Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
352 PM CDT Friday Sep 19 2014
Short term /tonight/...
issued at 351 PM CDT Friday Sep 19 2014
Strong mixing will persist through the late afternoon
hours before decoupling by around 23z to 00z tonight. Pressure
gradient should remain strong enough to keeps sustained winds at or above 12
knots before the weak cold front pushes across the forecast area
late this evening into the overnight hours. Very limited moisture
and forcing with the cold front...but latest hrrr/hopwrf bring the
best precipitation chances from 03z to 09z Saturday. Have highest probability of precipitation
going during this time frame. Low confidence with the severe weather
potential but cannot completely rule an isolated pulse severe storm
with some elevated instability available.
Long term /Saturday through Friday/...
issued at 351 PM CDT Friday Sep 19 2014
Thunderstorms trying to hang on for Saturday afternoon primarily
south and east of the Des Moines metropolitan area through the Iowa
Missouri border. Cold frontal passage main trigger for any storm development.
Problem is that the cold front is looking fairly weak and not a lot
of frontogenetical forcing indicated during the daytime. Models
showing bullseye of cape of 2000 j/kg over southern Iowa with around
30-40kts of 0-6km bulk shear. Models also showing this as a
looking at sounding profiles...any thunderstorms would likely be
elevated. Though precipitable waters are +1 to +2 Standard dev for early
Saturday...sounding profile looking very unsaturated. Aloft...the
right exit region of the jet streak will be right behind this
front so this will not support lift. In all...expecting storms
south...but severe threat will be minimal at best.
Models coming into good agreement with showing a sizable ridge
coming off The Rockies through the Dakotas Sunday into Monday. This
will keep the dmx County Warning Area in cold air advection through Monday morning. Strong
subsidence with this ridge should clear skies out for Sunday and
Monday mornings. 850mb temperatures appear to be in the +8c to +10c range
for the morning hours. Which should be supportive of lows in the middle
40s to low 50s. Ridge to propagate through Iowa Monday
afternoon...placing the County Warning Area in southerly flow late Monday into
Tuesday. This will allow for some slight warm air advection for Tuesday and Wednesday...along
with a return of probability of precipitation.
Wednesday into Thursday...models showing a decent slug of moisture
transport being advected into Iowa during the morning by 25-30kt
winds at 850mb. GFS trying to push a vorticity maximum through during this
time frame. GFS also showing decent middle to upper level q-vector
convergence to help generate lift. At 1 to 1.25 inches...precipitable water
values return to 0 to +1 Standard dev above normal. Enough support to
keep sounding profile saturated aloft. Putting it all
together...will buy into probability of precipitation for Wednesday am into the PM. Question is
how long to sustain them as the progression of this pattern
becomes murky from Thursday into the weekend.
00z models hinted at an Omega block setting up across the Continental U.S..
Euro and GFS 500mb analysis both had pattern trying to organize and
are placing Iowa on the western edge of the high...which is where
the deformation zone is typically located. If this setup becomes
more organized...could be a prolonged period of scattered precipitation
into and throughout the weekend. The 12z runs retrograded this
shortwave/trough and dissolved it by the weekend...which seems
suspect. Hopefully tomorrow/S model runs will be able to offer a
better solution. Regardless...it appears 850mb temperatures will be in the
+12c to +14c range which would suggest temperatures near to
slightly above normal for next week.
issued at 1234 PM CDT Friday Sep 19 2014
VFR conditions anticipated through the taf period with lesser
potential for MVFR cigs/vis. Weak front will move through Iowa b/T
06z to 10z tonight and kept thunderstorms in the vicinity mentioned but lower confidence
with any widespread or lengthy precipitation potential.