Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois
1122 PM CDT Wednesday Oct 22 2014

Synopsis...
issued at 323 PM CDT Wednesday Oct 22 2014

12z upper air analysis has and upper high over the Great Lakes. A weak low level jet
ran from the High Plains of New Mexico into Nebraska that was
pulling moisture north ahead of shortwave exiting The Rockies.
Satellite trends show the clouds in central Iowa slowly evaporating
as they move east but new clouds were developing across western Iowa.

18z surface analysis has the surface ridge from the Great Lakes into the
lower Mississippi Valley. A cold front ran from the eastern Dakotas
into northeast Colorado. Dew points were in the 30s and 40s across
the Great Lakes and Ohio Valley with a plume of dew points in the
50s and low 60s from the Southern Plains into the eastern Dakotas.

&&

Short term...(this evening through thursday)
issued at 323 PM CDT Wednesday Oct 22 2014

Quiet and dry conditions will continue across the area late this
afternoon through midnight as high pressure slowly moves into the
east Great Lakes.

After midnight...moisture and clouds will slowly move eastward from
the western half of Iowa. Forcing will increase with light rain
moving into the western third of the area by sunrise. The trends in
the models have been to slow down the arrival of the rain so the
potential is there for further slowing. If this occurs...then rain
may only be getting into the extreme northwest and west areas around
sunrise.

The prolonged period of clear skies will allow temperatures to drop
into the middle 30s in the far northeast/east areas with the
possibility of temperatures near freezing in favored low lying areas.

On Thursday...a band of rain will move across the area from west to
east. Based on the overall forcing and moisture...the threat of rain
will only be for about 6 hours at any given location with about 3-4
hours of that time with the highest potential for rain.

As such...roughly the eastern half of the area will stay dry until
late morning or early afternoon. By early afternoon...the rain will
end across the western quarter to third of the area and areas west
of the Mississippi should be dry by middle afternoon. Rain will linger
over the far eastern areas into late afternoon and end by sunset
Thursday.

Rainfall amounts will be at best a quarter of an inch over the
western third of the area. Areas east of there may see 0.1 to 0.2
inches of rain.

Long term...(thursday night through wednesday)
issued at 323 PM CDT Wednesday Oct 22 2014

Thursday night...any showers will be exiting far eastern County Warning Area around
00z or 7pm if not earlier as shortwave and attendant forcing quickly
shifts east. This will be followed by continued clearing skies.
Despite decrease in clouds... light southwesterly winds and wet
ground should prevent big drop off in temperatures with lows most places
actually warmer from Wednesday night and mainly in the range of 42-49f.

Friday and Friday night...some changes to forecast as models now
converging on scenario of warm frontal passage during the afternoon
and evening. Good low to middle level Theta-E advection with decent
low level moisture gradient with dewpoints in lower 50s to lower 60s to
aid in transient band of low and middle cloudiness overspreading the
County Warning Area on Friday and exiting west to east Friday night with the passage
of the front. Moisture is fairly shallow with dry air aloft favorable
for drizzle and some models are hinting at some very light non-measurable
amounts of precipitation. Main changes were to increase sky cover... which
may need further raising... and tweak highs down a bit especially west
of the Mississippi River where clouds to arrive sooner. May need to
further tweak highs down as model trends seem to suggest slowing of warm
front and attendant clouds. For now though have general range of highs
from middle 60s to around 70 with warmest far west/southwest where trying
to push front through and bring some decrease in clouds by middle to late
afternoon. Clouds should continue shifting east Friday night exiting
by late evening with warm frontal passage. This will be followed by
cool frontal passage overnight exiting southeast early Saturday morning.
Lows Friday night coolest northwest County Warning Area (mid 40s) to warmest southeast
County Warning Area (l50s).

Saturday through Sunday...some changes made to forecast for these periods
as well as models have continued slowing the movement of the high pressure
area over the weekend. This does not mean any change to nice weekend being
advertised... but it does mean temperatures not quite as warm with absence of
southerly winds and while cooler it will still be around normal to even
a bit above with highs mainly in the 60s. Winds will also be much less of
a factor for Sunday with the high just exiting Sunday morning... but should
see 10-20 miles per hour during afternoon but direction from east and southeast which
not favorable for much warmup. Min temperatures Saturday night have been lowered
a couple degrees but with ideal radiational cooling setup these may need to
be further adjusted downward with more widespread upper 30s possible
central and northeast with 850 mb temperatures around 7-9c. Some patchy shallow
fog will also be possible late Saturday night with steep low level inversion.

Early next week...all models support a period of wet weather for the
County Warning Area with longwave trough traversing the Midwest. The main challenge is
timing of rain chances and amounts as models offer considerable amount of
variance with strength of energy and timing. Hi-res European model (ecmwf) continues to be
slower and deeper and would suggest after some rain chances late Sunday night
northern County Warning Area ahead of warm front then mainly dry and quite mild warm sector
for Monday with highs in the 70-75f range... with rain not arriving until
later Monday night through Wednesday in the form of period of frontal rain
followed by secondary swath of moderate to locally heavy rains with closing
southern stream energy lifting through the middle Mississippi Valley. Gem and GFS
more progressive with trough and weaker with embedded energy and supportive
of Sunday night through Monday as main window of rain with mainly light
to moderate amounts then cooler/drier Tuesday into Wednesday. Forecast is
somewhat blended approach with rain chances beginning Sunday night northern
County Warning Area and entire County Warning Area Monday into Tuesday. Temperatures mild Monday with highs around 70
to l70s then trending cooler rest of period.

&&

Aviation...(for the 06z tafs through 06z Thursday night)
issued at 1121 PM CDT Wednesday Oct 22 2014

Clear skies and unlimited visibilities will last through very
early in the period...followed by increasing middle clouds as a slow
moving front arrives from the west. This front will bring rain
showers to the region Thursday. This light rain activity will
provide a mainly VFR event...with visibilities expected to remain
around 6 miles or better through the event. The best chance for
rains will be between 15z and 21z Thursday. South to southeast
winds will continue tonight and Thursday around 10 kts...but will
switch to southwest as the rain ends Thursday afternoon. Ervin



&&

Dvn watches/warnings/advisories...
Iowa...none.
Illinois...none.
MO...none.
&&

$$

Synopsis...08
short term...08
long term...05
aviation...Ervin

National Weather Service Glossary of Abbreviations