Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 202348
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
548 PM CST Mon Feb 20 2017

.SHORT TERM.../Tonight through Tuesday/
Issued at 328 PM CST Mon Feb 20 2017

Tremendous moisture transport north from the gulf, enhanced by a
pronounced trough extending from southern Canada to northern Mexico,
has brought record PWAT values to the Midwest which are over 400
percent of seasonal values. Widespread rainfall amounts today of one
half to one inch measured. Mainly light rain still ongoing over the
forecast area this afternoon. Instability has weakened this
afternoon as some cooling has occurred around 800 mb. A surface
boundary moving into western Iowa may provide the focus for
additional instability later this after and early evening as it
moves across central Iowa. Therefore, a few thunderstorms may
redevelop before the system exits to the east. Much drier air will
follow overnight and good subsidence will develop as a ridge axis
arrives. Fog potential is a concern considering the rainfall today
and then the clearing skies and light winds arriving overnight. Have
added fog to the forecast and expect an area of dense fog may
develop later tonight into Tuesday morning.

Tuesday will be mild once again with temperatures again near record
highs. Any fog that does develop may linger, especially over
northeast Iowa for much of the morning and may have an impact on
temperatures.  Otherwise, highs will be in the 60s to lower 70s.

.LONG TERM.../Tuesday night through Monday/
Issued at 328 PM CST Mon Feb 20 2017

Tue Night through Fri Night...Confidence: Medium-High

Two main story lines within this time frame. First, warm, near
record, temps will remain Wed. Warm southerly flow will be aided by
a passing surface low to the north. 850mb temps in good agreement in
the 12-15 deg C range from N to S, and will push temps back into the
60s to mid-70s with good mixing expected. Abnormally warm temps of
late then begin to come to an end as trailing cold front moves
through, dropping temps back into the 40s/50s for Thur.

Second, main show through the forecast remains a trough digging out
of the SW US and accompanying surface low to develop in the TX/OK
Panhandle regions. By Thur night, large surge of theta-e
advection/isentropic lift will move into Iowa and initiate rain. As
the low deepens Thur night, track should take it across SE Iowa,
providing an increasingly likely opportunity for a significant
snowfall event across portions of N/NW Iowa. Periods of blowing snow
appear likely as well with tightly wrapped low and sustained 25+ kt
winds. Have bumped up winds over initial guidance, but may need to
be bumped up a few more kts.

Further S/SE, initial precip will be rain with a few thunderstorms
mixed in as suggested by MUCAPE values around 100-400 J/kg. As
surface low continues progression NE, strong cold front and winds
will pass through the state turning precip all snow. With frontal
passage, sustained winds around 25kts and gusts in excess of 35 kts
will be possible Fri afternoon and into the evening. Temps will
plummet into the teens/low 20s for lows Sat morning and 20s/low 30s
highs. Changeover to snow should be quick with entire column rapidly
cooling below freezing, though some concern for a freeze on roads of
melted/melting snow dependent on how quickly ground temps react. A
couple other things of note, PWats very high for this time of the
year in the warm sector around 1-1.1 in and even to an extent in the
cold sector around 0.60-0.75 in. As a result, periods of heavy
rainfall appear increasingly likely in SE Iowa and heavy snowfall
back over NW/N Iowa. DGZ depth has backed off a bit over previous
runs, curbing some concern for sustained heavy snowfall in the wrap
around/deformation zone though. While models are in fairly good
agreement into Friday, GFS somewhat curiously hangs the surface low
up in SE IA/NW IL, causing it to be about 6-12 hrs slower than the
Euro in exiting Iowa. Given the lack of blocking features to the
east, have trended POPs toward Euro, but possibly not enough.

Saturday through Monday...Confidence: Low

Large scale pattern remains in fair agreement between Euro/GFS, with
500 mb flow relatively zonal, however details in waves moving
through the flow differ significantly. GFS consistently more
aggressive with precipitation chances Sun and Mon and Euro only
creeping chances in late Mon. Temps significantly different as well
due to handling of waves, with GFS significantly cooler than the
Euro. Have kept with a blend of the two for the time being.

&&

.AVIATION.../For the 00Z TAFS through 00Z Tuesday evening/
Issued at 546 PM CST Mon Feb 20 2017

Extremely variable conditions anywhere from LIFR to VFR exist
across the forecast area ahead of an approaching surface trough
just west of the MO River at 00z. Generally the lower end of that
range exists in the KMCW/KALO areas. These conditions should
gradually improve west to east as the trough moves through
tonight. High pressure, clearing skies and light/variable winds
will follow quickly on its heals however which may lead to
radiation fog development early Tue morning as low level moisture
lingers, especially north and east. VFR conditions are
anticipated to return area wide by very late morning however with
SW winds and some sunshine.

&&

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

&&

$$

SHORT TERM...Donavon
LONG TERM...Curtis
AVIATION...Small



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