Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 202347
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
547 PM CST Mon Nov 20 2017

...Updated for 00z Aviation Discussion...

.SHORT TERM.../Tonight through Tuesday/
Issued at 324 PM CST Mon Nov 20 2017

A cold front will enter northwest Iowa by 06Z this evening and push
through the state by 12Z Tuesday morning.  High pressure at the
surface will build into the state for the rest of the day. Aloft, a
pretty strong shortwave will swing through the state.  A narrow area
of lift will accompany this shortwave moving across northern Iowa in
the morning then dropping through the eastern third to half of the
state for the rest of the day.  Strong winds will will occur behind
the front and will be enhanced as this area of lift drops across
northern then eastern Iowa.  Winds post frontal will increase across
northern Iowa to 15g25kts by or shortly after 09z then increase with
that area of lift to around 30g40kts for about a 4 to 6 hour window.
Pretty high confidence that northern Iowa will see borderline to
advisory criteria wind speed/gusts.  So as a result will issue a
wind advisory for northern Iowa from 12Z to 19Z.  As the shortwave
quickly passes east the area of lift mainly grazes the eastern third
or so of the state and while winds will likely increase, confidence
is somewhat less that we will see advisory criteria.  Winds will be
hit hard in the forecast and HWO but the advisory can be expanded
with later shifts as confidence grows.  Moisture is woefully lacking
with this system so other than some mid/high clouds we will not see
precip...just wind and cold.

Regarding temps...we will be in strong cold advection during the day
Tuesday so expect highs to be some 20 to 30 degrees colder than
today.

.LONG TERM.../Tuesday night through Monday/
Issued at 324 PM CST Mon Nov 20 2017

Little change to trends over the next 7 days. Cold conditions will
persist Tuesday night as ridge of high pressure passes across the
state. Overnight lows will dip into the mid to upper teens at most
locations. Warm advection will arrive by Wednesday afternoon as the
high pressure system moves east and low pressure enters the Dakotas.
Despite the warm advection Wednesday, temperatures will remain cool
with highs in the mid 30s to lower 40s. Much warm temperatures
arrive for the end of the week as the thermal ridge arrives with
highs in the 40s and 50s Thursday and the 50s and 60s on Friday. An
inversion around 900 mb Thursday will keep the temperatures cooler
despite very warm temperatures aloft followed by deeper mixing on
Friday. A few things of note on Friday. Guidance is trending a
little faster with a boundary entering at least northern Iowa by
Friday afternoon. Associated clouds associated with the boundary
will have an impact on temperatures. The initial cold advection
though should be countered by deeper mixing with the frontal passage
with the stronger cold advection arriving Friday night. If the sun
persists over central and southern Iowa into Friday afternoon, then
forecast highs may still be too low and may approach record highs.

Guidance is trending farther east with the long wave trough over the
weekend which would keep Iowa a little warmer over the weekend
though still much cooler than Friday. Ridging will begin to move
back into the region early next week that will bring a return to
near to above normal temperatures by Monday. Little to no
precipitation is expected through the forecast period.

&&

.AVIATION.../For the 00Z TAFS through 00Z Tuesday evening/
Issued at 547 PM CST Mon Nov 20 2017

Gusty winds are the primary aviation concern through the TAF
period. Winds will decrease this evening, but will remain around
10 knots through much of the night. A strong cold front will move
through the area later tonight into early tomorrow morning
causing the winds to become from the northwest and very gusty. The
strongest winds are expected to last several hours after the
front passes with gusts during this time between 25 and 35 knots.
Cloud bases down to FL100 as the front moves through the state are
expected and included in this TAF issuance. However, it is
possible lower clouds closer to MVFR criteria may occur over the
northern terminals around the front with cross sections showing a
plume of higher moisture around 850mb/between FL025 and FL035. At
this point, confidence was not high enough to include this in the
forecast.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Wind Advisory from 6 AM to 1 PM CST Tuesday for IAZ004>007-016-
017.

&&

$$

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



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