region jump to over 2 to 2.25 inches. Have bumped up QPF vs
guidance and if this setup holds, next shifts may have to bump
notably further. Similar setup for Wednesday evening into Thursday,
though the axis of may be oriented more NW-SE than W-E as is
expected Tue PM-Wed AM. Ultimately portions of NW to North-central
Iowa may receive moderate to heavy rainfall two nights in a row.
Reports of corn starting to tassel in that area suggests ground
will be able to support higher rainfall amounts...hindering flash
flood potential.


&&

.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1255 PM CDT Sun Jul 16 2017

Main concern will be potential convection/MVFR cigs. Front sitting
over southern Iowa will lift back north/northeast Monday. Meanwhile
agitated cu filed over southeast Iowa now firing thunderstorms.
Present hires models suggest the focus of any storms will be south
of TAF sites through period. However...there remains a rather
strong signal for some MVFR fog development late tonight and
lowering cigs as moisture pools over southern to western Iowa.
Models are not in agreement with exact placement and may need to
refine forecast...but trends suggest MVFR cigs from about 10 to
15z Monday before mixing helps break up lower cigs. /rev


&&

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

$$

SHORT TERM...REV
LONG TERM...Kotenberg
AVIATION...REV



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