Johnston, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

705
FXUS63 KDMX 142327
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
527 PM CST Thu Nov 14 2024

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

.KEY MESSAGES...

-Dry into Friday, with patchy fog (some dense) mainly south and west
late tonight into early Friday

-Quiet with mainly dry weather into the weekend, outside of minimal
chances (around 10%) of rain southeast late Saturday into Sunday

-Multiple chances for precipitation through next week, much
 cooler towards the latter half

&&

.DISCUSSION...
Issued at 317 PM CST Thu Nov 14 2024

Dreary conditions remain across the greater majority of Iowa, as
thick cloud cover continues to blanket all but the southwest portion
of Iowa early this afternoon as the low pressure system previously
overhead from yesterday departs east. Temperatures have been a bit
slow to increase through the day so far with the overall lack of
sunshine, which will likely lead to slightly cooler temperatures
than what were expected, so values have been bumped down at least a
degree or two over portions of the areas where clouds remain. Highs
in the upper 40s to low 50s are expected this afternoon. These
clouds should gradually exit at least the southwestern two-thirds of
Iowa into the evening, though some cloud cover looks to return into
Friday morning. Lingering low level moisture and light winds over at
least portions of western into southern Iowa into Friday morning per
model guidance such as the HREF, NAM, HRRR has placed patchy areas
of fog, some areas even dense. However, other models dry out the
lower levels and lessen the overall potential, but given how
conditions are playing out so far, would lean more towards the
higher likelihood that this fog will gradually develop. Further
confidence is needed however for potential headline
consideration, so will be continuing to monitor trends over the
next several hours and adjust the forecast as needed. Overnight
lows are expected to be on the chilly side, with values in the
mid to upper 30s.

Upper level ridging moving into the central US into Friday and at
least the start of the weekend will lead to dry and quiet weather
across Iowa. Increasing low southwest flow, especially into
Saturday, will lead to warmer air pushing into the state, with highs
expected to reach into the upper 50s to low 60s, several degrees
above average for high temperatures given the time of year. Winds
will be breezy as a result with gusts upwards of 20-30 mph,
especially north. A shortwave breaking away from a larger trough
tracking across the southwestern CONUS and lifting northeast into
the Dakotas looks to develop a frontal boundary setting up over at
least southeastern Iowa late Saturday into early Sunday. Though
moisture per model soundings is present in the low levels with
moderate warm air advection, weaker forcing for lift through much of
the column looks to keep conditions dry, so have currently left out
any rain mention in the forecast at this time and will continue to
monitor for potential changes.

All eyes remain focused on next week as major changes are expected
to occur. The week starts out with the larger trough mentioned above
over the southwestern CONUS deepening more significantly, then
ejecting north/northeastward into the Central Plains and gradually
into Iowa by Tuesday. Long range guidance per latest runs seem to be
in better agreement on widespread rain chances increasing from south
to north through Monday morning, with the state within the warm
sector of the strengthening low pressure as it continues to lift
north through Tuesday. Periods of heavier rainfall remain in play
given the strong surge of warm and moist air from the south paired
with widespread ascent within an area of moderate frontogenesis, as
signaled with this system per GFS guidance, with ensemble mean
rainfall totals generally depicting at least 1-2 inches over much of
the state. Cannot rule out some potential for thunderstorm activity
given increasing dewpoints reaching into the low 50s and warmer
temperatures for the time of year as well in the 50s paired with the
stronger forcing, but the overall potential remains lower end at
this time and will continue to watch closely. As the low stalls over
the Upper Midwest and circulates overhead per GFS guidance with
increasing NW flow, could see the remaining potential for some
introduction of winter precipitation. The Euro has a different take
on the evolution of this system as another area of low pressure
developing within the larger trough lifts into the Central Plains
and becomes ingested into the earlier week system, creating a much
more defined low pressure with the deformation zone right over Iowa,
which given enough moisture and stronger forcing for ascent suggests
much more impactful precipitation over the area, especially as
temperatures are generally expected to be much cooler with lows in
the 30s and highs in the upper 30s to low 40s into the latter work
week. Still too far out to get into more specific details on
expected weather and impacts, but definitely quite dynamic with
different possibilities producing significant differences in
outcomes. Will be watching trends over the next several days as more
becomes better known as more model members pick up these systems
through next week.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 527 PM CST Thu Nov 14 2024

Stratus continues to slowly pull east this evening with overcast
MVFR ceilings giving way to VFR unrestricted ceilings. However,
recent rainfall and good radiational cooling may allow for the
development of fog overnight impacting all TAF sites with MVFR
visibilities for a few hours on either side of sunrise. The fog
is expected to burn off gradually on Friday morning as mixing
slowly increases. Surface winds are light and variable overnight
becoming southeast with a modest increase in speed by midday
Friday.

&&

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

&&

$$

DISCUSSION...Bury
AVIATION...Cogil