Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
548 PM CST sun Mar 1 2015
Short term /tonight/...
issued at 350 PM CST sun Mar 1 2015
Little sensible weather tonight. Any effects of Dakotas/Minnesota short
wave will be past Iowa tonight with mostly clear skies anticipated.
Associated cold advection behind its passage is not significant but
another Arctic high pressure area will move through Iowa overnight.
This will still be sufficient to drop temperatures back into the lower
teens to single digits again.
Long term /Monday through Sunday/...
issued at 350 PM CST sun Mar 1 2015
Primary focus for the long term period will be on system moving
through the region late Monday through Tuesday. Warm air advection
commences behind the departing surface high on Monday with a few
light snow showers possible across northwest Iowa along the
leading edge of the Theta-E advection Monday afternoon. Profiles
rapidly saturate below 700 mb from southwest to northeast between
00 and 06z. With soundings nearly isothermal between 0 and -3 c
and surface temperatures initially below freezing...expect freezing
rain...mixing in at times with rain and snow...along and south of
Interstate 80 through 12z. This region of freezing rain shifts off
to the east central County Warning Area between 12 and 18z as the profiles in the
south rise above freezing and the precipitation switches to rain. Total
ice accumulations are expected to be less than a tenth of an inch.
Model discrepancy arises during the day on Tuesday as the surface
cyclone ejects northeastward across the state Tuesday afternoon.
The NAM is the furthest north while the ec the more southern
solution with the GFS in the middle...so leaned towards the GFS
for the thermal profile forecast. This resulted in lowering snow
ratios over the north with storm total snowfalls now only expected
to be in the one to three inch range. Dgz forcing is potent...but
narrow and short lived over northern Iowa. Would not be surprised
to see a few areas with over three inches of snow...but given the
rather uncertain location of this forcing...have shifted the
forecast towards the overall trend of lesser snowfall. Dry slot
looks to punch up across the central and southern portions of the
County Warning Area between 12 and 18z...so have lowered probability of precipitation and quantitative precipitation forecast in the south
and west. This may have to be further lowered in upcoming shifts.
While snowfall amounts over the north are not as high as
previously forecast...the wind and blowing snow threat during the
late morning and afternoon hours remains quite high. Arctic
boundary races southeastward behind the departing low with winds
veering to the northwest and increasing to 25 to 30 kts with
higher gusts. These winds look to set in behind the falling
snow...but given the fresh snowpack on top of the 4 to 6 inches
of snow that fell the previous week...confidence is increasing for
a ground blizzard over the northern County Warning Area. The peak in winds will
be between 18z and 00z with the best pressure gradient over
northern Iowa. 0 to 1 km lapse rates quickly steepen to near
adiabatic at this same time with weak convective
instability...thus have left probability of precipitation in the high chance to low
likely pop for northern Iowa with the expectation of some snow
showers developing in the cold air advection. In coordination with surrounding
offices have not hoisted any headlines at this time...which will give US
time to fine tune the location of highest threat.
Beyond Tuesday looks relatively inactive but with somewhat of a
change in temperature regime. The pattern looks very familiar at
onset with broad mean trough moving through the central Continental U.S.
Resulting in yet another iteration of Arctic high pressure. This
will likely produce another round of widespread single digit
above and below zero mins Wednesday night...even sub -10f northeast.
Moderation will commence after this point however with heights
beginning to rise as the westerlies retreat northward. This will
eventually boost temperatures back to around normal into next
weekend. The forecast in the extended remains dry with no evidence
of any substantial forcing or adequate moisture.
issued at 547 PM CST sun Mar 1 2015
Few clouds over the area tonight as ridge approaches from the
West. Ridge moves east of the region with southeast flow
returning. Middle level warm air advection with middle level ceilings by
15z over most of the area. VFR conditions expected.