Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 221132

Area Forecast Discussion
National Weather Service Des Moines IA
632 AM CDT Tue May 22 2018

.SHORT TERM.../Today through Tonight/
Issued at 323 AM CDT Tue May 22 2018

A large bank of dense fog has developed from northwestern/northern
Iowa down through east central/southeastern Iowa, within the axis of
a surface high pressure ridge. A dense fog advisory has been issued
to account for this, with gradual dissipation expected after
sunrise. Meanwhile, a few small showers and thunderstorms are
drifting in from eastern Nebraska, generated by a weakening LLJ and
modest warm air advection aloft. Some high-res models bring this
convection eastward toward central Iowa later this morning, while
more solutions dissipate it or hold it in western Iowa. Confidence
is low, but will maintain slight chance POPs in our western counties
through the day to account for this. In any event no real impacts
are foreseen and severe weather is not expected. It will be warmer
today, but not as warm as the remainder of the week discussed in the
long term discussion below.

Tonight the aforementioned surface high pressure ridge will move off
to the east, allowing low-level flow to come around to southeast
with more robust warm air advection aloft and the development of a
somewhat stronger LLJ. As the nose of the LLJ impinges on the zone
of strongest warm air advection we will see thunderstorms develop
overnight, mainly across northwestern and northern Iowa after 03Z.
The severe weather threat is low but locally heavy rain may be
possible. Have maintained categorical POPs up in our far north
around 09Z tonight when we should see maximum convective coverage.

.LONG TERM.../Wednesday through Monday/
Issued at 323 AM CDT Tue May 22 2018

Convection will be ongoing at the beginning of the period across
northern Iowa in area of good moisture transport near the nose of
the low level jet. This convection will weaken and dissipate through
the day as convergence weakens and focuses farther north.
Otherwise, the forecast area will be in the warm sector with
temperatures well into the 80s and dewpoints into the mid 60s,
therefore it will feel much more like summer. This will be a
harbinger of this to come with little push of colder air over the
next several days as upper ridging remains in place in rather
stagnant pattern. Therefore, the very warm conditions on Wednesday
will continue into Thursday and Friday. An upper wave approaching on
Friday will push a weak boundary toward the state. Initially,
convection across the Plains will approach northwest portions of the
forecast area late Thursday night as the low level jet veers into
the state, however the bulk of this convection should remain west of
Iowa overnight. The front will ease into the state on Friday/Friday
night, although with the upper system already nearly past the state,
the push with this boundary will be weak. However, sufficient
convergence should allow for some convective development during the
afternoon into the evening although with weak shear, overall
convection will be somewhat disorganized and likely cold pool

With little frontal push, the airmass will remain virtually
unchanged into the weekend and early next week with continued above
normal temperatures and limited precipitation threats.


.AVIATION.../For the 12Z TAFS through 12Z Wednesday morning/
Issued at 632 AM CDT Tue May 22 2018

Areas of fog affecting most of the terminals early this morning
will dissipate fairly quickly in the next couple hours.
Thereafter, VFR conditions are expected through the rest of today
and into tonight. Thunderstorms are forecast late tonight, after
about 03Z, across northern Iowa and may affect FOD/MCW/ALO toward
the latter part of the TAF period. Have handled this with just
VCTS for now due to low confidence in details, but expect
refinement in subsequent TAF issuances.


Dense Fog Advisory until 9 AM CDT this morning for IAZ004>007-



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