Scientific Forecaster Discussion
fxus63 kdmx 161959
Area forecast discussion
National Weather Service Des Moines Iowa
259 PM CDT Mon Sep 16 2019
Discussion.../tonight through Monday/
issued at 258 PM CDT Mon Sep 16 2019
Surface analysis showing some kind of weak boundary across Iowa with
southerly flow west into central and southeast flow east into
central. Dewpoints range from the low to mid 60s east with upper
60s to lower 70s west. Overall convergence along this boundary is
weak with surface flow at 5 to 10 kts and none of the short term
models show any convection/precip so I continued the dry forecast
through the evening. Some potential for fog again tonight but
confidence is low given a more shallow moisture layer and stronger
Upper ridging in place for Tuesday but it will be shifting east as a
deep trough pushes into The Rockies/northern plains. The trough
will send a couple shortwaves riding up the backside of the ridge
but for Tuesday, this will mainly impact Minnesota and the Dakota's.
Tuesday night however, the trough advances further east, pushing the
upper ridge axis into the Great Lakes. Theta-E advection increases
across the area ahead in southwest flow across the state. Decent
forcing along a frontal boundary enters northwest Iowa around 06z
and advances across the state through 15z Wednesday but weakens as
it does so.
Expect storms to develop in this regime Tuesday night and continue
across the northwest half or so of the state into Wednesday morning.
For now the trend is for diminishing storms in the morning but a
strong instability axis sits SW to NE across Iowa through the day
and it is not inconceivable to have storms festering all day. Shear
overall is pretty weak but low level shear is decent enough to
possibly see a few strong storms with marginally severe hail.
Storms redevelop Wednesday night...primarily west of the state as
another shortwave approaches. As the shortwave lifts across Iowa
mainly late night Wednesday night into Thursday morning, the storms
will overspread Iowa but again, shear parameters are pretty weak.
Pwat's however increase to 1.8+ inches of water so heavy rainfall
would be likely especially with stronger storms.
Beyond Thursday models vary considerably with smaller details
lending to uncertainty in storm chances Thursday night and Friday.
The GFS advances the western trough a little faster than the Euro
thus pushing a shortwave into western Iowa Friday where the Euro
and Canadian keeps this further west. Friday night into Saturday
the trough deepens and develops a closed low advancing the initial
trough into Canada and driving a surface low/cold front through
Iowa. It will be another wet and stormy Saturday from the looks of
things at this point. Mid level flow and thus, deep shear will be
better in the Friday night/Saturday time frame so severe potential
will also be better. Questions remain about timing etc. So severe
threat with respect to threats/timing will be better determined once
some clarity in the models occur.
On Sunday the second trough, formed by the deepening low passes east
with additional isolated precip chances before an upper ridge builds
in. Beyond Sunday night, models are clueless ranging from a weak
ridge with shortwaves riding across Iowa with varying intensities to
another deep low dropping out of Canada. Just left the model blends
in for that period since there is no consensus whatsoever.
Aviation.../for the 18z tafs through 18z Tuesday afternoon/
issued at 1221 PM CDT Mon Sep 16 2019
Fog has burned off this morning but low cigs remain over the
northeast. These should also lift in the next hour or two. VFR
conditions expected through the evening with patchy fog likely once
again overnight into the mid morning hours. Low confidence in the
areal extent due to stronger surface flow, so I did not include in
taf's at this point. Fog may just be in low lying areas.