Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

FXUS63 KDMX 302036

Area Forecast Discussion
National Weather Service Des Moines IA
336 PM CDT Sun Apr 30 2017

.SHORT TERM.../Tonight through Monday/
Issued at 326 PM CDT Sun Apr 30 2017

Low pressure continues to lift north through northeast Kansas and 
central Missouri this afternoon as a deep upper level low moves 
north into southeast Kansas. A broad area of intense theta-e 
advection is arriving over Iowa ahead of the system. Another wave of 
showers and a few embedded thunderstorms has developed over the 
central portion of the state in response to the theta-e advection. 
This region will continue to lift north through mid evening. The 
overall instability and mid level lapse rates have remained modest 
enough that any severe weather potential is near NIL. Gusty 
northeast winds with a few gusts over 45 mph have also developed 
this afternoon over mainly west central Iowa and in the region of 
strongest pressure gradient. The gradient will begin to weaken as 
the low pressure arrives and the wind should drop off substantially 
in vicinity of the surface low overnight. 

The main region of any convection will be closely tied to the theta-
e advection surge through mid evening. The dry slot will arrive 
though there will be pockets of enough moisture for a few showers 
within this area mixed with drizzle. Areas of fog are possible and 
will be most favored in the regions where the wind diminishes the 
most, likely just east of the low. A period of light snow is 
possible over the far northwest late tonight into Monday morning 
though not expecting any accumulations as profiles battle dry air 
intrusion throughout this period. The wind will switch from the 
northwest during the day and may approach wind advisory criteria by 
the afternoon as mixing deepens and modest cold advection develops. 
The precipitation will end from west to east through the day.

.LONG TERM.../Monday night through Sunday/
Issued at 326 PM CDT Sun Apr 30 2017

Tue AM-Wed AM...
Confidence: High

Sunshine and blue skies are expected to return to the DMX CWA on 
Tuesday. By 12z Tue, the system largely responsible for our cloudy 
and rainy past weekend will have pushed towards the Great Lakes. 
Strong pressure rises and a still somewhat tightened pressure 
gradient should keep up gusty winds at least during early Tuesday 
morning. By 18z-21z Tue, pressure gradient notably relaxes which 
should help decrease winds. Models show large-scale subsidence and 
dry air entrainment to help clear up atmos...which is further 
verified by looking at current vissat that shows distinct clearing 
behind this low. 850mb temps look to range within a few degrees of 
+0C (-1 to -2 std dev) within a weak WAA though there 
will be more blue skies than clouds, am not optimistic on sfc temps 
making it much into the 60s, if at all.

Wed PM-Thu PM...
Confidence: Medium

Our sunshine and rain-free conditions are interrupted by another 
wave slated to pass through the upper Midwest on Wednesday. Models 
are coming into good agreement with ejecting an impulse off the 
upper low spinning near the Gulf of Alaska. By 18z Wed, the trough 
axis of this wave should be extending southward near the IA/NE 
border...propagating to the east as the wave makes a southeasterly 
progression. The 12z Sun GFS develops a sfc cyclone over west-
central MN to accompany this wave...and shows weak low-level
frontogenetical forcing to accompany this boundary passage. 
Timing of this boundary will be key, as available energy goes away
with daytime heating. Largely uncapped with 0-6km MUCape values 
near 500 J/kg (very minimal) and 0-6km shear approaches 20 to 25 
kts (very minimal). In all, appears enough to get thunderstorms 
over western to NW CWA, but as of now, will be pulse thunderstorms
at the very best. Models are trying to push through another band 
of precip on Thursday, but under strong NW flow, with 20+ degree 
sfc dwpt depressions, could just see 4000ft-based cu with virga.

Fri & Beyond...
Confidence: Medium

We are still on track for a major pattern change, with an omega 
block trying to set up across the CONUS by next weekend. By 
Saturday, the low discussed above should be closed and deepened, 
located over the southeastern CONUS. Meanwhile, another upper low 
should be coming onshore near California...with the ridging in 
between these two lows. As of now, it appears the DMX CWA should be 
in the dry section of the block...setting us up for a sunny & warm 
weekend. At this point, tough to justify modifying temps from 
guidance in either direction to where value can be have 
stuck with guidance. Instincts are that once the models settle on 
when we will be settled under the ridge, temps will need to be 


.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1241 PM CDT Sun Apr 30 2017

Widespread LIFR to IFR conditions will persist for much of the
forecast period. Periods with showers and a few thunderstorms can
be expected through this evening then becoming less widespread
overnight. A few areas of fog may develop overnight in vicinity of
surface low pressure moving through Iowa and where the wind will
be lightest. The wind will become gusty from the northwest on




National Weather Service Glossary of Abbreviations