Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
641 PM CDT Tuesday Mar 11 2014
Short term /tonight/...
issued at 330 PM CDT Tuesday Mar 11 2014
Tonight...forecast concern focused on deepening surface low in
Kansas that will track east across Missouri tonight into
Wednesday morning. 12z model runs are in much better agreement
than 06z runs...but the GFS/sref are slightly further north with
the heaviest snow band than NAM/ECMWF. Leaned more toward the
southern NAM/European model (ecmwf) solution...with utilizing the 11.15z hopwrf for
finer details in the pop/weather trends and snow amounts.
Significant pressure falls continue across Missouri into far
southeast Iowa this afternoon...while the tight pressure gradient in
southwest Iowa through Kansas looks to continue to cause gusty
northeast winds through this evening. With current winds across
southwest Iowa and higher confidence with winds remaining strong
through tonight as the cold air advection develops behind the surface low...leaned
toward a Wind Advisory in the southwest through 03z.
Next issue is with sharp deformation zone to develop on the backside
of the surface low late this evening into the early morning hours
Wednesday. Strong isentropic upglide along the 295 to 305k surfaces
and moisture transport pumps in precipitable water values of 0.5 to 0.75 inches
over southeast Iowa...which creeps into a few counties of the
forecast area. A cross section from northeast Missouri to central
Iowa shows impressive frontogenetical forcing and decent negative
epv in the far southeast corner of the County Warning Area. Plus...with the strong
Omega in the dendritic layer...potential for significant snowfall
rates along the Missouri border. However...the issue is timing of
rain changing to snow as the window of snow accumulations looks to
be short. Confident that with the snowfall rates will cool the
column to the wet bulb temperature and switch precipitation to all snow between
03z to 05z. As a result...increased quantitative precipitation forecast amounts along the MO/Iowa
border and thus increased snow amounts in this area. With the higher
snowfall rates and winds atop the mixed layer ranging from 3o to 35
knots...leaned toward a Winter Weather Advisory from 03-09z
Wednesday. Generally looking at a sharp 2-4 inch band in this area
and visibility restrictions.
Long term /Wednesday through Tuesday/...
issued at 330 PM CDT Tuesday Mar 11 2014
In the near term...northwest flow is forecast into the weekend.
The 12z GFS/European model (ecmwf) are in decent agreement with the mass fields
with a northwest flow shortwave moving through the upper Midwest
on Friday. Cold advection on Wednesday behind tonight/S system
with the surface high move through central Iowa by afternoon. Warm
advection gets going already Wednesday and increases on Thursday
ahead of the approaching shortwave. 850 mb temperatures are expected to
reach around 4-6 c by 00z Friday which supports rapid warmup compared
to Wednesday. The 12z GFS/ecwmf are close with respect to the
timing of a fast moving cold front which will cross central Iowa
Thursday. Given the speed and position of the wave and the dry
airmass...do not expect precipitation from the system Thursday
night and Friday...just a dry frontal passage.
For the extended...colder temperatures look in store with
northwest flow continuing with low model agreement concerning the
details. The European model (ecmwf) is colder keeping Iowa in the cold air through
the extended while the GFS has another significant warmup ahead of
another front Monday into Tuesday. Current forecast does not have
a much temperature variability as suggested by the GFS. Kept small
probability of precipitation Saturday night and again on day seven...but significant
precipitation is not expected.
Will need to watch the rivers in northern Iowa again late this
week for ice jam potential...especially on Thursday and Friday.
issued at 638 PM CDT Tuesday Mar 11 2014
Main concern will occur through this evening as system continues
to organize over Missouri. Precipitation will expand across
southern Iowa with rain and snow transitioning to all snow as cold
air pushes in from the north. However...the snow will only last a
few hours before ending shortly after midnight in the southeast.
The remainder of the area will see VFR conditions through
Wednesday although some areas of MVFR conditions will occur on
Wednesday as flurries and lower clouds pass through the state with
a weak shrotwave. Winds will be very strong from the north
overnight with some gradual weakening late tonight.
Wind Advisory until 10 PM CDT this evening for Adair-Adams-
Winter Weather Advisory from 10 PM this evening to 4 am CDT
Wednesday for Appanoose-Davis-Wayne.