Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 182039
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
339 PM CDT Mon Sep 18 2017

.SHORT TERM.../Tonight through Tuesday/
Issued at 337 PM CDT Mon Sep 18 2017

A vigorous short wave trough moving across the region today with
kinematic forcing lifting through Iowa and Minnesota. This has
resulted in showers and thunderstorm moving across central Iowa
today. The precipitation has moved to mainly east of Interstate 35
by 3 pm though some lingering isolated to scattered activity may
remain west through 6 pm. Much drier mid-level air is already
morning into the western portion of the state and is associated with
the subsidence region of the upper level system. The precipitation
will continue to end overnight as the dry mid level air continues to
spread southeast.

The primary challenge overnight into early Tuesday is the potential
for stratus to persist and the potential for fog development. A weak
area of high pressure will keep the wind flow light and with the
recent precipitation, fog development is likely.  How persistent the
stratus remains overnight will play a role in the chances for dense
fog development. For now, just have areas of fog mentioned due to
cloud uncertainties but will mention dense potential, especially
central and north in the HWO. Tuesday will be warmer with highs in
the upper 70s north to mid 80 south. Given the mixing potential, any
fog should erode by mid morning.

.LONG TERM.../Tuesday night through Monday/
Issued at 337 PM CDT Mon Sep 18 2017

We will remain in an active pattern through the forecast period with
several chances for thunderstorms.  For the first half of the week,
a low over the Pacific Northwest will pivot a trough through the
Northern Plains as it lifts into Canada.  This will flatten the
ridge over the region putting Iowa in a more southwesterly flow.
This will bring in warmer temps and a little more moisture. At the
surface a low over the Northern Plains will lift northeast as well
dropping a quasi-stationary warm front across the northern half of
Iowa.  By late day Tuesday and especially Tuesday night, MUCAPES
around 3000 J/Kg will move up into the western third of Iowa with a
ribbon of theta-e advection across the north. We could potentially
see storms west and north but I have focused precip chances more
where the front will be and where the theta-e advection ribbon is
likely to be.  That feature pivots through by mid morning Wednesday,
stalling across the east.  By late Wednesday, during peak heating
and into the evening, south central to eastern Iowa will destabilize
with MUCAPES of 4000+.  There will be a sharp delineation of
precip/no precip as high pressure builds into the northwest third or
so of the state but in eastern iowa and perhaps across the southeast
portion of our forecast area, we could see a period of strong to
severe storms before they shift east.

Thursday through Saturday a deep trough develops over the west and
Rockies sharpening the ridge to our east and putting us in deep
southwest flow.  Temps will become even warmer during this period
with highs expected now in the lower 90`s Friday and maybe even
Saturday.  The models hint at a shortwave riding up the backside of
the ridge which, given our instability and moisture could trigger at
least isolated storms Thursday afternoon.  Otherwise this should be
a dry period.

By later Saturday the upper trough makes it`s way into the Rockies
and western Plains.  As energy ejects out of the base of the trough
and up the backside of the ridge, thunderstorm chances will
increase.  The increase will be seen mainly over far western Iowa
late day Saturday and Saturday night but not until Sunday night and
Monday for central and eastern Iowa so this will be a slow moving
system given many parts of the state a prolonged period ( a day or
two) of rain chances.  This is welcome since we have not seen rain
in some time.  At this time qpf looks to be a half inch to three
quarters of an inch for any one period though locally higher amounts
are certainly likely. Overall the risk of ponding water/localized
flooding is low.

As for the severe risk...given the instability, boundary position
and potential for that ribbon of theta-e advection to all be in the
vicinity of west central into northern Iowa late Tuesday night and
again with peak heating Wednesday afternoon into the evening, there
is at least a marginal risk for some of the storms to become severe
with damaging wind and some large hail a threat.  The caveat is that
the better shear remains to the north and west of Iowa, though shear
is still sufficient over the northwest quarter to half.  Later in
the week there are still too many uncertainties to determine severe
potential...especially with timing details being problematic and the
best shear appearing to be well northwest of the state.

&&

.AVIATION.../For the 18Z TAFS through 18Z Tuesday afternoon/
Issued at 1245 PM CDT Mon Sep 18 2017

Scattered showers and thunderstorms moving across central Iowa
this afternoon with cigs lowering to the west and dropping to
MVFR. The precipitation will be ending by late afternoon and be
mostly out of the area by early evening. Potential for IFR/LIFR
fog or stratus developing overnight. If the clouds do break, could
be dense fog. Have now have stayed with stratus but may need to
transition to fog mention with future updates.

&&

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

$$

SHORT TERM...Donavon
LONG TERM...FAB
AVIATION...Donavon



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