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Area forecast discussion 
National Weather Service Des Moines Iowa
537 am CST Tuesday Jan 27 2015

Short term /today/...
issued at 355 am CST Tuesday Jan 27 2015

Main concerns today will be temperatures and cloud cover through the
morning hours...and perhaps a portion of the afternoon east. To
varying degrees...models all hint at some clouds moving southeast
along the eastern edge of the h700 ridge caught in the northwest
flow early today. However...none of the models is doing very well.
The hrrr and nmm WRF may have a slightly better handle on the
situation. Examining the upper level h700 ridge through the day
today shows that it is likely to continue building east through the
afternoon hours...ending any cyclonic portion of circulation which
is associated with the leftover system from yesterday. At 00z Tuesday
the h850 analysis showed a pronounced wave tracking through eastern
Iowa back north toward Minneapolis...but northwest of there the flow
becomes more anticyclonic over the eastern Dakotas. Another upstream
trough is already developing with an attendant warm front over the
western Dakotas. Some clouds over Saskatchewan and Manitoba may
track southeast toward the region this afternoon but likely only
affect the far eastern counties with middle to high level clouds. Based
on clouds and later trajectories...cooler highs will be had over the
northeast half of the forecast area while the southwest half will
remain mild for January...but not as warm as yesterday. With a
rather weak ridge axis passing over the area today winds will remain
light. Highs will range from the upper 30s northeast to the upper
40s southwest.

Long term /tonight through Monday/...
issued at 355 am CST Tuesday Jan 27 2015

Am expecting maximum temperatures for Wednesday to be very similar to monday's highs.
A warm front will cut across the middle of the County Warning Area...leading to a
notable temperature gradient...from northwest to southeast. Low 40s expected in NE County Warning Area
and upper 50s in SW County Warning Area. NAM more aggressive with low level cloud
cover and thus is a cool outlier. NAM also still thinks there is
snow across the County Warning Area so am discarding NAM solution.

A very fast moving shortwave to impact County Warning Area Thursday morning. Through
water vapor imagery...this shortwave can be easily picked up coming
onto the California coast at 06z Tuesday. Models initializing well with
placement of this low as it comes onshore.

Additionally...models converging on solution of putting the wave
directly over central Iowa at 06z Thursday. There is a sizable dry air
pocket from around 850mb to 500mb which will prevent cloud
thickness from exceeding 5 kft and...subsequently...keep this
system from producing anything beyond light precipitation. Have bumped
up temperatures from pvs shift. Kmcw to kest GFS soundings support mainly
dz/-ra...with a very narrow window of -ip to -sn possible from
around 09z to 12z. Profile during this time is below zero until
near surface...which is not enough warm layer depth to fully melt all
snow flakes. By 12z Thursday...this low will be over northern Illinois
and strong cold air advection plus subsidence will be advected into our County Warning Area.
Though sunshine expected...daytime temperatures to only warm around 3 to
5 degrees from overnight mins due to cold air advection.

Thursday's winds also Worth mentioning. Models showing 25 knots to 30
knots winds right off the deck...approaching 35 knots to 40 knots to the top
of the mixed layer. Surface pressure gradient of 3mb/100km to support
strong winds at the surface. In all...may approach Wind Advisory
criteria starting early Thursday am and headline may be warranted if this
solution holds. A strong surface high will move through the Midwest
Friday...keeping Friday quiet and helping to decouple winds heading
into Saturday.

For this weekend...the 00z runs are holding as they are showing an
Alberta clipper type system looks to pass through the Great Lakes
region. There will be an attendant cold front to this low which is
being prognosticated to propagate through the County Warning Area Sunday morning. The
timing has changed some from the 12z Monday model runs to the 00z Tuesday
runs...with the Euro and GFS dropping back frontal passage until around 06z
to 12z sun. Models saturate the skies from top down in advance of
this system...starting Saturday morning. Pushing timing back seems
reasonable...as low levels not saturated until after 00z sun. Thus
have pushed back probability of precipitation slightly. Sounding profile completely
saturated and well below freezing Sunday morning. Have begun to
nudge Sunday morning probability of precipitation upward...with further increases
certainly possible if the model solutions continue to hold.

The past few model runs have certainly been trending downward with
temperatures behind this front. Ensembles have 850 mb temperatures down to
the -12c to -15c range across the County Warning Area. Single digit temperatures
appear very possible Monday and Tuesday morning.

&&

Aviation...27/12z
issued at 537 am CST Tuesday Jan 27 2015

Main concerns through the period are low clouds. Clouds trapped
within a weak upper level ridge continue to drift slowly southwest
into the northeast and northern portions of the forecast area at
1130z. Meanwhile the upper level h700 ridge is expected to slowly
drift east...forcing the deck of IFR/MVFR conditions to slide east
as well. The rap and 06z GFS suggest the deck will hold on longer
..especially northeast at kmcw and Kalo. Have lengthened cloud
residence time through 18z with gradual scattered conditions returning
after 18z there. At this time...will continue VFR conditions at kdsm
and kotm with brief period of MVFR ceilings at kfod between 15 and
17z. Remainder of period stronger southeast flow returns with 12kt
sustained and gusts after 07z west and north sites. /Rev

&&

Dmx watches/warnings/advisories...
none.
&&

$$

Short term...rev
long term...kotenberg
aviation...rev

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