Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Des Moines Iowa
627 am CDT Tuesday Sep 30 2014

..updated aviation discussion...

Short term /today/...
issued at 336 am CDT Tuesday Sep 30 2014

Today...forecast concerns dealt with timing of precipitation and maximum
temperatures today. Large and stubborn 500 mb ridge will slow the
eastward moving of the incoming 500mb vorticity maximum/low pressure system
into the state until late this afternoon. The hrrr/hopwrf/arw/nam12
all keep precipitation out of western Iowa until after 18-19z today and
then into the forecast area by around 21-22z time frame. Cut back on
the eastern extent of probability of precipitation as well as lowered probability of precipitation to chances until
after 20z across the far west. NAM/GFS/rap crl sounding have rain
falling b/T 20-23z depending the model. Plus the strong moisture
transport and Theta-E advection finally enter the western portion of
the forecast area middle to late afternoon. Higher confidence with the
delayed onset of precipitation today.

Maximum temperatures were tricky today with the lesser eastward extent of
precipitation but still anticipating an abundance of middle to high cloud
cover through much of the day. The partial strong warm air advection as southeast
flow increases throughout the day should help warm temperatures in the
central to south into the middle-upper 70s. Leaned toward the European model (ecmwf) for
temperatures today. NAM seemed way too cold but the GFS was more in
line with the European model (ecmwf) for temperatures.

Long term /tonight through Monday/...
issued at 336 am CDT Tuesday Sep 30 2014

Active weather pattern returning to the area after rather quiescent
period. For now h500 ridge remains dominant across the central
Continental U.S. With large surface high anchored over southern Canada just north
of Lake Superior. Subjective h500 analysis at 00z shows weak lobe
of heat falls moving across western Iowa and eastern NE associated
with first lobe of vorticity moving northeast out of the central
rockies. Main upper low continues to drift north-northeast out of central Colorado
with surface shortwave over northeast Colorado with attendant trough
back northwest into central Wyoming. By 00z Wednesday...the h500 short wave
will continue to move north northeast while the h500 vorticity maximum
over Nebraska interacts with a ribbon of h850 moisture extending
from the Dakotas south to Texas. This should slowly increase
thunder chances across Nebraska today with some activity reaching
the western half of the forecast area toward 00z. The advancement
of the band of showers and storms will be complicated somewhat by
the upper level ridge and weak Omega block across the central
Continental U.S.. along with that...reinforcing dry air continues to advect
southwest at the surface from the eastern Great Lakes toward Iowa
where dew points of the middle 40s from eastern Wisconsin give way to
dew points in the middle 50s to lower 60s across western Iowa.
Current trends and model guidance suggest that the precipitation
should slowly work east overnight as the dry air and persistent
northeast flow today gradually become more southeast with Wednesday
morning. Have trimmed back early period pop after 00z this evening
and increased overnight more in line with 12z/00z Euro based on
analysis and current trends. All models continue to indicate
lessening instability as the area of storms crosses the region
tonight. So...have trimmed back wording from Cat thunder to scattered
thunder and then isolated after 04z tonight west. Since this is a several
part system with the second wave projected to cross the area on
Wednesday...pop chances will again increase over the west/central
areas early and then east through the day with higher probability of precipitation
expected. Once again with somewhat limited instability thunder of
the isolated to scattered variety is expected. With the slower evolution of
the entire system and the synoptic scale models suggesting the
western most over northwest British Columbia...will
have a longer time to dig southeast through Wednesday...and should
track through Iowa Thursday and Thursday evening. This should
reduce pop chances Wednesday evening and overnight except for the south.
The current 00z Gem is following the earlier 12z Euro evolution
while the NAM is woefully too far north with its forcing Wednesday night
and Thursday. The GFS lies inbetween...but is leaning more toward
the 00z Gem and 12z Euro solutions. Given our shift to northwest
flow and more energetic fall systems...preference will be given to
the slower solutions that are digging this subsequent wave across
the central Continental U.S.. rain chances should diminish Wednesday night
into early Thursday...only to increase again as the final wave
deepens over the middle Mississippi River valley on Thursday and a
developing full fall cyclone emerges. Given the potential for a
def zone rain axis over the northwest and thunderstorms over the
east/southeast...will increase rain and thunder chances accordingly.
Outlook for severe is the next item of discussion...we continue
outlooked for a slight risk across the southwest on day2...but
this is predicated on a rather ambitious warming of the NAM
solution in the afternoon hours across much of the Central Plains.
Given the uncertainty of how much if any sun we will have during
the day Wednesday...conditions remain rather conditional during the day.
After sunset...potential for some nocturnal convection which may
lead to wind or hail threats across the far southwest/south. With
the passage of the fall cyclone Thursday afternoon...temperatures will
begin a significant downward trend. H850 temperatures on Friday with a
closed h500 low amplifying south into the region lowers our
daytime h850 temperatures to 0 to -2c. With -30c prognosticated at h500...cold
pool should cloud area over...perhaps bringing showers and
stronger mixing across the region as well during the daytime
hours. By Friday night and early Sat morning...cold air settles
into the area. Frost may be possible depending on how quickly the
winds decouple overnight. For now surface gradient looks strong enough
to support some flow through Saturday morning. By late Sat
afternoon Euro h850 temperatures recover to 2c west to -2c east. GFS is
too progressive and warming too fast. Highs Saturday will likely
slowly recover a few degrees to the middle to upper 50s with a crisp
fall day expected. A decent push of warm air advection Sunday
morning should bring a few clouds...and a milder day with highs
back toward 70. With west-northwest flow now persisting into early next
week...expect some fast moving clippers to track through the
region...keeping temperatures near seasonal norms.


issued at 622 am CDT Tuesday Sep 30 2014

IFR/LIFR ceilings will continue through much of the morning. The
latest hrrr has the low stratus burning off b/T 14-17z this
morning. Pressure gradient will continue to strengthen and allow
for the east-southeast winds to increase through the day.
Introduced MVFR ceilings this evening and then precipitation roughly 3-4
hours after ceilings move into central Iowa.


Dmx watches/warnings/advisories...none.



Short term...podrazik
long term...rev

National Weather Service Glossary of Abbreviations