Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 221126
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
526 AM CST Wed Nov 22 2017

...Updated for 12z Aviation Discussion...


.SHORT TERM.../Today through Tonight/
Issued at 346 AM CST Wed Nov 22 2017

Bottom Line up Front...
A chilly, but hazard-free forecast is in store for Iowa today.
The biggest noticeable weather impacts will be from wind chill
temperatures in the teens through single digits for most of the
morning hours, and the potential for flurries/wintry precip for
portions of east-central/northeast Iowa this afternoon and
evening.

Temperatures...
09z Water vapor image picking up on large ridge building over the
intermountain west beginning to reach into southern Canada.
Additionally, there is an upper low over the eastern Hudson bay,
with the accompanying longwave trough digging into the Ohio Valley
region. This large-scale setup will have Iowa socked into
northwesterly flow aloft once again today. Closer to home, there
is an 1032mb sfc high pushing southeastward into northeastern
Kansas. This sfc high should make it into eastern/southeastern
Missouri by this afternoon, gradually pushing Iowa into S/SW flow
at the sfc.

Yesterday, 850mb temps fell to the -4C across the south to -10C
range across northern Iowa, which is the range they are close to for
this morning. Though 850mb temps will notably rise later today as
the WAA/return flow arrives into Iowa as the thermal ridge begins to
crash over the state, the arrival will be a little too late to boost
temps very much after the noon hour, as temperature values should
stay fairly flat, especially across northeastern Iowa, where there
will likely be more extensive, thicker cloud cover...


Wintry precip...
Models are picking up on a PV anomaly/ill-formed shortwave
interjected into the mix in ahead leading edge of our thermal
ridge. Models are also picking up on a 1015mb or so sfc reflection
low propagating E/SE into western MN by 00z Thu. There is
generally good clustering with this feature... though worth noting
the hi-res models are trending slightly faster with this push.
Leading this sfc low is a very well-defined band of 850mb theta-e
advection oriented from N-S that makes it all the way down to
I-80. This band looks to pass through Iowa from 18z Wed to 00z
Thu. Properly captured right behind this band of theta-e advection
is a slug of moisture, phased with ascent through the dendritic
growth zone. The only mitigating factor keeping this from being a
noteworthy precip event is the lack of near-sfc moisture. From
around 800mb to the sfc, there is a clear lack of saturation,
which will push to evaporate a lot of the precip before it reaches
the sfc. Ultimately, cloud thickness will have no problem
supporting sufficient droplet growth, but it will be a rough
journey upon cloud departure.

In terms of precip type, fcst soundings early on clearly suggest
flurries. Leaned slightly towards HRRR sounding, though the RAP and
general consensus okay enough to use as well. As 00z approaches, WAA
will have reached KMCW and the rest of our CWA, suggesting a trend
towards sleet/fz rain. However, by 00z Thu, hi-res models showing
the best ascent wast of the DMX CWA. Thus, have wintry precip
chances ending somewhat abruptly before sensible wintry precip
impacts are felt. Did not want this flurry/wintry precip potential
to go unmentioned, but hazardous impacts from this should
effectively be nil.


.LONG TERM.../Thursday through Tuesday/
Issued at 346 AM CST Wed Nov 22 2017

Fairly warm pattern expected through much of the mid/long range,
with main issue being winds Friday into Saturday and again Monday
into Tuesday. Broad upper ridge in place across the western US with
westerly to northwesterly upper level flow across the region to begin
the period. Area of high pressure in control at the surface with waa
for Thanksgiving Day. H85 temps around +5C to +9C which should
result in highs in the 40s/lower 50s as sfc winds begin to shift
around to the south late Thursday. Southerly flow and WAA to
increase late Thursday night into Friday ahead of the next cold
front expected to drop through the state mid Friday into Friday
night associated with an area of low pressure moving through
Ontario. May still see a shower clip the far north, but again low
confidence the area will see anything more than an increase in
clouds with the frontal passage. Bigger issue is the winds behind
the front with strong CAA, tightened sfc pressure gradient and
higher pressure rises. BUFKIT winds showing 35-40 kts at the top of
the mixed layer with a decent period of strong/gusty winds possible
in the afternoon/early evening hours. Winds expected to be gusty
again Saturday with strong flow throughout the lower levels, with
sunny skies and a large area of high pressure building into the
central Plains. With mixing could get some of the stronger winds
aloft to mix down, however expect winds to begin to diminish toward
late afternoon/early evening as the high builds into Iowa and the
flow aloft begins to decrease.

Western US upper ridge to then shift eastward into the central US
during the upcoming holiday weekend. The sfc high will shift off to
the south and east as return flow sets up across the state into
Monday. An upper trough digs into the intermountain west with
southerly to southwesterly flow increasing ahead of it with a strong
push of much warmer air into the state for Monday. H85 temps
increase to around +15C by late Monday with our warmest
southwesterly flow in place. For now have gone slightly above
guidance grids with temps into the 50s to around 60. Would not be
surprised though if temps ended up substantially warmer with much of
the area into the mid 50s to mid 60s or warmer. Will be monitoring
in the coming days and adjust temps upward if current solutions
still expected. The western US upper trough to eject eastward into
Tuesday/Wednesday with the sfc low lifting through IA during the day
Tuesday. May see some precip chances on the backside of this system
across the far north, otherwise majority of precip associated with
this system to remain to the north/west of the state. Strong cold
front to drop through late Tuesday which will send temps plummeting
with highs by Wednesday only in the mid 30s to mid 40s again.

&&

.AVIATION.../For the 12Z TAFS through 12Z Thursday morning/
Issued at 524 AM CST Wed Nov 22 2017

High confidence in VFR conditions throughout entire TAF period.
Largest concern will be chance of flurries/light wintry mix to
impact KMCW and KALO... with KFOD and KOTM likely just missing
out. Am not expecting sensible impacts to aviation activities, as
flurries should be light and not accumulate on surfaces, for the
short duration they may occur.


&&

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

$$

SHORT TERM...Kotenberg
LONG TERM...Beerends
AVIATION...Kotenberg



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