Scientific Forecaster Discussion
fxus63 kdmx 232031
Area forecast discussion
National Weather Service Des Moines Iowa
331 PM CDT Mon Oct 23 2017
Short term.../tonight through Tuesday/
issued at 330 PM CDT Mon Oct 23 2017
The primary concern into Tuesday remains strong winds. Water vapor
imagery and objective model guidance nicely shows a short wave/pv
anomaly along the western Iowa/MO border at 19z. Any precip produced
by this feature looks to be driven by high based DPVA and
frontogenetical forcing and should shortly exit the forecast area.
Winds may continue to surge a bit into the early evening hours, even
beyond sunset, with the objective analysis still showing the surface
isallobaric Max upstream from South Dakota into Minnesota. The sustained winds may
diminish slightly, but rap, NAM and GFS soundings with projected
mixing all suggest gusts may reach 40+kts across northern Iowa with
lesser gusts farther south.
After a minor lull into the late evening hours, forecast soundings
again show strong mixing and gust potential into the lower 40s north
06z-15z, which is the same time the mslp gradient strengthens.
Consensus MOS guidance depicts sustained speeds of 25 to 35 mph
through the early afternoon hours, which is also supported by sref
mean winds and 25+kt probabilities. NAM and GFS .5-1km layer winds
also depict gust potential in at least the lower 40s. These factors
have all led to medium to high confidence in advisory criteria wind
conditions. The soundings and winds aloft suggest the advisory may
be a tad long, but peak wind potential likely in the 09z-18 window.
The mixing may be tempered slightly however as deep forcing
increases lift on the back side of the Great Lakes cyclone
tomorrow, which may brush eastern sections with light rain early
Tuesday. Tuesday will likely be at or near the coolest daytime
temperatures of the season to date with highs only anticipated to
reach the middle 40s to lower 50s.
Long term.../Tuesday night through Monday/
issued at 320 PM CDT Mon Oct 23 2017
Long range forecast begins Tuesday night with steadily relaxing
winds as a deep surface low pushes northeast from the Great Lakes
region. Quick round of warm air advection Wednesday as a thermal ridge swiftly
moves east across the Central Plains. This will lead to at least
one day of near to above normal highs in the low to mid 60s. A
robust pv anomaly dives southeast out of Canada late Wednesday and
digs into the northern plains, pushing a cold front through the
County Warning Area Thursday. Strong cold air advection and a tight pressure gradient behind the
front will create blustery conditions. BUFKIT soundings indicate
35-40+ kt winds at the top of the mixed layer, so it's reasonable
to assume that we will approach Wind Advisory criteria. Non-
diurnal temp trends are also possible, but that will be highly
dependent on frontal passage timing. GFS/NAM bring the front
through Thursday morning with falling temps through the afternoon.
Euro/Canadian are ~12 hours slower with mild afternoon temps.
This has the temp forecast for Thursday in flux and will need to
be refined once timing issues are resolved. Not expecting any
precip associated with the frontal passage as fronts earlier in
the week scour out most available low level moisture.
Little change to the Friday/Saturday forecast. The trough digs south
and becomes highly amplified over the central Continental U.S.. highs only in
the 30s/40s remain on track for Friday and blustery northwest winds
will make temperatures feel even cooler. Still a chance we could see
some light rain/snow in Iowa in this time frame. The best moisture
and lift will likely stay north of the surface low tracking across
southern Minnesota. 12z model runs advertise light snow as far south as
dsm Saturday morning, but no mention was added to the forecast due
questions regarding dry air and run-to-run inconsistencies. Below
normal temperatures continue through the weekend as the upper
level low slowly pushes east. A hard freeze and end to the growing
season may occur Fri/Sat nights as temps drop into the 20s.
Aviation.../for the 18z tafs through 18z Tuesday afternoon/
issued at 1253 PM CDT Mon Oct 23 2017
The primary concern through the period will be gusty northwest
winds and associated cross wind effects, especially at kfod, kmcw
and Kalo. Gusts may reach well through the 30s at times in these
locations, mainly into early this evening and then again early
Tuesday. At least MVFR stratus may also cross the MS river into Iowa early
Tuesday morning and continue past sunrise before returning to VFR
again toward the end of the valid period or just beyond.
Wind Advisory until 7 PM CDT this evening for iaz057-070.
Wind Advisory from 7 PM this evening to 6 PM CDT Tuesday for
Wind Advisory until 6 PM CDT Tuesday for iaz004-015-023-033-034-