Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 190507

Area Forecast Discussion
National Weather Service Des Moines IA
1207 AM CDT Mon Mar 19 2018

...Updated for 06z Aviation Discussion...

.SHORT TERM.../Tonight through Monday/
Issued at 318 PM CDT Sun Mar 18 2018

There is little sensible weather of concern through the period again
outside of fog and cloud trends. A notable short wave is present to
our SW across OK with weak instability spreading northward into KS
and NE, but the combination of a steady fetch of 1.5-2km dry air and
the southerly short wave track should keep our forecast mainly dry
through 00z Tue outside of token slight chances far west and SW.
There will be a compact QG rise/fall couplet passing to our south
across MO, but models are showing little hope of adequate phasing of
moisture and lift this far north.

Overall model guidance as a whole keeps stratus in place into
Monday, especially west and south, but is less bullish with fog
potential. Have added mention east however where recent visible
satellite imagery has shown some clearing. Winds will remain light
so radiation fog development will once again be possible in these
locations with current temp/dew point spreads generally 8F or less
at 20z.

There should be continued cloudiness tomorrow, but model soundings
suggest some lifting of ceilings with associated mixing boosting
temps to more seasonal values in most locations.

.LONG TERM.../Monday night through Sunday/
Issued at 318 PM CDT Sun Mar 18 2018

From Monday night through Tuesday a broad 500 mb shortwave
trough will move slowly across the region, weakening as it begins
to move away. The broad forcing for ascent associated with this
feature will generate a field of thick clouds and light
precipitation which will primarily impact our forecast area around
Tuesday morning. There is some question as to the onset of
precipitation given the dry air in place ahead of the system, but
it should begin by sunrise Tuesday in our far west/northwest
counties and slowly advance eastward through the morning before
fizzling out as the shortwave weakens by Tuesday evening. Forecast
temperatures and soundings indicate the precipitation will mostly fall
as light snow initially, then change over to or mix with rain
across most of the area as temperatures warm during the day. In
far northern Iowa, up around Mason City and Estherville, the
precipitation could remain snow all day but with the snow
tapering off in the afternoon only light accumulations are
anticipated, ranging up to perhaps 2 inches in our far
northwestern counties. This should have very little impact,
although the Tuesday morning commute could be mildly affect in
parts of rural northwest Iowa.

By midweek a large thermal ridge will have established itself over
the southern Rockies and High Plains, pushing toward Iowa on
Wednesday. As the ridge approaches, there is some signal that a
compact shortwave impulse riding over it may cross Iowa around
Wednesday night into Thursday, producing a brief stripe of
precipitation somewhere in our forecast area roughly around
Thursday morning. However, given the speed and subtle nature of
the parent feature there is a large degree of spread in model
solutions and the timing/location/p-type details remain unclear.
Any accumulation will be fairly light and depending on time of day
may fall as rain, but there is a small possibility that we could
see lightly accumulating snowfall during the Thursday morning
commute. We will be refining this forecast as the week progresses.

From Thursday night onward forecast confidence steadily decreases.
By Friday morning the broad trough axis should be roughly down the
Missouri River valley, with a large low pressure gyre spinning
off the Pacific Northwest coast. A series of waves/vorticity
maxima will spin off this gyre and eastward across the Midwest,
overtopping and flattening out the ridge and bringing one or more
rounds of precipitation to Iowa around the end of the work week
and into early next week. However, the variability in sensible
weather solutions is so high that it is difficult to hang your hat
on anything concrete at this time. Given the general synoptic
pattern it is likely that the first round of precipitation will
come around Thursday night/Friday and primarily in the form of
rain, with a relative break around Saturday followed by another
round or two of precipitation between Sunday and Tuesday of next
week. It appears temperature profiles would be supportive of rain
in these latter events as well, but at this range and given the
variability of the pattern this is a low confidence forecast.


.AVIATION.../For the 06Z TAFS through 06Z Monday night/
Issued at 1204 AM CDT Mon Mar 19 2018

Slight updates from previous TAF to timing of IFR to MVFR. Am
keeping MVFR at KDSM... will have to keep an eye on fog as KIKV
down to 1/4 of a mile and LIFR CIGs. KALO likely to remain fog
free due to relatively higher/increasing winds. If winds fall
off, then fog will need to be added to KALO. Will be keeping an
eye on advection fog in NW Iowa. KEST has tanked... but confidence
high that this fog will not make it down to KFOD.

Winds above 10 kts at most TAF sites through this afternoon, and
gradually decreasing overnight tonight.




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