Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
1049 PM CDT Tuesday Jul 7 2015
Short term /tonight/...
issued at 320 PM CDT Tuesday Jul 7 2015
Canadian high pressure will continue to influence the weather
overnight in Iowa with cool temperatures and quiet conditions.
Currently...cumulus field has overspread much of the eastern half of Iowa
with daytime heating. These clouds will dissipate rapidly this
evening as sunset approaches with clear skies for the remainder of
the evening. However...high clouds will begin to spread back into
the state from the south late tonight as system in Texas begins to
move northward. Otherwise...temperatures will begin to drop rapidly
after sunset under the clear skies and weakening winds. Anticipate
lows into the 40s in the north and possibly the Nishna valleys of
the west and have gone underneath guidance in these favored
locations. May begin to see showers approach the far south toward
daybreak but any threat from those will wait until Wednesday.
Long term /Wednesday through Tuesday/...
issued at 320 PM CDT Tuesday Jul 7 2015
High pressure will continue to push east through the day Wednesday
with low pressure system in Oklahoma beginning to lift north.
Models have continued southern trend with this system...keeping
low south of Iowa...lifting northeast across Missouri. Given
strength of retreating high and dry air expected to be in place
across most of state...agree with trend of keeping precipitation
chances further to the southeast through the day...and have
continued to cut back on probability of precipitation...keeping them confined to the
southeast in the afternoon and overnight period...with best
chances coming in the late afternoon and early evening hours.
Concern will then shift to Thursday night as boundary lifts north
from southern state. This will bring best chances for
precipitation in the extended...especially Thursday night into
Friday as boundary lifts north. Models are not in good agreement
with the timing of this system...though generally have a slower
solution...with NAM and GFS being most aggressive...and European model (ecmwf) and
Gem being slower. In addition...slower solutions keep best
precipitation south of Iowa again as system lift north. Have
opted to keep precipitation trends as is...with chances across
central and southern Iowa Thursday night into Friday...though with
slightly slower trend...may see best chances very early Friday.
Could see chances for heavy rain Thursday night with good warm
cloud depths and precipitable water values near 1.8 inches.
System will lift north with ridge beginning to build into southern
states. However...strength and location of ridge is still somewhat
uncertain in models with systems pushing east across Canada
helping to limit northern extent of ridge. This will be the main
player through the end of the extended...driving precipitation
chances and temperatures. GFS is most aggressive with ridge...and
brings in hot and humid conditions for the weekend...with guidance
indicating highs in the 90s across much of the area.
However...European model (ecmwf) and Gem keep more active pattern across central
Iowa with various waves pushing through flow...and therefore
slightly cooler temperatures. Given such uncertainty at this
length...have trended towards a blend of the two...with highs in
the upper 80s and low 90s late in the period...and occasional
precipitation chances...especially in the north.
issued at 1049 PM CDT Tuesday Jul 7 2015
Mainly VFR conditions will prevail for most of the taf period. Low
clouds may produce ceilings around fl035-045 at times overnight and
into Wednesday...but expect they will remain above fl030. By
Wednesday afternoon an area of rain showers/thunderstorms and rain will move from NE MO into
southeast Iowa and may affect otm...but at this time appears it will stay
just southeast of otm thus have not included in tafs yet. By Wednesday
evening more low clouds and br may develop...with MVFR conditions
becoming more likely after dark and toward the end of the 06z taf