Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
543 PM CST Thursday Jan 29 2015
Short term /tonight/...
issued at 315 PM CST Thursday Jan 29 2015
Main forecast concern dealt with lingering stratus deck
and temperatures as a result of the lingering cloud cover. Leaned
toward the NAM/rap/hrrr for cloud cover and low level relative humidity this
evening. Soundings suggest the inversion continues through Friday
morning and potentially trapping the low level moisture/stratus over
central Iowa during much of the night. There is a decent break in
the cloud deck over north-central Iowa and into southern Minnesota
as of 21z...and the hrrr has somewhat picked up on this trend and
slowly erodes the stratus by around 05z across Iowa/Minnesota. Have
stratus slowly eroding during the evening hours with the northeast
slowest to clear out.
Cold air advection continues throughout much of the night before the surface ridge
slides into northern Iowa Friday morning and finally allowing the
winds to become light and variable. So any radiational cooling will
be closer to sunrise. With the temperatures remaining fairly cold
this afternoon...leaned toward the rap for lows tonight as it seemed
to have a better handle on temperature and cloud cover trends.
Long term /Friday through Thursday/...
issued at 315 PM CST Thursday Jan 29 2015
Much of the long term forecast will be dominated by broad
northwest flow and a series of waves moving through the flow.
Surface ridging will be across the state at the beginning of the
longer term tomorrow morning with Friday being the best day out of
remainder of the forecast. Warm advection will increase by later
in the day as southerly surface flow resumes and readings once
again above normal. A decent pressure gradient keeping winds up on
Friday night along with increasing cloudiness will help keep
temperatures quite warm overnight. May begin to see some light
precipitation in the far southwest toward daybreak but the bulk of
the system will come in later periods.
Moving into the weekend...all focus shifts to a shortwave topping
the western ridge and its interaction with a very moist upper low
across the southwest United States. The 12z models are beginning
to cluster around a solution of dropping the northern shortwave
southeast with sufficient amplitude to bring the moisture plume
far enough north to impact Iowa...at least the southern half or
so. Initially...a shortwave that passes through the Great Lakes
area Friday night will send a cold front southward through the
forecast area on Saturday. Meanwhile..increasing isentropic lift
and moisture advection will lead to precipitation development
across southern Iowa on Saturday morning ahead of the boundary.
The precipitation will likely be rain in far southern Iowa with
temperatures in the middle to upper 30s. However...a transition to
snow is likely by i80 or hwy30 as colder air is encountered with
the southward moving boundary. Forcing will sustain and increase
into Saturday night with decent middle level frontogenetical forcing
across the southern Iowa. Any rain in the south on Saturday will
transition to snow in the evening as cold air continues to deepen
and spread southward behind the boundary. By late Saturday night
into Sunday...decent middle level forcing from approaching southeast
moving shortwave will help snow to continue during the day.
Eventually...the forcing will shift to the east by late in the day
with the snow ending. There will be a tight gradient of snowfall
on northern side of precipitation shield and therefore the snow
amounts that accompany this system. At this point...the gradient
appears near the i80 corridor although this will likely move with
subsequent information. Given the long duration of this event...it
doesnt appear warning criteria will be met...although the
combination of winds and snow may be enough to boost US into
warning. Given the changes in model runs over the past 36
hours...will wait on any headlines but begin to highlight more in
the severe weather potential statement and other products. Amounts of around 6 inches seem
possible near the MO border with 2-5 in central Iowa...and an inch
or less across the north.
Much colder air will filter into the state for next week with a
series of shortwaves passing through the region. This will bring
the threat of snow Monday night into Tuesday with reinforcing
shots of colder air.
issued at 542 PM CST Thursday Jan 29 2015
Widespread MVFR ceilings across central Iowa will gradually diminish
overnight as VFR conditions move south out of Minnesota. A brief
period with ceilings below 2 kft may occur early this evening. Gusty
northwest winds will also diminish overnight and become south to
southwest at 10 miles per hour or less on Friday.