Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 161737
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1237 PM CDT Mon Oct 16 2017

.SHORT TERM.../Today through Tonight/
Issued at 323 AM CDT Mon Oct 16 2017

GOES-16 Nighttime Microphysics shows a serene scene with nary a
cloud over central Iowa early this morning. At the surface, a zone
of high pressure is sprawled over the central Plains with Iowa on
the northern side of this high. Winds were light from the southwest
if not calm. Temperatures were in the 30s over many parts of
northern and western Iowa with temperatures several degrees either
side of 40 over the southeastern part of the state a little near
3am. The Frost Advisory area at this point looks well placed with
temperatures ranging from 34 to 38 with several hours for them to
drop further.

Quiet weather will prevail as northwest flow persists aloft today
and tonight. After the first part of the weekend had precipitable
water (PW) values 2 to 4 standard deviations above normal, PW values
under a half inch today will be around 2 standard deviations below
normal. 00z KOAX sounding as well as forecast soundings show the
dearth of moisture in the atmosphere. As the nocturnal inversion
breaks down due to daytime heating and mixing of the boundary layer
this morning, will see drier air aloft mix down to the surface.
Dewpoints were adjusted downward as initial forecast guidance was
too high given forecast soundings. Thus, lowered dewpoints a few
degrees by mid to late morning and at least several degrees this
afternoon yielding dewpoints mainly in the 30s today. Winds were
also adjusted a bit upward as the pressure gradient tightens over
the state later this morning and afternoon using CONSMOS to yield
these higher sustained wind speeds. 850mb temperature change from
Sunday afternoon to Monday afternoon will be 6 to 7 degrees C
higher. This will result in surface temperatures peaking 7 to 10
degrees higher than yesterday and near the long term normals in the
low and middle 60s.

Tonight, a weak shortwave may bring some passing high clouds across
the state. Not expecting these clouds to have any significant impact
on keeping lows up, but low temperatures will be about 10 degrees
higher than Monday morning.

.LONG TERM.../Tuesday through Sunday/
Issued at 323 AM CDT Mon Oct 16 2017

The only window for any weather of note through the period will
be toward the weekend. Fast zonal flow will be in place across the
region at onset with the jet still to our north. This will keep
warm and dry weather in place through the work week. Have nudged
max temps up toward the warm end of guidance with favorable
conditions for warming/mixing during the day. Soundings are quite
dry. There will be a very weak wave passage Tue Night into Wed,
and then the more potent short wave across southern Canada Wed,
but neither will have sufficient forcing or lift for precip in
Iowa. The latter will drop a weak boundary into the state Wed
Night with southerly flow following to end the work week as upper
level ridging builds into the central CONUS.

The zonal flow will buckle into the weekend but stay fairly
progressive with a long wave trough crossing the Rockies. This
will induce theta-e advection and low end PoPs Fri Night, mainly
driven by moisture advection with minimal baroclinicity in place
as Iowa will already be well into the warm sector. The potential
for surface based convection will increase along a front reaching
the western forecast border during peak heating Saturday with a
narrow sliver of 1000+ j/kg MLCAPEs by 00z. Even if true surface
based convection struggles, deep convergence and QG forcing will
likely blossom precip into the night aided by weak frontogenetical
forcing and cold front aloft. Thus have likely to categorical
PoPs going Saturday Night fed by 40kts of inflow and moisture
transport along the 300K isentropic surface. MUCAPEs should
gradually weaken into the night with soundings going to moist
adiabatic lapse rates. The severe potential seems minimal as shear
appears fairly weak along and ahead of the front. Weaker showers
may linger into Sunday before precip ends for the weekend as the
northern stream trough exits. 00Z NAEFS Precipitable waters,
specific humidities and integrated water vapor transport are not
extreme but in the 90th plus percentile so moderate rains, likely
in the half to one inch range, are possible. Warm cloud depths
will be over 10k ft.

The outlook beyond the valid period into next week calls for a
return to little or no precipitation and seasonal temperatures
with Iowa staying in dry NW flow on the southern fringe of the
westerlies.

&&

.AVIATION.../For the 18Z TAFS through 18Z Tuesday afternoon/
Issued at 1236 PM CDT Mon Oct 16 2017

Excellent aviation weather is expected with unrestricted ceilings
and visibilities. Surface winds will be from the southwest to
south through the period.

&&

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

&&

$$

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



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