Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 171121

Area Forecast Discussion
National Weather Service Des Moines IA
621 AM CDT Tue May 17 2022

...Updated for the 12z Aviation Discussion...

.SHORT TERM.../Today through Wednesday/
Issued at 245 AM CDT Tue May 17 2022

Forecast Impacts for the week:

...Storm and some severe chances ramp up tonight
...Second round of thunderstorms Thursday night after warm day
...Much cooler over the weekend

Confidence Short Term: Medium to High

Rather weak synoptic features at the surface with 03z subjective
analysis showing a ridge of high pressure over the Northern Plains
while a weak cool front has backed into Iowa from the northeast this
evening as low pressure moves into southern lower Michigan.  Farther
west a rather disorganized area of weak lows stretches from New
Mexico north to Montana along the lee of the Rockies.  Aloft, the
H850 pattern is better defined with a trough stretching from
southern Mexico north to a closed low in western Colorado with a
northern trough across the Canadian Rockies.  With a pronounced
ridge of high pressure from the eastern Dakotas south across Iowa
Monday, conditions remained dry and sunny. With a developing warm
front tonight over northwest South Dakota southeast to Kansas, weak
return southerly flow and a ribbon of moisture to our west has
allowed for an area of showers and isolated thunderstorms to develop
north of the boundary from South Dakota into southwest Minnesota at
05z. These showers will tend to weaken with time early today
before another area develops in eastern Nebraska in the early
afternoon and spreads east into the afternoon hours. Tonight the
deeper H850 moisture is from the Gulf northwest into Texas/western
Oklahoma northwest into eastern Colorado. Through early morning
the increasing thetae advection across the warm front at H850 will
increase an area of showers into northern Iowa with more spotty
showers moving into central Iowa between 12z and 18z today. The
main channel of H850 moisture will arrive over central and western
Iowa between 18z and 00z Wednesday with a gradual increase in
more thunderstorms over the western forecast area at that time.
Almost all of the deterministic models, both synoptic and CAMs
keep the stronger instability gradient in far western Iowa eastern
Nebraska through 00z with the better chances for stronger storms
in a narrow window between 23z and 02z in our far west. By then an
increasing H850 low level jet and general southeast progression
of a warm front associated with a surface low over eastern Utah
now...moving into the Oklahoma Panhandle by 00z will accelerate
development of stronger convection from southeast Nebraska/eastern
Kansas/far southwest Iowa east southeast across Missouri into
Wednesday morning. For our area, it appears the better chances for
any stronger storms will be mainly in the far west southwest and
possibly along the Iowa Missouri border from later in the evening
into the overnight hours. Though the NAM solution is farthest north,
if the model bias is removed, the stronger convection should
track moreso as the HRRR, Euro and GFS deterministic suggest. With
PWATs increasing to 1.25 to 1.5 inches by 00z west and southwest
of our area; then to 1.75 to 2 inches by 06z mainly south of Iowa,
there will be some risk of efficient rainfall in the southwest and
far south tonight as warm cloud depths increase to 11.4 kft by
06z. With the bulk of the forcing mainly over Missouri, rainfall
total will be shaded higher in the south with potentially up to 1
to 2 inches along the border back toward west central Iowa compared
to a quarter to half inch north to the I80 corridor and less farther
north. Prior to the arrival of the storms later today, temperatures
should warm to the upper 70s in the south where some sun will be
present yet into the afternoon, while the north will be subject to
more clouds this morning and afternoon which should keep temperatures
down to the lower 70s today. Tonight with more widespread shower
and thunderstorm coverage, lows will fall to the lower 50s north
to the lower 60s in the south. Wednesday will see a return to
milder temperatures in the north/northwest as sunshine returns
during the afternoon once the first wave pulls east of the area.
Elsewhere, highs will be similar to Tuesday with mid to upper 70s
in the south.


.LONG TERM.../Wednesday Night through Monday/
Issued at 245 AM CDT Tue May 17 2022

Confidence: Medium

The Canadian portion of the western trough that is present early
this morning will finally edge southeast into northern Minnesota
with time by 00z Thursday. With a slightly more zonal fetch from
the Rockies east into Iowa, warm air advection will once again
begin to push H850 temperatures back into the mid to upper teens
by Thursday afternoon. At the same time, more shortwave energy
over western Canada will drop into the Northern Plains by 12z
Thursday and further amplify southerly flow into the Western Plains
with increasing moisture flow from the Southern Plains and the
Gulf. After lows in the 50s Wednesday night, the strong warm air
advection Thursday should push afternoon highs into the upper 80s
to around 90 south while the north warms to the lower to mid 80s.
By 18z Thursday, much of the area will be back in PWATs of 1.25
inches with moisture convergence into the late evening hours increasing
an area of PWATs of 1.5 to 1.75 over the southeast half of the
forecast area. There remains large variation with regard to warming
aloft between the GFS and Euro. The GFS suggests little cap in
the afternoon hours and CAPE increasing to 3500 to 4000 j/kg...and
has some storms firing along the approaching boundary in northwest
Iowa. Meanwhile the Euro is much warmer at H700 with temperatures
increasing into the evening hours to +10 to +12C over the region
ahead of the boundary. The one consistency for both models is that
the better bulk shear will reside behind the boundary. This should
help limit the extent of any severe storms over the region and
likely promote storms with more efficient rainfall. Questions
remain as to what area(s) would have the better chances for any
storms into Thursday evening and overnight into Friday morning.
Our current forecast has quite the widespread PoP chances over the
region, but that may need further refinement in the next couple
of days. Once the cool front moves through into Friday morning, a
more fall like airmass will return to the region from Friday into
the weekend. Northerly flow returns to much of the region with
H850 temperatures falling back into the single digits by late
Friday. Farther north in Minnesota, it even looks cold enough to
snow again in the northern portion of the state with another weak
short wave tracking through the Arrowhead region. As the front
hangs up to our east Saturday morning, some lingering showers may
still be present in eastern sections prior to moving out of Iowa.
Highs Saturday into Sunday will only reach the 50s to lower 60s on
Sunday with lows in the 40s Friday night and 30s and 40s Saturday
night. The cool Canadian high responsible for the weekend chill
will move east by Monday with some return flow from the west.
Combined with a weak push of moisture from the Southern Plains,
more showers are expected to break out into the region by Monday
with highs remaining in the 60s.


.AVIATION.../For the 12Z TAFS through 12Z Wednesday morning/
Issued at 621 AM CDT Tue May 17 2022

Few if any showers through 18z. Increase in coverage 20-00z then
more widespread coverage 00z through 12z. Confidence on location
and timing still needs some refinement, so have gone with VC
wording for now. Cigs will lower to MVFR/IFR mainly aft 00z west
overspreading region by 08z. Ambient wind generally under 12kt.




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