Scientific Forecaster Discussion
fxus63 kdmx 200450
Area forecast discussion
National Weather Service Des Moines Iowa
1150 PM CDT Thu Sep 19 2019
..updated 06 aviation...
Discussion.../tonight through Thursday/
issued at 345 PM CDT Thu Sep 19 2019
The main forecast concern was focused on saturday's severe weather
and heavy rain potential. Other concerns were the low severe
threat across the west late Friday night, continued heat and
humidity Friday, and the cooler weather coming into the state by
Tonight into Friday...large upper level ridge over the southeast
U.S. Continues to place the state within south to southwest flow
during this period. Theta-E advection and warm air advection increase late tonight
into Friday morning and looks to be enough to spawn some showers
and thunderstorms, mainly along and north of Interstate 80 to
the Minnesota border. Expecting this activity to cease by around midday
Friday over much of the County Warning Area other than some lingering storms in
the northeast. Should be another well above normal and very humid
day with the breezy south winds developing once the rain ceases.
Nudged up wind gusts a bit in the afternoon and lowered pops
considerably past 18z Friday.
Friday night into Saturday night...surface dew points remain in
the middle 60s to lower 70s and suspect these to not deviate
much until a cold front and trough move through the region late
Saturday night. Concerned with the heavy rain threat Saturday
morning along the nose of the low level jet. Deep warm-layer cloud depths of
around 4000m combined with moderately strong moisture transport
and Theta-E advection into the state from 06-18z Saturday and even
to 00z Sunday. Very high precipitable water values around 2
inches, which is well above the daily Max for this time of year
per oax sounding climatology. The focus for the heavy rain looks
mainly over the southern half of the forecast area where the
better moisture convergence sets up and corfidi vectors are nearly
oriented with the storm motion. Thus, there's a good potential
for training of storms. Confident is fairly high a Flash Flood
Watch will be needed on Saturday and is likely to be handle with
future forecast updates.
The other concern is with the severe weather Saturday into
Saturday night. Shear is strong throughout the column and veers
from the surface to around 1-2km across southern Iowa. Instability
remains the question mark as the morning convection could hinder
the atmosphere from destabilization in the afternoon hours.
Sunday into Wednesday...a cooler airmass filters into the region
by Sunday and the state finally will see below normal
temperatures for the this time of year. The NAM, European model (ecmwf) and GFS are
slow to move the convection southeast of the forecast area Sunday
morning and have pops lingering through 18z. Surface high
pressure builds into the state through Tuesday providing a nice
dry and mild period. The European model (ecmwf) and GFS suggest a weak shortwave to
cut across the region Tuesday into Wednesday, but differ
considerably with timing and location.
Aviation.../for the 06z tafs through 06z Friday night/
issued at 1147 PM CDT Thu Sep 19 2019
Main concerns will be low-end MVFR and possibly brief IFR cigs at
mcw. A few isolated showers are possible across northern and
northeastern Iowa, but coverage not expected to be very wide.
Thus will leave vcsh in at mcw and alo. Patchy fog may reduce
visibility across the north and northeast. However higher clouds
moving in from Nebraska will likely limit radiational cooling
overnight. Winds will begin to pickup Friday afternoon.