Johnston, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

432
FXUS63 KDMX 220527
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1127 PM CST Sat Dec 21 2024

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

.KEY MESSAGES...

- Freezing drizzle chances in the north Sunday night into Monday
  morning (30-40% confidence for any given location).
  Accumulations will be limited to glazing at this point, but
  this will still be impactful.

- Rain chances over the southeast early Christmas Day.

- Precipitation possible (60% confidence) at times over the
  region after Christmas and into next weekend. Continue to
  monitor the forecast.

&&

.DISCUSSION...
Issued at 323 PM CST Sat Dec 21 2024

Morning objective analysis showed a ridge at H850 over eastern Iowa.
This is what helped keep stratus overhead for most of the day for
parts of the state. The ridge axis is departing this afternoon and
allowing steering flow of the clouds to veer, pushing the clouds
back to the northeast. Meanwhile, more stratus (just higher up) has
coated northern Iowa this afternoon. Highs there will be held to
near 20 and even just the teens for spots like Mason City.
Confidence is increasing for persistent overcast skies through
majority of the next 3 days, if not almost the whole period, most
especially for the northern half of the state (70% confidence in
this outcome). Not only do we have the weak December sun, now right
at its minimum from the Winter Solstice today, but also in part due
to the series of waves arriving this week, advecting more moisture
to generate clouds (and perhaps some advection fog some mornings).
The next wave passes just to the north Sunday night. The past few
issuances have mentioned that most of the precipitation will be to
the north and east. Now that this event is in the Hi-res window,
there are a few solutions trying to produce QPF along the trailing
cold front that tails across central Iowa, most all of these
solutions condensing this activity nearest the low center. A look at
soundings still shows that disconnect between low and midlevel
moisture, making ice introduction difficult to achieve in Iowa; the
peak lift occurring in that dry layer. This is why models
aren`t producing much at all in the way of QPF, but there could
still be impactful wintry precip in the north in the form of
freezing drizzle. WAA and moisture advection collect at the
surface low and saturate the lowest 3kft, or so of the
atmosphere. Condensation pressure deficits are low and some
weak on the order of a microbar or two in the same layer can be
found, most of the lift developing east of the CWA. Have added
precipitation mentions generally north of Highway 20 for Sunday
night into Monday morning. Confidence is up to 40% for any given
point to see freezing drizzle, some snow may mix in as well
nearest the IA-MN border. A glaze of ice light enough to have no
measured accumulation is still enough to cause problems on
untreated surfaces, especially as surfaces have been well below
freezing for several days. Monitor the forecast for updates to
locations of freezing drizzle as confidence in the southern and
western extent remain in question.

Zonal flow remains for at least the first half of the week and
southerly flow will try and preserve the 40 degree mark for highs
nearby. Clouds, advection fog and precipitation chances make the
high temperature forecast challenging. More precipitation chances
return early Christmas morning in the southeast from a passing
southern stream trough. Over 60% of ensemble members have measurable
rainfall in southeast Iowa from that system, so confidence has
increased and precipitation chances will increase in future
issuances if this trend holds. Precipitation chances broaden, but
are legitimate for the holiday weekend. Multiple waves will
propagate along the southern stream in the central US, but with
some uncertainty as to where their impacts will be. If you`re
traveling in the region after Christmas, it will be worth
monitoring the forecast.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1127 PM CST Sat Dec 21 2024

MVFR stratus remains at the edge of KMCW however the site has
remained VFR so have kept that solution in the TAFs. VFR cloud
cover is expected into Sunday with south winds becoming breezy
in the afternoon, gusting 20 kts. Winds will diminish west to
east into the evening, lasting through late tomorrow night at
KOTM.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Jimenez
AVIATION...Hagenhoff