Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 210950

Area Forecast Discussion
National Weather Service Des Moines IA
450 AM CDT Wed Mar 21 2018

.SHORT TERM.../Today through Tonight/
Issued at 421 AM CDT Wed Mar 21 2018

The light snow flurries have tapered off across the area over the
past several hours, with an area of high pressure situated just to
the west of the state. Low to mid level moisture remains in place
with some resultant cloud cover across the western half of the
forecast area. The cloud cover is expected to remain in place
through much of the morning and begin to break up through the late
morning into the afternoon hours especially across the southern
portions of the forecast area. The north will remain under the
influence of a weak wave moving from central ND into MN today which
will allow clouds to remain in place through much of the day into
tonight. Another weak upper wave will eject eastward out of the
Rockies today and move toward the state tonight. This will allow
cloud cover to move back into the area and bring a chance of light
rain toward 09-12Z Thursday across the southwest. Have kept with the
mention of some slight chance to low chance pops, however soundings
indicate deeper moisture/better saturation available after 12z, thus
could see a delay in precip onset until that time as we progress
through the day. As for temps, kept it a bit cooler still in the
north where clouds to remain most of the day, also worried temps
might be a bit warm with more northerly flow and colder air
remaining in place. However mid/late March sun is gaining in
strength so didn`t alter too much from the previous forecast

.LONG TERM.../Thursday through Tuesday/
Issued at 421 AM CDT Wed Mar 21 2018

The forecast is rather messy through the period. Thursday and
Thursday night, an upper ridge will shift out of the Rockies an
into the Upper Midwest. A subtle shortwave drops down the upper
ridge while at the surface a |low develops over the Rockies with a
weak warm front extending across Kansas and Nebraska into
Missouri. As the weak disturbance passes Thursday some weak
forcing and theta-e advection will exist across southern into
central Iowa and this will be enough for some light rain to
develop. If the precip develops early enough Thursday morning then
there may be a period of snow or a rain/snow mix. QPF will be
light so there should not be much precip with this system.

A more significant system will impact Iowa Friday through
Saturday. A strong shortwave will break down the upper ridge and
deepen the surface low as it shifts to the east while enhancing
the warm front. Deeper moisture will be associated with this
system and will result in some moderate to locally heavy rainfall
across the area. This could impact local streams. Some instability
works into southern Iowa and while it is somewhat weaker and
further south with today`s model runs, I still kept the mention of
isolated thunderstorms in far southern Iowa...mainly south of
highway 34. This time of year with the instability and the amount
of shear over southern Iowa, it would not be out of the question
for some small hail to occur with stronger storms but with the
uncertainty of how much instability we will see I left any mention
of hail out of the forecast and products.

Further north, cold air will work into the state Friday night
through Saturday morning and with the amount of moisture available
and the strong forcing moving across from late Friday night
through Saturday morning, moderate to locally heavier snow will be
possible somewhere across northern central into northeast Iowa. I
have ramped up wording in the hazardous weather outlook product
and weather story. If heavier snow continues to look likely across
this area in the next couple of model runs messaging will have to
be ramped up as well. Right now it looks as though temps will warm
to above freezing Saturday afternoon so there be a relatively
short window for this moderate to locally heavy snowfall to occur.
This needs to be better resolved as well as it will have an impact
on just where the heavier snow will fall but this system will bear
watching as it will have the biggest impact to the area.

On Sunday a deep trough will move onshore in the Pacific Northwest
pushing the weakened upper ridge towards the Great Lakes. This
will also act to push the surface low east thus ending the precip
threat by Saturday evening. However another surface low will
develop over the Rockies and move into the Plains by late Sunday
spreading more precip back into western Iowa by late Sunday night.
The precip will then move across the rest of the state on Monday
further as north central into northeast and east central Iowa. As
for pytpe, it should be the typical snow at night into early
morning Sunday night into Monday, changing to all rain by Monday
afternoon and east of the area by Tuesday.


.AVIATION.../For the 06Z TAFS through 06Z Wednesday night/
Issued at 1134 PM CDT Tue Mar 20 2018

Little change from thinking at 00z. MVFR to isolated IFR
conditions in flurries and patchy fog are in place along and west
of a KCNC-KDSM-KMCW line at 06z with VFR conditions farther east.
MVFR or less conditions may persist near KFOD and points west but
confidence is not high. Farther east confidence is greater in VFR
conditions through the period.




SHORT TERM...Beerends
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