Area Forecast Discussion
Issued by NWS Des Moines, IA
FXUS63 KDMX 281240
Area Forecast Discussion
National Weather Service Des Moines IA
608 AM CST Tue Feb 28 2017
.SHORT TERM.../Today through Tonight/
Issued at 415 AM CST Tue Feb 28 2017
Surface low over Southeast Nebraska will slowly lift East/Northeast
through this evening. For today the low will meander into South
Central Iowa with a warm front extending roughly to Waterloo from
the low. While we may see some periods of light rain in the morning
especially north of the warm front...instability doesn`t really
arrive until late morning/afternoon. I do however keep at least a
slight chance for thunder through the morning along the boundary.
In addition, a nice ribbon of theta-e advection sets up as well
this afternoon so a few robust storms are possible across the
Southeast this afternoon. Current SPC Outlooks keep the far
Southeast in a marginal risk for hail and given that a couple of
storms overnight pulsed up and gave pea to nickel hail to some
depth, I cannot argue with that. There will be a wide range of
temps again this afternoon. North of the boundary highs should only
be in the lower 40`s and possibly less over snow covered areas of
the far North. South of the boundary highs will be in the upper 60`s
and I would not be surprised to see a couple of 70 degree readings.
For tonight the low slides into Illinois with an upper trough
pushing into the Plains. Colder air will be pushing into the state
behind the surface low and some deformation zone precip sets up
across the Northwest quarter of the state. There will be a
changeover from rain to snow especially after midnight and some
locations across the North central and Northeast parts of the
state could see light snow accumulations over night...extending
into the Wednesday morning period. Locations East of the I-35
corridor and North of the Highway 20 corridor could see a couple
inches out of this before the low finally moves off to the East.
Temps tonight will be considerably colder...especially South. Mid
20`s Northwest the to upper 30`s Southeast will be some 5 to 10
degrees colder than what we have seen.
.LONG TERM.../Wednesday through Monday/
Issued at 345 AM CST Tue Feb 28 2017
A couple fast moving systems targeting Iowa early in the period
then trending warmer and drier into the weekend. The initial
system is showing up as a vigorous PV anomaly cluster over the
Pacific Northwest this morning. This energy will race towards the
region on Tuesday night and still will be impacting the area on
Wednesday. Low and mid-level cold advection will keep the
precipitation as mostly light snow over the north while warmer low
level temperatures will lead to a mix of rain and snow south. Some
accumulations over an inch are possible especially over the
northeast. Subsidence and drying will arrive by mid to late
morning as the upper level energy passes by. The subsidence
combined with differential heating will create good low to mid-
level lapse rates and the potential for very good mixing by
Wednesday afternoon. This will lead to breezy to windy conditions.
The good lapse rates could lead to weak instability as well by
the afternoon if enough moisture lingers though does not appear to
be much of a forcing mechanism available by that time to cause
any shower activity.
The second strong PV anomaly will arrive late Wednesday night into
Thursday. This system will have a surface low pressure reflection
moving southeast across Iowa. Still some uncertainty of the exact
track of this low with the NAM having it along the far eastern
portion of the forecast area and the EC west closer to the Des
Moines metro. This low will have a band of snow moving to the
northeast of its track. In addition, some convective instability
looks possible as low level lapse rates steepen once again. This
time though enough moisture for convective development along a
cold front sinking through the state, looks to be in place. In
addition, gusty northerly winds are likely.
High pressure will move across the state Thursday night then warm
advection will commence on Friday as return flow develops as the
high departs off to the southeast. The upper level flow during
this time will transition to a more zonal to weak ridging flow
that will help usher in another round of very mild temperatures.
Temperatures will lag on Friday due to warm advection clouds.
Stronger warm advection will arrive Friday night into Saturday.
This will help push high temperatures into the 60s south.
Temperatures will lag with the snow cover north. A better surge of
higher dew points will arrive on Sunday into Sunday night. These
dew points will accelerate the snow melt north. Highs on Sunday
should be in the 50s and 60s again with lows Sunday night possibly
remaining in the upper 30s to upper 40s. A cold front will pass
through on Monday with colder temperatures yet still mild for
early March. Light rain is possible ahead of the boundary Sunday
night into early Monday.
.AVIATION.../For the 12Z TAFS through 12Z Wednesday morning/
Issued at 608 AM CST Tue Feb 28 2017
periods of rain and isolated thunderstorms are expected over TAF
locations today. Best locations for storms will be southeast of a
ALO to LWD line. Expect mainly IFR cigs to spread over the state by
early afternoon with some breaks. Some storms far SE this aftn will
be capable of hail. IFR conditions continue tonight with rain
changing to snow everywhere but OTM. Sfc wind will be Southerly at 5-
15kts then switch to the NW and increase to 15-18kts.