Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

000 
FXUS63 KDMX 282342
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
642 PM CDT Tue Mar 28 2017

.SHORT TERM.../Tonight through Wednesday/
Issued at 340 PM CDT Tue Mar 28 2017

An upper level closed low within a deep trough will shift East 
across the Southern Plains beginning tonight through beyond the 
Wednesday forecast period.  At the surface, a low will travel from 
Texas into Oklahoma with a warm front extending into Missouri by the 
end of the forecast period.  For Iowa, an inverted trough will nose 
into the West and South as this low travels East.  Soundings 
indicate some dry air in the mid levels to overcome but low levels 
will be saturated enough to keep the stratus going across the 
Southern half or so of Iowa.  Across the Northern third, where skies 
are currently clear or clearing, mid/high clouds will move back in 
this evening.

As for precip...given the dry air to overcome with no real forcing 
arriving in the CWA until around 12Z...I have a hard time believing 
that we will see much precip before then.  However, in that 09Z to 
12Z period, as the forcing comes in and the trough advances, the 
South and West should quickly saturate and some rain is expected 
there by 12Z.  By 18Z an area of theta-e advection works into the 
South and by then we will have deep saturation and pretty strong 
forcing in place and it should be raining across much of that area, 
and moderately at times.  It is unclear how far North the saturation 
will occur but if the entire CWA isn't saturated by 17-18Z, it 
shouldn't take much into the early afternoon to accomplish this. We 
will then see a fairly steady rain through the remainder of the 
period.  Instability works into the Southern quarter or so of the 
forecast area in the mid-afternoon and evening so I did add some 
slight chance thunder wording to the forecast there.  

I went cooler on MaxT for Wednesday then we had going.  With clouds 
and precip occurring we will not see much of a diurnal trend in 
temps.  Add to that an Easterly wind at 10 to 20 mph by late morning 
and it will be a pretty cold, wet day. 

.LONG TERM.../Wednesday night through Tuesday/
Issued at 340 PM CDT Tue Mar 28 2017

20z water vapor imagery shows a longwave trough over Arizona and New 
Mexico providing strong lifting for extensive thunderstorm 
development over the southern Plains this afternoon. By Wednesday 
evening, this trough will have swung eastward over the southern 
Plains with rain scattered to widespread across central Iowa in 
the warm conveyor belt and deformation zone. Over southern Iowa, 
elevated instability will be rather limited, but may be able to 
get a few peals of thunder, especially early in the evening. 
Farther north, forecast soundings over the far north indicate 
thermal profiles just above freezing, though isolated pockets of 
brief rain/snow mix may occur where heavier precipitation falls. 
As the low pressure passes near or north of St. Louis and into the
Ohio Valley on Thursday, rain will end by midday over western 
Iowa and by mid-afternoon over the eastern part of the CWA per a 
GFS/ECMWF/CMC solution. The NAM is slower and holds precipitation 
into Friday, which seems rather unlikely. Have placed temperatures
on the cooler side of MOS guidance due to the clouds and 
rainfall on Thursday

Behind the departing trough, ridging builds into the region. 
Temperatures from the end of this week into early next week will be 
at or slightly above normal. A new trough of low pressure will enter 
the western US and dig into the southwestern US, which will bring 
chances of mainly rain later Saturday and moreso Sunday. As this 
system passes south of Iowa on Monday, most of the rain will be 
focused south of the state, though it may not be an entirely dry 
day. Another system in relatively fast moving northwest flow will
bring another chance of precipitation Tuesday.

&&

.AVIATION.../For the 00Z TAFS through 00Z Wednesday evening/
Issued at 641 PM CDT Tue Mar 28 2017

MVFR stratus is in place roughly across the southern half of Iowa
at 00z with mostly clear/VFR conditions north. These conditions
should not change much initially, but eventually the stratus will
begin advancing northward again early Wednesday reaching all 
sites with MVFR or IFR stratus. This will be accompanied by 
showers crossing the MO River as well with visibility also
contributing to those restrictions.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

National Weather Service Glossary of Abbreviations