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Area forecast discussion 
National Weather Service Des Moines Iowa
534 PM CST Wednesday Nov 25 2015

Short term /tonight/...
issued at 414 PM CST Wednesday Nov 25 2015

Forecast concern for tonight will continue to be precipitation
chances and precipitation type across northwestern County Warning Area. Moisture
remains slow to reach central Iowa...with increasing low level
moisture to the south and west throughout the day...though not
sufficient for -ra or dz...especially with limited forcing. While
moisture and forcing continue to slowly increase...have very low
confidence in probability of precipitation through beginning of period. Models continue to
trend slower with moisture and slightly slower with boundary
dropping southeast across County Warning Area tonight...though expect precipitation
chances to continue to increases throughout the overnight hours and
best chances will be after midnight across the area...especially the
south. Further northwest temperatures will cool behind boundary and
may see wintry mix near 12z...with rain changing to freezing
rain...then changing to sleet or snow. Expect any periods of mix to
be brief and only light accumulations. Along boundary high res
models indicate potential for fog...and current observation across northwest
support this trend. Therefore have kept fog...and may see dense fog
at times.

Long term /Thursday through Wednesday/...
issued at 414 PM CST Wednesday Nov 25 2015

Forecast is focused on the Thursday/Thursday night system that
will impact the area. Thermal profiles will be key regarding
wintry mix types and amounts. Many other factors to review as
well. A boundary will be sinking south through the period and
begin to bring colder air into the central Iowa. As the boundary
shifts will be ahead of warmer air aloft though 925 mb
temperatures do cool then mixed to the surface. Warm layer
temperatures aloft will be in the 3-5c range initially before
cooling therefore the melting of hydrometeors will occur. Question
is what is the potential for freezing rain. Surface high pressure will
be settling south over the Western Plains and the overall
trajectory for reinforcing cold advection to counter act latent
heat release and mixing should delay temperature drop to below
freezing. Road temperatures today are in the upper 40s to upper
50s with 2 inch sub surface temperatures in the low to middle 40s.
Metropolitan Road forecasts for the state do show temperatures falling
with precipitation onset then will be slow to fall below freezing
even as air temperatures fall to near to slight below freezing.
Elevated surfaces such as tops of trees and higher power lines may
accumulate faster with temperatures aloft at those locations a few
degrees cooler than at the surface. This is what makes the
forecast difficult with more ice and potential power outages if
ice accumulations aloft become more significant than expected.

Overall...the period of freezing rain looks limited to 1 to 3
hours over northern Iowa and as the precipitation nearing and
end...when rates decrease. Review of the simple ice accumulation ratios when taking into account wind and heat
Transfer...will be in the 0.50 to 0.75 range so the full quantitative precipitation forecast will
not be converted to ice. Therefore...will issue a Winter Weather
Advisory for portions of northern and west central Iowa on
Thursday for a combination of light snow and ice accumulations.
Have left portions of southwest Iowa in a Winter Storm Watch after
00z in collaboration with wpc/oax/eax for potential for one
quarter inch of ice. Farther east into central Iowa including the
Des Moines metropolitan...have left headline free with less certainty and
that it would be a 3rd period advisory. Will continue to mention
minor ice accumulations and light snow in the severe weather potential statement.

The remainder of the forecast will feature a large upper low
pressure lifting out to the southwest Continental U.S. And reach the area
around Tuesday. This pattern will continue to keep Iowa in a cool
temperature regime with light chances for rain/snow through the


issued at 534 PM CST Wednesday Nov 25 2015

Conditions will steadily deteriorate overnight as ceilings lower and
-dz/br develop and produce visibility restrictions. This is already
being observed along and behind a cold front approaching from northwest
Iowa...and as it moves slowly across Iowa these conditions will
spread across the terminals mainly by midnight tonight...except at
otm where it will be more toward sunrise Thursday. Once the
ceilings/visibilities lower they will not improve appreciably for the
remainder of the taf period. Dz will become a steadier and at
times moderate rain on Thursday...changing to freezing rain/snow toward the
end of the period at mcw/fod though there is uncertainty in the
timing of that transition. Amendments will likely be necessary
through much of Thursday and Thursday night.


issued at 326 am CST Wednesday Nov 25 2015

Still some potential for minor river flooding southeast a few days
following the rainfall...given the previous snowmelt north and the
combination of additional rainfall tonight/Thursday.


Dmx watches/warnings/advisories...
Winter Weather Advisory from noon Thursday to midnight CST
Thursday night for Audubon-Boone-Bremer-Butler-Carroll-Cerro

Winter Weather Advisory from 6 am to 9 PM CST Thursday for
Calhoun-Crawford-Emmet-Hancock-Humboldt-Kossuth-Palo Alto-

Winter Storm Watch from Thursday evening through late Thursday
night for Adair-Adams-Cass-Ringgold-Taylor-Union.



Short term...awb
long term...donavon

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