Johnston, Iowa 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

890
FXUS63 KDMX 301953
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
253 PM CDT Sun Mar 30 2025

.KEY MESSAGES...

-Rain/snow mix into the early evening across central and north
 Iowa.

-Rain/snow in north Iowa Tuesday with potential for strong to
 severe storms late Tuesday night into Wednesday morning. The
 primary threat is large hail.

-Active weather is trending south of Iowa by later in the work
 week with seasonal temperatures.


&&

.DISCUSSION...
Issued at 253 PM CDT Sun Mar 30 2025

Upper low is currently passing over Iowa this afternoon with
deformation precipitation over northern Iowa.  Cold advection has
been ongoing across much of the area through the day with
temperatures holding steady in the 30s to mid 40s.  The
precipitation has been a mixture of rain and snow although where the
heavier precipitation has been occurring in the northwest and west
central, snow has become the predominant precipitation type. As
the low pulls northeast into the evening, this deformation
precipitation slides eastward for the remainder of the afternoon
and evening and should depart by late evening across the
northeast. Any accumulations of snowfall are expected to under
an inch and remain on grassy areas with melting on most road
surfaces as pavement temperatures are holding above freezing.
However, after sunset, some minor accumulations may occur on
less traveled roads but remain relatively un-impacted.
Otherwise, winds tend to gradually diminish as the pressure
gradient relaxes toward daybreak on Monday with quiet weather
through the day.

Warm advection intensifies into Monday night with strengthening
isentropic lift after midnight.  Clouds overspread the state with
temperatures likely bottoming out somewhat early then remaining
steady or even slowly rising as southeast winds increase after
midnight.  Condensation pressure deficits crash on Tuesday morning
across northwest Iowa with rain and snow developing during the
morning and spreading north and east with the lift during the
afternoon.  Much of this precipitation is likely to remain north of
Highway 20 in Iowa where lift and saturation is maximized.
Otherwise, surface winds are forecast to be strong throughout the
day on Tuesday with a tightening gradient as robust low level flow,
some of which mixes down to the surface.

By Tuesday night into Wednesday morning, a warm front in Kansas and
Missouri lift quickly northward ahead of an approaching upper low. A
50kt+ low level jet points into Iowa during the night with strong
moisture transport/theta-e advection into the state.  Convection is
expected to develop overnight as the instability is released
spreading rapidly northeast into the state during the night.
Soundings indicate this convection/storm inflow remains elevated in
a deeply sheared atmosphere, although much of the direction shear
remains below the inflow level.  Hail production remains the main
concern with the convection given the elevated nature although some
gusty winds may be able to punch to the surface as the the cool
layer becomes more shallow with the approach of the warm front.  A
dry slot rapidly pushes into the state on Wednesday morning shutting
down convection from west to east through the morning with only
limited threat of showers in the afternoon across the north but any
severe threat remains south and east of the area by Wednesday
afternoon.  The pattern remains active in the central United State
into the second half of the work week but model trends have been
keeping precipitation mostly south of Iowa during this time.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1242 PM CDT Sun Mar 30 2025

Low ceilings and restrictions to visibilities are expected
across the northern half of Iowa this afternoon into the early
evening as areas of drizzle and light snow are likely to
persist. The precipitation comes to an end by mid to late
evening with improving conditions toward daybreak on Monday. VFR
conditions will become widespread by the end of the forecast
period as ceilings improve. Surface winds remain strong from the
northwest this afternoon and this evening but slowly weaken
into Monday morning.

&&

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

&&

$$

DISCUSSION...Cogil
AVIATION...Cogil