Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
644 am CDT Tuesday Apr 21 2015
Short term /today/...
issued at 339 am CDT Tuesday Apr 21 2015
Several nuances with the forecast today with large upper low
rotating across southern Ontario. Iowa will be in broad northwest
flow during the day with shortwave passing through the state later
today. This wave is evident in satellite imagery currently crossing
from Canada into North Dakota early this morning. There is some
light returns on radar associated with the clouds although only one
locations reporting some light snow with virga in most locations.
Soundings are initially very dry with good moistening in middle levels
with rather decent forcing and lift as the wave approaches this
afternoon. Anticipate clouds to overspread portions of central Iowa
this afternoon and may see a few isolated showers from near Denison
and Atlantic to the Oskaloosa and Ottumwa areas during the
afternoon. May see some gusty winds with these showers as well with
evaporation in dry air near surface producing strong downward moving
air in the already strong airflow across the state.
In northern Iowa...wind profiles in the late morning into middle
afternoon are rather impressive with deep mixing. Winds at top of
mixed layer from 40-50kts although its debatable as too how much can
mix toward the surface. Increasing clouds may inhibit downward
transport as well as the decent lift...although this is maximized
farther south into central and southern Iowa. It still appear
sustained winds of 30 miles per hour will be possible today across the
northeast third to half of the forecast area with gusts of 45 miles per hour.
Have included a Wind Advisory in collaboration with surrounding
sites for the strong winds.
Temperatures will be problematic as well with 850mb temperatures generally
warmer than yesterday...especially in central and southern Iowa.
However...increasing clouds and the threat of a few showers by
afternoon may limited daytime highs and any evaporative cooling from
showers will quickly squash readings. Have generally cut highs
slightly from previous forecast given the increased clouds.
Long term /tonight through Monday/...
issued at 339 am CDT Tuesday Apr 21 2015
Little has changed with the forecast for the 00z run of the forecast
models. The shortwave rounding the large upper low today will pass
through southeast Iowa this evening. Colder and drier air will
shift into Iowa for the overnight along with a north wind at 10 to
20 miles per hour which will make it feel more like late fall than early
Spring. After looking at things I do not think frost will be much
of a concern tonight. The stiffer winds remaining overnight along
with some very dry air moving in will not be conducive to widespread
For Wednesday into Thursday surface high pressure shifts over the
upper Midwest while all models depict some semblance of a ridge
moving in aloft. The Euro and Canadian models appear to be more
bullish on the ridge but in any event...more dry weather on tap and
very cool for late April. Frost is more of a concern Wednesday
night and Thursday night because winds are much lighter and
soundings are not as dry...especially Thursday night.
However...offsetting that will be increasing clouds...especially
Friday the surface high moves off to the east. Aloft the models all
bring a low out of the Desert Southwest but are all over the place
with intensity of the low and whether its a closed low as the GFS
suggests or if its more of a strong shortwave as the rest of the
models suggest. Also... the models develop a shortwave over Canada
and the northern rockies which...downstream over US...develops some
upper level short wave ridging. None of the models agree on precipitation
potential with this but it would seem that southern Iowa would have
some chance at showers and not so much for northern Iowa. This would
be in the Friday night/early Saturday time frame with upper ridging
becoming established for much of the weekend. Instability is
minimal at this point so I have removed thunder from the forecast.
Late Sunday and Monday the models all want to bring another wave
into Iowa. While location and intensity vary...western Iowa appears
to have the best chance for precipitation late Sunday pushing into eastern
Iowa Monday morning. Upper level ridging re-develops over the west
putting the area back into northwest flow but while temperatures will
remain on the cooler side...some moderation in highs will occur over
the weekend and into next week so highs will be more seasonal.
issued at 639 am CDT Tuesday Apr 21 2015
Shortwave will drop across Iowa by this afternoon spreading mainly
VFR ceilings over the taf sites. Isolated shower activity is
expected this afternoon...primarily affecting kdsm and kotm and
with some evaporative cooling with the falling precipitation...
strong and erratic winds will be possible. Otherwise...deep mixing
and strong wind profile favors strong northwest winds at all sites
with kmcw and Kalo seeing the highest winds today. Wave passes
this evening with widespread VFR conditions overnight along with
much lighter winds.
Wind Advisory from noon today to 6 PM CDT this evening for Black